Study On Creep Properties Of Sandy Mudstone In Open-Pit Coal Mines Under Freeze-Thaw Cycles

More than 80% of China’s open-pit coal mines are located in cold regions above38° north latitude,where significant freeze-thaw phenomenon occurs in the slope rocks due to diurnal and seasonal temperature differences.Especially in the soft rock slopes of open-pit coal mines,the freeze-thaw damage effect of rock is more obvious.Under the long-term action of overlying static loads on the slopes,large-scale landslide disasters are prone to occur.Starting from the engineering requirement of long-term stability of soft rock slopes in cold regions open-pit coal mines,this thesis takes the sandy mudstone commonly found on open-pit coal mine slopes as the research object.Freeze-thaw cycles tests,SEM tests,uniaxial compression tests and uniaxial creep tests were conducted to study the freeze-thaw damage on the microscopic structure,physical and mechanical properties,and creep properties of the sandy mudstone under the influence of freeze-thaw cycle number and freezing temperature.A freeze-thaw creep model considering freeze-thaw damage was established based on creep theory and damage mechanics theory.The main conclusions obtained are as follows:(1)Measurements were taken on the mass and P-wave velocity of the sandy mudstone before and after freeze-thaw cycles.And the microscopic structural features of the sandy mudstone were analyzed through SEM test.The results showed that as the freeze-thaw cycle number increases and the freezing temperature decreases,the loss rates of mass and P-wave velocity of the sandy mudstone increased nonlinearly in an exponential manner,and the growth amplitude gradually decreased.The microscopic structure of the sandy mudstone changed from dense to loose,and the microscopic surface became rough instead of smooth.Microcracks and pore defects gradually developed,and freeze-thaw damage increased gradually.(2)Uniaxial compression test was conducted on sandy mudstone samples that had undergone different freeze-thaw cycles.The changes in the uniaxial mechanical properties and failure characteristics of the sandy mudstone were analyzed.The results showed that as the freeze-thaw cycle number increases and the freezing temperature decreases,the uniaxial compressive strength and elastic modulus of the sandy mudstone decreased nonlinearly in an exponential manner,and the reduction amplitude gradually decreased.The peak strain increased nonlinearly in an exponential manner,and the increase amplitude gradually decreased.Meanwhile,the uniaxial failure mode of the sandy mudstone changed from splitting failure to shear failure,and the number of cracks and degree of fracturing increased significantly.(3)An analysis of the evolution characteristics of cracks in sandy mudstones during uniaxial compression was carried out based on the acoustic emission test.The results showed that the peak value of acoustic emission ringing count,cumulative ringing count of acoustic emission,crack initiation stress,and crack damage stress all decreased with the increase in the number of freeze-thaw cycles and the decrease in the freezing temperature.The sandy mudstone gradually exhibited plastic deformation characteristics from brittleness as a whole.However,the ratio of the initial cracking stress and crack damage stress to the peak stress did not change significantly with the number of freeze-thaw cycles and the freezing temperature.(4)Creep test was conducted on sandstone mudstones treated with different freezethaw cycles,and the creep characteristics and failure feature changes of sandstone mudstones were analyzed.The results showed that,under the same stress level,the instantaneous strain,creep strain,and steady-state creep rate of sandstone mudstones increase as the freeze-thaw cycle number increases and the freezing temperature decreases.The ratio of long-term strength to uniaxial compressive strength remains relatively stable and is not significantly affected by the freeze-thaw cycle number and freezing temperature,which is consistent with the crack initiation stress.At the same time,the creep failure mode of sandstone mudstones gradually changes from shear failure to a complex tension-shear composite mode,with a significant increase in the number of main cracks and a significant reduction in rock sample integrity.(5)Based on the creep theory and damage mechanics theory,a freeze-thaw-creep model considering freeze-thaw damage was established,and the parameters in the freeze-thaw-creep model were identified and verified.It was found that the fitting curve based on the freeze-thaw-creep model was highly consistent with the experimental data,and can well reflect the creep deformation characteristics of sandstone mudstones under different freeze-thaw conditions.The research results of this thesis can provide a theoretical basis and engineering guidance for the stability researches of open-pit mine soft rock slopes in cold regions.