Study On True-triaxial Unloading Behaviors And Failure Mechanism Of Damaged Sandstone

The study on excavation unloading behaviors of underground rock mass is improtant for deep mining engineering.Before excavation,the underground rock mass may be affected by other excavation works or man-made disasters,such as blasting,mechanical vibration and mine fire,which induce pre-damage in adjacent rock masses to be excavated.This kind of damage is often neglected in experimental investigations,which may cause the misunderstanding of mechanical characteristics of rock masses in practical excavation engineering.In this study,both load and high temperature are considered as the factors inducing the pre-damage in sandstone samples.Based on the GCTS Triaxial Rock Testing System(RTX-3000),a series of true-triaxial unloading tests involving prior loading damage and thermal damage in sandstone samples were carried out,which revealed the strength and deformation characteristics and failure modes.Then,a damage constitutive model of thermally treated sandstone under true-triaxial unloading conditions was proposed and validated by the experimental results.Furthermore,discrete element simulations were conducted to capture the micro-scale responses and fracture mechanism of sandstone under true-triaxial unloading conditions.The research works and findings are as follows.(1)The sandstone samples were collected from field and proessed into rectangular prism shape with dimensions of 50 mm×50 mm×100 mm.The influences of maximum principal stress level and prior cyclic loading damage on true-triaxial unloading behaviors of sandstone samples were identified by experimental investigations.The results show that as the level of?₁ at the unloading point increases,the peak strength(?_p)and peak strain(?_p)gradually increase,while the minimum principal stress decreasing parameter(?)first decreases slowly and then shows a sharp decline,and the failure mode of sample changes from mixed tensile-shear failure to shear failure.As the number of prior cyclic loads increases,the peak strain(?_p)and the ratio(?)of crack damage stress to peak strength(?_p)decrease,while the peak strength(?_p)and Young's modulus(E_d)first increase and then decrease.The dominant fractures contributing to rock failure changes from tensile fractures to mixed tensile-shear fractures,then to shear fractures.(2)The sandstone samples were thermally treated with a temperature range of25??950?,and then the scanning electron microscopy(SEM)and other testing techniques were used to reveal the microstructure and physical properties of sandstone.Subsequently,the mechanical behaviors of sandstone samples under true-triaxial unloading conditions were studied in depth.The results show that high-temperature treatment weakens the microstructure and physical properties of sandstone.As the treatment temperature increases,the peak strain(?_p),minimum principal stress decreasing parameter(?)and the dissipated energy ratio(?)at peak strength point gradually increase,while the peak strength(?_p),accumulated absorbed strain energy(U₁),elastic strain energy(U_e)and dissipated energy(U_p)at peak strength point first increase and then decrease,and the failure mode changes from shear failure to tensile failure.(3)Based on the damage mechanics theory and Weibull distribution,a damage evolution equation related to the thermally treated sandstone was derived,and then a damage constitutive model for thermally treated sandstone under true-triaxial unloading conditions was developed by combining the damage evolution equation with the Drucker-prager strength criterion which could predict the strength of micro-unit.Moreover,the theoretical expressions for the constitutive model were determined by means of multiple functions differentiation.The stress-strain curves predicted by the constitutive model generally presented a good agreement with the experimental curves,which proved that the constitutive model could well reflect the stress strain behavior of thermally treated sandstone under true-triaxial unloading conditions.(4)Based on the discrete element theory,true-triaxial unloading simulation tests on sandstone samples with different maximum principal stress levels were performed by the commercially available Particle Flow Code in three-dimensions(PFC3D),and the micro-scale responses and fracture mechanism were revealed.The results show that both the accumulative micro-cracks at the unloading point and micro-crack generation rate during the unloading phase exhibit an increasing trend with the increase in the level of?₁.As the axial strain increases,the coordination number gradually decreases,and the anisotropic degree of contact normal force and anisotropic degree of contact shear force decrease at the post peak stage.Due to material inhomogeneity and less constraint in the direction of?₃,most of the micro-cracks randomly appearing at the initial stage of experiment show a nearly vertical dip and a dip direction coincident with the direction of?₃.Previously generated micro-cracks induce stress concentration in neighboring cements,causing the micro-crack clusters to form in the sample.A further development of micro-crack clusters results in a preferential cracking region,which finally evolves into the macro fracture band.When?is relatively low,the formation of that approximately vertical fracture is associated with the superiority of tensile contacts in number and force during the unloading phase.(5)In light of the lithology conversion of sandstone after thermal treatment,true-triaxial unloading simulation tests on sandstone samples treated with different temperatures were performed using PFC3D,and the micro-scale responses and fracture mechanism were revealed.The results show that with the increase of treatment temperature,the accumulative micro-cracks at the unloading point and micro-crack generation rate during the unloading phase first increase and then decrease.As the axial strain increases,the coordination number gradually decreases,while the anisotropic degree of contact normal force and anisotropic degree of contact shear force first increase and then decrease.At the post peak stage,stress concentration occurs alternately in somewhere of the macro fracture band or nearby.When the treatment temperature exceeds 500°C,a dense region can be formed in the interior of sample due to low stiffness of mineral components.The displacements of grains surrounding the dense region are larger than those inside the dense region,making more micro-cracks appear around the dense region,which finally causes the macro tensile fracture to occur along the sides of rock sample.