Research On Meso-damage Mechanism And Uniaxial Failure Characteristics Of Freeze-thawed Fractured Sandstone

The meso-damage expansion of rock slope in cold region is caused by cyclic freeze-thaw(F-T),and the macroscopic performance is weakened by natural joints.In addition,the amount of water stored in fractures affects the effect of F-T:there is an essential difference between saturated F-T and ice-filled F-T.The latter can cause more serious frost damage,which poses a threat to the design,construction,operation and maintenance of rock mass engineering in cold regions.However,no research has explored the degradation effect and damage mechanism of saturated/ice-filled fractures during cyclic F-T from a mesoscopic level;on the other hand,mesoscopic behavior interpretation is particularly important for macroscopic fracture evaluation,but few reported studies Macro-and mesoscopic correlations of frozen-thawed rocks.Based on the above considerations,this paper conducts experimental research on the meso-scale to reveal the rock F-T damage mechanism and load failure characteristics.For this reason,CT scanning was carried out on saturated intact,saturated fractured and ice-filled fractured sandstone with different F-T times and stress states.Through the three-dimensional reconstruction image,the meso-parameter evolution analysis and fractal geometry quantitative description are carried out,and then the strain localization characteristics of the loading process are evaluated by the digital volume correlation method.The conclusions are as follows:(1)The representative elementary volume(REV)of fractured rock can be measured by fractal dimension,and the size of rock bridge and fracture region are 260 and 580 voxels respectively.Due to F-T damage,the REV increases with the increase of the F-T times.(2)During F-T cycling,pore characteristics vary according to size:micropores account for>60%of pores and increase in number more than pores of other sizes.Mesopores gradually decrease in number but their volume fraction remains>70%.Macropores have the greatest increase in volume fraction.Early F-T cycling induces the connection of pore throats,while later cycling accelerates their expansion.The intact rock mainly forms annular F-T damage,while the damage concentration area of the fractured rock is located near the prefabricated fractures with low dip angle.Increases in hydrostatic pressure indirectly caused by water-ice phase transformation are the leading cause of cracking in ice-filled fractures,and then the specimen forms a tree-like fracture network under the action of cyclic F-T.Fractal dimension and tortuosity fractal dimension are suitable for the quantitative characterization of porosity and absolute permeability,respectively,and can be correlated by cubic polynomial function and logarithmic function.Cyclic F-T can expand the self-similar range of rock fracture structure;however,fractal features will be destroyed after a certain threshold is exceeded.(3)During the loading process of the frozen-thawed sandstone,the micropores and mesopores are compacted and closed,and the macropores are interconnected to form extralarge pores.The intact sandstone cracks in the middle of the specimen to form a single fracture,while the fractured sandstone is first broken in the F-T damage concentration area to form a complex fracture network.Among them,the strain field of the former is mainly shown as a shear zone with an inclination of about 75°,but strain concentration occurs in the edge area;the latter mainly occurs strain localization in the region of prefabricated fractures with low dip angle,and the strain is larger along the step slip direction.The statistical frequencies of equivalent strain and normal strain are lognormal distribution and normal distribution respectively.F-T damage of intact rock directly induces strain concentration and guides the development of secondary fractures.The failure characteristics of fractured rocks are determined by the arrangement of the prefabricated fractures,and F-T damage is weakly related to the strain development.Therefore,the strain distribution of saturated/ice-filled sandstone is similar,but F-T damage can be used as the inducement for the formation of wing cracks.To sum up,this paper systematically analyzes the F-T and loading process of sandstone from the mesoscopic level combined with CT technology.The research methods and conclusions can provide a new reference for the F-T damage mechanism and failure prediction method of rock slopes in cold regions.