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26 pages, 12645 KiB

Open AccessArticle

Development and Application of Methodology for Quantification of Overbreaks in Hard Rock Tunnel Construction

by Zoran Berisavljević, Dragoljub Bajić and Vojkan Jovičić

Appl. Sci. 2023, 13(3), 1379; https://doi.org/10.3390/app13031379 - 20 Jan 2023

Cited by 9 | Viewed by 2655

A methodology for determining overbreaks in hard rock tunnel construction using the drill-and-blast technique is presented in this paper. The methodology was developed for and applied to crystalline medium- to thick-bedded limestone, but it can be used in any jointed hard rock mass. [...] Read more.

A methodology for determining overbreaks in hard rock tunnel construction using the drill-and-blast technique is presented in this paper. The methodology was developed for and applied to crystalline medium- to thick-bedded limestone, but it can be used in any jointed hard rock mass. Overbreaks are inevitable in hard rock tunnelling in a low-confinement environment (shallow tunnels up to several hundred meters deep) as a result of wedge failures along unfavourably oriented discontinuities caused by blasting. It is widely accepted in engineering practise that overbreaks will be inevitable even if smooth contour blasting is applied. If not controlled, overbreaks can result in extreme financial and time costs; and determining, predicting, and mitigating them is the key to successful tunnel construction in hard rock. Technological overbreaks, which are caused by the inappropriate use of drill-and-blast excavation, are not easily distinguished from the inevitable overbreaks dictated by the geological conditions with which they interfere and overlap. A methodology was developed with the aim of distinguishing the two causes of overbreaks, which can be applied in any phase of tunnel construction for evaluation or mitigation. The analysis of key inputs, including geological face mapping, shear strength tests along discontinuities of the rock mass, and their spatial orientation relative to tunnel advancement and survey overbreak measurements, is presented in this paper. Due to the stochastic and statistical nature of the problem, a probabilistic concept was also applied as part of the method so that the probability of failure around unprotected tunnel sections could be determined. The so-called stability criterion is introduced to distinguish between stable and unstable sections in terms of the probabilistic safety factor. The quantification of overbreaks, including the threshold value distinguishing technological from geological overbreaks, is proposed. The application of the methodology, demonstrated on an 8.1 km long section of a 12 km long pressure tunnel in hard rock, is presented in the paper. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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17 pages, 6670 KiB

Open AccessArticle

Strength Characteristics and In Situ Stress Measurement of Rock Cores from a Large Section Underground Station: A Case Study

by Diyuan Li, Aohui Zhou, Lichuan Wang, Yuda Chen and Chunde Ma

Appl. Sci. 2022, 12(21), 10873; https://doi.org/10.3390/app122110873 - 26 Oct 2022

Cited by 1 | Viewed by 2069

Abstract

Chongqing is a mountain city in western China with a complex geological environment, which brings many difficulties to the construction of large sections of underground projects. In order to avoid serious problems in the construction of Guobo Station with large sections, intensive coring [...] Read more.

Chongqing is a mountain city in western China with a complex geological environment, which brings many difficulties to the construction of large sections of underground projects. In order to avoid serious problems in the construction of Guobo Station with large sections, intensive coring and laboratory tests were carried out to obtain the rock mass properties in detail. The boreholes reveal that the lithology of the rock mass in the project area consists mainly of mudstone and a small area of sandstone. The uniaxial compressive strength (UCS) and the Brazilian tensile strength (BTS) of mudstone are highly scattered, which are 4.43~42.43 MPa and 0.66~4.30 Mpa, respectively, but they all show a lognormal distribution. The average UCS of sandstone is 65.31 MPa, and the BTS is 3.06 MPa. Meanwhile, the softening coefficients of mudstone and sandstone are 0.33 and 0.73, which indicate that the water content has a great influence on the mechanical properties of the rock. For the in situ stress field, the relationship between the three principal stresses is as follows: the maximum horizontal principal stress > minimum horizontal principal stress > vertical principal stress, and the lateral pressure coefficient is 2.11~5.71. Based on the present experimental results, it is proposed that the potential risks of excavation include: (1) bias pressure (2) high in situ stress; (3) surrounding rock deterioration. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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18 pages, 3373 KiB

