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Editorial

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9 pages, 186 KiB

Open AccessEditorial

Recent Advances in Fluid Flow in Fractured Porous Media

by Richeng Liu, Sha Lou and Yujing Jiang

Processes 2019, 7(5), 255; https://doi.org/10.3390/pr7050255 - 2 May 2019

Cited by 3 | Viewed by 2799

Abstract

The fluid flows through both the fractures and pores in deep-seated rock masses, which is of special importance for the performance of underground facilities such as groundwater use and nuclear waste disposal [...] Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

9 pages, 213 KiB

Open AccessEditorial

Special Issue: Fluid Flow in Fractured Porous Media

by Richeng Liu and Yujing Jiang

Processes 2018, 6(10), 178; https://doi.org/10.3390/pr6100178 - 1 Oct 2018

Cited by 2 | Viewed by 3556

Abstract

The fluid flow in fractured porous media plays a significant role in the characteristic/assessment of deep underground reservoirs such as CO₂ sequestration [1–3], enhanced oil recovery [4,5] and geothermal energy development [...] Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

Research

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16 pages, 5564 KiB

Open AccessArticle

Effect of Nitric Acid Modification on Characteristics and Adsorption Properties of Lignite

by Bo Huang, Guowei Liu, Penghui Wang, Xiang Zhao and Hongxiang Xu

Processes 2019, 7(3), 167; https://doi.org/10.3390/pr7030167 - 22 Mar 2019

Cited by 60 | Viewed by 6109

Abstract

The objective of this research was to explore the changes of the pore structure and surface properties of nitric-modified lignite and base the adsorption performance on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, several lignite samples [...] Read more.

The objective of this research was to explore the changes of the pore structure and surface properties of nitric-modified lignite and base the adsorption performance on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, several lignite samples were treated with different concentrations of nitric acid in order to get different pore structure and surface chemistry adsorbent levels. A common heavy metal ion contaminant in water, Pb²⁺, served as an adsorbate probe to demonstrate the change of modified lignite adsorption properties. The pore structure and surface properties of lignite samples before and after modification were characterized by static nitrogen adsorption, X-ray diffraction, Scanning electron microscope, Fourier transform infrared spectroscopy, zeta potential, and X-ray photoelectron spectroscopy. The experimental results showed that nitric acid modification can increase the ability of lignite to adsorb Pb²⁺. The adsorption amount of Pb²⁺ increased from 14.45 mg·g⁻¹ to 30.68 mg·g⁻¹. Nitric acid reacted with inorganic mineral impurities such as iron dolomite in lignite and organic components in coal, which caused an increase in pore size and a decrease in specific surface areas. A hydrophilic adsorbent surface more effectively removed Pb²⁺ from aqueous solution. Nitric acid treatment increased the content of polar oxygen-containing functional groups such as hydroxyl, carbonyl, and carboxyl groups on the surface of lignite. Treatment introduced nitro groups, which enhanced the negative electrical properties, the polarity of the lignite surface, and its metal ion adsorption performance, a result that can be explained by enhanced water adsorption on hydrophilic surfaces. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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24 pages, 37150 KiB

Open AccessArticle

DEM Investigation of the Influence of Minerals on Crack Patterns and Mechanical Properties of Red Mudstone

by Shuai Zhang, Dongsheng Zhang, Qiang Zhao, Mingbo Chi, Wei Zhang and Wei Yu

Processes 2019, 7(3), 162; https://doi.org/10.3390/pr7030162 - 15 Mar 2019

Cited by 10 | Viewed by 3635

Abstract

Rocks are natural heterogeneous materials. It is common for a rock to have several kinds of minerals, which will have a significant effect on its mechanical behavior. The purpose of the numerical simulation study in this paper is to explore the effects of [...] Read more.

Rocks are natural heterogeneous materials. It is common for a rock to have several kinds of minerals, which will have a significant effect on its mechanical behavior. The purpose of the numerical simulation study in this paper is to explore the effects of minerals on the crack patterns and mechanical properties of rocks. First, the corresponding calculation model is established by using the discrete element method (DEM), whereby the mechanical parameters of the blocks and joints in the Tyson polygon procedure are fitted with the rock properties obtained in the laboratory. Then, various combination models of different mineral sizes and ratios are established to study the effects of mineral size, position, and ratio on the fracture distribution and mechanical properties of rock samples. The results indicate that with increased circle size of the center mineral and the mineral ratio, the elastic modulus and uniaxial compression strength (UCS) of the model gradually increase. The drop degree of post-peak stress decreases, and the integrity and bearing capacity increase. It is found that there is a quartic polynomial relationship between elastic modulus and mineral circle radius, with R² ≥ 0.94. The minerals located in the crack propagation path will effectively block the crack and change the propagation direction. When the mineral position is close to the model boundaries, especially the lateral boundaries, it has a significant influence on the crack initiation position, causing crack initiation to occur at the upper boundary of the mineral circle and propagate to the middle of the model. With increased mineral percentage and small-size mineral circle proportion, the depth of extension of the crack from boundary to center is reduced, the crack has wide development in the boundary area, the number of central cracks in the rock specimen decreases, and the degree of fragmentation decreases. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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23 pages, 8001 KiB

Open AccessArticle

Experimental Development of Coal-Like Material with Solid-Gas Coupling for Quantitative Simulation Tests of Coal and Gas Outburst Occurred in Soft Coal Seams

by Xingkai Wang, Wenbing Xie, Zhili Su and Qingteng Tang

Processes 2019, 7(3), 155; https://doi.org/10.3390/pr7030155 - 13 Mar 2019

Cited by 9 | Viewed by 3536

Abstract

Solid-gas coupling coal-like materials are essential for simulating coal and gas outbursts and the long-term safety study of CO₂ sequestration in coal. However, reported materials still differ substantially from natural coal in mechanical, deformation and gaseous properties; the latter two aspects are [...] Read more.

Solid-gas coupling coal-like materials are essential for simulating coal and gas outbursts and the long-term safety study of CO₂ sequestration in coal. However, reported materials still differ substantially from natural coal in mechanical, deformation and gaseous properties; the latter two aspects are common not considered. There is a lack of a definite and quantitative preparation method of coal-like materials with high similarity for future reference. Here, 25 groups of raw material ratios were designed in the orthogonal experiment using uniaxial compression, shearing and adsorption/desorption tests. Experiment results indicated that the coal-like materials were highly similar to soft coals in properties mentioned above. And range analysis revealed the key influencing factors of each mechanical index. The gypsum/petrolatum ratio controls the density, compressive strength, elastic modulus, cohesion and deformation characteristic. The coarse/fine coal powder (1–2 and 0–0.5 mm) controls the internal friction angle and is the secondary controlling factor for compressive strength and elastic modulus. The effect of coal particle size on the sample strength was studied using scanning electron microscope (SEM). When the gypsum/petrolatum ratio increased, the deformation characteristics changed from ductile to brittle. The different failure modes in the samples were revealed. The coal powder content is a key in the gas adsorption/desorption properties and an empirical formula for estimating the adsorption capacity was established. Based on the range analysis of experimental results, a multiple linear regression model of the mechanical parameters and their key influencing factors was obtained. Finally, a composition closely resembling the natural coal was determined, which differs by only 0.47–7.41% in all parameters except porosity (11.76%). Possible improvements and extension to similar materials are discussed. The findings of this study can help for better understanding of coal and gas outburst mechanism and stability of CO₂ sequestration in soft coal seams. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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26 pages, 7441 KiB

Open AccessArticle

Experimental Investigation of Pore Structure and Movable Fluid Traits in Tight Sandstone

by Dengke Liu, Wei Sun and Dazhong Ren

Processes 2019, 7(3), 149; https://doi.org/10.3390/pr7030149 - 8 Mar 2019

Cited by 21 | Viewed by 4427

Abstract

Whether the variation of pore structures and movable fluid characteristics enhance, deteriorate, or have no influence on reservoir quality has long been disputed, despite their considerable implications for hydrocarbon development in tight sandstone reservoirs. To elucidate these relationships, this study systematically analyzes pore [...] Read more.

Whether the variation of pore structures and movable fluid characteristics enhance, deteriorate, or have no influence on reservoir quality has long been disputed, despite their considerable implications for hydrocarbon development in tight sandstone reservoirs. To elucidate these relationships, this study systematically analyzes pore structures qualitatively and quantitatively by various kinds of direct observations, indirect methods, and imaging simulations. We found that the uncertainty of porosity measurements, caused by the complex pore-throat structure, needs to be eliminated to accurately characterize reservoir quality. Bulk water was more easily removed, while surface water tended to be retained in the pores, and the heterogeneity of pore structures was caused by the abundance of tiny pores. The rates of water saturation reduction in macropores are faster than those for tiny pores, and sandstones with poor reservoir quality show no marked descending of lower limits of movable pore radius, indicating that the movable fluid would advance exempted from the larger pores. This study suggests that the deterioration of reservoir quality is strongly affected by the reduction of larger pores and the aqueous phases tended to remain in the tiny pores in the forms of surface water. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 6457 KiB

Open AccessArticle

Development of 3D Finite Element Method for Non-Aqueous Phase Liquid Transport in Groundwater as Well as Verification

by Wei Yu and Hong Li

Processes 2019, 7(2), 116; https://doi.org/10.3390/pr7020116 - 25 Feb 2019

Cited by 2 | Viewed by 4369

Abstract

Groundwater contamination previously occurred at a broad range of locations in present-day China. There are thousands of kinds of contaminants which can be divided into soluble and insoluble categories in groundwater. In recent years, the non-aqueous phase liquid (NAPL) pollution that belongs to [...] Read more.

Groundwater contamination previously occurred at a broad range of locations in present-day China. There are thousands of kinds of contaminants which can be divided into soluble and insoluble categories in groundwater. In recent years, the non-aqueous phase liquid (NAPL) pollution that belongs to the multi-phase seepage flow phenomenon has become an increasingly prominent topic due to the challenge brought by groundwater purification and its treatment. Migrating with seepage flow and moving into the potable water sources, these contaminants directly endanger people’s health. Therefore, it is necessary to research how these contaminants not only migrate, but also are then accordingly remedied. First, as an analysis means, an effective numerical method is necessary to be built. A three-dimensional finite element method program for analyzing two-phase flow in porous media, which can be applied to the immiscible contaminant transport problem in subsurface flow has been developed in this paper. The fundamental theory and numerical discretization formulations are elaborated. The numerical difficulty brought about by the distinct non-linearity of the temporal evolution of saturation-dependent variables is overcome by the mixed-form formulation. The effectiveness of simultaneous solution (SS) method and its improvement in efficiency are explained. Finally, two computational examples are given for verifying the correctness and demonstrating the preliminary applicability. In addition, the function of two-phase immiscible flow, especially in Fast Lagrangian Analysis of Continua (FLAC) is used to simulate the same examples and the results are compared to further verify the correctness of the numerical development. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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17 pages, 23504 KiB

Open AccessArticle

The Application of a Three-Dimensional Deterministic Model in the Study of Debris Flow Prediction Based on the Rainfall-Unstable Soil Coupling Mechanism

by Shuangshuang Qiao, Shengwu Qin, Junjun Chen, Xiuyu Hu and Zhongjun Ma

Processes 2019, 7(2), 99; https://doi.org/10.3390/pr7020099 - 15 Feb 2019

Cited by 21 | Viewed by 4583

Abstract

As debris flow is one of the most destructive natural disasters in many parts of the world, the assessment and management of future debris flows with proper forecasting methods are crucial for the safety of life and property. So increasing attention has been [...] Read more.

As debris flow is one of the most destructive natural disasters in many parts of the world, the assessment and management of future debris flows with proper forecasting methods are crucial for the safety of life and property. So increasing attention has been paid to the forecasting methods on debris flows. A debris flow forecasting method based on the rainfall-unstable soil coupling mechanism (R-USCM) is presented in the current study. This method is based on the debris flow formation mechanism. The density of sediment is introduced as an evaluation index to determine the susceptibility of debris flow occurrence. The forecasting method includes two phases: (1) rainfall and soil coupling and (2) runoff and unstable soil coupling. Scoops3D, a three-dimensional (3D) model for analyzing slope stability, was introduced into the debris flow forecasting method. In order to test the forecasting accuracy of this method, Jiaohe County was selected as a research area, and the serious debris flow disasters attributed to strong rainfall on 20 July 2017 were taken as the research case. By comparing the forecasting results with the debris flow distribution map for Jiaohe County, the method based on the R-USCM is feasible for forecasting debris flows at the regional scale. The application of the Scoops3D model can more reasonably analyze the slope stability than the traditional two dimensional (2D) method and improve the forecasting ability of debris flows. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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17 pages, 5861 KiB

Open AccessArticle

Model for the Patterns of Salt-Spray-Induced Chloride Corrosion in Concretes under Coupling Action of Cyclic Loading and Salt Spray Corrosion

by Ruixue Liu, Huiguang Yin, Lianying Zhang, Bing Li and Xianbiao Mao

Processes 2019, 7(2), 84; https://doi.org/10.3390/pr7020084 - 7 Feb 2019

Cited by 5 | Viewed by 4095

Abstract

In this study, the patterns of chloride ion erosion of unsaturated concrete subjected to the coupling action of cyclic loading and salt spray corrosion were experimentally studied, and Fick’s Second Law was used to fit the variation patterns of chloride concentration to obtain [...] Read more.

