Research On Unloading Mechanical Properties Of The Sandstone Under Different Temperature And Pore Water Pressure

With the increasing construction of water conservancy projects and underground traffic in recent years,the problems of safe excavation and effective utilization of underground space have attracted more and more a ttention.The rock deformation caused by excavation is more obvious than the conventional loading.At the same time,the change of the environment around the excavation project will also affect the deformation of the rock.At present,there are relatively few studies on rock unloading.Therefore,based on the previous unloading research results,this paper considers the unloading of sandstone(constant principal stress difference unloading confining pressure)mechanical properties test under the environment of temperature and water pressure,which focus on the characteristics of deformation,strength and damage of sandstone under different environments.At the same time,the constitutive equations of sandstone unloading under different temperatures and water pressures are derived.The main research content and conclusions of this paper are as follows:1)Single and triaxial tests of saturated sandstone were carried out using RMT-150 C rock mechanics rigid servo test machine.Based on this,three axes tests under different water pressure and unloading levels,which mainly studies the characteristics of deformation,strength,expansion and damage,were conducted by the TOP INDUSTRIE automatic rock triaxial test machine.The results show that the water pressure and unloading level can have a positive effect on the peak strain,that is,the higher the water pressure and unloading level,the larger the corresponding peak strain.The higher the water pressure,the smaller the cohesion.The higher the unloading level,t he greater the cohesive force.The effect of water pressure and unloading level on the internal friction angle is opposite.The higher the water pressure and unloading level,the greater the compliance of the strain(circumferential and axial)confining pressure,indicating that the strain(circular and axial)responds more quickly to the confining pressure reduction under the action of water pressure and unloading level.In other words,strain(circular and axial)is more sensitive to confining pressure reduction.After the unloading failure of sandstone,most of the rock samples show shear failure.When under the low confining pressure,the rock samples are mostly tensile spitting cracks.When under the high confining pressure,the rock samples are mostly ductile shearing cracks.2)The triaxial unloading test under different temperature and water pressure was carryed out by the TOP IN DUSTRIE automatic rock triaxial test machine based on the test under water pressure,mainly studying its deformation,strength,expansion and other characteristics.The results show that the higher the temperature,the smaller the deformation modulus,Poisson's ratio,peak strain,cohesion and the decreased parameters of confining pressure,while the lateral plastic strain and internal friction angle increase with the increase of temperature.As the stress ratio increases,its volumetric strain increases first and then decreases.The higher the temperature,the smaller the volumetric strain.It fully demonstrates that the higher the temperature,the more obvious the lateral expansion.3)Based on the unloading characteristics of sandstone under temperature-water pressure-stress,the stress-strain curve is simplified into three sections: nonlinear elastic stage,unloading yield stage and brittle softening stage.The constitutive equation was established with the improved Duncan-Chang model in the nonlinear elastic stage.The peak strength was calculated by the Mohr-Coulumb principle.Based on the assumption that the brittle softening segment is linear,the corresponding linear constitutive equation is established.Comparing the experimental curves with the theoretical curves,it is consistent with the test results.