Search Results (28)

As Earth Pressure Balance (EPB) shield machine crossed with steep slopes beneath an existing highway in sandstone–mudstone alternating strata, case studies of changes in vertical displacement, settlement trough evolution, and tunnel stress induced by shield tunnel construction were investigated. The quality of synchronous grouting was evaluated using ground penetrating radar (GPR) technology. The results showed that highway settlement could be categorized into four stages: initial settlement, uplift, secondary settlement, and stabilization. The secondary settlement caused by shield tail detachment was significantly greater than the initial settlement induced by distant shield construction. The settlement trough evolved throughout construction; the maximum settlement point shifted from the tunnel centerline but it consistently remained within 3 m. During the early phase of shield tail detachment, the circumferential stress of shield tunnel changed rapidly. The circumferential stress was primarily compressive, tensile stress was observed at some monitoring points. The tensile stress at the monitoring points gradually transitioned to compressive stress. After the tunnel undercrossed, the circumferential stress gradually stabilized. The GPR detection revealed that in groundwater-rich strata, poor grouting quality areas were prone to appear at the tunnel crown, while grouting quality in other areas performed better. This engineering case can serve as a positive reference for similar EPB shield tunnels passing in close proximity beneath existing highways. Full article

Synchronous grouting slurry is widely used in shield tunnel construction to fill the gaps between stratum and shield tail segments. However, as grout is nearly liquid in the initial stages, the tunnel lining segments recently separated from the shield tail are easily affected by the upward buoyancy generated by grout, causing issues such as longitudinal misalignment and opening of ring joints. Therefore, studying the upward buoyancy characteristics of synchronous grout is crucial. In this study, floating characterisation parameters of grout were investigated using buoyancy model tests, orthogonal tests, and comprehensive tests. The floating characterisation parameters are affected by distribution ratio and types of each grout component. The relationship between the floating characterisation parameters of grout and buoyancy was established. The results show that density, flow index, and shear strength can be used as the floating characterisation parameters. Binder–sand and water–binder ratios have the largest impact on the density. The bentonite–water ratio exerts a primary influence on the flow index, while the water–binder ratio contributes a secondary effect. In addition, bentonite–water and binder–sand ratios have the greatest effect on the shear strength. Furthermore, the particle size of sand and type of bentonite considerably influence the flow index and shear strength. A high-shear grout using well-graded fine sand and a high mesh of sodium bentonite was considered in this study. When the content of bentonite exceeds 7% (P2.2), Archimedes’ law is not applicable for calculating the upward buoyancy of grout. Buoyancy supply rate exhibits gradual enhancement with flow index elevation, yet with diminishing growth rates. Full article

To prevent the occurrence of excavation face instability incidents during shield tunneling, this study takes the Bailuyuan tunnel of the ‘Hanjiang-to-Weihe River Water Diversion Project’ as the engineering background. A three-dimensional discrete element method simulation was employed to analyze the tunneling process, revealing the displacement response of the excavation face to various tunneling parameters. This led to the development of a risk assessment method that considers both tunneling parameters and geological conditions for deep-buried shield tunnels. The above method effectively overcomes the limitations of finite element method (FEM) studies on shield tunneling parameters and, combined with the Analytic Hierarchy Process (AHP), enables rapid tunnel analysis and assessment. The results demonstrate that the displacement of the excavation face in shield tunnel engineering is significantly influenced by factors such as the chamber earth pressure ratio, cutterhead opening rate, cutterhead rotation speed, and tunneling speed. Specifically, variations in the chamber earth pressure ratio have the greatest impact on horizontal displacement, occurring predominantly near the upper center of the tunnel. As the chamber earth pressure ratio decreases, horizontal displacement increases sharply from 12.9 mm to 267.3 mm. Conversely, an increase in the cutterhead opening rate leads to displacement that first rises gradually and then rapidly, from 32.1 mm to 121.1 mm. A weighted index assessment model based on AHP yields a risk level of Grade II, whereas methods from other scholars result in Grade III. By implementing measures such as adjusting the grouting range, cutterhead rotation speed, and tunneling speed, field applications confirm that the risk level remains within acceptable limits, thereby verifying the feasibility of the constructed assessment method. Construction site strategies are proposed, including maintaining a chamber earth pressure ratio greater than 1, tunneling speed not exceeding 30 mm/min, cutterhead rotation speed not exceeding 1.5 rpm, and a synchronous grouting range of 0.15 m. Following implementation, the tunnel construction successfully passed the high-risk section without any incidents. This research offers a decision-making framework for shield TBM operation safety in complex geological environments. Full article

