Study On Mechanical Characteristics Of Triaxial Unloading Of Cross-jointed Sandstone Under Circulating Pore Water Pressure

With the continuous construction and development of large-scale hydropower projects,with the normal operation of the dam,a certain range of reservoir water cutting zone will be formed on the slope of the reservoir bank.After the completion of the Three Gorges Project,by 2022,a total of 175 m experimental water storage in the Three Gorges Reservoir Area will be carried out 12 times,and the rock mass structure of the bank slope will deteriorate due to the fluctuation of water level circulation.In the Three Gorges Reservoir area,large-scale cutting joint fractures developed and the number of cross-joints was large,and the geometric parameters of joints were the main factors affecting the mechanical characteristics of jointed rock mass.The later construction of the reservoir area and the occurrence of landslides will lead to the stress redistribution of the rock mass on the reservoir bank due to unloading,which will eventually cause the deterioration of the rock mass properties.Based on this,this paper takes the sandstone on the bank slope of the Three Gorges Reservoir area as the research object,and carries out the unloading mechanical characteristics of cross-joint sandstone with different inclination angles under the action of circulating water pressure,and the conclusion is as follows:(1)Based on the triaxial compression test of cross-joint sandstone,it is found that the strength of the cross-joint rock sample is mainly affected by the inclination angle of the main joint,the inclination angle of the main joint increases from 0 ° to 90 °,and the peak intensity increases sequentially.Under the inclination angle of the same main joint,the inclination angle of the secondary joint was from 0° to 90°,and the peak intensity showed a trend of decreasing first and then increasing,and the overall change range was small.The cracking stress,elastic modulus and deformation modulus increased sequentially with the increase of the inclination α of the main joint,and showed a trend of decreasing first and then increasing with the increase of the β of the inclination angle of the secondary joint.(2)Based on the triaxial unloading test of cross-joint sandstone,it is concluded that the confinement pressure drop decreases first and then increases with the β of the sub-joint inclination angle of the same main joint inclination.When the inclination angle of the main joint α is 0°,the circumferential peak strain and axial peak strain of the rock sample are the smallest.The variation trend of ambient strain confining pressure flexibility and axial strain confining pressure flexibility is roughly the same,and the circumferential strain confining pressure and axial strain confining pressure flexibility in the plastic deformation stage are greater than those in the elastic deformation stage.(3)According to the triaxial unloading test of cross-joint sandstone under the action of circulating pore water pressure,it is shown that with the continuous increase of the number of hydraulic pressure cycles,the axial strain and ring strain continue to increase,and the increasing speed shows a trend of increasing first and then decreasing,the deterioration speed of the first five hydraulic cycles is faster,and the deterioration degree shows a gradual decrease trend after the six hydraulic cycles.The increment of axial strain and circumferential strain is the largest when α=60°,and the increment of axial strain and circumferential strain is the smallest when α=90°.α=60°joint specimen has the largest degree of wave velocity attenuation,secondary porosity,elastic modulus deterioration and deformation modulus deterioration.The circulating pore water pressure effect has the most obvious deterioration effect on 60°-60° joint inclination samples.(4)Based on the theory of damage mechanics,the damage variables are determined,and the constitutive model of cross-jointed sandstone and the triaxial unloading damage constitutive model of cross-jointed sandstone under circulating pore water pressure are established by combining the generalized Hooker’s theorem,Misses yield criterion and Weibull statistical distribution theory.