Research On Mechanical Properties And Constitutive Model Of Anisotropic Layered Sandstone Under The Thermal-mechanical Action

In the western region of China,many tunnels which are long and deeply buried have the characteristics of high geo-stress,high geothermal and traversing layered rock mass.In such an environment,high geo-stress,high geothermal,anisotropy will have a strong influence on tunnel surrounding rock deformation and failure,this poses a great threat to the safety of the tunnel and underground engineering,thus study the different temperature action of rock mass mechanical properties and establish the constitutive relationship is extremely important.In this paper,a series of tests on layered sandstone such as high temperature triaxial compression test,high temperature triaxial unloading test,acoustic emission testing,electron microscope scanning,acoustic test and X-ray diffraction are carried out,deformation and mechanical properties of the anisotropic layered sandstone under the action of thermal – mechanical are studied from the angles of macroscopic and microscopic.Based on this,the constitutive model of anisotropic layered sandstone with the thermal – mechanical coupling is put forward.The following results were obtained:(1)With the increase of angle,the acoustic wave velocity of specimens decrease linearly,which show the anisotropy of Propagation of acoustic wave in layered sandstone.(2)The failure process of rock has experienced the compaction stage,the elastic stage,the micro crack growth stage,the crack growth stage,the stress drop stage and the residual strength stage.The rock shows obvious brittle failure when the temperature changes from 20?~120?,and shows plastic failure characteristic when the temperature is 150?,presenting brittle-plastic damage.(3)With the increase of the angle,the elastic modulus shows a decreasing trend,the Poisson's ratio first decreased and then increased.Crack stress,expansion stress,and peak stress also first decreased and then increased.With the increase of temperature,the elastic modulus first increases and then decreases,but the Poisson's ratio shows the opposite trend.The crack stress,expansion stress and peak stress present a trend of slight growth as a whole under the temperatures change from 20? to 120?.The crack stress,expansion stress and peak stress greatly reduced when the temperature is 150?.Therefore,it can be deduced that there is a critical temperature value between 120 ? and 150 ?,which makes the mechanical properties of sandstone abrupt.The peak value of strength shows weak growth trend when the temperature changes from 20?~120?.(4)The degree of anisotropy of layered sandstone is belongs to the lower anisotropy.The anisotropy degree of sandstone changes with the temperature slightly from 20? to 120?,while it increases under the temperature between 120? and 150?.(5)Ring count,energy and the cumulative ring count,total energy increases with the rise of temperature between 20? and 120?,but decreases when the temperature is 120 ? ~150 ?.With the increase of the angle,the cumulative energy and accumulative count presents the trend of decreasing first and then increasing.(6)Rock samples are controlled by bedding Angle or rock matrix,mainly presents shear failure.Individual samples show ballooning appearance and a tensile failure when the temperature is 120? and the sample shape obviously when the temperature is 150?,which mainly controlled by temperature.The damage form of rock samples change from shear failure to tensile failure with the increase of temperature.There is an obvious angle effect in the failure mode of layered sandstone:Shear failure which is skewed towards the bedding planes,mainly appears in rock samples with bedding angles of 0°,45°,60°,and 90°.Shear failure which is along bedding planes,mainly appears in rock samples with the angle of 30°and its intensity is the lowest.It reveals that the angle of the sandstone is 30 °and it is the most unfavorable angle of mechanical properties,strength characteristics and fracture pattern to the sandstone.(7)Compared with the high temperature triaxial test,elastic modulus in high temperature unloading test is greater than that in the triaxial test,and peak intensity is smaller,the reason is that unloading confining pressure will weaken the lateral confinement of the rock,the duration is short,and the stress-strain curve is deviated from the straight line segment into the crack propagation stage in advance.The degree ofrock anisotropy Rc and anisotropic parameters were greater than that in triaxial test,which shows that the unloading confining pressure can enhance the anisotropy of the rock.Acoustic emission activity and energy release degree also significantly higher than that in the high temperature triaxial test.When the rock is damage in the unloading test,the bulge shape is small and the tensile cracks are small.(8)The large number of calcite,illite are the important component of calcium cementation and clay cementation between mineral grains,When the temperature rises to a certain degree,internal structure of cement will change,and the cementation decreased,even the mineral particles and cement are detached,which result in the decrease of strength of sandstone and the change of failure mode.(9)Through the electron microscope scanning analysis,the microscopic form of the rock fracture are mainly the intergranular fracture and transcrystalline rupture,and the rupture forms is shear failure when the temperature is 20?-90?,and the microscopic form of fracture are tearing fracture,transcrystalline fracture and cleavage fracture when the temperature is 120?-150?,the rupture forms are tensile failure and tensile-shear failure type.In contrast with the fracture photos of the samples in triaxial compression test,the fractures of rock in unloading test are rougher and have more rock powder as well as mineral fragments.(10)To establish the statistical damage evolution equation of layered sandstone under the action of thermal – mechanical considering the effect of temperature and angle,the intensity of the element inside the rock is assumed to obey Weibull distribution of three parameters,strength criterion of layered rock mass is introduced as a failure criterion of the element inside the rock.Also,according to the theory of strain equivalence and the principle of effective stress,a statistical damage constitutive model is established with the introduction of damage variable correction factor.The constitutive model is verified combined with the results of the triaxial test.The results indicate that: Stress-strain curves of rock are fitted well by this constitutive model,and the model can reflect the characteristics of the stress-strain process effectively,the value peak of strength,strain and residual strength in the theoretical curve is consistent with the test results.