Open AccessArticle

Determination of the Length of the Rock Bolts for Tunnels with Consideration of the Nonlinear Rheological Behavior of Squeezing Rock

by Yu Jiang, Ning Li, Hao-Hong Jiang, Ming-Liang Zhou and Jiao-Long Zhang

Appl. Sci. 2022, 12(17), 8560; https://doi.org/10.3390/app12178560 - 26 Aug 2022

Viewed by 2641

Abstract

An accurate model for the prediction of the rheological behavior of surrounding rocks is essential to the designing of rock bolts for tunnels under squeezing conditions. Our understanding of the state of the art suggests that the rheology of rock consists of the [...] Read more.

An accurate model for the prediction of the rheological behavior of surrounding rocks is essential to the designing of rock bolts for tunnels under squeezing conditions. Our understanding of the state of the art suggests that the rheology of rock consists of the transient, the steady-state, and the accelerated regimes. Thus, a visco-elastic-plastic rheological model, namely the Komamura-Huang-Bingham model, was developed. The model used the Drucker-Prager yield criterion in order to consider the effects of the intermediate principal stress on the strength of the rock mass. The developed model was implemented in the framework of finite element simulations. It was validated by comparing the simulation results with the on-site monitoring data. The focus of the simulations was on the mechanical behavior of the rock bolts and the surrounding rock mechanics during the construction of the tunnel. A sensitivity analysis was performed with respect to the length of the bolts and the stress-to-strength ratio of the rock was performed. It was shown that increasing the length of the bolts up to 9 m results in a reduction in rock deformation. This critical value of the bolts’ length is approximately equal to the diameter of the investigated tunnel. A further increase in the length results in an insignificant reduction in the deformation of the surrounding rock. In the case of the Class A and B squeezing conditions suggested by Hoek, the deformation of the surrounding rock is insensitive to the length of the bolts. The elongation of the bolts does not change considerably with time, which is in contrast to the Class C and D squeezing conditions. The extent of the plastic zone is related to time and in situ stress but is independent of the length of the bolts. This is consistent with the characteristics of the nonlinear rheology of rock mass. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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14 pages, 4377 KiB

Open AccessArticle

An Improved Wavelet Threshold Denoising Method for Health Monitoring Data: A Case Study of the Hong Kong-Zhuhai-Macao Bridge Immersed Tunnel

by Xinghong Jiang, Qing Lang, Qiang Jing, Hui Wang, Juntao Chen and Qing Ai

Appl. Sci. 2022, 12(13), 6743; https://doi.org/10.3390/app12136743 - 3 Jul 2022

Cited by 15 | Viewed by 3597

Abstract

Tunnels generally operate underground or underwater in a complex environment. As a result, the health monitoring system is inevitably affected by various environmental factors, which introduces noise to the system. However, the noise contained in the monitoring sequence may disrupt structural damage identification [...] Read more.

Tunnels generally operate underground or underwater in a complex environment. As a result, the health monitoring system is inevitably affected by various environmental factors, which introduces noise to the system. However, the noise contained in the monitoring sequence may disrupt structural damage identification and health state assessment as the real structural response may be overwhelmed by the noise. To properly eliminate the noise in an objective way, this study proposed an improved wavelet threshold denoising method. Firstly, it adopts a quantitative factor, namely the Sparse Index, to assist the selection of the best wavelet basis in numerous wavelet packages. Then, the decomposition layer and threshold are optimized by a comprehensive evaluation based on a variation coefficient method. At last, the application of the concrete strain health monitoring data of the Hong Kong-Zhuhai-Macao Bridge immersed tunnel verified the effectiveness of the proposed method. It is found that the combination of sym12 and five decomposition layers can obtain the best denoising results within the selected wavelet families and decomposition levels. Moreover, the proposed method achieves good denoising results under different fluctuation levels. Thus, the proposed method is reliable, can solve the problem of optimal parameter selection such as decomposition level and wavelet basis in wavelet denoising, and can be applied in the structural health monitoring of critical infrastructures. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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29 pages, 74483 KiB

