Research On Strength And Deformation Characteristics Of Fractured Sandstone With Different Water Content Under Freeze-Thaw Action

The degradation of the mechanical properties of the rock mass is one of the important factors causing the instability of underground engineering in cold regions.The freeze-thaw cycle,the water content and the existing fissures in the rock mass have an important impact on the mechanical properties of the rock mass.Therefore,it is of great significance to study the strength and deformation characteristics of fractured sandstones with different water contents under freeze-thaw action for the safety of underground engineering in cold regions.At present,the researches on the mechanical properties of fractured sandstone under the action of freezing and thawing were insufficient.In this paper,the mine sandstone of Jiangzhuang Coal Mine in northern China was selected as the research object.In addition,standard sandstone samples were made after different freeze-thaw cycles,different immersion times,and prefabricated fractures at different angles.Conventional uniaxial compression and uniaxial compression acoustic emission tests were carried out,and the effects of freeze-thaw,water content and dip angle on the mechanical properties,crack types,and acoustic emission characteristics of fractured sandstone were explored through laboratory tests.Combined with discrete element numerical simulation technology,the whole process of freeze-thaw water-saturated fractured sandstone under compression was analyzed,and the strength and deformation characteristics of fractured sandstone with different water contents under freeze-thaw action were jointly revealed.The main research results of this paper are as follows:(1)The freeze-thaw cation reduced the mechanical properties of sandstone,and the higher the water content,the more significant the freeze-thaw damage effect.After 20 freeze-thaw cycles,the peak strength and elastic modulus of sandstone decreased significantly,and the decrease in peak strength of low-angle(0°?45°)fractured sandstone was higher than that of high-angle(60°?90°)fractured sandstone.Freeze-thaw action enhanced the brittleness of sandstone,and the degree of fragmentation and the number of cracks in sandstone failure under freeze-thaw action were higher than those of sandstone without freeze-thaw action.Freeze-thaw enhanced the activity of sandstone acoustic emission events,making acoustic emission ring count rate peak and cumulative ring count increase.(2)Moisture can weaken the mechanical properties of sandstone.With the increase of water content,the peak strength and elastic modulus of sandstone decreased linearly.The peak strength of fractured sandstone with a fracture dip angle of 45° was the largest.The increase of water content reduced the brittleness of sandstone and improved the ductility of sandstone.The degree of fragmentation and the number of cracks at the time of failure gradually decreased.High water content inhibited the activity of acoustic emission,and the acoustic emission ringing count rate peak,the cumulative ringing count rate,the distribution density and the distribution range of acoustic emission amplitude of sandstone without freeze-thaw action gradually decreased with the increase of water content.(3)Compared with the intact sandstone,the mechanical properties of the fractured sandstone were greatly reduced,but the increase of the fracture dip angle could optimize the mechanical properties of the fractured sandstone.The fracture dip angle increases from 0° to 90°,the peak stress of the fractured sandstone increased between 168.2%and 274.5%,the elastic modulus increased between 117.7%and 232.5%.The failure mode of fractured sandstone without freeze-thaw action changed splitting failure to shearing failure and finally to splitting failure with the increase of fracture inclination angle.However,under the freeze-thaw action,the brittleness of the fractured sandstone was improved,so the failure modes of the fractured sandstone under the freeze-thaw action ware all splitting failures.The increase of fracture dip angle enhanced the activity of acoustic emission,and the ringing count rate,accumulative ringing count rate,amplitude distribution range and amplitude distribution density in the early stage of loading increased accordingly.(4)The discrete element numerical simulation results of the peak stress and elastic modulus of the fractured sandstone with saturated water content under the freeze-thaw action showed an increasing trend as the inclination angle of the fracture increases,which was in good agreement with the laboratory test results.After encrypting the fracture dip angle,the peak stress,elastic modulus and crack initiation stress of the fractured sandstone showed a fluctuating increase with the increase of the fracture dip angle.The development degree and number of cracks gradually increased with the increase of the fracture dip angle.The inclination angle of cracks had an important influence on the stress distribution around the cracks during the loading process.The mechanical properties of the fractured rock mass under the combined action of freezethaw cycles and moisture were significantly reduced.The strength and deformation degradation laws of the fractured rock mass with different water contents revealed in this paper are an important reference for the design and construction of underground engineering in cold regions and provide important theoretical support and guidance for the stability analysis of supporting structures during the operation of underground engineering in cold regions.