Estimating the actual evapotranspiration (ET_{c act}) of cropland in arid areas, exploring the time trend, and analyzing periodic variation are the key to long-term assessment of water resource availability and regional drought. The Penman formula has a strong ability to characterize reference crop evapotranspiration (ET_o). However, the application of this formula may be limited in the absence of a complete set of climate data. While previous studies have investigated K_{c act} in China, few have employed localized K_c values to systematically analyze long-term periodic fluctuations in ET_{c act} under climate variability conditions. Therefore, this study aimed to evaluate the applicability of nine ET_o estimation models in the Loess Plateau of China, calculate actual crop coefficients (K_{c act}) for spring maize and winter wheat, and examine the temporal trend and periodicity of ET_{c act} for long-term (1961–2018) continuous cropping of spring maize and winter wheat in the study area. The Mann–Kendall test and continuous wavelet transform (CWT) were used to obtain the temporal trend and periodicity of ET_{c act}. The results were as follows: (1) Priestley–Taylor (Prs–Tylr), based on radiation, and the 1985 Hargreaves–Samani (Harg), based on temperature, can be used when meteorological data are limited. It should be noted that among the models evaluated in this study, except for FAO56-PM, only the Harg equation is compatible with K_c-ET_o due to established conversion factors. (2) The K_{c act} of spring maize at the seeding–jointing stage and the earning–filling stage was 12% and 10% lower than the value recommended by FAO, respectively. For K_{c act} of winter wheat, it was 65% higher, 31% lower, and 85% higher than the FAO experience values in the rejuvenation–jointing stage, heading–grouting stage, and grouting–harvest stage. (3) Winter wheat, through its ET_{c act} cycle synchronized with precipitation and excellent water balance, can effectively alleviate regional drought. It is recommended to be included in the promotion of drought resistance policies. Full article

Frequently, the viscous mixture from shield operations is disposed of because its significant water ratio and the presence of polymers like foaming agents result in subpar structural qualities, contributing to the unnecessary consumption of land and the squandering of soil assets. Therefore, these problems urgently need to be solved economically and effectively. This study relies on the shield sludge produced by Qingdao Metro Line 6 project, and sand and shield sludge were used as the raw materials for synchronous grouting. By applying the basic principles of geopolymerization, ingredients like shield sludge and ground granulated blast furnace slag (GGBS) were mixed with sodium hydroxide, serving as the activating agent, in the preparation of the simultaneous grout formulas. A broad range of laboratory tests was conducted to evaluate the performance of these grout formulations. The effects of varying material ratios on key performance indicators—namely, fluidity, water secretion rate, setting time, and 3-day unconfined compressive strength (UCS)—were systematically analyzed. Based on these findings, the optimal material ratios for shield sludge-based synchronous grouting materials were proposed. Subsequently, component geopolymer was prepared from the activated shield sludge and shield sludge without adding any additional alkaline activators by simply adding water. A geopolymer with a 28-day compressive strength of 51.08 MPa was obtained when the shield sludge dosing was 60 wt%. This study aims to provide a reference for the preparation of synchronous grouting materials for the resource utilization of shield sludge. Full article

