Study On Damage Properties And Loading-unloading Mechanical Behavior Of Sandstone Under Freeze-thaw Action

The damage and deterioration of rocks caused by freeze-thaw action is an important problem faced by geological engineering in cold regions.Experts and scholars at home and abroad have carried out a lot of experimental analysis and theoretical research on the mechanical properties of freeze-thaw rocks.Insufficient research on the mechanical properties of freeze-thaw damage of rock and less research on mechanical properties of freeze-thaw sandstone under unloading path,so it is of great theoretical and practical significance to study the freeze-thaw damage characteristics of rock and the mechanical properties of freeze-thaw rocks under different stress paths in alpine areas.In this paper,the sandstone with large porosity and susceptible to freeze-thaw is taken as the research object.The freeze-thaw test,physical properties test(porosity,permeability,ultrasonic test)of sandstone and the compression test under different stress paths are carried out.At the same time,the particle flow discrete element software PFC was used to simulate the mechanical test of freeze-thaw rock.The conclusion is as follow:(1)Segmentation of stress-strain curve in conventional triaxial test,including pore compaction stage,elastic stage,plastic stage,and strain softening stage.Freeze-thaw action did not change the division of the stress-strain curve of sandstone.Freeze-thaw action reduces the elastic modulus of saturated sandstone,the peak strength decreases,the peak strain increases,and the mechanical properties deteriorate.With the number of freeze-thaw cycles increases,the cohesion of the sandstone and the internal friction angle deteriorate to varying degrees.Among them,the cohesion force is more sensitive to the number of freeze-thaw cycles and the effect is more significant.(2)After freeze-thaw the porosity of sandstone increases.With the process of freeze-thaw cycle,the porosity inside the rock increases significantly,and it can be considered that the increment of porosity in a single freeze-thaw cycle is almost equal.As the number of freeze-thaw cycles increases,the pores are more connected and the permeability of sandstone is enhanced.The permeability of rock shows a monotonous increasing trend.After 60 freeze-thaw cycles,the permeability of sandstone increases by 1.51 times.The increment of each freeze-thaw of sandstone increases with the number of freeze-thaw cycles,indicating that the freeze-thaw cycle causes permeability increase speed up.(3)After the freeze-thaw cycle,the sandstone wave speed gradually decreases.Among them,the P-wave velocity almost linearly declined.Combined with the definition of dynamic elastic modulus,the relationship between sandstone freeze-thaw damage factor D and porosity and P-wave velocity is established.According to the comparison between the model and the test results,the model shows that the damage effect of sandstone is better,and the compressive strength of the rock can be predicted by the damage model.(4)In the particle flow discrete element PFC software,there is a quantitative relationship between the meso-structure parameter of the parallel bonding model and the macro-parameters.The damage and degradation of the rock caused by freeze-thaw can be equivalent to the meso-structure parameter of the bonding model.By analyzing the corresponding relationship between macro and meso-structure parameters,the general determination method of meso parameters of freeze-thaw sandstone is summarized.At the same time,according to the characteristics of stress-strain curve of freeze-thaw rock,the constitutive equation of broken-line freeze-thaw sandstone was established.(5)Under the freeze-thaw action,the stress-strain curves of sandstone under different unloading stress paths exhibit different mechanical characteristics.Freezing and thawing reduced the peak strength of the rock,and the unloading capacity of sandstone during unloading failure was significantly reduced.The more freeze-thaw cycles,the more sensitive the radial strain is to changes in confining pressure.Freezing and thawing reduces the initial deformation modulus,and the deformation modulus of the sample decreases linearly during the confining pressure relief.The more freeze-thaw cycles,the faster the decrease.The dilatancy angle of unloading failure of sandstone under different stress paths is reduced by the effect of freeze-thaw.