In this study, the patterns of chloride ion erosion of unsaturated concrete subjected to the coupling action of cyclic loading and salt spray corrosion were experimentally studied, and Fick’s Second Law was used to fit the variation patterns of chloride concentration to obtain the chloride diffusion coefficient. Accordingly, we have established a mathematical model that describes chloride transport in unsaturated concrete and accounts for the effects of gas flow, water migration, convection diffusion, and capillary action. This model is composed of three equations—the gas flow equation, the solution flow equation, and the solute convection–diffusion equation. The COMSOL numerical analysis software was subsequently used to obtain solutions for this model, based on parameters such as porosity and the chloride diffusion coefficient. Subsequently, the saturation, relative permeability, and the chloride ion concentration during the first corrosion cycle were analyzed. The numerical results were consistent with the experimental values and were therefore superior to the values obtained using Fick’s Second Law. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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23 pages, 6948 KiB

Open AccessArticle

Experimental Study on Electric Potential Response Characteristics of Gas-Bearing Coal During Deformation and Fracturing Process

by Zhonghui Li, Yue Niu, Enyuan Wang, Lanbo Liu, Honghao Wang, Mingfu Wang and Muhammad Ali

Processes 2019, 7(2), 72; https://doi.org/10.3390/pr7020072 - 1 Feb 2019

Cited by 22 | Viewed by 3353

Abstract

Coal mass is deformed and fractured under stress to generate electrical potential (EP) signals. The mechanical properties of coal change with the adsorption of gas. To investigate the EP response characteristics of gas-bearing coal during deformation and fracture, a test system to monitor [...] Read more.

Coal mass is deformed and fractured under stress to generate electrical potential (EP) signals. The mechanical properties of coal change with the adsorption of gas. To investigate the EP response characteristics of gas-bearing coal during deformation and fracture, a test system to monitor multi-parameters of gas-bearing coal under load was designed. The results showed that abundant EP signals were generated during the loading process and the EP response corresponded well with the stress change and crack expansion, and validated this with the results from acoustic emission (AE) and high-speed photography. The higher stress level and the greater the sudden stress change led to the greater EP abnormal response. With the increase of gas pressure, the confining action and erosion effect are promoted, causing the damage evolution impacted and failure characteristics changes. As a result, the EP response is similar while the characteristics were promoted. The EP response was generated due to the charge separation caused by the friction effect etc. during the damage and deformation of the coal. Furthermore, the main factors of the EP response were different under diverse loading stages. The presence of gas promoted the EP effect. When the failure of the coal occurred, EP value rapidly rose to a maximum, which could be considered as an anomalous characteristic for monitoring the stability and revealing failure of gas-bearing coal. The research results are beneficial for further investigating the damage-evolution process of gas-bearing coal. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 13132 KiB

Open AccessArticle

Experimental Study on Mixed Mode Fracture Behavior of Sandstone under Water–Rock Interactions

by Wen Hua, Jianxiong Li, Shiming Dong and Xin Pan

Processes 2019, 7(2), 70; https://doi.org/10.3390/pr7020070 - 1 Feb 2019

Cited by 29 | Viewed by 3785

Abstract

Water–rock interactions can significantly deteriorate the physical and mechanical properties of rocks, and it has been identified as one of the significant factors influencing the stability and safety of structures in rock–soil engineering. In this study, the fracture mechanical properties of sandstone under [...] Read more.

Water–rock interactions can significantly deteriorate the physical and mechanical properties of rocks, and it has been identified as one of the significant factors influencing the stability and safety of structures in rock–soil engineering. In this study, the fracture mechanical properties of sandstone under periodic water–rock interactions and long-term immersion have been studied with central cracked Brazilian disk specimens. The degradation mechanism of water–rock interactions was also studied using a scanning electron microscope (SEM). Finally, the generalized maximum tangential stress and generalized maximum tangential strain criteria were adopted to evaluate the experimental results. The results show that periodic water–rock interactions can remarkably affect the fracture resistance of sandstone. With the increase in the number of cycles, the pure mode I, pure mode II, and mixed mode fracture toughness decreases greatly, however, the values of K_If/K_IC and K_IIf/K_IC decrease slightly. Furthermore, the fracture resistance of sandstone influenced by cyclic wetting–drying is more significant than long-term immersion. Moreover, the fracture criteria, which considers the effect of T-stress, can reproduce the test results very well. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 6054 KiB

Open AccessArticle

The Effects of Backfill Mining on Strata Movement Rule and Water Inrush: A Case Study

by Jian Hao, Yongkui Shi, Jiahui Lin, Xin Wang and Hongchun Xia

Processes 2019, 7(2), 66; https://doi.org/10.3390/pr7020066 - 29 Jan 2019

Cited by 14 | Viewed by 3468

Abstract

Backfill mining is widely used to control strata movement and improve the stress environment in China’s coal mines. In the present study, the effects of backfill mining on strata movement and water inrush were studied based on a case study conducted in Caozhuang [...] Read more.

Backfill mining is widely used to control strata movement and improve the stress environment in China’s coal mines. In the present study, the effects of backfill mining on strata movement and water inrush were studied based on a case study conducted in Caozhuang Coal Mine. The in-situ investigation measured abutment pressure distribution (APD), roof floor displacement (RFD), and vertical stress in the backfill area. Results are as follows: (i) The range and peak of APD, RFD, and vertical stress in the backfill area are smaller than in traditional longwall mining with the caving method. (ii) Backfill mining could change the movement form and amplitude of overburden and improve the ground pressure environment. (iii) Floor failure depth (FFD) is much smaller in backfill mining. Backfill mining can be an effective method for floor water inrush prevention. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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17 pages, 6076 KiB

Open AccessArticle

Study on the Preparation and Hydration Properties of a New Cementitious Material for Tailings Discharge

by Yunbing Hou, Pengchu Ding, Dong Han, Xing Zhang and Shuxiong Cao

Processes 2019, 7(1), 47; https://doi.org/10.3390/pr7010047 - 17 Jan 2019

Cited by 6 | Viewed by 4136

Abstract

Blast furnace slag (BFS) is often used as a cement-based raw material for underground filling and surface cemented paste discharge of tailings during mining processes. This paper studied a new cement-based material (NCM) with BFS to replace ordinary Portland cement (OPC). A uniaxial [...] Read more.

Blast furnace slag (BFS) is often used as a cement-based raw material for underground filling and surface cemented paste discharge of tailings during mining processes. This paper studied a new cement-based material (NCM) with BFS to replace ordinary Portland cement (OPC). A uniaxial compressive strength (UCS) experiment was used to test the mechanical strength of samples; X-ray diffraction and thermal gravity experiments were used to test the crystalline phases and amount of hydration products by samples; a scanning electron microscope experiment was used to observe the influence of the hydration products morphology by samples; mercury intrusion porosimetry experiment was used to analyze the pore size distribution of samples. The samples with NCM had an optimum UCS; the crystalline phases of the hydration products were similar in OPC and NCM. However, the amount of product formed in OPC was less than that in NCM at the same curing time; more ettringite and calcium silicate hydrate were produced in samples with NCM, which filled the pores and enhanced the UCS of the samples. The final mercury intrusion volume of the samples with NCM were lower than the samples with OPC at the same curing time, which showed that samples with NCM had lower porosities. For the samples with NCM and OPC cured from 7 days to 28 days, the mercury intrusion volume was reduced by 18% and 13%, and the most common pore size of the samples reduced by 53% and 29%, respectively. This showed after 21 days curing time, the pores of all the samples getting smaller; however, the samples with NCM were more compact. The main ingredients of the NCM were clinker, lime, gypsum and BFS, and its ratio was 14:6:10:70. The content of additives to NCM was 0.4%, and the ratio of sodium sulfate: alum: sodium fluorosilicate was 2:1:1. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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22 pages, 8205 KiB

Open AccessArticle

Simulating the Filtration Effects of Cement-Grout in Fractured Porous Media with the 3D Unified Pipe-Network Method

by Zizheng Sun, Xiao Yan, Weiqi Han, Guowei Ma and Yiming Zhang

Processes 2019, 7(1), 46; https://doi.org/10.3390/pr7010046 - 16 Jan 2019

Cited by 17 | Viewed by 5132

Abstract

In grouting process, filtration is the retention and adsorption of cement-grout particles in a porous/fractured medium. Filtration partly/even completely blocks the transportation channels in the medium, greatly decreasing its permeability. Taking into account filtration effects is essential for accurately estimating the grout penetration [...] Read more.

In grouting process, filtration is the retention and adsorption of cement-grout particles in a porous/fractured medium. Filtration partly/even completely blocks the transportation channels in the medium, greatly decreasing its permeability. Taking into account filtration effects is essential for accurately estimating the grout penetration region. In this paper, the 3D unified pipe-network method (UPM) is adopted for simulating 3D grout penetration process in a fractured porous medium, considering filtration effects. The grout is assumed to exhibit two-phase flow, and the filtration effects depend on not only the concentration and rheology of the grout but also the porosity and permeability of the fractured porous medium. By comparing the model with the experimental results, we firstly verify the proposed numerical model. Then sensitivity analysis is conducted, showing the influences of grout injection pressures, the water–cement ratios of grout (W/C) and the grout injection rates on filtration effect. Finally, the grout filtration process in a complex 3D fractured network is simulated, indicating that the size of the grout penetration region is limited due to filtration. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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18 pages, 11063 KiB

Open AccessArticle

Numerical Investigation of Hydraulic Fracture Propagation Based on Cohesive Zone Model in Naturally Fractured Formations

by Jianxiong Li, Shiming Dong, Wen Hua, Xiaolong Li and Xin Pan

Processes 2019, 7(1), 28; https://doi.org/10.3390/pr7010028 - 8 Jan 2019

Cited by 30 | Viewed by 5373

Abstract

Complex propagation patterns of hydraulic fractures often play important roles in naturally fractured formations due to complex mechanisms. Therefore, understanding propagation patterns and the geometry of fractures is essential for hydraulic fracturing design. In this work, a seepage–stress–damage coupled model based on the [...] Read more.

Complex propagation patterns of hydraulic fractures often play important roles in naturally fractured formations due to complex mechanisms. Therefore, understanding propagation patterns and the geometry of fractures is essential for hydraulic fracturing design. In this work, a seepage–stress–damage coupled model based on the finite pore pressure cohesive zone (PPCZ) method was developed to investigate hydraulic fracture propagation behavior in a naturally fractured reservoir. Compared with the traditional finite element method, the coupled model with global insertion cohesive elements realizes arbitrary propagation of fluid-driven fractures. Numerical simulations of multiple-cluster hydraulic fracturing were carried out to investigate the sensitivities of a multitude of parameters. The results reveal that stress interference from multiple-clusters is responsible for serious suppression and diversion of the fracture network. A lower stress difference benefits the fracture network and helps open natural fractures. By comparing the mechanism of fluid injection, the maximal fracture network can be achieved with various injection rates and viscosities at different fracturing stages. Cluster parameters, including the number of clusters and their spacing, were optimal, satisfying the requirement of creating a large fracture network. These results offer new insights into the propagation pattern of fluid driven fractures and should act as a guide for multiple-cluster hydraulic fracturing, which can help increase the hydraulic fracture volume in naturally fractured reservoirs. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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19 pages, 5791 KiB

Open AccessArticle

Characterization of Pores and Fractures in Soft Coal from the No. 5 Soft Coalbed in the Chenghe Mining Area

by Pan Wei, Yunpei Liang, Song Zhao, Shoujian Peng, Xuelong Li and Ran Meng

Processes 2019, 7(1), 13; https://doi.org/10.3390/pr7010013 - 31 Dec 2018

Cited by 14 | Viewed by 3481

Abstract

The characteristics of the pore structure and gas migration in soft coalbeds are the premise of evaluating gas discharge in soft coalbeds. To explore the pore structure characteristics of soft coal masses, the No. 5 soft coalbed in the eastern zone of Chenghe [...] Read more.

The characteristics of the pore structure and gas migration in soft coalbeds are the premise of evaluating gas discharge in soft coalbeds. To explore the pore structure characteristics of soft coal masses, the No. 5 soft coalbed in the eastern zone of Chenghe Mining Area, was investigated and compared with the No. 5 hard coalbed in the western zone. By using a mercury intrusion method, low-temperature liquid nitrogen adsorption, and scanning electron microscopy (SEM), the pore structure characteristics of the No. 5 coalbed were explored. Moreover, based on fractal theory, the pore structure of coal was characterized. The results showed the pores in soft coal mainly appeared as small pores and micropores in which the small pores accounted for nearly half of the total pore volume. Mesopores and macropores were also distributed throughout the soft coal. The mercury-injection and mercury-ejection curves of soft coal showed significant hysteresis loops, implying that pores in coal samples were mainly open while the mercury-injection curve of hard coal was consistent with its mercury-ejection curve, showing no hysteresis loop while having an even segment, which indicated that closed pores occupied the majority of the pore volume in the coal samples. The curves of low-temperature nitrogen adsorption of soft coal all follow an IV-class isotherm. Moreover, the fractal dimensions of soft coal are respectively larger than the fractal dimensions of hard coal. It can be seen that the characterization of pores and fractures of the soft coal was different from the hard coal in the western distinct of the old mining area. The gas prevention and control measures of soft coal should be formulated according to local conditions. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 5504 KiB

Open AccessArticle

2D Plane Strain Consolidation Process of Unsaturated Soil with Vertical Impeded Drainage Boundaries

by Minghua Huang and Dun Li

Processes 2019, 7(1), 5; https://doi.org/10.3390/pr7010005 - 21 Dec 2018

Cited by 8 | Viewed by 4240

Abstract

The consolidation process of soil stratum is a common issue in geotechnical engineering. In this paper, the two-dimensional (2D) plane strain consolidation process of unsaturated soil was studied by incorporating vertical impeded drainage boundaries. The eigenfunction expansion and Laplace transform techniques were adopted [...] Read more.