To investigate the mechanism by which clay shock slurry fills excavation gaps and reduces ground layer deformation during shield tunneling, we conducted a study using the project example of Beijing Metro Line 19 from Youanmenwai Station to Niujie Station, which passes through Guang’anmennei Station to CaiShiKou Station of Beijing Metro Line 7 at a close distance. We employed physical and mechanical testing, numerical simulation calculations, and other methods to examine the deformation law and mechanism of the clay shock method in shield tunneling construction. Our results indicate that (1) as the mass concentration of clay shock slurry increases, its permeability decreases significantly; at a mass concentration of 400 kg/m³, clay shock slurry can prevent synchronous grouting slurry from flowing forward, providing optimal filling and support for excavation gaps. (2) Clay shock slurry can reduce friction between the shield shell and soil body by 50%, avoiding super-consolidation, shear damage, and volumetric expansion of the surrounding soil body. (3) Radial grouting with a two-fluid slurry of cement–water glass at a 1:1 ratio within 15 rings after shield tail removal effectively reduces settlement of the existing tunnel. (4) Numerical simulations demonstrate that using clay shock slurry to fill shield tunnel gaps not only significantly reduces construction settlement but also effectively inhibits strata displacement along the tunnel axis. Full article

During shield tunnel construction, waste mud is a significant source of urban construction waste. However, the disposal of waste mud has always been a challenge in engineering. Addressing the challenge of harmlessly disposing of, or repurposing, mud cakes formed after pressure filtration of shield mud remains a pressing issue for many cities. To address the challenge of shield mud disposal and explore the utilization technology of this resource, this study focuses on shield mud obtained from the Shenzhen subway tunnel. Calcined shield mud powder (CSMP) was prepared by activating its potential pozzolanic properties through a calcination process. Compressive strength tests revealed that, while CSMP exhibits some pozzolanic activity, its performance is limited. When 30% of the cement is replaced, the mortar’s maximum strength activity index (SAI) is only 82.6%, which makes it unsuitable as a supplementary cementitious material for concrete applications. At the same time, CSMP was also evaluated as a partial replacement for fly ash in the formulation of synchronous grouting materials, with performance metrics including fluidity, bleeding rate, hardening rate, setting time, and compressive strength systematically tested. The experimental results showed that, while CSMP reduces the fluidity of grouting, it significantly improves volumetric stability, shortens setting time, and enhances mechanical performance. Compared to the fly ash used in the study, CSMP exhibited better pozzolanic reactivity, promoting the formation of C-S-H and C-A-S-H phases, optimizing the pore structure, and increasing the density and overall performance of the grouting material. When the substitution rate is below 60%, the performance of grouting meets standard requirements, indicating the strong feasibility of utilizing CSMP to replace fly ash in synchronous grouting materials. Full article

Shield tunneling generates a massive amount of muck, and achieving the on-site reuse of muck is an urgent need in the field of shield tunneling. This study, based on a section of the Xianyang diversion tunnel in a loess stratum, aims to optimize the mix ratios of loess muck grouting materials to meet specific performance requirements. Laboratory tests were conducted to analyze the effects of the bentonite content and water–solid ratio on the properties of grout. The engineering properties, cost, and environmental impact of the optimized loess muck grouting materials were compared with those of traditional grouting materials. Additionally, XRD, SEM, and CT were employed to investigate the solidification mechanism of loess muck grouting materials. The results show that the bleeding rate, setting time, fluidity, and consistency of loess muck grouting materials decreased with increasing bentonite content, while these properties increased as the water–solid ratio rose. The compressive strength reached 0.26 MPa and 1.05 MPa at 3 d and 28 d, respectively. Compared to traditional grouting materials, the economic cost and carbon emissions of loess muck grouting materials were reduced by 49.46% and 37.17%, respectively. As the curing time increased, gel filling and particle agglomeration reduced the number of pores. The dense microstructure is the primary factor for the improvement of strength. Full article

