Experimental Study On Mining-induced Mechanics And Seepage Characteristics Of Sandstone Under True Triaxial Stress

Since the reform and opening up,with the rapid increase of human underground activities such as resource exploitation,the depth of underground work is getting deeper.At the same time,it also brings a series of deep rock engineering problems represented by "three heights and one disturbance".In other words,the stress environment of rock mass presents the characteristics of high in-situ stress,high pore pressure and high ground temperature.In addition,engineering activities and tectonism will significantly change the surrounding rock environment of rock mass,and the stress state presents obvious anisotropy(?1>?2>?3),and the environment of rock mass presents fluid-solid coupling state even multi-field coupling state.Based on the self-developed "multi-functional true triaxial geophysical(TTG)apparatus",this paper studies the true triaxial mechanical properties,mining dynamic properties,mechanism of mining dynamic disaster and seepage characteristics of sandstone,emphasis is put on the strength,deformation,acoustic emission and failure characteristics of sandstone under different mining depth stress conditions;the influence of different unloading rates on the strength,deformation and failure characteristics of sandstone;and the permeability,deformation and stress sensitivity of sandstone under the condition of true triaxial stress and gas-solid coupling.The main research results are as follows:? The dynamic characteristics of sandstone under different mining depth stress are studied experimentally,the results showed that with the increase of mining depth,the peak stress of sandstone failure under displacement type disturbance and stress type disturbance increases gradually,and the strength of sandstone under stress type disturbance is much higher than that under displacement type disturbance.Under the displacement type load,the prepeak dilatancy of the specimen decreases with the increase of mining depth,the failure mode gradually changes from tensile failure to shear failure,and the crack morphology changes from complex and diverse tension-shear associated cracks to single shear crack.Under the action of stress type disturbance,the specimens show a tendency of accelerated expansion.?True triaxial mechanical tests of sandstones with different mining depths show that the acoustic emission behavior of sandstones presents obvious stages under the action of mining stress,that is,there is barely acoustic emission at the pore compaction and elastic stage,but a large number of acoustic emission occurs at the yield stage or at the moment of failure.With the increase of mining depth,the time required to generate a large number of acoustic emission from the beginning of loading lagged behind,and the larger ?2??3 is,the greater ?1 required to generate a large number of acoustic emission.? The mining dynamic disaster is a process of elastic energy release,and the decrease rate of stress along the working face(unloading)direction increases first and then decreases,and the rock mass shows accelerated expansion along the unloading face.With the increase of mining depth,the failure mode is gradually from tensile failure to shear failure,the number of cracks is reduced,the rock fragments are increased and the integrity is better.?In the true triaxial mechanical tests with different unloading rates,the unloading speed of the maximum horizontal principal stress becomes larger,the peak stress in the vertical direction of the sandstone specimen is smaller,the principal strain in the X direction becomes smaller,the principal strain in the Y direction becomes larger,and the earlier the expansion phenomenon occurs,the expansion capacity increases.The failure mode of rock samples changes from shear failure to tensioning failure,and the tensile crack near the unloading surface is prominent.When the minimum horizontal principal stress unloading speed increases,the vertical peak stress of the sandstone specimen decreases,the maximum horizontal principal strain gradually increases,the minimum horizontal principal strain gradually decreases,and the volume strain gradually decreases,the expansion capacity increases.The crack gradually shifted from the main plane in the Z direction to the main plane in the X direction,and the failure mode changed from tensile-shear failure to tensile failure.? In the true triaxial seepage test,the intermediate principal stress and the intermediate principal stress coefficient b have a significant influence on the permeability and deformation of sandstone.The larger the intermediate principal stress coefficient,the increase rate of the maximum principal strain decreases with the increase of the maximum principal stress,the intermediate principal strain turns from negative to positive and the amount of deformation gradually increases,the amount of deformation of the minimum principal strain also gradually increases,and the rate of decrease in normalized permeability increases.?By comparing,fitting and analyzing the test data with many classic permeability models,the results show that compared with the porosity model,the S&D model and the TTP model have a better fit of the evolution law of sandstone permeability under true triaxial stress conditions.Moreover,the S&D model does not consider the anisotropy of sandstone,so the TTP model describes the evolution of sandstone permeability more accurately.?Based on the fitting results of the TTP model,the change rule of the Cf value is analyzed,and the results show that the stress sensitivity of sandstone decreases with the increase of the maximum principal stress,but its decline rate becomes slower.At the same time,the increase of the intermediate principal stress and the intermediate principal stress coefficient will cause the stress sensitivity to decrease in different degrees.