The consolidation process of soil stratum is a common issue in geotechnical engineering. In this paper, the two-dimensional (2D) plane strain consolidation process of unsaturated soil was studied by incorporating vertical impeded drainage boundaries. The eigenfunction expansion and Laplace transform techniques were adopted to transform the partial differential equations for both the air and water phases into two ordinary equations, which can be easily solved. Then, the semi-analytical solutions for the excess pore-pressures and the soil layer settlement were derived in the Laplace domain. The final results in the time domain could be computed by performing the numerical inversion of Laplace transform. Furthermore, two comparisons were presented to verify the accuracy of the proposed semi-analytical solutions. It was found that the semi-analytical solution agreed well with the finite difference solution and the previous analytical solution from the literature. Finally, the 2D plane strain consolidation process of unsaturated soil under different drainage efficiencies of the vertical boundaries was illustrated, and the influences of the air-water permeability ratio, the anisotropic permeability ratio and the spacing-depth ratio were investigated. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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17 pages, 5891 KiB

Open AccessArticle

Investigation of Deep Mine Shaft Stability in Alternating Hard and Soft Rock Strata Using Three-Dimensional Numerical Modeling

by Xiaoming Sun, Gan Li, Chengwei Zhao, Yangyang Liu and Chengyu Miao

Processes 2019, 7(1), 2; https://doi.org/10.3390/pr7010002 - 20 Dec 2018

Cited by 50 | Viewed by 6672

Abstract

The problem of shaft instability has always been a major difficulty in deep mining practices. The shaft fracture has a high probability of being located near the aquifers and the soft–hard rock contact zone. This paper describes the deformation and stress characteristics of [...] Read more.

The problem of shaft instability has always been a major difficulty in deep mining practices. The shaft fracture has a high probability of being located near the aquifers and the soft–hard rock contact zone. This paper describes the deformation and stress characteristics of surrounding rock and the shaft lining under the interactive geological conditions under soft and hard rock strata in Anju coal mine, Shandong Province, China. Using the Method of Geological Strength Index (GSI ) and considering the rock-softening characteristics of water, the parameters of rock mass are calibrated. By means of the 3DEC-trigon method, the variation characteristics of surrounding rock and the shaft lining are simulated. After shaft excavation, under the condition of no support, shear failure and tensile failure occur in shallow surrounding rock shafts, and a pressure relief zone is formed. Shear failure is the main destruction mode in deep surrounding rock. Because of the different strengths of the surrounding rock, the deformation of the surrounding rock is significantly different. After the surrounding rock is softened by water absorption, the difference is magnified. The maximum shear stress and plastic zone appear near the interface between soft and hard rock. Under the condition of shaft lining support, uneven deformation of surrounding rock surely leads to nonlinear variation of pressure on the shaft lining. Under the action of an inhomogeneous pressure field, partial shear failure occurs in the shaft lining, and the shear failure area increases after the surrounding rock is softened by water. Because of the nonlinear deformation of the shaft lining, it is easy to produce stress concentration and bending moment near the interface between hard and soft strata. The control methods of advance grouting and pressure relief excavation are proposed to improve the stability of the shaft, and a good effect is gained. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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12 pages, 1713 KiB

Open AccessArticle

Experimental Study on Compression Deformation and Permeability Characteristics of Grading Broken Gangue under Stress

by Yu Zhang, Wei Zhou, Ming Li and Zhanqing Chen

Processes 2018, 6(12), 257; https://doi.org/10.3390/pr6120257 - 9 Dec 2018

Cited by 9 | Viewed by 3800

Abstract

As the important raw material for backfill mining, broken gangue’s deformation and permeability characteristics directly affect the deformation of the overlying strata above the filling space. In this paper, through lateral compression and pressed seepage tests, the deformation and permeability characteristics of broken [...] Read more.

As the important raw material for backfill mining, broken gangue’s deformation and permeability characteristics directly affect the deformation of the overlying strata above the filling space. In this paper, through lateral compression and pressed seepage tests, the deformation and permeability characteristics of broken gangue as a function of the stress level and grading features were studied. This research indicates that the stress of broken gangue increases exponentially with an increase in strain, and the compression modulus and compression rate present a positive correlation. The samples with discontinuous grading are more difficult to compress than the continuous grading samples, and the discontinuous grading samples are tighter in accordance with the increase in compression rate. At the same time, the change range of the seepage velocity and permeability of the broken gangue decreases. Positive correction between the grading index of the broken gangue and the effect of reducing the permeability of samples is more obvious under axial compression, and less axial stress is needed to achieve the same permeability level for discontinuous grading. This paper can provide an important test basis for the design of grading parameters and the prediction of filling effects of broken gangue on backfill mining. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 4283 KiB

Open AccessArticle

Effects of Cyclic Wetting-Drying Conditions on Elastic Modulus and Compressive Strength of Sandstone and Mudstone

by Shiyuan Huang, Junjie Wang, Zhenfeng Qiu and Kai Kang

Processes 2018, 6(12), 234; https://doi.org/10.3390/pr6120234 - 22 Nov 2018

Cited by 45 | Viewed by 5195

Abstract

The influence of water on the mechanical properties of rock is vital for determining the rock stability when subjected to changes of water conditions. In this paper, a series of uniaxial compression tests were conducted to investigate effects of cyclic wetting and drying [...] Read more.

The influence of water on the mechanical properties of rock is vital for determining the rock stability when subjected to changes of water conditions. In this paper, a series of uniaxial compression tests were conducted to investigate effects of cyclic wetting and drying on the mechanical properties of sandstone and mudstone collected from Chongqing city, China. The results showed that both elastic modulus and uniaxial compressive strength of sandstone and mudstone were reduced by wetting and drying cycles, and that the degradation rate of the two mechanic parameters of mudstone was always larger than sandstone. The parameters, including water adsorption, degradation degree of elastic modulus, degradation degree of uniaxial compressive strength, increase with the increase of the wetting-drying cycles (N). The relationship between these three parameters and the value of N + 1 could be well fitted by logarithmic curves. The average degradation degree was also used to describe the degradation of per time wetting-drying cycles. It is found that the average degradation degree of elastic modulus and uniaxial compressive strength decrease with the increase of wetting-drying cycles. Moreover, the relationships between the mechanical properties and the porosity are presented, which can be fitted by linear curves. In the cyclic wetting-drying process, the elastic modulus and the uniaxial compressive strength decreased with the porosity increasing, and the degradation rates of sandstone mechanic parameters were higher than those of mudstone. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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18 pages, 8441 KiB

Open AccessArticle

Feasibility of Using Gangue and Fly Ash as Filling Slurry Materials

by Hao Wang, Jian Jiao, Yumin Wang and Weisheng Du

Processes 2018, 6(12), 232; https://doi.org/10.3390/pr6120232 - 22 Nov 2018

Cited by 17 | Viewed by 3430

Abstract

When used as filling aggregates, the physical and mechanical properties of gangue and fly ash are crucial for the design of filling and transportation systems. The mineral composition of gangue and fly ash affects the filling body’s strength, and the fluidity of the [...] Read more.

When used as filling aggregates, the physical and mechanical properties of gangue and fly ash are crucial for the design of filling and transportation systems. The mineral composition of gangue and fly ash affects the filling body’s strength, and the fluidity of the slurry affects the mining engineering process. In this study, gangue and fly ash samples were characterized by X-ray diffraction to ensure their suitability as filling materials. Tests were carried out with an Intelligent Torque Rheometer, and the optimal ratio of the slurry’s components was determined. After fitting the data using ORIGIN software, the following curves were obtained: (1) rheological parameters versus slurry mass fraction; (2) rheological parameters versus component ratio. On the basis of the curves, the recommended ratio of the components in the paste-like slurry was determined. We found that the mass concentration of the slurry must be kept strictly below 78%, and the optimal slurry composition includes coal gangue, fly ash, and a gelling agent at a weight ratio of 8:3:1. In order to illustrate the effectiveness of the filling technology, surface sink curves from different filling techniques in two closely situated positions of the Xinyang Coal Mine were compared. The results showed that the use of a paste-like slurry can effectively reduce surface subsidence. Furthermore, it will lead to cost-effective engineering designs for the sustainable development of coal mines. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 3308 KiB

Open AccessArticle

The Effect of Collision Angle on the Collision and Adhesion Behavior of Coal Particles and Bubbles

by Qiming Zhuo, Wenli Liu, Hongxiang Xu, Xiaopeng Sun, He Zhang and Wei Liu

Processes 2018, 6(11), 218; https://doi.org/10.3390/pr6110218 - 5 Nov 2018

Cited by 21 | Viewed by 4296

Abstract

The collision and adhesion behavior of particles and bubbles is the key to flotation. Many scholars have investigated the collision and adhesion law of regularly shaped and homogeneous particles (glass beads, glass fiber), but the particles in flotation cells are irregular and heterogeneous. [...] Read more.

The collision and adhesion behavior of particles and bubbles is the key to flotation. Many scholars have investigated the collision and adhesion law of regularly shaped and homogeneous particles (glass beads, glass fiber), but the particles in flotation cells are irregular and heterogeneous. Therefore, it is necessary to take actual coal samples as the research object. First, based on previous research, a particle–bubble collision and adhesion behavior measurement device was set up to study free falling coal particles with different surface properties colliding and adhering to a bubble (db = 1.0 mm). Then bituminous coal from Inner Mongolia was taken as the test object, and the collision and adhesion process of a large amount of coal particles was traced. The entire process is photographically recorded by a camera and analyzed frame by frame through a self-designed software. Finally, the relationship between collision angle and initial settlement position (initial), particle velocity (process), and adhesion efficiency (result) was studied by taking the collision angle as the cut-in point. It was found that both the distribution range of the initial settlement position and the particle central distribution interval are expanding outward with the increase of collision angle. The resistance layer has an important influence on the velocity of particles. The collision angle had an effect on adhesion efficiency and the adhesion efficiency of low-density particles was higher than that of high-density particles. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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13 pages, 1782 KiB

Open AccessArticle

Experimental Study on Feasibility of Enhanced Gas Recovery through CO₂ Flooding in Tight Sandstone Gas Reservoirs

by Fengjiao Wang, Yikun Liu, Chaoyang Hu, Yongping Wang, Anqi Shen and Shuang Liang

Processes 2018, 6(11), 214; https://doi.org/10.3390/pr6110214 - 2 Nov 2018

Cited by 22 | Viewed by 4181

Abstract

The development of natural gas in tight sandstone gas reservoirs via CH₄-CO₂ replacement is promising for its advantages in enhanced gas recovery (EGR) and CO₂ geologic sequestration. However, the degree of recovery and the influencing factors of CO₂ [...] Read more.

The development of natural gas in tight sandstone gas reservoirs via CH₄-CO₂ replacement is promising for its advantages in enhanced gas recovery (EGR) and CO₂ geologic sequestration. However, the degree of recovery and the influencing factors of CO₂ flooding for enhanced gas recovery as well as the CO₂ geological rate are not yet clear. In this study, the tight sandstone gas reservoir characteristics and the fluid properties of the Sulige Gasfield were chosen as the research platform. Tight sandstone gas long-core displacement experiments were performed to investigate (1) the extent to which CO₂ injection enhanced gas recovery (CO₂-EGR) and (2) the ability to achieve CO₂ geological storage. Through modification of the injection rate, the water content of the core, and the formation dip angle, comparative studies were also carried out. The experimental results demonstrated that the gas recovery from CO₂ flooding increased by 18.36% when compared to the depletion development method. At a lower injection rate, the diffusion of CO₂ was dominant and the main seepage resistance was the viscous force, which resulted in an earlier CO₂ breakthrough. The dissolution of CO₂ in water postponed the breakthrough of CO₂ while it was also favorable for improving the gas recovery and CO₂ geological storage. However, the effects of these two factors were insignificant. A greater influence was observed from the presence of a dip angle in tight sandstone gas reservoirs. The effect of CO₂ gravity separation and its higher viscosity were more conducive to stable displacement. Therefore, an additional gas recovery of 5% to 8% was obtained. Furthermore, the CO₂ geological storage exceeded 60%. As a consequence, CO₂-EGR was found to be feasible for a tight sandstone gas reservoir while also achieving the purpose of effective CO₂ geological storage especially for a reservoir with a dip angle. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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19 pages, 5337 KiB

Open AccessArticle

The Impact of Oriented Perforations on Fracture Propagation and Complexity in Hydraulic Fracturing

by Liyuan Liu, Lianchong Li, Derek Elsworth, Sheng Zhi and Yongjun Yu

Processes 2018, 6(11), 213; https://doi.org/10.3390/pr6110213 - 1 Nov 2018

Cited by 31 | Viewed by 6017

Abstract

To better understand the interaction between hydraulic fracture and oriented perforation, a fully coupled finite element method (FEM)-based hydraulic-geomechanical fracture model accommodating gas sorption and damage has been developed. Damage conforms to a maximum stress criterion in tension and to Mohr–Coulomb limits in [...] Read more.