This study explores the deformation characteristics of surrounding rock during tunnel construction through fault fracture zones. A numerical model is established using ABAQUS to analyze the interaction between the shield machine, support system, and geotechnical materials. The model incorporates key factors, including palm face support force, grouting pressure, and the friction between the shield shell and surrounding rock. The results show that the plastic zone of the surrounding rock is concentrated within the fault zone and at the junction with normal rock, propagating along the contact surface. In the loosening zone, stress and strength are significantly reduced, leading to crack expansion and plastic slip. Without adequate support, these conditions can result in tunnel destabilization. The displacement of the surrounding rock is most prominent during the detachment of the shield tail and the synchronized grouting phase. These findings provide valuable insights for improving tunnel construction safety and stability in fault fracture zones, where the integrity of the surrounding rock is compromised by fractures and fissures. However, the constructed models may restrict the ability to capture all complex material behaviors and interactions that could arise in actual field conditions. Full article

As shield tunnels increase, managing shield muck strains construction and the environment. To mitigate this problem, shield muck replaced bentonite in silty clay to improve synchronous grouting slurry. Initially, the physical attributes and microstructural composition of shield muck were obtained, alongside an analysis of the effects of the muck content, particle size, and general influencing factors on the slurry properties through standardized tests and regression models. Subsequently, leveraging three-dimensional response surface methodology, admixture interactions and multiple factor impacts on the slurry were explored. Finally, utilizing the SQP optimization technique, an optimal slurry blend ratio tailored for actual project needs was derived for improved muck slurry. The findings reveal with the decreasing bleeding rates as the muck content rises, the particle size diminishes. An inverse relationship exists between the muck content and slurry fluidity. At soil–binder ratios below 0.6, a decrease in the soil–binder ratio intensifies the influence of the water–binder ratio on the slurry density, bleeding rate, and setting time. The fly flash–cement ratio inversely correlates with the slurry bleeding rate, while the ratio greater than 0.6 is positively correlated. For muck particle sizes under 0.2 mm, the fly flash–cement ratio inversely impacts the density, while over 0.2 mm, it correlates positively. The optimal proportion for silty clay stratum synchronous grouting slurry, substituting muck for bentonite, includes a water–binder ratio of 0.559, binder–sand ratio of 0.684, fly flash–cement ratio of 2.080, soil–binder ratio of 0.253, particle size under 0.075 mm, and water-reducing admixture of 0.06. Full article

In urban shield tunneling, reducing the disturbance of underground construction to the surrounding environment is important for both tunnel engineers and researchers. Among other factors, the quality of synchronous grouting is one of the crucial factors affecting the safe construction of shields. In order to determine a reasonable grouting pressure and grout amount during shield construction, the relationships among synchronous grouting pressure, grout amount and shield chamber pressure are analyzed using field monitoring data. Based on the tunnel face pressure and the ultimate yield conditions of the soil at the gap edge, a method for calculating the grouting pressure considering the overburdening load of the tunnel was proposed. Then, by linking the grout amount and the grouting pressure, an accurate calculation method for the simultaneous grout amount in shield construction was proposed. These methods were then used in the construction of the Jurong shield tunnel. The results show that the adopted grouting pressure and grout amount calculated by the proposed method, which considered the change of the overburdening load of the tunnel, can well control the ground deformation caused by the shield construction and significantly reduce the uneven settlement of the surface buildings. The proposed methods in this paper may provide a reference for other shield construction projects. Full article

The purpose of this study is to investigate the feasibility of replacing the fly ash in synchronous grouting material by reusing the shield mud produced in the clay stratum during the shield construction of Wuhan Rail Transit Line 11. The test utilizes the shield mud from the clay stratum to replace the fly ash material in synchronous grouting at percentages of 20%, 40%, 60%, 80%, and 100%, and research and analyses are conducted on the fluidity, stability, strength, and resistance to water dispersion of the slurry after the replacement; at the same time, improvements in the undesirable phenomenon produced by the synchronous grouting slurry are also examined after the replacement. The results show that, when the fly ash is replaced by shield mud at 80%, the mortar still has good stability and strength performance, but, at the same time, the initial value of consistency and the phenomenon of flow time loss is too large. Through the adjustment of the water–binder ratio and the addition of an appropriate amount of a polycarboxylate superplasticizer agent, the adverse phenomenon of the slurry is effectively improved, and the compressive strength and ease of the slurry are also improved. At the same time, when adding an appropriate amount of hydroxyethyl methyl cellulose (HEMC), the slurry has good water dispersion resistance, but, with the gradual increase in HEMC, the fluidity of the slurry deteriorates and the compressive strength decreases. The test proves that the shield mud in the clay stratum can be used to replace most of the fly ash in an appropriate proportion, which not only solves the problem of the shield mud being difficult to work with, but also provides more valuable insights for tunneling projects. Full article

