Model test study on surrounding rock deformation and failure mechanisms of deep roadways with thick top coal

doi:10.1016/j.tust.2014.12.013

Tunnelling and Underground Space Technology

Volume 47, March 2015, Pages 52-63

https://doi.org/10.1016/j.tust.2014.12.013 Get rights and content

Highlights

•
A combined type large-scale geomechanical model test system applicable to deep roadways with thick top coal was developed.
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Model test typical deep roadway with thick top coal was conducted.
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The surrounding rock deformation and failure mechanisms of deep roadways with thick top coal were analyzed.
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The deformation and failure on the roof of the deep roadways with thick top coal are more serious.

Abstract

The deep roadway with thick top coal is a typical roadway difficult to support in deep mining project, and resolving this sophisticated problem to control the stability of surrounding rock is of great significance in safety production of coal mines. In order to explore the surrounding rock deformation and failure mechanisms of such deep roadways, with Zhaolou coal mine in Juye mining area of China as the engineering background, a large-scale geomechanical model test was carried out. The displacement and stress evolution laws of surrounding rock supported by the pressure relief anchor box beam system were researched. Meanwhile, the related results validate and analyze main failure characteristics and mechanisms of the surrounding rock by comparing with field test.

Graphical abstract

Introduction

China produces the most coal in the world. With the rapid development of the national economy and state higher attention to energy strategies, the demand for coal, primary energy in China, has significantly increased in recent years. As shallow resources decrease, coal mining gradually tends to deep resources. Under the influences of high in-situ stress, high permeability, high geotherm, and strong mining disturbances in deep complex geological conditions, roadway supporting in such seriously broken surrounding rock is of huge cost and difficulty. In fact, at the beginning of the 20th century, deep soft rock supporting as a worldwide difficulty was brought up and discussed. In particular, roadway supporting is always a major problem in coal mine production and construction, as presented by He and Sun (2004). Data shows that the thick coal seam accounts for approximate 45% of China’s total coal reserves and production. As a typical roadway difficult to support in the thick coal seam, the deep roadway with thick top coal is easy to produce roof accidents endangering coal mining safety owing to the thick top coal and the resulting wide range of the surrounding rock failure zone, long-lasting deformation, and serious roof separation or convergence after roadway excavation. Therefore, it is significant to study the failure mechanism of surrounding rock in deep roadways with thick top coal and propose advanced roadway supporting technologies to resolve the surrounding rock control problem in such roadways are of great significance in coal mine safety production.

In general, the following approaches to analyzing the stability of underground caverns have been widely adopted: theoretical analysis, numerical simulation, geomechanical model test, and engineering analogy. Theoretical analysis (Sharan, 2003, Cohen et al., 2009, Fraldi and Guarracino, 2010) and numerical simulation (Jing and Hudson, 2002, Addenbrooke et al., 1997, Eberhardt, 2001) are more popular than model tests in economy and convenience. However, in case of complex geotechnical problems, they need to be simplified but then fail to fully describe the complex field geological conditions while geotechnical model tests can directly reflect the physical and mechanical phenomena of the under-study system and quantitatively or qualitatively reflect the rock mechanical characteristics and interaction effects with related engineering structures. Hence, the model test can make up the inadequacy of theoretical analysis, numerical simulation and field tests, and is a classic method in underground engineering research. Meguid et al. (2008) proposed the experiment method and principle in view of the geological model test on underground tunnels in soft rock; Tetsuo et al. (1996) researched the mechanical behaviors of jointed rock masses around an underground opening under excavation using a large-scale geological model; Zhu et al. (2010) made a true triaxial model by piling up and bonding precast model blocks and conducted model tests on the stability of Shuangjiangkou hydropower station cavern groups; Li et al. (2005) analyzed the stability of Xiluodu hydropower station cavern groups through three-dimensional geomechanical model tests and compared the results with numerical simulation results; Huang et al. (2012) systematically studied the instability and failure patterns of tunnels with weak interlayers through model tests; Chen et al. (2014) researched the dynamic failure process around a circular cavern in hard and brittle rock under the high and increasing natural stress conditions. However, the experimental studies mentioned above focused on traffic tunnels, hydropower stations and so on. The model tests on the surrounding rock deformation and failure mechanisms and supporting technologies of deep roadways with thick top coal have not been conducted.

