Experimental Study On Mechanical Properties And Gas Permeability Of Sandstone Under True Triaxial Loading And Unloading

The study of rock mechanics and seepage characteristics under the condition of mining stress is of great significance to the safe and efficient production of mining,underground space,civil engineering,transportation,water conservancy and other projects.Under the influence of tectonic stress,most of the rocks are in the true three axial stress state.Based on the self-developed "multi-functional true triaxial fluid-solid coupling experiment system",the experimental study of mechanical properties and seepage law of sandstone under true three axial stress is carried out in this paper with sandstone as the research object.This paper probe into the influence of intermediate principal stress on mechanical properties of sandstone under loading;research the mechanical properties and seepage law of sandstone under the influence of intermediate principal stress,the minimum principal stress,loading and unloading rate when loading and unloading.Based on the above experiments,the main results are demonstrated as follows:?1?The mechanical properties of sandstone under different intermediate principal stress conditions are studied.Within the test stress interval,with the increase of the maximum principal stress,the maximum principal strain increases gradually,the intermediate principal strain and the minimum principal strain decreases gradually,and the minimum principal strain is greater than the intermediate principal strain;The maximum principal strain and the minimum principal strain increase with the increase of the intermediate principal stress,and the intermediate principal strain decreases with the increase of the intermediate principal stress.The compressive strength of sandstone goes up with the increase of intermediate principal stress.Besides,the curve of sandstone strength and the intermediate principal stress can be described by quadratic term.?2?The experimental study on mechanical properties of sandstone under loading and unloading conditions with different intermediate principal stress and different minimum principal stress was carried out.Within the test stress interval,under the condition of the same initial stress,the stain capacity of each direction in the loading and unloading failure of the sandstone is smaller than that of the loading failure,the breaking strength of sandstone increases go up with the increase of intermediate principal stress and minimum principal stress;The fitting curve of the breaking strength and the intermediate principal stress of sandstone can be expressed by quadratic terms and there is a good linear relationship between the breaking strength and the minimum principal stress.In this experiment,the minimum principal stress plays a leading role in the failure strength of sandstone.?3?The mechanical properties of sandstone under different loading and unloading rate were studied.Within the test stress interval,with the increase of the loading rate of the maximum principal stress,the breaking strength of sandstone increased gradually,the fitting curve could be represented by a power function;the maximum principal strain,the intermediate principal strain and the minimum principal strain are increased;With the increase of unloading rate,the breakdown strength of sandstone decreases gradually,the fitting curve can be described by linear function;the maximum principal strain and the intermediate principal strain are decreased.?4?The experimental results demonstrate that the permeability of sandstone decreases firstly and then increased variation under the condition of loading and unloading,which is similar with the experiment sandstone loading.Same with the experimental rule of gas free sandstone,the maximum principal stress increases with the increase of the intermediate principal stress,the minimum principal stress and the loading rate.The breakdown strength of sandstone under unloading is reduced because of the existence of gas pressure.The gas pressure influence coefficient is decreased with the increase of the intermediate principal stress,the minimum principal stress and the maximum principal stress loading rate.?5?Improved the octahedral shear stress strength criterion(?_oct=a+b?_m,2+c?_?),and it has better applicability in this paper.