To better understand the interaction between hydraulic fracture and oriented perforation, a fully coupled finite element method (FEM)-based hydraulic-geomechanical fracture model accommodating gas sorption and damage has been developed. Damage conforms to a maximum stress criterion in tension and to Mohr–Coulomb limits in shear with heterogeneity represented by a Weibull distribution. Fracturing fluid flow, rock deformation and damage, and fracture propagation are collectively represented to study the complexity of hydraulic fracture initiation with perforations present in the near-wellbore region. The model is rigorously validated against experimental observations replicating failure stresses and styles during uniaxial compression and then hydraulic fracturing. The influences of perforation angle, in situ stress state, initial pore pressure, and properties of the fracturing fluid are fully explored. The numerical results show good agreement with experimental observations and the main features of the hydraulic fracturing process in heterogeneous rock are successfully captured. A larger perforation azimuth (angle) from the direction of the maximum principal stress induces a relatively larger curvature of the fracture during hydraulic fracture reorientation. Hydraulic fractures do not always initiate at the oriented perforations and the fractures induced in hydraulic fracturing are not always even and regular. Hydraulic fractures would initiate both around the wellbore and the oriented perforations when the perforation angle is >75°. For the liquid-based hydraulic fracturing, the critical perforation angle increases from 70° to 80°, with an increase in liquid viscosity from 10⁻³ Pa·s to 1 Pa·s. While for the gas fracturing, the critical perforation angle remains 62° to 63°. This study is of great significance in further understanding the near-wellbore impacts on hydraulic fracture propagation and complexity. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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17 pages, 3015 KiB

Open AccessArticle

Experimental Development Process of a New Fluid–Solid Coupling Similar-Material Based on the Orthogonal Test

by Shiliang Liu and Weitao Liu

Processes 2018, 6(11), 211; https://doi.org/10.3390/pr6110211 - 1 Nov 2018

Cited by 34 | Viewed by 3961

Abstract

Similar-material, composed of different raw materials with different properties, is similar to the physical and mechanical properties of geotechnical media, in which raw material proportioning is an important means to control the performance of similar-material in physical simulation. On this basis, a new [...] Read more.

Similar-material, composed of different raw materials with different properties, is similar to the physical and mechanical properties of geotechnical media, in which raw material proportioning is an important means to control the performance of similar-material in physical simulation. On this basis, a new fluid–solid coupling similar-material was developed through proportioning tests, in which similar-material is mixed with river sand, calcium carbonate, talc powder, white cement, vaseline, antiwear hydraulic oil. The optimum proportioning test development process was established. First, the proportioning test scheme was designed based on the orthogonal test. Subsequently, test specimens were produced to obtain parameters such as density, compressive strength, tensile strength, and permeability coefficient. Then, by increasing the ingredients of the proportioning, the evolution law of parameters was obtained by range and variance analysis. Finally, four multiple linear regression equations between the parameters and similar-material ingredients were obtained, and the optimum proportioning of ingredients was further determined for different requirements. The results indicate that the selected raw materials and their proportioning method are feasible, and the results were also verified in a coal mine floor water inrush by physical simulation test. The experimental development process of a fluid–solid coupling similar-material can provide a reference for similar-material under different demand conditions. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 2658 KiB

Open AccessArticle

High Mixing Efficiency by Modulating Inlet Frequency of Viscoelastic Fluid in Simplified Pore Structure

by Meng Zhang, Yunfeng Cui, Weihua Cai, Zhengwei Wu, Yongyao Li, Fengchen Li and Wu Zhang

Processes 2018, 6(11), 210; https://doi.org/10.3390/pr6110210 - 1 Nov 2018

Cited by 7 | Viewed by 3766

Abstract

Fluid mixing plays an essential role in microscale flow systems. Here, we propose an active mixing approach which enhances the mixing of viscoelastic fluid flow in a simplified pore T-junction structure. Mixing is actively controlled by modulating the driving pressure with a sinusoidal [...] Read more.

Fluid mixing plays an essential role in microscale flow systems. Here, we propose an active mixing approach which enhances the mixing of viscoelastic fluid flow in a simplified pore T-junction structure. Mixing is actively controlled by modulating the driving pressure with a sinusoidal signal at the two inlets of the T-junction. The mixing effect is numerically investigated for both Newtonian and viscoelastic fluid flows under different pressure modulation conditions. The result shows that a degree of mixing as high as 0.9 is achieved in viscoelastic fluid flows through the T-junction mixer when the phase difference between the modulated pressures at the two inlets is 180°. This modulation method can also be used in other fluid mixing devices. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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21 pages, 4967 KiB

Open AccessArticle

Geotechnical Properties of Effluent-Contaminated Cohesive Soils and Their Stabilization Using Industrial By-Products

by Muhammad Irfan, Yulong Chen, Muhammad Ali, Muhammad Abrar, Ahmed Qadri and Osama Bhutta

Processes 2018, 6(10), 203; https://doi.org/10.3390/pr6100203 - 22 Oct 2018

Cited by 16 | Viewed by 6427

Abstract

The unchecked and unnoticed disposal of industrial leachates is a common malpractice in developing countries. Untreated effluents from industries drastically deteriorate the soil, altering nearly all of its characteristics. An increase in urbanization has led to construction on these deteriorated lands. In this [...] Read more.

The unchecked and unnoticed disposal of industrial leachates is a common malpractice in developing countries. Untreated effluents from industries drastically deteriorate the soil, altering nearly all of its characteristics. An increase in urbanization has led to construction on these deteriorated lands. In this study, the chemical impact of two industrial effluents, dyeing (acidic) and tannery (basic), is studied on two cohesive soils, i.e., high plastic clay (CH) and low plastic clay (CL). Properties such as liquid limit, plasticity index, specific gravity, maximum dry density, unconfined compressive strength, swell potential, swell pressure, and compression indices decrease with effluent contamination, with the exception of the basic effluent, for which the trend changes after a certain percentage. This study also examines the time variation of properties at different effluent percentages, finding that unconfined compressive strength of both soils increases with time upon dyeing (acidic) contamination and decreases with tannery (basic). The stabilizing effect of two industrial by-products, i.e., marble dust and ground granulated blast furnace slag (GGBFS) have been evaluated. Unlike their proven positive effect on uncontaminated soils, these industrial by-products did not show any significant stabilization effect on leachate-contaminated cohesive soils, thereby emphasizing the need to utilize special remediation measures for effluent treated soils. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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18 pages, 7436 KiB

Open AccessArticle

Changes in Particle Size Composition under Seepage Conditions of Reclaimed Soil in Xinjiang, China

by Zizhao Zhang, Wanghua Sui, Kaikai Wang, Guobin Tang and Xiaoping Li

Processes 2018, 6(10), 201; https://doi.org/10.3390/pr6100201 - 20 Oct 2018

Cited by 7 | Viewed by 3378

Abstract

The distribution of reclaimed soil particle size under seepage conditions after the management period will directly determine the success or failure of reclamation work. The geotechnical experimental method was used in this paper to study the changes in the granulometric composition of soil. [...] Read more.

The distribution of reclaimed soil particle size under seepage conditions after the management period will directly determine the success or failure of reclamation work. The geotechnical experimental method was used in this paper to study the changes in the granulometric composition of soil. The results show that the granulometric composition of the reclaimed soil varied obviously at different depths. The granulometric composition of the soil at a depth of 10 cm was not much different from undisturbed reclaimed soil (URS). At a depth of 30 cm, as the sharp decrease of the content of fine particles resulted in coarser reclaimed soil, the soil became more uniform, with an increase in porosity and water content. At a depth of 50 cm, the fine particle content was generally slightly lower than that of URS. At a depth of 70 cm, the fine particle content of the soil greatly exceeded that of the URS, with the finest soil particles and lowest porosity. The main reason for the above-mentioned changes of granulometric composition in the reclaimed soil was the seepage in soil caused by irrigation during the management period. The research results can provide a reference for management after land reclamation at non-metallic mines in Xinjiang, China. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 5895 KiB

Open AccessArticle

Effect of Dry-Wet Cycling on the Mechanical Properties of Rocks: A Laboratory-Scale Experimental Study

by Xiaojie Yang, Jiamin Wang, Dinggui Hou, Chun Zhu and Manchao He

Processes 2018, 6(10), 199; https://doi.org/10.3390/pr6100199 - 19 Oct 2018

Cited by 43 | Viewed by 7041

Abstract

Taking Nanfen open-pit iron mine in Liaoning Province as the engineering background, this study analyzes the effect of water-rock circulation on the mechanical properties of rock through a combination of macro-mechanical experiments and microstructure tests in the laboratory. Uniaxial compression experiments and acoustic [...] Read more.

Taking Nanfen open-pit iron mine in Liaoning Province as the engineering background, this study analyzes the effect of water-rock circulation on the mechanical properties of rock through a combination of macro-mechanical experiments and microstructure tests in the laboratory. Uniaxial compression experiments and acoustic wave tests are used to determine the degradation law of the mechanical properties of chlorite under the periodic action of water. The experimental results show that dry-wet cycles have a continuous and gradual effect on the rock sampled: Its uniaxial compressive strength, elastic modulus, and acoustic velocity all decrease gradually with an increase in the number of cycles. After 15 wet-dry cycles, the uniaxial compressive strength and elastic modulus of the rock decreased by 34.21% and 44.63%, respectively. Electron microscope scans of the rock indicate that the particle size, characteristics, and pore distribution at the rock surface had changed significantly after water-rock interaction. Finally, a drainage system and sliding force monitoring devices have been arranged at the mine site that can effectively reduce the impact of water-rock interaction on the stability of the mine. This combination of macro-experiments and micro-analysis allowed the weakening effect of dry-wet cycles on slope rock to be studied quantitatively, providing a theoretical reference for stability evaluation in geotechnical engineering. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 5043 KiB

Open AccessArticle

Rheological Behavior and Modeling of a Crushed Sandstone-Mudstone Particle Mixture

by Zhenfeng Qiu, Ting Cao, Yongsuo Li, Junjie Wang and Yulong Chen

Processes 2018, 6(10), 192; https://doi.org/10.3390/pr6100192 - 16 Oct 2018

Cited by 11 | Viewed by 3347

Abstract

A mixture of sandstone and mudstone particles is often used as the main filling material for many agriculture-related and civil engineering projects, including rock-fill dams and foundations. The long-term rheological deformation experienced by rock-fill dams and foundations filled with this mixture is much [...] Read more.

A mixture of sandstone and mudstone particles is often used as the main filling material for many agriculture-related and civil engineering projects, including rock-fill dams and foundations. The long-term rheological deformation experienced by rock-fill dams and foundations filled with this mixture is much bigger than that of coarse-grained and cohesive soils, due to the deterioration and softening of the mudstone particles. This study focuses on the rheological deformation of a sandstone-mudstone particle mixture, prepared by mixing sandstone and mudstone particles, based on the content by weight of four mudstone particle types. Confined uniaxial compression tests were performed to test the rheological deformation of 24 samples of the mixture, and a stress-strain curve was obtained for each test. On the basis of compression curves, the rheological process of the mixture was divided into four phases: linear, attenuation rheological, secondary attenuation rheological and stable phases. The three defining features of the curve, namely the rheological attenuation factors, attenuation rheology critical strain and limited rheological strain, were then determined and modeled. A segmented rheological model was then proposed, based on a modified attenuation rheological constitutive model for coarse-grained soil. The modelled results compared well with the experimental data, and the modelled compression-curve prediction was able to describe the two-stage attenuation rheology features (attenuation rheological and secondary attenuation rheological phases) of the sandstone-mudstone particle mixture. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 7658 KiB

Open AccessArticle

Experimental Investigation on the Law of Grout Diffusion in Fractured Porous Rock Mass and Its Application

by Donghai Jiang, Xianzhen Cheng, Hengjie Luan, Tongxu Wang, Mingguang Zhang and Ruiyun Hao

Processes 2018, 6(10), 191; https://doi.org/10.3390/pr6100191 - 16 Oct 2018

Cited by 21 | Viewed by 3522

Abstract

Because of the limitation of mining techniques and economic conditions, large amounts of residual coal resources have been left in underground coal mines around the world. Currently, with mining technology gradually developing, residual coal can possibly be remined. However, when residual coal is [...] Read more.

