Study On Loading And Unloading Mechanical Properties Of Sandstone Under The Pore Water Pressure

Human engineering activities,such as the excavation of slopes,underground caverns,and the repeated rise and fall of reservoir water levels,will break the balance between the original stress field and seepage field of rock mass,resulting in the change of stress environment and water environment of rock mass.Therefore,it is very significant to study the variation of mechanical properties of rock mass under the action of excavation unloading and pore water pressure.Based on this,take the sandstone in the three Gorges reservoir area as the research object in this paper,and study the loading and unloading mechanical properties of the sandstone under the pore water pressure through laboratory tests.The main research contents and conclusions are as follows:1)The triaxial loading test of sandstone is carried out,the linear relation equations between peak strength,residual strength,peak strain,modulus and confining pressure are fitted.The strength characteristics of rock samples are described by Mohr-Coulomb strength criterion,and the constitutive model of conventional triaxial compression test is established.2)The unloading test of sandstone under different unloading threshold stress levels were carried out.The results show that the peak value of deviant stress of unloading rock sample is strictly linear with the unloading threshold stress level at the same confining pressure.There is a linear relationship between the peak strain and the starting stress level of unloading.With the unloading of confining pressure,the Secant modulus of rock samples gradually decreased with the increase of unloading amount.However,the higher the threshold stress level of unloading,the lower the Secant modulus of rock samples.3)The results of conventional triaxial loading test and unloading test are compared.The results show that the peak strength of rock sample under unloading test is smaller than that under loading test,and the damage to rock mass caused by unloading is greater.In turn,rock samples are easier to destroy and more sudden when unloading.4)The triaxial compression tests of rock samples under pore water pressure have been carried out.The results show that the stress-strain curve is concave due to the influence of pore water pressure.With the increase of pore water pressure,the triaxial compression stress-strain curve of rock samples gradually shifts to the right with the increase of pore water pressure,and the peak stress of rock samples decreases and the peak strain increases gradually.Under the same confining pressure,the peak strength of rock samples decreases linearly with the increase of pore water pressure.Meanwhile,the peak strength of rock sample is affected more obviously by pore water pressure under low confining pressure.When the confining pressure is low,the peak strain of rock sample is larger,which indicates that pore water pressure has great influence on rock mass deformation under low confining pressure.5)The unloading test of sandstone under different pore water pressure were carried out.The results show that when the pore water pressure is constant,the unloading amount of confining pressure decreases with the increase of unloading threshold stress level.The larger the pore water pressure is,the smaller the confining pressure unloading amount when the samples are destroyed.When the pore water pressure is constant,the peak stress increases linearly with the increase of the unloading threshold stress level.During the unloading of confining pressure,the Secant modulus of rock samples decreases with the increase of confining pressure unloading amount.The lower the unloading stress level is,the faster the Secant modulus of rock sample decreases when the confining pressure is discharged.6)when the pore water pressure is constant,the failure degree of rock specimen increases with the increase of unloading threshold stress level.At the same time,rock samples are more easily destroyed under pore water pressure.With the increase of pore water pressure,the failure of rock samples will eventually form "X" or "Y" type conjugate cracks.At the same unloading threshold stress level,the larger the pore water pressure,the greater the failure degree of rock samples.7)Under the action of pore water pressure,the effective peak strength of sandstone in unloading test is obviously lower than that of the loading test,which indicates that the damage degree of rock samples in unloading state is greater.Therefore,it is necessary to study the mechanical properties of rock mass under unloading condition when excavating in rock mass.