Study On Permeability Evolution Law Of Coal Pillar Dam Under Creep Water Action

The underground reservoir of coal mine provides an effective technical way to solve the problem of mining water resources protection in ecologically fragile mining areas in Western China.Among them,the stability of coal pillar dam is particularly important because of mining.The author's team has systematically studied the mechanical action of water coal rock under uniaxial loading,but the law of water migration in the coal pillar dam and the distribution of water seepage damage area in the coal pillar have not been studied in depth.Therefore,the study of the creep water influence of the evolution of coal permeability law is of great significance for the analysis of the long-term stability of the coal pillar dam.In this paper,laboratory experiments,theoretical calculation and numerical simulation are combined(1)Under triaxial stress,with the increase of moisture content,the coal sample changes from tensile failure to shear failure.Compared with the uniaxial compression failure of water bearing coal sample,the shear failure characteristics of water bearing coal sample after triaxial compression failure are stronger;With the increase of water content,the decrease of residual strength after peak decreases obviously,and the plastic characteristics of post peak curve become more obvious;Compared with the results of uniaxial test,the weakening effect of water under triaxial stress on the mechanical properties of coal and rock mass is limited.(2)Under constant confining pressure,when the axial stress is loaded to the peak strength of coal sample about 40%,the coal sample can enter the steady state of creep.With the increase of water content,the proportion of accelerated creep strain value to the total strain value and the residual deformation increase,the steady creep rate and ultimate strain rate decrease.(3)With the increase of water content,the gas phase effective permeability of coal samples decreases logarithmically under constant effective stress,while the decrease rate of permeability in steady creep is decreasing first and then increasing.Meanwhile,the cutting compression coefficient of coal samples increases by 109% with the increase of water content;With the increase of effective stress,the gas phase effective permeability of coal samples decreases nonlinearly under the conventional triaxial compression,but the decrease value of permeability of high water content coal samples is lower.The permeability of coal samples decreases to stable value under steady creep state after stress loading,and the permeability will rise step by step after entering the accelerated creep stage.(4)According to the creep experiment results,the Boggs model can describe the whole process of accelerated creep of coal samples with different water content.Based on the three-way strain balance equation,the effective stress evolution equation is derived,and the permeability theoretical calculation model is obtained by the S-D model.The results of the model calculation can better fit the experimental data.At the same time,the law of water migration in the dam body of coal pillar is compared under static and dynamic permeability by numerical simulation.The seepage process in elastic area of coal column dam body is divided into "fast seepage period" and "steady seepage period".Among them,a coal body damage area will be formed near the water side of coal column during "fast seepage period",In ideal state,the water seepage rate in the coal body is very slow in "steady seepage period",which has little influence on the stability of the coal pillar.The development width of coal damage area has a positive correlation with the water pressure in the underground reservoir and the initial water saturation in the coal pillar,and a negative correlation with the initial air pressure in the coal pillar.There are 55 figures,6 tables and 73 references in this paper.