Because of the limitation of mining techniques and economic conditions, large amounts of residual coal resources have been left in underground coal mines around the world. Currently, with mining technology gradually developing, residual coal can possibly be remined. However, when residual coal is remined, caving areas might form, which can seriously affect the safety of coal mining. Hence, grouting technology is put forward as one of the most effective technologies to solve this problem. To study the grouting diffusion in fractured rock mass, this paper developed a visualization platform of grouting diffusion and a three-dimensional grouting experimental system that can monitor the grout diffusion range, diffusion time and grout pressure; then, a grouting experiment is conducted based on this system. After that, the pattern of the grouting pressure variation, grout flow and grout diffusion surface are analyzed. The relationship among some factors, such as the grouting diffusion radius, compressive strength of the grouted gravel, porosity, water-cement ratio, grouting pressure, grouting time, permeability coefficient and level of grout, is quantitatively analyzed by using MATLAB. The study results show that the flow pattern of the grout in fractured porous rock mass has a parabolic shape from the grouting hole to the bottom. The lower the level is, the larger the diffusion range of the grout is. The grouting pressure has the greatest influence on the grouting diffusion radius, followed by the grouting horizon and water-cement ratio. The grouting permeability coefficient has the least influence on the grouting diffusion radius. The grout water-cement ratio has the greatest influence on the strength of the grouted gravel, followed by the grouting permeability. The grouting pressure coefficient has the least amount of influence on the grouting diffusion radius. According to the results, the grouting parameters are designed, and a layered progressive grouting method is proposed. Finally, borehole observation and a core mechanical property test are conducted to verify the application effect. This grouting technology can contribute to the redevelopment and efficient utilization of wasted underground coal resources. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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26 pages, 11948 KiB

Open AccessArticle

Numerical Investigation of the Failure Mechanism of Transversely Isotropic Rocks with a Particle Flow Modeling Method

by Xu-Xu Yang, Hong-Wen Jing and Wei-Guo Qiao

Processes 2018, 6(9), 171; https://doi.org/10.3390/pr6090171 - 17 Sep 2018

Cited by 15 | Viewed by 4531

Abstract

Transversely isotropic rocks are commonly encountered in rock engineering practices, and their strength and failure behavior is often governed by the property of anisotropy. The particle flow modeling method was utilized to investigate the failure mechanism of transversely isotropic rocks subject to uniaxial [...] Read more.

Transversely isotropic rocks are commonly encountered in rock engineering practices, and their strength and failure behavior is often governed by the property of anisotropy. The particle flow modeling method was utilized to investigate the failure mechanism of transversely isotropic rocks subject to uniaxial compressive loading. The details for establishing transversely isotropic rock models were first presented, and then a parametric study was carried out to look into the effect of interface properties on the failure mode and strength of transversely isotropic rock models by varying the interface dip angle. The smooth joint model was incorporated to create interfaces for the completeness of establishing transversely isotropic rock models with the particle flow modeling method. Accordingly, three failure modes observed in transversely isotropic rock models with varying dip angles were tensile failure across interfaces, shear failure along interfaces, and tensile failure along interfaces. Furthermore, the interface mechanical parameters were found to differently influence the failure behavior of transversely isotropic rock models. The bonded joint cohesion and bonded joint friction angle that contribute to the shear strength of interfaces have considerable influence on the uniaxial compressive strength (UCS) values, while the joint coefficient of friction and joint tensile strength have a slight influence on the UCS values. The findings in this paper indicated the importance of interfaces in estimating failure behavior of transversely isotropic rocks. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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18 pages, 5752 KiB

Open AccessArticle

Temporal Mixing Behavior of Conservative Solute Transport through 2D Self-Affine Fractures

by Zhi Dou, Brent Sleep, Pulin Mondal, Qiaona Guo, Jingou Wang and Zhifang Zhou

Processes 2018, 6(9), 158; https://doi.org/10.3390/pr6090158 - 5 Sep 2018

Cited by 13 | Viewed by 4038

Abstract

In this work, the influence of the Hurst exponent and Peclet number (Pe) on the temporal mixing behavior of a conservative solute in the self-affine fractures with variable-aperture fracture and constant-aperture distributions were investigated. The mixing was quantified by the scalar [...] Read more.

In this work, the influence of the Hurst exponent and Peclet number (Pe) on the temporal mixing behavior of a conservative solute in the self-affine fractures with variable-aperture fracture and constant-aperture distributions were investigated. The mixing was quantified by the scalar dissipation rate (SDR) in fractures. The investigation shows that the variable-aperture distribution leads to local fluctuation of the temporal evolution of the SDR, whereas the temporal evolution of the SDR in the constant-aperture fractures is smoothly decreasing as a power-law function of time. The Peclet number plays a dominant role in the temporal evolution of mixing in both variable-aperture and constant-aperture fractures. In the constant-aperture fracture, the influence of Hurst exponent on the temporal evolution of the SDR becomes negligible when the Peclet number is relatively small. The longitudinal SDR can be related to the global SDR in the constant-aperture fracture when the Peclet number is relatively small. As the Peclet number increases the longitudinal SDR overpredicts the global SDR. In the variable-aperture fractures, predicting the global SDR from the longitudinal SDR is inappropriate due to the non-monotonic increase of the longitudinal concentration second moment, which results in a physically meaningless SDR. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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13 pages, 12157 KiB

Open AccessArticle

Shear-Flow Coupled Behavior of Artificial Joints with Sawtooth Asperities

by Cheng Zhao, Rui Zhang, Qingzhao Zhang, Zhenming Shi and Songbo Yu

Processes 2018, 6(9), 152; https://doi.org/10.3390/pr6090152 - 1 Sep 2018

Cited by 12 | Viewed by 4342

Abstract

The coupling between hydraulic and mechanical processes in rock joints has significantly influenced the properties and applications of rock mass in many engineering fields. In this study, a series of regular shear tests and shear-flow coupled tests were conducted on artificial joints with [...] Read more.

The coupling between hydraulic and mechanical processes in rock joints has significantly influenced the properties and applications of rock mass in many engineering fields. In this study, a series of regular shear tests and shear-flow coupled tests were conducted on artificial joints with sawtooth asperities. Shear deformation, strength, and seepage properties were comprehensively analyzed to reveal the influence of joint roughness, normal stress, and seepage pressure on shear-flow coupled behavior. The results indicate that the shear failure mode, which can be divided into sliding and cutting, is dominated by joint roughness and affected by the other two factors under certain conditions. The seepage process makes a negative impact on shear strength as a result of the mutual reinforcing of offsetting and softening effects. The evolution of hydraulic aperture during the shear-flow coupled tests embodies a consistent pattern of four stages: shear contraction, shear dilation, re-contraction, and stability. The permeability of joint sample is considerably enlarged with the increase of joint roughness, but decreases with the addition of normal stress. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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18 pages, 5367 KiB

Open AccessArticle

Analysis of Overlying Strata Movement and Disaster-Causing Effects of Coal Mining Face under the Action of Hard Thick Magmatic Rock

by Quanlin Wu, Quansen Wu, Yanchao Xue, Peng Kong and Bin Gong

Processes 2018, 6(9), 150; https://doi.org/10.3390/pr6090150 - 1 Sep 2018

Cited by 25 | Viewed by 4663

Abstract

When the hard and thick key strata are located above the working face, the bed separation structure is easy to be formed after mining because of the high strength and integrity of the hard and thick key strata and the initial breaking step [...] Read more.

When the hard and thick key strata are located above the working face, the bed separation structure is easy to be formed after mining because of the high strength and integrity of the hard and thick key strata and the initial breaking step is large. After the hard, thick strata are broken, the overburden will be largely collapsed and unstable in a large area and the dynamic disaster is easily induced. In this study, considering the fundamental deformation and failure effect of coal seam, the development law of the bed separation and the fractures under hard and thick magmatic rocks and the mechanism of breaking induced disaster of hard and thick magmatic rocks are studied by similar simulation tests. The results of the study are as follows: (1) The similar material ratio of coal seam is obtained by low-strength orthogonal ratio test of similar materials of coal seam, that is, cement:sand:water:activated carbon:coal = 6:6:7:1.1:79.9. (2) The magmatic rocks play a role in shielding the development of the bed separation, which makes the bed separation beneath the magmatic rock in an unclosed state for a long time, providing space for the accumulation of gas and water. (3) The distribution pattern of the fracture zone shows different shapes as the advancing of working face and the fracture zone width of the rear of working face coal wall is larger than that of the front of the open-off. (4) The breaking of magmatic rocks will press the gas and water accumulated in the bed separation space below to rush towards the working face along the fracture zone at both ends of the goaf. The above results are verified through the drainage borehole gas jet accident in the Yangliu coal mine. The research results are of great significance for revealing the occurrence process of dynamic disasters and adopting scientific and reasonable preventive measures. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 5465 KiB

Open AccessArticle

A Numerical Study of Stress Distribution and Fracture Development above a Protective Coal Seam in Longwall Mining

by Chunlei Zhang, Lei Yu, Ruimin Feng, Yong Zhang and Guojun Zhang

Processes 2018, 6(9), 146; https://doi.org/10.3390/pr6090146 - 1 Sep 2018

Cited by 32 | Viewed by 5609

Abstract

Coal and gas outbursts are serious safety concerns in the Chinese coal industry. Mining of the upper or lower protective coal seams has been widely used to minimize this problem. This paper presents new findings from longwall mining-induced fractures, stress distribution changes in [...] Read more.

Coal and gas outbursts are serious safety concerns in the Chinese coal industry. Mining of the upper or lower protective coal seams has been widely used to minimize this problem. This paper presents new findings from longwall mining-induced fractures, stress distribution changes in roof strata, strata movement and gas flow dynamics after the lower protective coal seam is extracted in a deep underground coal mine in Jincheng, China. Two Flac^3D models with varying gob loading characteristics as a function of face advance were analyzed to assess the effect of gob behavior on stress relief in the protected coal seam. The gob behavior in the models is incorporated by applying variable force to the floor and roof behind the longwall face to simulate gob loading characteristics in the field. The influence of mining height on the stress-relief in protected coal seam is also incorporated. The stress relief coefficient and relief angle were introduced as two essential parameters to evaluate the stress relief effect in different regions of protected coal seam. The results showed that the rock mass above the protective coal seam can be divided into five zones in the horizontal direction, i.e. pre-mining zone, compression zone, expansion zone, recovery zone and re-compacted zone. The volume expansion or the dilation zone with high gas concentration is the best location to drill boreholes for gas drainage in both the protected coal seam and the protective coal seam. The research results are helpful to understand the gas flow mechanism around the coal seam and guide industry people to optimize borehole layouts in order to eliminate the coal and gas outburst hazard. The gas drainage programs are provided in the final section. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 5768 KiB

Open AccessArticle

Experimental Study on the Reinforcement Mechanism of Segmented Split Grouting in a Soft Filling Medium

by Zhipeng Li, Shucai Li, Haojie Liu, Qingsong Zhang and Yanan Liu

Processes 2018, 6(8), 131; https://doi.org/10.3390/pr6080131 - 17 Aug 2018

Cited by 26 | Viewed by 4392

Abstract

Subsection split grouting technology can effectively improve the grouting efficiency and homogeneity of grouting in a target reinforcement area. It is therefore necessary to clarify the reinforcement mechanism and characteristics of the soft filling medium under the condition of split grouting. A three-dimensional [...] Read more.

Subsection split grouting technology can effectively improve the grouting efficiency and homogeneity of grouting in a target reinforcement area. It is therefore necessary to clarify the reinforcement mechanism and characteristics of the soft filling medium under the condition of split grouting. A three-dimensional grouting simulation test of segmented split grouting in a soft filling medium was conducted. The distribution characteristics and thicknesses of the grouting veins were obtained under the condition of segmented grouting. The mechanical mechanism of segmented split grouting reinforcement, based on the distribution characteristics of different grouting veins, was revealed. After grouting, a uniaxial compression test and an indoor permeation test were conducted. Based on the method of the region-weighted average, the corresponding permeability coefficient and the elastic modulus of each splitting-compaction region were obtained. The quantitative relationship between the mechanical properties and the impermeability of the soft filling medium before and after grouting was established. The results revealed that three different types of veins were formed as the distance from the grouting holes increased; namely, skeleton veins, cross-grid grouting veins, and parallel dispersed grouting veins. The thicknesses of the grouting veins decreased gradually, whereas the number of grouting veins increased. Moreover, the strikes of the grouting vein exhibited increased randomness. The reinforcement effect of segmental split grouting on soft filling media was mainly confirmed by the skeleton support and compaction. The elastic modulus of the grouting reinforcement solid increased on average by a factor that was greater than 100, and the permeability coefficient decreased on average by a factor that was greater than 40 in the direction of the parallel grouting vein with the most impermeable solid. The research results may be helpful in the investigation of the split grouting reinforcement mechanism under the condition of segmented grouting. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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22 pages, 4492 KiB

Open AccessArticle

A Strain-Based Percolation Model and Triaxial Tests to Investigate the Evolution of Permeability and Critical Dilatancy Behavior of Coal

by Dongjie Xue, Jie Zhou, Yintong Liu and Sishuai Zhang

Processes 2018, 6(8), 127; https://doi.org/10.3390/pr6080127 - 13 Aug 2018

Cited by 19 | Viewed by 4944

Abstract

Modeling the coupled evolution of strain and CH₄ seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on [...] Read more.

