| With the implementation of China’s "One Belt And One Road" and "Western Development" strategies,a large amount of infrastructure has been invested and constructed in the northeast and northwest regions.Rock,as the carrier of engineering,will suffer damage to its internal structure after years of freezing and thawing weathering.Most of the rocks in nature are in unsaturated state,and the saturation of the rocks is different in different regions and different burial depths,which will be affected by freezing and thawing in cold regions.Aquifer rocks in low temperature environment,however,its internal water phase change,volume increases by about 9%,lead to internal crack extending,continue to development,at the same time the new crack produce results in the decrease of bearing capacity,cause certain threat to the engineering safety,such as: rock side slope body surface of the rock spalling,frosty cold regions tunnel leakage after freezing and thawing.In this paper,the red sandstone of the slope at the entrance of Liulin No.1 Tunnel which is susceptible to freeze-thaw action is selected as the research object,which is vulnerable to freeze-thaw action,is selected to explore the damage and deterioration law of physical and mechanical properties of red sandstone under different freeze-thaw conditions and different saturation conditions by using indoor freeze-thaw test and conventional uniaxial compression test with different saturations.At the same time,a scanning electron microscopy(SEM)test was carried out on the sample damaged section after freeze-thaw loading.The fractal characteristics of the sample after freeze-thaw loading were studied,and the fractal dimension of the image was obtained.The relationship between the change of the sample microstructure and the macroscopic mechanical properties was analyzed.The freeze-thaw load damage constitutive model of red sandstone with different saturation was established based on the fractal characteristics of the failure section.The main conclusions of this paper are as follows:(1)Before freeze-thaw,the longitudinal wave velocity and thermal conductivity of red sandstone increase with the increase of saturation,while the peak strength decreases with the increase of saturation.In the process of freeze-thaw cycle test,the mass loss rate and volume change rate of red sandstone increase with the increase of saturation and freeze-thaw times.The longitudinal wave velocities of red sandstone with 40% and 60% saturation decrease with the increase of freeze-thaw cycles,and the decreasing rate increases gradually.After 100 freeze-thaw cycles,the decreasing rates of wave velocities are 5.32% and 7.72%,respectively.During the freeze-thaw cycles of the samples with 80% and 100% saturation,the decreasing rate of wave velocity increases first and then tends to be flat.(2)The uniaxial compressive strength of red sandstone decreases with the increase of saturation and freeze-thaw times.The loss rate of peak strength of red sandstone with 40% saturation is 14.96% after 100 freezing-thawing times,and the loss rate of strength of fully saturated sandstone is 48.92% after 100 freezing-thawing times.The elastic modulus of red sandstone with different saturation decreases exponentially with the number of freeze-thaw times,but the decreasing rate decreases with the increase of the number of freeze-thaw times.(3)The freeze-thaw failure mode of red sandstone with different saturation is crack and spalling mode,and the failure mode of the samples after freeze-thaw loading is axial splitting,"skew" type failure and "X" type shear failure.When the saturation is low and the number of freeze-thaw is small,most of the rocks are axial splitting failure;when the saturation is high and the number of freeze-thaw is large,the rocks are seriously damaged,most of the samples are shear failure and the fracture surface is increased.(4)The essence of freeze-thaw damage of red sandstone is that the water inside the rock changes phase and expands volume under low temperature environment,which makes the original cracks in the rock continue to develop and expand and the width of the cracks increase.When the temperature rises,water migrates inside the rock and has a water-rock physical and chemical interaction with mineral particles,leading to the weakening of the degree of cementation among mineral particles,which is macroscopically manifested as the decrease of rock strength.According to the test results,the more saturated the sample is,the more obvious freeze-thaw damage is.(5)There is a relationship between the fractal dimension of the specimen failure section and the macro failure mode.When the fractal dimension of the specimen failure section is large,it indicates that the structural plane is rough and the overall fluctuation is large.With the increase of saturation and the number of freeze-thaw,the fractal dimension of the cross section is small.SEM image shows that the crack width increases,and the secondary crack occurs as shear failure mode and the fracture surface increases under uniaxial loading.(6)The freeze-thaw damage model of red sandstone with different saturation based on damage mechanics is in good agreement with the curve obtained from the test,and the error between the peak strength calculated by the damage model and the test value is small.It can be seen from the damage model calculation curve that the total damage of the sample with 40% saturation is 0.13 after 100 freeze-thaw cycles.After 100 freeze-thaw cycles,the total damage of the 100% saturated sample reached0.68,indicating that saturation and freeze-thaw cycles had a great influence on the damage of the red sandstone.It can be seen from the calculated damage evolution curve that the greater the saturation,the greater the number of freeze-thaw cycles,the smaller the damage rate under load. |