Open AccessArticle

Behaviour of Horseshoe-Shaped Tunnel Subjected to Different In Situ Stress Fields

by Wael R. Abdellah, Abdel Kader A. Haridy, Abdou Khalaf Mohamed, Jong-Gwan Kim and Mahrous A. M. Ali

Appl. Sci. 2022, 12(11), 5399; https://doi.org/10.3390/app12115399 - 26 May 2022

Cited by 7 | Viewed by 4702

Abstract

At great depths, tunnel openings experience a tectonic stress field rather than overburden stress. This paper aims to examine the impact of different in situ stress ratios and multiple tunnel depths below the surface on the excavation induced-stresses and displacements around tunnel openings. [...] Read more.

At great depths, tunnel openings experience a tectonic stress field rather than overburden stress. This paper aims to examine the impact of different in situ stress ratios and multiple tunnel depths below the surface on the excavation induced-stresses and displacements around tunnel openings. Thus, a series of models has been built, using a two-dimensional elasto-plastic finite-elements code, RS2D, to conduct parametric stability analysis. The performance of tunnel opening is examined by evaluating the induced stress-deformation around the opening. The results indicate that ratio of wall convergence, roof sag and floor heave increase as in situ stress ratio and tunnel depth below surface increase. Additionally, the induced-stresses increase as depth and state of in situ stress increase. In addition, the extent of yielding zones into rock mass around tunnel roof and floor deteriorates as tunnel depth and in situ stress ratio increase. Moreover, the normal stress along rock joints is sharply dropped when joints pass in the vicinity of tunnel opening (e.g., centre of opening). As well, the direction of shear stress along joints is reversed. Consequently, inward shear displacement of rock, on the underside of the weakness plane, is produced as a result of slip occurrence. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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12 pages, 3463 KiB

Open AccessArticle

Characteristics of Dynamic Safety Factors during the Construction Process for a Tunnel-Group Metro Station

by Qingfei Li, Ruozhou Li, Weiguo He, Xin Gao, Xupeng Yao, Yong Yuan and Jiaolong Zhang

Appl. Sci. 2022, 12(10), 4900; https://doi.org/10.3390/app12104900 - 12 May 2022

Cited by 2 | Viewed by 2043

Abstract

Dynamic safety factors during the construction of an overlapping tunnel-group metro station were studied in the framework of the strength-reduction finite element method. Based on the equivalent plastic strain and displacement of surrounding rock, its damage mode under typical excavation conditions was investigated. [...] Read more.

Dynamic safety factors during the construction of an overlapping tunnel-group metro station were studied in the framework of the strength-reduction finite element method. Based on the equivalent plastic strain and displacement of surrounding rock, its damage mode under typical excavation conditions was investigated. The aim of this investigation was to provide information for the design activities concerning the supporting system of the station and the pre-reinforcement of its surrounding ground. The accuracy of the model was assessed by comparing the ground settlements obtained from on-site monitoring with those from the numerical model. The analysis results show that the safety factor reaches the minimum when the No. 3 guide hole of the station hall is excavated. Thus, this is the most dangerous construction step. During this step, the plastic zone penetration phenomenon occurs in the surrounding rock, which is sandwiched between the hall and the platform of the station. In this case, both the deformation of the surrounding rock and the internal forces of the lining increase. The surrounding rock in the sidewall loses its stability. Thereafter, the primary support plays a role of stabilizing the guide hole. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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17 pages, 5046 KiB

Open AccessArticle

Analysis of Vertical Earth Pressure Acting on Box Culverts through Centrifuge Model Test

by Inyeop Chu, Sang-Kyun Woo, Sang Inn Woo, Joonyoung Kim and Kicheol Lee

Appl. Sci. 2022, 12(1), 81; https://doi.org/10.3390/app12010081 - 22 Dec 2021

Cited by 6 | Viewed by 4225

Abstract

Due to the lack of surface space, most structures are heading underground. The box culvert is underground infrastructure and serves to protect the buried structure from the underground environments, but it has a different characteristic from other structures in that the inner space [...] Read more.