Modeling the coupled evolution of strain and CH₄ seepage under conventional triaxial compression is the key to understanding enhanced permeability in coal. An abrupt transition of gas-stress coupled behavior at the dilatancy boundary is studied by the strain-based percolation model. Based on orthogonal experiments of triaxial stress with CH₄ seepage, a complete stress-strain relationship and the corresponding evolution of volumetric strain and permeability are obtained. At the dilatant boundary of volumetric strain, modeling of stress-dependent permeability is ineffective when considering the effective deviatoric stress influenced by confining pressure and pore pressure. The computed tomography (CT) analysis shows that coal can be a continuous medium of pore-based structure before the dilatant boundary, but a discontinuous medium of fracture-based structure. The multiscale pore structure geometry dominates the mechanical behavior transition and the sudden change in CH₄ seepage. By the volume-covering method proposed, the linear relationship between the fractal dimension and porosity indicates that the multiscale network can be a fractal percolation structure. A percolation model of connectivity by the axial strain-permeability relationship is proposed to explain the transition behavior of volumetric strain and CH₄ seepage. The volumetric strain on permeability is illustrated by axial strain controlling the trend of transition behavior and radical strain controlling the shift of behavior. A good correlation between the theoretical and experimental results shows that the strain-based percolation model is effective in describing the transition behavior of CH₄ seepage in coal. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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11 pages, 3331 KiB

Open AccessArticle

Deformation and Hydraulic Conductivity of Compacted Clay under Waste Differential Settlement

by Sifa Xu, Cuifeng Li, Jizhuang Liu, Mengdan Bian, Weiwei Wei, Hao Zhang and Zhe Wang

Processes 2018, 6(8), 123; https://doi.org/10.3390/pr6080123 - 8 Aug 2018

Cited by 6 | Viewed by 4166

Abstract

Landfill is still the most important process to dispose of municipal solid waste in China, while landfill closure aims for pollution control, security control, and better land reuse. However, uneven settlement of landfill cover system is very likely to cause deformation and cracking. [...] Read more.

Landfill is still the most important process to dispose of municipal solid waste in China, while landfill closure aims for pollution control, security control, and better land reuse. However, uneven settlement of landfill cover system is very likely to cause deformation and cracking. The objective of this paper is to examine the effects of geogrid reinforcement on the deformation behaviour and hydraulic conductivity of the bentonite-sand mixtures that are subjected to differential settlement. The laboratory model tests were performed on bentonite-sand mixtures with and without the inclusion of geogrid reinforcement. By maintaining the type and location of the geogrid within the liner systems as constant, the thickness of the bentonite-sand mixtures is varied. The performation of the liner systems with and without the inclusion of geogrid reinforcement was assessed by using jack to control differential settlement. Un-reinforced bentonite-sand mixtures of 100 mm and 200 mm thickness were observed to begin cracking at settlement levels of 2.5 mm and 7 mm, respectively. When settlement reached 25 and 42.5 mm, cracks for 100 mm and 200 mm thick bentonite-sand mixtures without geogrid penetrated completely. The settlement levels for bentonite-sand mixtures of 100 mm thickness with and without geogrid reinforcement was found to be 10 mm and 15 mm, respectively, when its hydraulic conductivity was around 5 * 10⁻⁷ cm/s. In comparison, geogrid reinforced bentonite-sand mixtures was found to sustain large deformation with an enhanced imperviousness. The results from the present study can provide theory evidence of predicting deformation and hydraulic conductivity of the landfill cover system. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 3957 KiB

Open AccessFeature PaperArticle

A Coupled Thermal-Hydraulic-Mechanical Nonlinear Model for Fault Water Inrush

by Weitao Liu, Jiyuan Zhao, Ruiai Nie, Yuben Liu and Yanhui Du

Processes 2018, 6(8), 120; https://doi.org/10.3390/pr6080120 - 7 Aug 2018

Cited by 13 | Viewed by 4323

Abstract

A coupled thermal-nonlinear hydraulic-mechanical (THM) model for fault water inrush was carried out in this paper to study the water-rock-temperature interactions and predict the fault water inrush. First, the governing equations of the coupled THM model were established by coupling the particle transport [...] Read more.

A coupled thermal-nonlinear hydraulic-mechanical (THM) model for fault water inrush was carried out in this paper to study the water-rock-temperature interactions and predict the fault water inrush. First, the governing equations of the coupled THM model were established by coupling the particle transport equation, nonlinear flow equation, mechanical equation, and the heat transfer equation. Second, by setting different boundary conditions, the mechanical model, nonlinear hydraulic-mechanical (HM) coupling model, and the thermal-nonlinear hydraulic-mechanical (THM) coupling model were established, respectively. Finally, a numerical simulation of these models was established by using COMSOL Multiphysics. Results indicate that the nonlinear water flow equation could describe the nonlinear water flow process in the fractured zone of the fault. The mining stress and the water velocity had a great influence on the temperature of the fault zone. The temperature change of the fault zone can reflect the change of the seepage field in the fault and confined aquifer. This coupled THM model can provide a numerical simulation method to describe the coupled process of complex geological systems, which can be used to predict the fault water inrush induced by coal mining activities. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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25 pages, 7717 KiB

Open AccessArticle

Numerical Simulation of Hydraulic Fracture Propagation in Coal Seams with Discontinuous Natural Fracture Networks

by Shen Wang, Huamin Li and Dongyin Li

Processes 2018, 6(8), 113; https://doi.org/10.3390/pr6080113 - 1 Aug 2018

Cited by 43 | Viewed by 8435

Abstract

To investigate the mechanism of hydraulic fracture propagation in coal seams with discontinuous natural fractures, an innovative finite element meshing scheme for modeling hydraulic fracturing was proposed. Hydraulic fracture propagation and interaction with discontinuous natural fracture networks in coal seams were modeled based [...] Read more.

To investigate the mechanism of hydraulic fracture propagation in coal seams with discontinuous natural fractures, an innovative finite element meshing scheme for modeling hydraulic fracturing was proposed. Hydraulic fracture propagation and interaction with discontinuous natural fracture networks in coal seams were modeled based on the cohesive element method. The hydraulic fracture network characteristics, the growth process of the secondary hydraulic fractures, the pore pressure distribution and the variation of bottomhole pressure were analyzed. The improved cohesive element method, which considers the leak-off and seepage behaviors of fracturing liquid, is capable of modeling hydraulic fracturing in naturally fractured formations. The results indicate that under high stress difference conditions, the hydraulic fracture network is spindle-shaped, and shows a multi-level branch structure. The ratio of secondary fracture total length to main fracture total length was 2.11~3.62, suggesting that the secondary fractures are an important part of the hydraulic fracture network in coal seams. In deep coal seams, the break pressure of discontinuous natural fractures mainly depends on the in-situ stress field and the direction of natural fractures. The mechanism of hydraulic fracture propagation in deep coal seams is significantly different from that in hard and tight rock layers. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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22 pages, 6868 KiB

Open AccessArticle

A High-Order Numerical Manifold Method for Darcy Flow in Heterogeneous Porous Media

by Lingfeng Zhou, Yuan Wang and Di Feng

Processes 2018, 6(8), 111; https://doi.org/10.3390/pr6080111 - 1 Aug 2018

Cited by 10 | Viewed by 4099

Abstract

One major challenge in modeling Darcy flow in heterogeneous porous media is simulating the material interfaces accurately. To overcome this defect, the refraction law is fully introduced into the numerical manifold method (NMM) as an a posteriori condition. To achieve a better accuracy [...] Read more.

One major challenge in modeling Darcy flow in heterogeneous porous media is simulating the material interfaces accurately. To overcome this defect, the refraction law is fully introduced into the numerical manifold method (NMM) as an a posteriori condition. To achieve a better accuracy of the Darcy velocity and continuous nodal velocity, a high-order weight function with a continuous nodal gradient is adopted. NMM is an advanced method with two independent cover systems, which can easily solve both continuous and discontinuous problems in a unified form. Moreover, a regular mathematical mesh, independent of the physical domain, is used in the NMM model. Compared to the conforming mesh of other numerical methods, it is more efficient and flexible. A number of numerical examples were simulated by the new NMM model, comparing the results with the original NMM model and the analytical solutions. Thereby, it is proven that the proposed method is accurate, efficient, and robust for modeling Darcy flow in heterogeneous porous media, while the refraction law is satisfied rigorously. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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23 pages, 11311 KiB

Open AccessArticle

Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action

by Ziheng Sha, Hai Pu, Ming Li, Lili Cao, Ding Liu, Hongyang Ni and Jingfeng Lu

Processes 2018, 6(8), 110; https://doi.org/10.3390/pr6080110 - 1 Aug 2018

Cited by 21 | Viewed by 4595

Abstract

The seepage action of underground water accelerates the deformation of roadway surrounding rock in deep mines. Therefore, the study of creep characteristics of surrounding rock under seepage action is the basis for the stability control of roadway surrounding rock in deep water-rich areas. [...] Read more.

The seepage action of underground water accelerates the deformation of roadway surrounding rock in deep mines. Therefore, the study of creep characteristics of surrounding rock under seepage action is the basis for the stability control of roadway surrounding rock in deep water-rich areas. In this paper, a seepage-creep coupling test system for complete rock samples was established. Combined with a scanning electron microscopy (SEM) test system, the seepage-creep law of coal measures sandstone and the damage mechanism were revealed. The study results showed that the maximum creep deformation of sandstone under natural and saturation state decreased gradually with the increase of confining pressure, and the maximum creep deformation under saturation state was greater than the corresponding value under natural state when the confining pressure was same. When the confining pressure was constant, the creep deformation, the constant creep deformation rate and the accelerated creep deformation rate of sandstone increased rapidly with the increase of infiltration pressure. With the change of time, the change of permeability parameters went through three cycles; each cycle was divided into two stages, slow change stage and rapid change stage, and the rate of variation increased with the increase of the seepage pressure. Based on the macroscopic and microscopic characteristics of sandstone rupture, the connection between macroscopic and microscopic mechanism on sandstone rupture was established. The results in this paper can provide a theoretical basis for stability control of roadway surrounding rock in water-rich areas. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 3406 KiB

Open AccessArticle

Effect of Pore Fluid Pressure on the Normal Deformation of a Matched Granite Joint

by Qiang Zhang, Xiaochun Li, Bing Bai, Shaobin Hu and Lu Shi

Processes 2018, 6(8), 107; https://doi.org/10.3390/pr6080107 - 1 Aug 2018

Cited by 12 | Viewed by 5408

Abstract

The influence of pore fluid pressure on the normal deformation behaviors of joints is vital for understanding the interaction between hydraulic and mechanical processes of joints. The effect of pore fluid pressure on the normal deformation of a granite matched joint was investigated [...] Read more.

The influence of pore fluid pressure on the normal deformation behaviors of joints is vital for understanding the interaction between hydraulic and mechanical processes of joints. The effect of pore fluid pressure on the normal deformation of a granite matched joint was investigated by laboratory experiments. Experimental results indicate pore fluid pressure significantly affects the normal deformation of jointed sample, and the relative normal deformation of jointed sample during fluid injection consists of the opening of the joint and the dilation of host rock. The action of pore fluid pressure on the joint follows the Terzaghi’s effective stress law. The normal deformation of the joint can be well quantitated by the generalized exponential model. The relative normal deformation of host rock during fluid injection would have a linear relationship with pore fluid pressure, and if affected by gas is more pronounced than water. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 5901 KiB

Open AccessArticle

Coal Anisotropic Sorption and Permeability: An Experimental Study

by Yulong Chen, Xuelong Li and Bo Li

Processes 2018, 6(8), 104; https://doi.org/10.3390/pr6080104 - 30 Jul 2018

Cited by 9 | Viewed by 3532

Abstract

Knowledge of the bedding plane properties of coal seams is essential for the coalbed gas production because of their great influence on the inner flow characteristics and sorption features of gas and water. In this study, an experimental study on the anisotropic gas [...] Read more.

Knowledge of the bedding plane properties of coal seams is essential for the coalbed gas production because of their great influence on the inner flow characteristics and sorption features of gas and water. In this study, an experimental study on the anisotropic gas adsorption–desorption and permeability of coal is presented. The results show that during the adsorption–desorption process, an increase in the bedding plane angle of the specimen expands the length and area of the contact surface, thereby increasing the speed and quantity of adsorption and desorption. With an increase in the bedding angle, the number of pores and cracks was found to increase together with the volumetric strain. The evolution of permeability of coal heavily depended on stress–strain stages. The permeability decreased with the increase of stress at the initial compaction and elastic deformation stages, while it increased with the increase of stress at the stages of strain-hardening, softening and residual strength. Initial permeability increased with increasing bedding angle. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 7227 KiB

Open AccessArticle

Experimental Study on the Damage of Granite by Acoustic Emission after Cyclic Heating and Cooling with Circulating Water

by Dong Zhu, Hongwen Jing, Qian Yin and Guansheng Han

Processes 2018, 6(8), 101; https://doi.org/10.3390/pr6080101 - 25 Jul 2018

Cited by 66 | Viewed by 4823

Abstract

Hot dry rock is developed by injecting cold water into high-temperature rock mass. At the same time, cold water is heated in contact with the rock mass. With the continuous influx of cold water, the surrounding rock will undergo a rapid cooling process, [...] Read more.

