Loading And Unloading Mechanical Characteristics And Micro-failure Mechanism Of Weak Cemented Soft Rock Under Low Temperature

Problems related to frozen rock mechanics are inevitable in the development of artificial frozen strata,tunneling,and other infrastructure construction or resource development in deep strata,in cold regions.The weakly cemented strata in western China are mainly composed of sandstone and mudstone,characterized by lower strength,poor cementation and extremely unstable mechanical behavior.When the development of artificial frozen stratum goes cross the weakly cemented stratum,its deformation and stability of the excavation face are determined by the mechanical properties of the frozen rock.Therefore,we took the weakly cemented sandstone and mudstone in the typical mining areas of western China as the research objects,and applied the experimental,theoretical and numerical simulation methods to deal with the mechanical characteristics of loading and unloading and micro-failure mechanism of the weakly cemented soft rock under different minus temperature.In addition,the mechanical characteristics and micro?failure mechanism of unloading failure of frozen weakly cemented soft rock with different stress paths,the temperature-load coupled damage constitutive model,unloading damage strength criterion and constitutive model,freezing temperature field and stress field were studied as well.The research results are mainly shown as follows:(1)Through the uniaxial and triaxial compression tests of weakly cemented soft rock with different temperature,the deformation characteristics and strength characteristics of weakly cemented sandstones and mudstones under temperature-loading coupling are analyzed,and in turn the relationship between the compressive strength and temperature,and the confining pressure is established.The results show that with the decrease of temperature,the weakly cemented soft rock decreases in ductility while its brittleness increases.The peak strength and the negative temperature are exponentially related,while the shear strength parameters are approximately in linear relation with temperature.(2)The impact of temperature and confining pressure on the macro-and micro-scopic failure mechanisms of weakly cemented soft rock is significant.With the decrease of temperature and the increase of confining pressure,the failure mode of rock specimens has experienced damage from shear failure to pure shear failure,with the angle between the fracture surface and the maximum principal stress gradually increases.The fractures of weakly cemented soft rock at normal temperature occur mostly on clay cement or at the junction of clay cement and mineral particles,and a few occur on mineral particles.In negative temperatures,cracks are most likely to occur at the boundary of mineral particles,ice media,cement and other phase media,which may separate out of friction under shearing.Shearing pattern belt,like river-and step-shaped patterns,may appear on the fracture.The relationship between macroscopic failure strength,temperature and fractal dimension of weakly cemented soft rock is established by introducing the fractal theory.With the decrease of temperature and the increase of confining pressure,the fabric plane of the rock breaking fracture gets more complicated,and the fractal dimension enlarges correspondingly.In all,the impact of temperature and confining pressure on the fractal dimension of the microscopic fracture of the rock is significant.(3)Taking the damage of the rock under normal temperature and loading as the reference damage state,we construct the temperature-load coupled damage variable,establish the damage constitutive of weak cementation soft rock under temperature-load coupling using the statistical damage theory and the generalized strain equivalence principle.In addition,the reliability of the damage model is verified based on experimental data.(4)The evolution characteristics of energy during uniaxial cyclic loading and unloading of rock samples are studied in terms of energy,through the uniaxial cyclic loading and unloading test of weakly cemented soft rock and triaxial unloading test in different stress paths,and the macro and microscopic unloading failure mechanism for the frozen and weakly cemented soft rocks in different stress paths is analyzed.The results show that the unloading confining pressure generates remarkable lateral expansion deformation and volume expansion of the rock.Under the condition of the same initial confining pressure,the ultimate axial strain of the post-peak unloading confining pressure is the largest,and the ultimate lateral strain and ultimate volume strain value of the pre?peak constant axial displacement come to the top.The pre-peak unloading test is most sensitive to the unloading of the confining pressure.The failure mode of the post-peak unloading confining pressure depends on the previous loading history,but the effect of unloading confining pressure on the macroscopic failure characteristics is not significant.With the increase of the initial confining pressure,the shear failure characteristics of the pre-peak lifting shaft unloading confining pressure are more significant;The pre-peak constant axial displacement unloading damage is essentially tensile failure,and the macroscopic deformation failure state is controlled by the development of micro-fractures during the unloading of confining pressure.(5)Under lower confining pressure,the fractures of the frozen and weakly cemented sandstones and mudstones are tortuous,and the main failure mode is intergranular fractures,and transgranular fractures including localized step-like or lamellar tearing;and the fracture surface of rocks under higher confining pressure,characterized by relative flatness and a large amount of broken crystal debris adhering,tends to have shear slip.(6)Based on the triaxial unloading test data of confining pressure in different stress paths,we establish the nonlinear strength criterion by power function for the frozen and weakly cemented soft rock under unloading confining pressure conditions,and the expression of the stress invariant is given.Subsequently,the dynamic damage variable is introduced,and the rock elastoplastic constitutive damage equation under unloading confining pressure is derived.(7)Weakly cemented frozen formations have experienced a process of a slow change in temperature during the freezing process,to rapid decline to a rate that slows down,and then to keep stable step by step.The development trend of freezing pressure acting on shaft lining can be divided into three stages:rapid growth stage,slow growth stage and stable stage.Combined with the curve showing the temperature,we can see that the freezing pressure at the sidewall of wells with a lower temperature is relatively larger.Considering the unloading effect of frozen excavation,and the unloading elastoplastic model of weakly cemented frozen soft rocks,we discuss the deformation pattern of frozen wall and the distribution pattern of freezing pressure during freezing excavation using the reversed stress release method,and make a comparison between the result gained with the well-site test result,to verify the correctness and reliability of the unloading constitutive damage model.