Experimental Study On The Mechanism Of Freeze-thaw Damage In Sandstone With Different Saturation Based On CT Scan

In order to investigate the deformation stability of geotechnical engineering under freeze-thaw and load in different water-bearing states in cold regions,red and white sandstones from a place in Shaanxi Province were taken as the research objects,and research work was carried out with the aim of providing a theoretical basis for scientific evaluation of the long-term stability of rock engineering under freeze-thaw environment in cold regions.Freeze-thaw cycling tests,electron microscope scanning tests,CT scanning tests during freeze-thaw,and uniaxial compression tests after freeze-thaw were conducted on two sandstones with natural water content(20%),incomplete water saturation(70%)and complete water saturation(100%).To investigate the role of freeze-thaw environment on the fine structure,macroscopic physical characteristics and macroscopic mechanical indices of the two sandstones,to define freeze-thaw damage variables and establish damage evolution curves,to establish the link between macroscopic mechanical strength and fine structural parameters,and to compare and analyze the effects of freeze-thaw times and saturation on the deterioration mechanism of freeze-thaw damage of the two sandstones,the main research contents and innovative results are as follows.(1)The freeze-thaw damage characteristics of two sandstones with different saturations were revealed.Based on the freeze-thaw cycling tests,electron microscope scanning tests were completed to investigate the freeze-thaw damage pattern,mass,wave velocity and variables of freeze-thaw damage during the whole freeze-thaw process.The results show that:the degree of saturation under the freeze-thaw action determines the degree of freeze-thaw damage of rock samples,and the deterioration of damage is the most intense in fully saturated sandstone,and the degree of damage is the lowest in the natural water-bearing state,and the freeze-thaw damage of white sandstone is more serious than that of red sandstone,especially the fully saturated white sandstone has end drop and lateral fracture in order.The greater the degree of saturation,the faster the rate of reduction of rock sample mass and wave velocity and the growth rate of freeze-thaw damage variables,and the magnitude of change is positively correlated with the size of saturation.The changes of physical quantity and freeze-thaw damage variables of white sandstone are generally larger than those of red sandstone,except for the exponential changes of freeze-thaw damage variables of fully saturated white sandstone,all other saturated rock samples have linear changes.It is also concluded that the differences in the internal fine structure of the rocks exacerbate the degree of freeze-thaw damage.(2)The fine structure damage evolution characteristics of freeze-thaw rocks with three saturations were investigated.Based on the freeze-thaw cycles of two sandstones with different saturations,high-precision CT scan tests were completed with corresponding saturations and freeze-thaw times,and the fine structural parameters such as porosity,permeability,pore parameters,throat parameters,fractal dimension and fine damage increment of the central cubic region of two sandstones with natural water content,incomplete water saturation and complete water saturation during freeze-thaw were quantitatively analyzed by combining 3D visualization techniques,equivalent sphere method and fractal theory.The results show that:the freeze-thaw action promotes the evolution of the fine structure inside the rock,and the growth rate of the fine structure parameters of the rock samples is proportional to their saturation magnitude.The fine structure parameters of white sandstone are faster than those of red sandstone,and increase linearly compared with other saturated rock samples,and the growth of fully saturated white sandstone is exponentially the fastest,compared with red sandstone,the fine structure parameters of white sandstone change significantly,and the pores are interconnected through the throat with strong space occupancy and obvious permeability.The freeze-thaw damage is mainly due to the increase of the original pore size and then rupture into small pores and the increase of the original throat length.The fractal dimension of red sandstone ranges from 2.18 to 2.25,and the fractal dimension of white sandstone ranges from 2.46 to 2.54.The fractal can quantify the dynamic process and law of the evolution of structural damage inside the rock body.The composition of the rock mass and the denseness of the skeletal structure cause differences in the fine structure freeze-thaw damage.(3)To study the mechanical properties of freeze-thaw rock samples with different saturations and to establish the link between macro-mechanical strength and fine structural parameters.Based on the freeze-thaw cycling tests of two sandstones with different saturations,the uniaxial compression tests with corresponding saturations and freeze-thaw times were completed to reveal the damage damage modes,macro-mechanical properties and macro-fine connections between the two rocks with different saturations under the dual action of freeze-thaw and load.The test results show that the freeze-thaw damage damage mode of red sandstone is mainly brittle damage,while that of white sandstone is mainly shear damage and shows good plasticity and ductility under both freeze-thaw and load.The strength and modulus of elasticity of both sandstones decreased linearly with the increase of freeze-thawing times,and after 150 freeze-thaws,the strength of fully saturated red sandstone decreased by 16.84%,which was 4.36 and 2.38 times of that of natural water and incomplete water,respectively,and the strength of fully saturated white sandstone decreased by 98.53%,which was 1.68 and 1.26 times of that of natural water and incomplete water,respectively.The uniaxial compressive strengths of the two sandstones with different saturations were negatively correlated with porosity and fractal dimension,which were basically consistent with their macroscopic physical quantities and macroscopic mechanical indices with the number of freeze-thaws.The changes of the fine structure of the rocks induced the deterioration of their macroscopic mechanical properties,and the fine structure was the key to influence their macroscopic mechanical properties.