Hot dry rock is developed by injecting cold water into high-temperature rock mass. At the same time, cold water is heated in contact with the rock mass. With the continuous influx of cold water, the surrounding rock will undergo a rapid cooling process, which results in several cycles of heating and cooling. However, there is little research on the influence of cycles of heating and cooling with circulating water on the mechanical properties of rock, which is of great importance to the stability of rock mass engineering in the process of energy development. In this paper, the effects of cyclic heating and cooling with circulating water on the damage of granite are studied using uniaxial compressive, Brazilian and acoustic emission (AE) tests. The results show that heat treatment temperature and number of cycles have important effects on the mechanical properties of granite as follows: (1) at the same treatment temperature, an increase in the number of cycles means that the distribution of physical and mechanical parameters of the granite show an almost exponential downward trend. The uniaxial compression of granite results in its transformation from brittle to plastic, and the failure mode changes from slipping of the shear surface to plastic failure. With increased cycles of heating and cooling with circulating water, the tensile strength of granite also decreases; temperature has an obvious influence on physical and mechanical parameters, cracking of samples, and plays a controlling role in the failure mode of samples. In addition, (2) at the same temperature, the heating and cooling numbers N have a significant influence on the AE distribution characteristics of the sample under uniaxial compression and the number of AE collisions, and the cumulative number of AE decreases with the increase of N. (3) The concepts of mechanical damage and high-temperature and cold-water shock damage during uniaxial compression of samples were proposed based on AE, and the damage equations were established respectively. The curve equations of damage value (D) and cycle numbers N after thermal shock damage of high temperature and cold water were overlaid. The cracking mechanism of high-temperature and cold water impact on granite was analyzed, and the thermal shock stress equation of high temperature and water cooling was established. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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19 pages, 7398 KiB

Open AccessArticle

Experimental Study of the Microstructural Evolution of Glauberite and Its Weakening Mechanism under the Effect of Thermal-Hydrological-Chemical Coupling

by Shuzhao Chen, Donghua Zhang, Tao Shang and Tao Meng

Processes 2018, 6(8), 99; https://doi.org/10.3390/pr6080099 - 24 Jul 2018

Cited by 5 | Viewed by 4566

Abstract

The microstructures of rock gradually evolve with changes in the external environment. This study focused on the microstructure evolution of glauberite and its weakening mechanism under different leaching conditions. The porosity were used as a characteristic index to study the effect of brine [...] Read more.

The microstructures of rock gradually evolve with changes in the external environment. This study focused on the microstructure evolution of glauberite and its weakening mechanism under different leaching conditions. The porosity were used as a characteristic index to study the effect of brine temperature and concentration on crack initiation and propagation in glauberite. The research subjects were specimens of

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3 × 10 mm cylindrical glauberite core, obtained from a bedded salt deposit buried more than 1000 m underground in the Yunying salt formation, China. The results showed that when the specimens were immersed in solution at low temperature, due to hydration impurities, cracks appeared spontaneously at the centre of the disc and the solution then penetrated the specimens via these cracks and dissolved the minerals around the crack lines. However, with an increase of temperature, the dissolution rate increased greatly, and crack nucleation and dissolved regions appeared simultaneously. When the specimens were immersed in a sodium chloride solution at the same concentration, the porosity s presented gradual upward trends with a rise in temperature, whereas, when the specimens were immersed in the sodium chloride solution at the same temperature, the porosity tended to decrease with the increase of sodium chloride concentration. In the process of leaching, the hydration of illite, montmorillonite, and the residual skeleton of glauberite led to the expansion of the specimen volume, thereby producing the cracks. The diameter expansion rate and the expansion velocity of the specimen increased with temperature increase, whereas, due to the common-ion effect, the porosity of the specimen decreases with the increase of sodium chloride solution concentration. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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20 pages, 13145 KiB

Open AccessArticle

The Fracturing Behavior of Tight Glutenites Subjected to Hydraulic Pressure

by Zhichao Li, Lianchong Li, Zilin Zhang, Ming Li, Liaoyuan Zhang, Bo Huang and Chun’an Tang

Processes 2018, 6(7), 96; https://doi.org/10.3390/pr6070096 - 20 Jul 2018

Cited by 8 | Viewed by 4855

Abstract

Tight glutenites are typically composed of heterogeneous sandstone and gravel. Due to low or ultra-low permeability, it is difficult to achieve commercial production in tight glutenites without hydraulic fracturing. Efficient exploitation requires an in-depth understanding of the fracturing behavior of these reservoirs. This [...] Read more.

Tight glutenites are typically composed of heterogeneous sandstone and gravel. Due to low or ultra-low permeability, it is difficult to achieve commercial production in tight glutenites without hydraulic fracturing. Efficient exploitation requires an in-depth understanding of the fracturing behavior of these reservoirs. This paper provides a numerical method that integrates the digital image processing (DIP) technique into a numerical code rock failure process analysis (RFPA). This method could consider the glutenite heterogeneities, including intrarock and interrock heterogeneities, and the practicability is verified through two numerical tests. Two-dimensional (2D) simulations show hydraulic fractures (HFs) can penetrate or deflect to propagate along the gravels, depending on the magnitude of stress anisotropy and gravel strength. Three-dimensional (3D) simulations with the consideration of gravel distribution orientation, gravel size and axial ratio show HFs could propagate past the gravel with no deflection, forming a bypass fracture that is not easy to observe in common laboratory experiments. HFs could also deflect to propagate along the gravels. The impacts of the gravel distribution orientation, gravel size and axial ratio are discussed in detail. The main propagation modes of HFs intersecting the gravels are summarized as: (1) penetrating directly; (2) deflecting to propagate along the gravels to form distorted HFs; (3) propagating to bypass the gravels; (4) a combination of (1) and (2), or (2) and (3). Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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14 pages, 1641 KiB

Open AccessArticle

A New Pseudo Steady-State Constant for a Vertical Well with Finite-Conductivity Fracture

by Yudong Cui, Bin Lu, Mingtao Wu and Wanjing Luo

Processes 2018, 6(7), 93; https://doi.org/10.3390/pr6070093 - 19 Jul 2018

Cited by 5 | Viewed by 4325

Abstract

The Pseudo Steady-State (PSS) constant b_Dps_s is defined as the difference between the dimensionless wellbore pressure and dimensionless average pressure of a reservoir with a PSS flow regime. As an important parameter, b_Dps_s has been widely used for decline [...] Read more.

The Pseudo Steady-State (PSS) constant b_Dps_s is defined as the difference between the dimensionless wellbore pressure and dimensionless average pressure of a reservoir with a PSS flow regime. As an important parameter, b_Dps_s has been widely used for decline curve analysis with Type Curves. For a well with a finite-conductivity fracture, b_Dps_s is independent of time and is a function of the penetration ratio of facture and fracture conductivity. In this study, we develop a new semi-analytical solution for b_Dps_s calculations using the PSS function of a circular reservoir. Based on the semi-analytical solution, a new conductivity-influence function (CIF) representing the additional pressure drop caused by the effect of fracture conductivity is presented. A normalized conductivity-influence function (NCIF) is also developed to calculate the CIF. Finally, a new approximate solution is proposed to obtain the b_Dps_s value. This approximate solution is a fast, accurate, and time-saving calculation. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 9317 KiB

Open AccessArticle

Critical Hydraulic Gradient of Internal Erosion at the Soil–Structure Interface

by Quanyi Xie, Jian Liu, Bo Han, Hongtao Li, Yuying Li and Xuanzheng Li

Processes 2018, 6(7), 92; https://doi.org/10.3390/pr6070092 - 18 Jul 2018

Cited by 27 | Viewed by 15434

Abstract

Internal erosion at soil–structure interfaces is a dangerous failure pattern in earth-fill water-retaining structures. However, existing studies concentrate on the investigations of internal erosion by assuming homogeneous materials, while ignoring the vulnerable soil–structure-interface internal erosion in realistic cases. Therefore, orthogonal and single-factor tests [...] Read more.

Internal erosion at soil–structure interfaces is a dangerous failure pattern in earth-fill water-retaining structures. However, existing studies concentrate on the investigations of internal erosion by assuming homogeneous materials, while ignoring the vulnerable soil–structure-interface internal erosion in realistic cases. Therefore, orthogonal and single-factor tests are carried out with a newly designed apparatus to investigate the critical hydraulic gradient of internal erosion on soil–structure interfaces. The main conclusions can be draw as follows: (1) the impact order of the three factors is: degree of compaction > roughness > clay content; (2) the critical hydraulic gradient increases as the degree of compaction and clay content increases. This effect is found to be more obvious in the higher range of the degree of soil compaction and clay content. However, there exists an optimum interface roughness making the antiseepage strength at the interface reach a maximum; (3) the evolution of the interface internal erosion develops from inside to outside along the interface, and the soil particles at the interface flow as a whole; and (4) the critical hydraulic gradient of interface internal erosion is related to the shear strength at the interface and the severity and porosity of the soil. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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14 pages, 5883 KiB

Open AccessArticle

Investigation of the Porosity Distribution, Permeability, and Mechanical Performance of Pervious Concretes

by Rentai Liu, Haojie Liu, Fei Sha, Honglu Yang, Qingsong Zhang, Shaoshuai Shi and Zhuo Zheng

Processes 2018, 6(7), 78; https://doi.org/10.3390/pr6070078 - 26 Jun 2018

Cited by 46 | Viewed by 7383

Abstract

Pervious concrete is a kind of porous and permeable material for pavements and slope protection projects, etc. In this paper, a kind of pervious concrete was prepared with Portland cement, silica fume (SF), polycarboxylate superplasticizer (SP), and limestone aggregates. The performance of concrete, [...] Read more.

Pervious concrete is a kind of porous and permeable material for pavements and slope protection projects, etc. In this paper, a kind of pervious concrete was prepared with Portland cement, silica fume (SF), polycarboxylate superplasticizer (SP), and limestone aggregates. The performance of concrete, such as its porosity, pore distribution, permeability coefficients, and mechanical properties, were studied through laboratory tests. The volumetric porosity was measured by the water displacement method, and the planar porosity and pore size distribution were determined using image processing technology. A permeameter with a transparent sidewall and an exact sidewall sealing method were used to measure the permeability coefficients accurately. The results show that the segregation index and flow values of pastes increased with the increase of SP and water cement ratio (W/C). The measured porosity (volumetric porosity and planar porosity) of pervious concrete with a single-size aggregate was closer to the design porosity than that with the blended aggregate. Compared with the design porosity selected in this study, aggregate size was the main factor influencing the pore distribution of pervious concrete. The standard deviation of the permeability coefficient was less than 0.03 under different hydraulic gradients. It was found that the relationships between the permeability coefficient and volumetric porosity (or effective pore size d₅₀) approximately obey polynomial function. Based on the test results, the optimized parameters were suggested for practical engineering: W/C of 0.26–0.30; 0.5% SP content; 5% SF content; 15–21% design porosity; and aggregate sizes of 4.75–9.5 mm and 9.5–16 mm. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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23 pages, 9425 KiB

Open AccessArticle

Deformation and Control Countermeasure of Surrounding Rocks for Water-Dripping Roadway Below a Contiguous Seam Goaf

by Changqing Ma, Pu Wang, Lishuai Jiang and Changsheng Wang

Processes 2018, 6(7), 77; https://doi.org/10.3390/pr6070077 - 25 Jun 2018

Cited by 17 | Viewed by 4178

Abstract

To solve the technical problem of supporting a water-dripping roadway below contiguous seams at the Tuanbai coal mine, the deformation law of surrounding rocks for the roadway was studied using Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) numerical simulation. Then, a [...] Read more.

To solve the technical problem of supporting a water-dripping roadway below contiguous seams at the Tuanbai coal mine, the deformation law of surrounding rocks for the roadway was studied using Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) numerical simulation. Then, a mechanical model of water-dripping rock using a bolt support was established, and further, technical countermeasures to control the deformation of the roadway with a bolt and cable support are proposed. The results show that the erosion of the water dripping on the roadway was substantial and showed notable changes over time during roadway excavation and mining work. These effects caused the road to heave slightly, but it tended to be stable during roadway excavation. Moreover, the erosion of the roof and two ribs increased exponentially, and the floor heave increased with significant displacement oscillation during mining. The anchoring length of bolts and the rock weakening from water dripping had noticeable effects on the surrounding rocks of the roadway. The logical parameters of the bolt spacing and tightening force (the bolt line spacing was 0.7–0.9 m and the tightening force exceeded 40 kN) of the bolt supports were studied and optimized. Finally, a support scheme for water dripping on the roadway at the Tuanbai coal mine is proposed. The observation data regarding the deformation of the surrounding rocks, monitoring of bolt and cable stress, endoscopy results of roof failure, and roof bed separation monitoring were used to verify the reasonableness of the scheme and ensure the requirements for support were met. The study results can serve as a reference regarding the support for water dripping on a roadway under similar conditions. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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16 pages, 13558 KiB

Open AccessArticle

Investigation on Reinforcement and Lapping Effect of Fracture Grouting in Yellow River Embankment

by Jian Liu, Zhi Wan, Quanyi Xie, Cong Li, Rui Liu, Mengying Cheng and Bo Han

Processes 2018, 6(7), 75; https://doi.org/10.3390/pr6070075 - 22 Jun 2018

Cited by 3 | Viewed by 4151

Abstract

Fracture grouting has been a mitigation measure widely used against seepage in the Yellow River Embankment. However, there is currently a lack of systematic investigations studying the anti-seepage effect of the fracture grouting employed in this longest river embankment in China. Therefore, in [...] Read more.