Due to the lack of surface space, most structures are heading underground. The box culvert is underground infrastructure and serves to protect the buried structure from the underground environments, but it has a different characteristic from other structures in that the inner space is empty. Therefore, in this study, the vertical earth pressure which is the most significant effective stress acting on a box culvert was measured by conducting a geotechnical centrifuge model test. A box culvert was installed following the embankment installation method, and the vertical earth pressure acting on it was measured considering the cover depth, gravitational acceleration, and loading and unloading conditions. The soil pressure measured was greater than the existing theoretical value under high cover depth and the unloading condition, which is considered as the variability of many soils or the residual stress acting under the loading condition. Finally, a goodness-of-fit test was conducted as a part of variability analysis. The measured earth pressure was found to be considerably larger than the existing theoretical value, and the variability was large as well. This means the existing theoretical equation is under-designed, which should be reflected in future designs. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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19 pages, 10332 KiB

Open AccessArticle

Mechanical Characteristics of Structures and Ground Deformation Caused by Shield Tunneling Under-Passing Highways in Complex Geological Conditions Based on the MJS Method

by Zikun Li, Jianbing Lv, Xiaodong Xie, Helin Fu, Juan Huang and Zhanrong Li

Appl. Sci. 2021, 11(19), 9323; https://doi.org/10.3390/app11199323 - 8 Oct 2021

Cited by 13 | Viewed by 2582

Abstract

This study defined the height ratio of soft-rock strata and established a numerical model for analyzing shield construction in upper-soft, lower-hard composite strata together with field monitoring data. In this way, the influence of shield tunneling while passing under the pile foundation of [...] Read more.

This study defined the height ratio of soft-rock strata and established a numerical model for analyzing shield construction in upper-soft, lower-hard composite strata together with field monitoring data. In this way, the influence of shield tunneling while passing under the pile foundation of the culvert at a short distance (the shortest distance is 1.4 m) in the typical upper-soft, lower-hard composite strata in Guangzhou can be examined. Moreover, the reinforcement effects of the ground, the bridges, and the culverts, using the strata-reinforcing plan dominated by the metro jet system (MJS) in a narrow space, are evaluated. Based on the results, (i) the maximum ground subsidence is found at the position in which the height ratio of the soft rock is 1.0. (ii) However, differential subsidence might be found in the subsequent shield construction when the soft-rock height ratio of the adjacent excavated surface ranges from 0 to 0.2 and from 0.5 to 1. (iii) The concentrated release of stress has a greater impact on the structure than the geological conditions of the shield tunneling face. (iv) Reinforcing with the MJS method contains the concentrated release of stress. This study can provide a reference for controlling the deformation of the under-crossing structure in the shield construction of the upper-soft, lower-hard composite strata. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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22 pages, 5620 KiB

Open AccessArticle

Strain-Softening Analyses of a Circular Bore under the Influence of Axial Stress

by Xuechao Dong, Mingwei Guo and Shuilin Wang

Appl. Sci. 2021, 11(17), 7937; https://doi.org/10.3390/app11177937 - 27 Aug 2021

Viewed by 1925

Abstract

Strain-softening analyses were performed around a circular bore in a Mohr–Coulomb rock mass subjected to a hydrostatic stress field in cross section and out-of-plane stress along the axis of the bore. Numerical procedures that simplify the strain-softening process in a step manner were [...] Read more.

Strain-softening analyses were performed around a circular bore in a Mohr–Coulomb rock mass subjected to a hydrostatic stress field in cross section and out-of-plane stress along the axis of the bore. Numerical procedures that simplify the strain-softening process in a step manner were employed, and on the basis of the theoretical solutions of the elastic–brittle–plastic (EBP) medium, the strain-softening results of the displacements, stresses and the plastic zones around the circular bore were obtained. The numerical solution was validated based on the fact that the strain-softening process became EBP when the softening slope was very steep and elastic-perfectly plastic (EP) when the softening slope was near zero. The results illustrated that the stresses and displacements in the rock mass surrounding the bore were affected by axial stress and that a proper consideration of out-of-plane stress was necessary. Moreover, the presented results can be used for the verification of numerical codes. Full article

(This article belongs to the Special Issue Advances in Developing Underground)

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