Experimental Study on the Fracture Distribution Characteristics of the Overlying Strata in an Abandoned Gob

A lot of gas resources remain in the abandoned gob. The overlying strata of the abandoned gob are the main places for gas storage and flow. The fracture distribution characteristics of the overlying strata have a significant impact on the gas migration. The mining similarity simulation test device of a plane stress was used to study the deformation and failure characteristics of overlying strata in an abandoned gob. The caved strata of the abandoned gob formed a trapezoidal distribution, and the caving range decreased gradually with an increase in distance from the coal seam. The strata collapsed in the caved zone, whereas the strata collapsed mainly on the bending subsidence in fractured zone. The subsidence curves of caved strata showed a lower concave shape, and the maximum subsidence existed in the middle of the abandoned gob. The caved strata subsidence decreased with an increase in distance from the coal seam. The horizontal fractures were dominant in the fractured zone. The abscission rate of the end mining position was greater than that of the start mining position. Large numbers of vertical fractures existed in the caved zone. The development degree of vertical fractures near the end mining position were larger than that of the start mining position, and the width of the gas-conducting fracture was more than three times that of the start mining position. The development degree, quantity and connectivity of the fracture in the end mining position were better than those in the start mining position.

Experimental Investigation on Hydraulic Properties of Granular Sandstone and Mudstone Mixtures

The caved zone during longwall mining has high permeability, resulting in a mass of groundwater storage which causes a threat of groundwater inrush hazard to the safe mining. To investigate the hazard mechanism of granular sandstone and mudstone mixture (SMM) in caved zone, this paper presents an experimental study on the effect of sandstone particle (SP) and mudstone particle (MP) weight ratio on the non-Darcy hydraulic properties evolution. A self-designed granular rock seepage experimental equipment has been applied to conduct the experiments. The variation of particle size distribution was induced by loading and water seepage during the test, which indicated that the particle crushing and erosion properties of mudstone were higher than those of sandstone. Porosity evolution of SMM was strongly influenced by loading (sample height) and SP/MP weight ratio. The sample with higher sample height and higher weight ratio of SP achieved higher porosity value. In particular, a non-Darcy equation, for hydraulic properties (permeability κ and non-Darcy coefficient ζ) calculation, was sufficient to fit the relation between the hydraulic gradient and seepage velocity. The test results indicated that, due to the absence and narrowing of fracture and void during loading, the permeability κ decreases and the non-Darcy coefficient ζ increases. The variation of the hydraulic properties of the sample within the same particle size and SP/MP weight ratio indicated that groundwater inrush hazard showed a higher probability of occurrence in sandstone strata and crushed zone (e.g., faults). Moreover, isolated fractures and voids were able to achieve the changeover from self-extension to interconnection at the last loading stage, which caused the fluctuation tendency of κ and ζ. Fluctuation ability in mudstone was higher than that in sandstone. The performance of an empirical model was also investigated for the non-Darcy hydraulic properties evolution prediction of crushing and seepage processes. The predictive results indicated that particle crushing and water erosion caused the increase of hydraulic properties, being the main reason that the experimental values are typically higher than those obtained from the predictive model. The empirical model has a high degree of predictive accuracy; however, κ has a higher predictive accuracy than ζ. Furthermore, the predictive accuracy of κ increases and ζ decreases with increasing weight ratio of SP.