Fracture grouting has been a mitigation measure widely used against seepage in the Yellow River Embankment. However, there is currently a lack of systematic investigations studying the anti-seepage effect of the fracture grouting employed in this longest river embankment in China. Therefore, in this work, laboratory and in situ experiments are carried out to investigate the reinforcement effect of fracture grouting in the Jinan section of the Yellow River Embankment. In particular, laboratory tests concentrate on studying the optimum strength improvement for cement–silicate grout by varying the content of backfilled fly ash and bentonite as admixtures. Mechanical strength and Scanning Electron Microscope photographs are investigated for assessing the strength and compactness improvement. Subsequently, based on the obtained optimum admixtures content, in situ grouting tests are carried out in the Jinan section of the Yellow River Embankment to evaluate the reinforcement and lapping effect of fracture grouting veins, where geophysical prospecting and pit prospecting methods are employed. Laboratory results show that, compared with pure cement–silicate grouts, the gelation time of the improved slurry is longer and gelation time increases as fly ash content increases. The optimum mixing proportion of the compound cement–silicate grout is 70% cement, 25% fly ash, and 5% bentonite, and the best volume ratio is 2 for the investigated cases. Geophysical prospecting including the ground penetrating radar and high-density resistivity method can reflect the lapping effect of fracture grouting veins on site. It shows that the grouting material mainly flows along the axial direction of the embankment. The treatment used to generate directional fracture is proved to be effective. The injection hole interval distance is suggested to be 1.2 m, where the lapping effect of the grouting veins is relatively significant. For the investigated cases, the average thickness of the grouting veins is approximately 6.0 cm and the corresponding permeability coefficient is averagely 1.6 × 10⁻⁶ cm/s, which meets the anti-seepage criterion in practice. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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15 pages, 7435 KiB

Open AccessArticle

Effects of Water Soaked Height on the Deformation and Crushing Characteristics of Loose Gangue Backfill Material in Solid Backfill Coal Mining

by Junmeng Li, Yanli Huang, Ming Qiao, Zhongwei Chen, Tianqi Song, Guoqiang Kong, Huadong Gao and Lei Guo

Processes 2018, 6(6), 64; https://doi.org/10.3390/pr6060064 - 30 May 2018

Cited by 32 | Viewed by 5278

Abstract

In solid backfill coal mining (SBCM), loose gangue backfill material (LGBM) is used to backfill the goaf after coal resources are exploited from the underground mines. Under certain geological conditions, LGBM with a certain height may be soaked in the water, and then [...] Read more.

In solid backfill coal mining (SBCM), loose gangue backfill material (LGBM) is used to backfill the goaf after coal resources are exploited from the underground mines. Under certain geological conditions, LGBM with a certain height may be soaked in the water, and then becomes saturated, significantly altering its mechanical properties. The confined compression experiments were used in this paper to analyze the deformation and the crushing characteristics of LGBM with varying water soaked heights in coal mines. The results showed that a large number of small holes that were distributed in the gangue blocks were the main reason why the material absorbed water and was softened. The crushing ratio and the maximum axial strain of LGBM samples gradually increased with the water soaked heights of the samples. In addition, there was a strong linear correlation between the crushing ratio and the maximum axial strain. When LGBM was used as a solid backfill material in SBCM, its deformation resistance would significantly decrease after it was soaked in the water. Higher water soaked height of LGBM led to lower deformation resistance and greater influence on the quality of backfilling. This research has great significance in getting a deep and better understanding of the mechanical properties of LGBM, as well as guiding engineering practice. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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14 pages, 31348 KiB

Open AccessArticle

Key Parameters of Gob-Side Entry Retaining in A Gassy and Thin Coal Seam with Hard Roof

by Shuai Yan, Tianxiao Liu, Jianbiao Bai and Wenda Wu

Processes 2018, 6(5), 51; https://doi.org/10.3390/pr6050051 - 7 May 2018

Cited by 33 | Viewed by 4393

Abstract

Gob-side entry retaining (GER) employed in a thin coal seam (TCS) can increase economic benefits and coal recovery, as well as mitigate gas concentration in the gob. In accordance with the caving style of a limestone roof, the gas concentration and air pressure [...] Read more.

Gob-side entry retaining (GER) employed in a thin coal seam (TCS) can increase economic benefits and coal recovery, as well as mitigate gas concentration in the gob. In accordance with the caving style of a limestone roof, the gas concentration and air pressure in the gob were analyzed, and a roof-cutting mechanical model of GER with a roadside backfill body (RBB) was proposed, to determine the key parameters of the GER-TCS, including the roof-cutting resistance and the width of the RBB. The results show that if the immediate roof height is greater than the seam height, the roof-cutting resistance and width of the RBB should meet the requirement of the immediate roof being totally cut along the gob, for which the optimal roof-cutting resistance and width of RBB were determined by analytical and numerical methods. The greater the RBB width, the greater its roof-cutting resistance. The relationship between the supporting strength of the RBB and the width of the RBB can be derived as a composite curve. The floor heave of GER increases with increasing RBB width. When the width of the RBB increased from 0.8 m to 1.2 m, the floor heave increased two-fold to 146.2 mm. GER was applied in a TCS with a limestone roof of 5 m thickness; the field-measured data verified the conclusions of the numerical model. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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13 pages, 19287 KiB

Open AccessArticle

Experimental Investigation of the Mechanical Behaviors of Grouted Sand with UF-OA Grouts

by Yuhao Jin, Lijun Han, Qingbin Meng, Dan Ma, Guansheng Han, Furong Gao and Shuai Wang

Processes 2018, 6(4), 37; https://doi.org/10.3390/pr6040037 - 19 Apr 2018

Cited by 8 | Viewed by 5361

Abstract

A detailed understanding of the engineering properties for grouted sand is a key concern in foundation engineering projects containing sand layers. In this research, experiments of grouting with various grain sizes of sand specimens using a new type of improved chemical material-urea formaldehyde [...] Read more.

A detailed understanding of the engineering properties for grouted sand is a key concern in foundation engineering projects containing sand layers. In this research, experiments of grouting with various grain sizes of sand specimens using a new type of improved chemical material-urea formaldehyde resin mixed with oxalate curing agent (UF-OA), which has rarely been used as grout in the reinforcement of soft foundations, were conducted on the basis of a self-developed grouting test system. After grouting tests, the effects on the mechanical behaviors of grouted sand specimens were investigated through uniaxial compression tests considering the grain size, the presence or absence of initial water in sand, and the curing time for grouted sand. Experimental results show that with the increase in the grain size and the presence of initial water in the sand specimen, the values of uniaxial compressive strength (UCS) and elastic moduli (E) of the grouted specimens decreased obviously, indicating that the increase of grain size and the presence of initial water have negative impacts on the mechanical behaviors of grouted sand; the peak strains (ε_c) were almost unchanged after 14 days of curing; no brittle failure behavior occurred in the grouted specimens, and desirable ductile failure characteristics were distinct after uniaxial compression. These mechanical behaviors were significantly improved after 14 days of curing. The micro-structural properties obtained by scanning electron microscopy (SEM) of the finer grouted sand indicate preferable filling performance to some extent, thereby validating the macroscopic mechanical behaviors. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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14 pages, 7225 KiB

Open AccessArticle

Laboratory Investigation of Granite Permeability after High-Temperature Exposure

by Lixin He, Qian Yin and Hongwen Jing

Processes 2018, 6(4), 36; https://doi.org/10.3390/pr6040036 - 19 Apr 2018

Cited by 28 | Viewed by 5950

Abstract

This study experimentally analysed the influence of temperature levels (200, 300, 400, 500, 600, and 800 °C) on the permeability of granite samples. At each temperature level, the applied confining pressure was in the range of 10–30 MPa, and the inlet hydraulic pressure [...] Read more.

This study experimentally analysed the influence of temperature levels (200, 300, 400, 500, 600, and 800 °C) on the permeability of granite samples. At each temperature level, the applied confining pressure was in the range of 10–30 MPa, and the inlet hydraulic pressure varied below the corresponding confining pressure. The results are as follows: (i) With an increase in the temperature level, induced micro-fractures in the granites develop, and the decrement ratios of both the P-wave velocity and the density of the granite increase; (ii) The relationship between the volume flow rate and the pressure gradient is demonstrably linear and fits very well with Darcy’s law. The equivalent permeability coefficient shows an increasing trend with the temperature, and it can be best described using the mathematical expression K₀ = A × 1.01^T; (iii) For a given temperature level, as the confining pressure increases, the transmissivity shows a decrease, and the rate of its decrease diminishes gradually. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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Review

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19 pages, 7557 KiB

Open AccessReview

Seepage Characteristics and Its Control Mechanism of Rock Mass in High-Steep Slopes

by Hong Li, Hongyuan Tian and Ke Ma

Processes 2019, 7(2), 71; https://doi.org/10.3390/pr7020071 - 1 Feb 2019

Cited by 4 | Viewed by 5160

Abstract

In Southwest China large-scale hydropower projects, the hydraulic conductivity and fracture aperture within the rock mass of a reservoir bank slope has dramatically undergone a time series of evolution during dam abutment excavation, reservoir impounding and fluctuation operation, and discharge atomization. Accordingly, seepage [...] Read more.

In Southwest China large-scale hydropower projects, the hydraulic conductivity and fracture aperture within the rock mass of a reservoir bank slope has dramatically undergone a time series of evolution during dam abutment excavation, reservoir impounding and fluctuation operation, and discharge atomization. Accordingly, seepage control measures by hydro-structures such as drainage or water insulation curtains should be guided by scientific foundation with a dynamic process covering life-cycle performance. In this paper, the up-to-date status of studying the evolution mechanism of seepage characteristics relating to fractured rock hydraulics from experimental samples to the engineering scale of the rock mass is reviewed for the first time. Then, the experimental findings and improved practice method on nonlinear seepage flow under intensive pressure drives are introduced. Finally, the scientific progress made in fractured rock seepage control theory and optimization of the design technology of high-steep slope engineering is outlined. The undertaken studies summarized herewith are expected to contribute to laying a foundation to guide the further development of effective geophysical means and integrated monitoring systems in hydropower station construction fields. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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Other

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12 pages, 3105 KiB

Open AccessTechnical Note

Experimental Study on the Shear-Flow Coupled Behavior of Tension Fractures Under Constant Normal Stiffness Boundary Conditions

by Changsheng Wang, Yujing Jiang, Hengjie Luan, Jiankang Liu and Satoshi Sugimoto

Processes 2019, 7(2), 57; https://doi.org/10.3390/pr7020057 - 22 Jan 2019

Cited by 7 | Viewed by 3159

Abstract

This study experimentally investigated the effects of fracture surface roughness, normal stiffness, and initial normal stress on the shear-flow behavior of rough-walled rock fractures. A series of shear-flow tests were performed on two rough fractures, under various constant normal stiffness (CNS) boundary conditions. [...] Read more.

This study experimentally investigated the effects of fracture surface roughness, normal stiffness, and initial normal stress on the shear-flow behavior of rough-walled rock fractures. A series of shear-flow tests were performed on two rough fractures, under various constant normal stiffness (CNS) boundary conditions. The results showed that the CNS boundary conditions have a significant influence on the mechanical and hydraulic behaviors of fractures, during shearing. The peak shear stress shows an increasing trend with the increases in the initial normal stress and fracture roughness. The residual shear stress increases with increasing the surface roughness, normal stiffness, and initial normal stress. The dilation of fracture is restrained more significantly under high normal stiffness and initial normal stress conditions. The hydraulic tests show that the evolutions of transmissivity and hydraulic aperture exhibit a three-stage behavior, during the shear process—a slight decrease stage due to the shear contraction, a fast growth stage due to shear dilation, and a slow growth stage due to the reduction rate of the mechanical aperture increment. The transmissivity and hydraulic aperture decreased, gradually, as the normal stiffness and initial normal stress increase. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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13 pages, 3232 KiB

Open AccessCase Report

Numerical Simulation on the Dynamic Characteristics of a Tremendous Debris Flow in Sichuan, China

by Yulong Chen, Zhenfeng Qiu, Bo Li and Zongji Yang

Processes 2018, 6(8), 109; https://doi.org/10.3390/pr6080109 - 1 Aug 2018

Cited by 7 | Viewed by 5192

Abstract

The mega debris flow that occurred on 13 August 2010 in Zoumaling Valley in Mianzhu County, China has done great damage to the local inhabitants, as well as to the re-construction projects in the quake-hit areas. Moreover, it is of high possibility that [...] Read more.

The mega debris flow that occurred on 13 August 2010 in Zoumaling Valley in Mianzhu County, China has done great damage to the local inhabitants, as well as to the re-construction projects in the quake-hit areas. Moreover, it is of high possibility that a secondary disaster would reappear and result in worse consequences. In order to maximize risk reduction of this problem, the local government planned to construct seven debris-resisting barriers across each ditch for mitigation of debris flow hazards in the future. In this paper, the numerical simulation fields of flow velocity, pressure, and mud depth of the Zoumaling debris flow had been computed by using finite volume method software based on computational fluid dynamics (CFD). The Bingham fluid was chosen as the constitutive model of this debris flow. The debris flow geometry model was a 3D model. The initial conditions, boundary conditions, control equations, and parameters were determined and adjusted by the actual conditions and analyses. The flow field data obtained from numerical simulations were substituted into the finite element software ANSYS. Then the calculations of fluid-solid coupling action between the flow and dam had been done. All these results of simulations and analyses could be the guide and suggestion for the design and construction of prevention engineering of Zoumaling debris flow. Full article

(This article belongs to the Special Issue Fluid Flow in Fractured Porous Media)

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