As a newly ascertained oversize coal field, Juye coal field retains the largest coal deposit in eastern China. Zhaolou coal mine is located in the central part of Juye coal field. The roadways in this mining area are usually driven along the coal seam floor. Taking the crossheading of working face 3302 as an example, it is a typical deep roadway with about 5 m thick top coal. It is pretty arduous to ensure the stability of this roadway given the complex field geological conditions and about 1000 m burial depth. By analyzing the deformation characteristics of deep roadways with thick top coal and existing problems about the anchor beam support system, Li et al., 2012, Wang et al., 2012a developed the pressure relief anchor box beam support system, designed different support schemes, and conducted numerical tests, field tests and theoretical analysis on the preceding crossheading. The results show that the pressure relief anchor box beam support system can better support such roadways and is an ideal structure form for supporting deep roadways with thick top coal. In this context and on the basis of preceding researches, this paper focuses on the research of surrounding rock stress and displacement evolution laws through large-scale geomechanical model tests and with the pressure relief anchor box beam support system, and compares the research results with field test results to finally obtain surrounding rock deformation and failure characteristics and mechanisms of deep roadways with thick top coal. This paper can be used as a reference for theory and support parameters of deep roadways with thick top coal.

Section snippets

Background

Located in the southwest of Shandong province, Juye coal field is an oversize coal field in eastern China. As an integrated coal mine with the largest reserves in eastern China, it has a total of 5.57 billion tons of ascertained geological reserves. The coal beds are buried in depths from 800 m to 1300 m with the average thickness of coal in main mineable coal seam 3# about 8 m. 7 pairs of mines, Longgu Coal Mine, Zhaolou Coal Mine, Guotun Coal Mine, Yuncheng Coal Mine, Wanfu Coal Mine, Pengzhuang

Determination and calculation of similarity scale

Analog scales are determined by considering field geological conditions, research objectives and performance of testing equipment. Specifically, the geometric similarity scale C_l and bulk density similarity scale C_γ of the experimental model is 15 and 1.5 respectively. Analog scales of other parameters can be deduced according to the analogy theory as follows: $C_{σ} = C_{E} = C_{c} = 22.5$ $C_{ε} = C_{f} = C_{μ} = 1$ where C_σ is similarity scale of stress, C_E is similarity scale of elastic modulus; C_c is similarity scale of

Large-scale model test system

A combined large-scale geomechanical model test system applicable to deep roadways with thick top coal is developed. The steel frame of test devices in this system is 0.5 m in width, 3.25 m in height and almost 5.2 m in length at the bottom boundary, which can be used to conduct tests on the model with a size of 2.4 m × 2.4 m × 0.5 m. In the central part, a transparent window with high-strength toughened glass is equipped to observe the process of excavating, supporting, the deformation and fracture of

Variation analysis of surrounding rock displacement

The laws of surrounding rock internal deformation and roadway convergence were analyzed based on FBG multi-point monitoring data and digital photography monitoring data. Fig. 13, Fig. 14, Fig. 15 show some monitoring results of the model internal multi-point extensometer measurement system on monitoring Section 2.

Fig. 16 shows the digital image of the roadway and displacement vector graph of surrounding rock during excavation, which are obtained by the Digital Photogrammetry & Deformation

Conclusions

This paper analyzes the deformation and failure characteristics and mechanisms of surrounding rock based on the conducted large-scale geomechanical model tests on deep roadways with thick top coal, compares model test results with field test results, and comes to the following conclusions:

(1)
The surrounding rock stress presents a step-wise constant decrease during roadway excavation; the effect of excavating on the stress decreases with the increasing of distance to the roadway surface; the stress

Acknowledgments

Much of the work presented in this paper was supported by the Natural Science Foundation of China (Nos. 51304125 and 51379114), the Open Fund of State Key Laboratory Open Fund of Geomechanics and Geotechnical Engineering (No. Z013004), the Innovation Fund for Postdoctor of Shandong Province (No. 201301004) and the Award Fund for Outstanding Young and Middle-Aged Scientist of Shangdong Province (BS2013NJ004). The authors would like to express appreciation to the reviewers for their valuable

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View full text

Model test study on surrounding rock deformation and failure mechanisms of deep roadways with thick top coal

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Background

Determination and calculation of similarity scale

Large-scale model test system

Variation analysis of surrounding rock displacement

Conclusions

Acknowledgments

Int. J. Solids Struct.

Int. J. Rock Mech. Min. Sci.

Int. J. Solids Struct.

Int. J. Rock Mech. Min. Sci.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Int. J. Rock Mech. Min. Sci.

The influence of pre-failure soil stiffness on the numerical analysis of tunnel construction

Geotechnique

Physical simulation on failure around a circular cavern in hard and brittle rock under the high and increasing natural stress conditions

J. Eng. Mech.

Mechanism and Design Method of Prestressed Anchor Cable