Freeze-thaw Deformation Characteristics And Thermal-mechanical Coupling Model Of Rocks Under Uniform & Unidirectional Freeze-Thaw Conditions

Cold regions are widely distributed in China,and it would result in greater deformation and damage of rock engineering which is subjected to frequent alternating freeze-thaw in cold regions.Most of the current research on freeze-thaw deformation and damage of rocks has been carried out under uniform freeze-thaw condition.However,in practical engineering,there are two different freeze-thaw conditions,namely,unidirectional freeze-thaw and uniform freeze-thaw condition.The deformation characteristics of rocks under uniform freeze-thaw and unidirectional freeze-thaw are significantly different and require in-depth comparative studies.In this study,freeze-thaw deformation experiments were carried out on sandstone under two different freeze-thaw conditions,and the microscopic pore-fracture evolution of the rocks is scanned before and after the freeze-thaw experiments to investigate the freezethaw deformation characteristics of rocks under unidirectional freeze-thaw and uniform freeze-thaw conditions and their relationship with microstructure evolution,and a thermal-mechanical coupling model of freeze-thaw deformation for rocks is proposed.The main conclusions are as follows:(1)Freeze-thaw deformation experiments on rocks under uniform freeze-thaw conditions reveal that the pore water of rock is the key factor in the deformation and damage development for rocks.Under uniform freeze-thaw condition,saturated rocks exhibit uniform freeze-thaw deformation with essentially equal freeze-thaw strains in different directions.The freeze-thaw deformation of saturated rocks in a single cycle can be summarized into eight stages: thermal contraction stage,frost heaving stage for the freezing process,frost shrinkage and then keeps stable,thermal expansion stage under subzero temperature,thawing shrinkage stage,thermal expansion and then keeps stable.At the equilibrium stage of thermal expansion,the strain is not completely recovered due to the plastic deformation of rocks,and the unrecovered strain is called residual strain.In the multi-cycle freeze-thaw process,the deformation of rocks increases rapidly during the first three cycles,then slows down and reaches a steady state after the fifth cycle.(2)Freeze-thaw deformation of rocks under unidirectional freeze-thaw conditions reveals that saturated rocks exhibit non-uniform freeze-thaw deformation,with the deformation in the parallel freeze-thaw direction being significantly greater than that in the vertical freeze-thaw direction,and the single-cycle deformation pattern is consistent with the uniform freeze-thaw deformation in eight stages.In the multi-cycle freeze-thaw process,the strains in the rocks remain increasing for the first few cycles and stabilize by the seventh cycle;at the same freezing and thawing temperatures,the residual strains in the unidirectional freeze-thaw process are smaller than those in the saturated rocks under uniform freeze-thaw conditions due to the slower freezing rate.(3)The microscopic scanning experiments of pore-fractures before and after the uniform and unidirectional freeze-thaw deformation experiments show that during uniform freeze-thaw condition,the main form of damage for rocks is the enlargement of the micropores,which increases the proportion of medium and large pores,and the random and uniform generation of microfractures around the pores,so that the freezethaw deformation measured at the same location is uniform.During unidirectional freeze-thawing,the microfractures are mainly generated in the vertical freeze-thawing direction,and the pores are connected to each other,resulting in microstructure after damage,leading to non-uniform deformation for rocks under unidirectional freezethawing conditions.(4)A thermal-mechanical coupling elastoplastic model of freeze-thaw deformation for rocks is developed,which couples a heat transfer equation that takes into account the unfrozen water content with an equilibrium equation based on the elastoplastic mechanics of porous media.The Mises yield criterion is chosen and the comparison of the pore ice pressure with the yield strength of the rock matrix is used as the key parameter to control the rock into the elastic and elastoplastic deformation phases.The model is used to carry out numerical simulations of freeze-thaw deformation of saturated rocks under uniform freeze-thaw condition in COMSOL software with thermal-mechanical coupling.The numerical results of temperature and deformation in single-cycle and multi-cycles are acquired,and comparisons between the results of thermal–mechanical numerical simulation based on the model and the experimental results show that the model can predict the temperature variation and freeze-thaw deformation process of porous rocks with desirable accuracy.