In response to the environmental pollution caused by transportation and accumulation of large-scale shield muck, the on-site reutilization of shield muck is an effective approach. This study explored the feasibility of silty clay muck to prepare muck grout. Through orthogonal experiments, the effects of cement, fly ash, shield muck, admixture, and the water–solid ratio on the fresh properties and mechanical properties of muck grout were studied. The performance prediction model was established Additionally, the intrinsic relationships between the compressive strength and microstructure of shield muck grouting materials were explored through multi-technology microstructural characterization. The results indicate that the content of muck and the water–solid ratio have a greater significant influence on the bleeding ratio, flowability, setting time, and volume shrinkage rate of muck grout compared to other factors. Cement has a greater significant influence on the compressive strength of muck grout than other factors. An optimal mix proportion (12% for cement, 18% for fly ash, 50% for muck, 0.465 for water–solid ratio, 19.5% for river sand, and 0.5% for bentonite) can produce grouting materials that meet performance requirements. The filling effect of cementitious substances and the particle agglomeration effect reduce the internal pores of grouting materials, improving their internal structure and significantly enhancing their compressive strength. Utilizing shield muck as a raw material for shield synchronous grouting is feasible. Full article

During shield construction in underground spaces, synchronous grouting slurry is poured between the surrounding rock and tunnel lining to ensure stability. For synchronous grouting slurries, few studies have investigated the relationship between the rheological parameters and physical properties, grout-segregation mechanism, and anti-segregation performance. Therefore, we explored the relationships between the slurry rheological parameters, segregation rate, and bleeding rate. Cement, sand, fly ash, and bentonite were used to prepare the slurry, and the effects of different polycarboxylate water-reducing agents and dispersible latex powder dosages were studied. The rheological parameters of 16 groups of uniformly designed slurries were tested, and the data were fit using the Herschel–Bulkley model. The optimal mix ratio lowered the slurry segregation rate, and its rheological behaviour was consistent with the Herschel–Bulkley fluid characteristics. High-yield-shear-stress synchronous grouting slurries with high and low viscosity coefficients were less likely to bleed and segregate, respectively. The optimised slurry fluidity, 3 h bleeding rate, 24 h bleeding rate, segregation rate, coagulation time, and 28 days compressive strength were 257.5 mm, 0.71%, 0.36%, 3.1%, 6.7 h, and 2.61 MPa, respectively, which meet the requirements of a synchronous grouting slurry of shield tunnels for sufficiently preventing soil disturbance and deformation in areas surrounding underground construction sites. Full article

The pressure distribution and the force on tunnel segments of synchronous grouting in the shield tail gap channel of shield tunnels are key to controlling the stability and surface settlement of the strata surrounding such tunnels. Based on the basic principles of fluid mechanics and the limit equilibrium method, this study establishes a mathematical model of synchronous grouting in shield tunnels, derives the expressions of the grouting pressure and the force on tunnel segments in the shield tail gap channel, and verifies them using an engineering case study. Studies have shown that the force on tunnel segments and the speed of shield excavation are increasing. An excessive shield excavation speed will cause the load on tunnel segments to increase, which exacerbates the uneven distribution of the grouting pressure. The force on tunnel segments and the grouting pressure also have a positive relationship with the thickness of the shield tail gap, but the impact is limited to a certain range. With an increase in the tunnel radius, the number of grouting holes should be appropriately increased to balance the water and soil pressure in the surrounding strata. These research results can provide a theoretical reference for the design of synchronous grouting for shield tunnels in the future. Full article