| During the operation of the Three Gorges Reservoir,the rock mass soaked for a long time below the flood control limit water level(145 m)experienced two stress conditions in the process of reservoir water lifting,tangential loading shear and normal unloading shear.The rock mass in the long-term soaking environment was affected by the water-rock action,and its mechanical properties also resulted in damage and deterioration.It is of great significance to study the influence of different soaking time on rock shear characteristics and analyze the difference of rock shear characteristics under different shear paths for the stability study of bank slope rock mass and the selection of rock shear strength parameters.In view of this,typical quartz sandstone samples were selected as research objects in this paper,which were soaked in the laboratory for different duration,and then tangential loading shear and normal unloading shear tests were carried out on the samples with different soaking times,so as to study the response of the rocks to the two shear paths after different soaking duration,and to reveal the microscopic mechanism of mechanical property damage combined with the micro-observation test.The main research contents and conclusions are as follows:(1)Tangential loading shear and normal unloading shear tests were carried out on typical quartz sandstone samples respectively,and the shear failure mode,deformation process and mechanical parameter characteristics of typical samples under different loading paths were analyzed.The tangential loading shear and normal unloading shear tests of typical samples were numerically simulated based on PFC2 D particle flow software.The models of crack development and force chain evolution under two different loading paths are studied,and the mechanism of different shear properties of sandstone under different test conditions is revealed.(1)The cohesion and internal friction Angle of quartz sandstone obtained by tangential shear test are 5.26 MPa and 50.66°.The cohesion of quartz sandstone obtained by normal unloading shear is 4.45 MPa and the Angle of internal friction is 62.87°.There are differences in the shear strength parameters of sandstone samples under the two loading paths.In practical engineering,it is of great significance to carry out corresponding shear test according to the specific stress path of rock mass to obtain the shear strength parameters,which is of great significance to improve the accuracy of calculation results.(2)In the process of normal unloading shear,the force chain of the tangential loading end of the sample gradually shifts from the diagonal direction to the shear direction,and tensile cracks develop in the middle of the sample,which is more likely to occur tensile shear failure.The actual shear plane deviates from the theoretical principal shear plane,leading to the increase of the effective shear plane,and the particle skeleton providing the internal friction Angle assumes greater shear stress.However,the filling particles that provide cohesion contribute less to the tensile shear failure,so the internal friction Angle of normal unloading shear is larger and the cohesion is lower than that of tangential loading shear.(2)Long-term immersion tests were carried out on typical quartz sandstone to analyze the changes of pore structure of samples and corresponding solution characteristics under different soaking cycles.Normal unloading and tangential loading shear tests were carried out on samples with different soaking times,and the evolution process of microstructure of samples during soaking was analyzed based on nuclear magnetic,electron microscopy and other technologies.The influence mechanism of long time soaking on shear properties of quartz sandstone was revealed.(1)For sandstone samples after different soaking times,the peak tangential load before failure decreases with the increase of soaking time,and the decreasing rate decreases gradually.Under normal unloading shear,the normal unloading load before failure decreases with the increase of soaking time,and the decreasing rate decreases gradually.Under different shear conditions,the main shear crack of sandstone samples presents three modes,namely,"I-shaped","S-shaped" and "M-shaped",but under normal unloading shear conditions,the rock fracture surface is more inclined to "S-shaped" and "M-shaped" modes.(2)Compared with the initial state,after 60 days of immersion,the cohesion of quartz sandstone samples decreased by 41% under tangential loading shear and 31% under normal unloading shear.The cohesion of samples was significantly affected by immersion.Under the two loading paths,the cohesiveness of samples showed the same trend with soaking time.It showed a gradual decrease but the rate of decrease slowed down.Compared with the cohesion,the internal friction Angle of the sample decreased by 1% and 7% respectively,and the influence of long-term soaking was small.(3)In the process of long-term immersion,the carbonate minerals in the cement are easy to react with water,and the dissolution and dissolution of the carbonate minerals are the reasons for the increase of pores in the sample.Based on the nuclear magnetic resonance test,the pore evolution law in the sample was quantitatively analyzed.During the first 20 days of immersion,the cemented minerals that are easy to react with water in the samples are dissolved and dissolved,resulting in the development of secondary pores.The internal porosity of the samples is increased rapidly,which is manifested as a rapid reduction of cohesion on the macro level.During the 20-60 days of immersion,the cemented minerals that are easy to react with water in the sample are reduced,and the particle skeleton of quartz and other minerals is relatively stable,and the porosity growth rate slows down.Macroscopically,the decreasing rate of cohesion decreases and tends to be stable.Compared with the samples soaked after 60 days,the internal pore structure of the samples soaked after 80 days changed less,and the properties of the samples tended to be stable after 60 days. |
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