Abstract

As a leading cause of injuries in underground coal mines, roof fall has been widely investigated with different methods. The majority of the research was conducted without considering the time factor. In fact, field observation show that the roof fall involves a time-dependent failure process. To investigate roof failure, UDEC was used to simulate the time-dependent fracturing process associated with creep of rock mass. The rock mass around an opening was meshed into small Voronoi blocks, and parameters for the joints between these blocks were calibrated to make uniaxial compressive strength of a 4m×8m rock block approximately equal to that of rock mass. The cracks caused by stress redistribution under different conditions can be captured by this method. After equilibrium, creep simulation was run to a predetermined time. Limited cracks forming within roof and intensive cracks forming within coal pillars are observed under different conditions, including high stress ratio, laminations, and smaller excavation width. When the yield condition and crack formation are combined, better simulation results can be obtained.

1. INTRODUCTION

According to MSHA (2014), improvements in roof control technology have significant decreased fatality, but roof fall accidents are still a leading cause of injuries in underground coal mines. This safety problem has been investigated for decades. Overbey et al. [1] considered surface fracture trace density, changes in roof rock type, and topographic unloading due to drainage the three major factors affecting the occurrence of roof fall, and proposed a technique to predict probable roof fall areas ahead of mining operations. Aggson [2] theoretically summarized the roof failure mechanisms under different conditions. Su and Peng [3] examined the intrinsic mechanisms of cutter roof failure by combining field investigation, laboratory testing, underground instrumentation, and numerical modeling, and found that high vertical stress coupled with a stiffer coal is the dominant factor in the formation of cutter roof. Gadde and Peng [4] proposed one approach using a strain-softening model to simulate the cutter initiation and propagation process. Then this method was used by Ray [5] to study the influence of cutting sequence on cutters and roof fall in underground mines. Gao and Stead [6] simulated the formation of cutter roof failure with 3DEC and PFC3Dwith respect to the orientation of major horizontal stress and the forward of excavation. Coggan et al. [7] used continuum, discontinuum, and hybrid finite element-discrete element codes to model the deformation of Coal Measure strata, and the modelled results demonstrated that the thickness of the relatively weak mudstone in the roof of the tunnel has a significant influence on the extent of failure.

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