Influence of fracture-induced weakening on coal mine gateroad stability
Introduction
The stability of roadways is a long-standing issue in underground coal mines, especially for gateroads that serve and ensure the safe production of longwall panels. Ground stability and failure mechanisms of roadways vary depending on stress, geological and geotechnical conditions. However, the difficulty of maintaining gateroad stability stems from the weak surrounding rock mass and continuous geotechnical disturbance. Serving the longwall panels, gateroads are always located in the coal seam, which generally has relatively weak properties compared to roof and floor rock formations. A database of coal mine rock mechanical properties, which consists of over 4,000 samples from fifty coal seams in ninety mines, was developed by Sun and Peng 1. According to their work, most coal seams have uniaxial compressive strength less than 34 MPa and tensile strength less than 2.7 MPa. Thus it can be expected that the weak properties of coal contribute significantly to the deformation of a gateroad once it is driven. Secondly, after coal in a longwall panel is extracted, and a void (goaf) behind the face is created, rock fracturing and yielding as well as stress concentration in the surrounding rock take place. Because of coal extraction, the gateroad is subjected to a complex loading pattern. In some cases such as gateroad entry layouts driven along goaf-edge, 2 the gateroad will furthermore undergo multiple mining influences.
Numerous studies have been done in different methods to estimate the stability of gateroads and design support schemes. Empirical methods 3, 4, 5 are often utilized based on the statistical analyses of adequate data collected in field practices. Many studies employ analytical methods 6, 7, 8 for better understanding of the failure mechanism and rock support performance. In recent years, an increasing number of studies have been done with different numerical methods. 9 Although numerical modelling is a state-of-the-art technique for rock mechanics study and different numerical methods are utilized, the constitutive models and parameters used to simulate the behavior of the rock mass ought to be rigorously considered and calibrated.
Many researchers analyzed gateroad stability with numerical modelling methods for the past few years. The most commonly used constitutive models are the elastic-perfectly plastic and strain-softening models, which use Mohr-Coulomb failure criterion. Shen 10 assessed the stability of a soft rock roadway with discrete element software UDEC whilst assuming strain-softening of the soft rock. In the study, the strain-softening parameters are determined based on previous simulation experiences for coal and surrounding strata. Zhang 11 evaluated rock bolt design under the effect of a longwall mining face with a strain-softening model for all rock strata. In this model, a plastic shear strain threshold of 0.3% and the residual cohesions are determined. In previous studies, 2, 12, 13 strain-softening model is applied for coal, and elastic-perfectly plastic model is chosen for other rock materials.
Due to the sedimentary effect of coal-forming process, the surrounding rock mass of underground coal mines exhibit a geologically stratified structure. Under this geological feature, tensile failure plays a dominant role especially when the surrounding rock is not subjected to high horizontal stress (ratio of horizontal-to-vertical stress >3). 14, 15 Rock failure due to underground coal mining activities is also demonstrated with field investigation 16 and numerical analysis. 10, 17 Since joints and other fractures in the rock can offer little or no resistance to tensile stresses, fractures will take place and develop in a brittle manner when the rock mass is subjected to tensile stress. 18 Other studies 19, 20, 21, 22 elaborate on the relation between the modulus of elasticity of rock mass and the occurrence of cracks and fractures, and corresponding formulas are proposed. In light of previous studies examining the post-peak behavior of surrounding rock, it is reasonable to take the variation of elastic modulus during tensile failure into consideration for stability analysis of openings in underground coal mines.
In the present study, tension-weakening model, which allows for the reduction in the rock mass stiffness due to fracture generation is developed and implemented with FISH, which is a programming language embedded within 3-dimensional explicit finite-difference software FLAC3D. The study examines gateroad stability at the Zhaogu No.2 mine. In the numerical model, the failure state of the rock mass is continuously monitored during the analysis, and the properties are weakened according to failure state to simulate the post-failure behavior. Numerical simulations with elastic-perfectly plastic model, strain-softening model and tension-weakening model are conducted for comparison with field measurements. In addition, a model parametrical study with respect to the tension-weakening model is carried out to examine the effect of tension-weakening and provide a basis for gateroad stability investigation and rock support design.
Section snippets
Geology and geotechnical overview of Zhaogu No.2 mine
Zhaogu No.2 mine is located in Xinxiang City, Henan Province, China. All panels in this mine are using retreating longwall method to extract coal seams. The coal seam is nearly horizontal with a mean thickness of 6.12 m.
The target gateroad for this case study is the tailgate of panel 11050 at a depth of 600 m. The panel is approximately 180 m wide along the dip and 2000 m long along the strike as illustrated in Fig. 1. A typical geological column based on core logging is shown in Fig. 2. The roof
Numerical simulation with elastic-perfectly plastic and strain-softening models
In the following sections, the tailgate of panel 11050 is numerically modelled to investigate its ground stability using finite-difference software FLAC3D.
Description of tension-weakening model
When elastic-perfectly plastic behavior is assumed, the mechanical properties remain constant after the material yields and fails. In terms of major and minor principal stresses, σ1 and σ3, the failure criterion of elastic-perfectly plastic model with the Mohr-Coulomb failure criterion can be expressed as
Eq. (1) indicates that the stress state does not change after failure took place, which obviously is not in accordance with the true characteristic of most
Mesh dependency
Since the implemented tension-weakening model achieves the stiffness weakening of rock mass due to tensile failure by examine the failure state and accordingly process the elastic modulus of each zone. Therefore, the mesh density of numerical model, especially for rock mass near gateroad, might be sensitive to simulation results. The mesh dependency of the tension-weakening model is investigated by two simulations which are identical to Section 4.3 with two models (Model #2 & #3)that only
Conclusion
A case study on gateroad stability at the Zhaogu No. 2 coal mine in China is carried out. According to field observations and monitoring results, the floor and two ribs of the gateroad deform more significantly than the roof. Severe fractures and significant deformation severely destabilize the surrounding rock, which leads to potentially high risk to personnel safety.
Based on the geotechnical characteristics of the gateroad, a tension-weakening model allowing for the stiffness weakening of
Acknowledgements
The research of this study was sponsored by the National Natural Science Foundation of China (Grant No. 51234005, 51434006, 51374139, 51574155) and the China Scholarship Council. The authors are grateful for their support.
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