Experimental Study On Water Permeability And Uniaxial Mechanical Cyclic Behaviours Of Bedding Coal

Being as a rock material with bedding structure,coal has obvious differences in permeability and mechanical properties on different bedding directions.This paper has gave a study of the layered coal based on the mechanical test and permeability test in order to make a further study about the change of mechanical properties and deformation laws of the coal under the repeated disturbance and the permeability of water in coal during the mining process on considering the influence of anisotropy of coal on its own properties.The main work and achievements are as follows:(1)Samples are made along the direction of vertical and parallel bedding respectively.The samples’ nuclear magnetic resonance test,tensile strength test and split test are carried out too.Experiment results show that the fracture structure of the coal and rock is dominated by micro pores,and the connectivity of large pores and fractures in parallel bedding samples is higher than that in vertical bedding samples.In this study,the uniaxial compressive strength of coal in the vertical bedding direction is about 1.91 times of that in parallel bedding direction.What’s more,the tensile strength of the former one is about 3.4 times of that in the latter.They have an obvious anisotropy.(2)This paper has made a water permeability test on coal samples in vertical and parallel bedding directions in order to study the seepage characteristics of water in coal.The results show that the water seepage in coal can be described by the Forchheimer type nonlinear seepage equation,and the seepage parameters in the direction of vertical and parallel bedding are fitted.Compared with the linear term and nonlinear term coefficient of Forchheimer formula,the volume flow hydraulic gradient curve of coal samples in the two bedding directions has found that the coal in the parallel bedding direction is more susceptible to the effect of inertial force than in the vertical direction.(3)The loading experiment indicates that the deformation of the two kinds ofbedding samples can be divided into three stages: compression,elastic and failure.The elastic modulus of the coal increases slightly under the condition of low stress,but decreases with increasing of stress amplitude.In addition,when the coal is compressed in vertical bedding direction,the brittle failure is the main failure mode,while the plastic failure mode appears when the coal is compressed in the parallel bedding direction;the damage variables of the two kinds of bedding samples are accumulated rapidly in the previous cycles,and then in a steadily way.(4)The test finds that the Poisson’s ratio increases with the increase of load levels and decreases with the increase of cycle numbers.Through the analysis of the anisotropy degree on two kinds of bedding samples before and after cycling,it has proved that the cyclic load has a weakening effect on the anisotropy of coal,and the effect is concentrated on the first 20 cycles.