Study On Damage Evolution Of Red Sandstone Under Freeze-thaw Cycles

With the development of one's national economy and one belt,one road,the demand for mineral resources has been increasing continuously.The resources in low altitude areas are decreasing.Mining and utilization of mineral resources in the cold regions of high altitude have gradually entered the field of vision.However,the rock mass is inevitably affected by the freeze-thaw cycle in the mining process of high altitude and cold area.Because of the periodic change of temperature,the rock mass will be damaged by freeze-thaw,accelerate the crack expansion and penetration,and then cause the failure of the rock mass structure.Therefore,it is of great significance to study the degradation mechanism and evolution law of rock freeze-thaw damage to ensure the safe and efficient mining of mineral resources in high altitude areas.In this paper,the uniaxial compression test and acoustic emission monitoring test of red sandstone samples under different freeze-thaw times are carried out.The rock micro structure test is used to analyze the mineral components of the samples before and after freeze-thaw and the changes of the micro morphology characteristics and pore structure of the samples under different freeze-thaw times.The process of rock freeze-thaw degradation is analyzed from the micro perspective The research contents are as follows.(1)In this paper,the freeze-thaw cycle test of red sandstone samples is carried out,and the three deterioration modes of the samples in the process of freeze-thaw are summarized: crack,particle falling off and flake falling.The change rules of the quality,porosity and elastic wave velocity of the samples in the process of freeze-thaw are analyzed.(2)The stress-strain curves of red sandstone samples under different times of freeze-thaw were obtained by uniaxial compression test.The change rules of strength characteristics,deformation characteristics and failure patterns with the increase of freeze-thaw times were studied,and the frost resistance of red sandstone was analyzed.According to the relevant principles of damage mechanics,the damage characteristics of red sandstone with the increase of freeze-thaw times are analyzed,and the damage model of freeze-thaw was established,which can predict the uniaxial compressive strength of rock under different freeze-thaw times.The acoustic emission signal of red sandstone was monitored during the uniaxial compression test of freeze-thaw samples,and the change rule of ring count rate,cumulative ring count and acoustic emission energy with the increase of thaw times was analyzed.(3)The change of mineral composition of red sandstone before and after freezing and thawing was analyzed by XRD.The micro morphology characteristics of red sandstone under different freeze-thaw times were analyzed by SEM test,and the relationship between macro and micro characteristics of samples with different freeze-thaw times was established during uniaxial compression failure.The influence of freezing and thawing on the pore structure of red sandstone is analyzed by nitrogen adsorption test.The results show that the freezing and thawing does not change the pore shape of rock,and the specific surface area of sample decreases with the increase of freezing and thawing times.Through the combination of nitrogen adsorption test and mercury injection method,the full pore size distribution characteristics of samples under different freeze-thaw times are obtained.The results show that the freeze-thaw cycle has a greater impact on the large pore size,and a relatively small impact on the middle and small pore size.(4)By idealizing the pores of rock into spherical elements,the deterioration mechanism of frost heave of rock is analyzed theoretically.The effective frost heaving force of rock due to pore water phase transition at low temperature is derived under the conditions of considering water migration and not considering water migration.When water migration is not considered,the maximum effective frost heave force produced in red sandstone is enough to crack and expand the pores of rock,which is suitable for the rock pores with low permeability.When considering water migration,the effective frost heave force model is derived by establishing temperature control equation,ice water mass conservation equation,phase change balance equation and considering the curvature effect with rock pores.Based on the maximum tensile strength criterion of rock,the expansion condition of rock pore in low temperature environment is established.