Study On Loading And Unloading Failure Mechanism Of Argillaceous Sandstone Under The Action Of Water-immersion

The study on loading and unloading failure mechanism of rock under the action of water-immersion have always been the focus of attention in the field of rock mechanics and it also has high practical value for the stability of human geotechnical engineering.This study has been done systematically on uniaxial compression,triaxial compression and constant axial compression confining pressure experiments focused exactly on the water-rock mechanical coupling action characteristics of argillaceous sandstone materials,the influence of different water bearing conditions on the meso-structural deterioration of rock specimens were investigated by using acoustic emission and nuclear magnetic resonance technology.Furthermore,a multi-linear strain softening constitutive model and a rock damage evolution model under water immersion are proposed.Based on the constitutive models and similarity principle between humidity and temperature fields,the distribution of humidity field and softening law of surrounding rock were obtained.Results are as follows:（1）The basic physical and mechanical characteristics and water content statistical characteristics of argillaceous sandstone samples were obtained through laboratory tests.The axial expansion strain experiment of rock specimens under confined expansion condition was carried out by using a new type of rock expansion tester which is research and develop independently,and the axial expansion coefficientβ=0.09896,the one-dimensional and three-dimensional unsteady expansion formula are derived.（2）Through uniaxial compression tests and SEM,AE,NMR observations methods were used to reveal the microstructure and physical properties of argillaceous sandstone specimens,which treated with different water content and different wetting-drying cycles.Experimental results show that the compressive strength and elastic modulus of rock decrease linearly,while the tensile strength decreases slowly with the increase of water content,and water has a relatively regular weakening effect on the mechanical characteristics of argillaceous sandstone.The effect of water content on the compressive strength of rock is obviously less than that of drying-wetting cycle,the dry-wet cycle is a cumulative loss process for the microstructure of rock specimens.（3）It can be obtained by co MParing loading and unloading experiments that:（1）The stress-strain curves of rocks in all experiments have obvious mechanical properties transition points.（2）The mechanical strength and elastic modulus of rock decrease linearly with the increase of water content,and the down-line relationship is better under low confining pressure.（3）With the increase of water content,the failure mode of rock samples gradually changes from tension-shear composite failure to shear failure Under the loading condition.Under the condition of unloading confining pressure,the fracture properties of rock are tensile,shear and composite fracture,and show the characteristics of progressive failure.And the failure of argillaceous sandstone is more severe during unloading tests.（4）From the perspective of energy,the U_e of rock shows a trend of rising first,then falling and then stabilizing in the triaxial compression experiment,and there is a threshold between 20MPa and 30MPa confining pressure,where the relationship between energy and water content changed from ambiguous to linear.As the unloading experiment goes on,the U_d maintaining sustained growth.（4）According to the mechanics experimental results under the action of water-rock,a unitive multilinear strain softening model is put forward,and established the"linear elastic-strain hardening-strain softening-residual strain"linear constitutive equation,and the uniaxial and triaxial and triaxial unloading condition curve fitting the stress-strain curve of rendering good adaptability.Based on the damage mechanics theory and Weibull distribution,and combined the damage evolution equation with the Drucker-prager strength criterion,a damage constitutive model of cement-bearing sandstone samples under the combined action of humidity and load is derived by considering the damage characteristics of rock mass caused by moisture degradation by water,which can well describe the experimental results of rock samples,and the variation of stress parameters at each turning point with water content can well reflect the deterioration of rock strength caused by water immersion.（5）Combined with the differential equation of temperature field and humidity field theory,the thermal conductivity coefficient(k=8.3813×10^-3W/m·℃)and thermal expansion coefficient(β’=1.109×10^-5/℃)of rock are obtained by optimization inversion of ANSYS thermal stress module.According to thermal stress Direct coupling method,the spatio-temporal evolution law of stress and deformation of surrounding rock of water-bearing roadway is obtained which considering the effect of Moisture diffusion and rock expansion,and the simulation results can effectively guide the support and treatment of surrounding rock in coal mine.