Study On The Physical And Mechanical Properties And Degradation Model Of Gray Sandstone Under Freeze-thaw Cycle

With the sustained economic development of China,the development and utilization of resources in cold areas is particularly important.However,with the increase of the quantity and scale of geotechnical engineering in cold areas,the problem of freeze-thaw disasters also appears,which seriously affects the stability and safety of buildings in cold areas.Therefore,based on the hydrogeological and meteorological investigation results in Jiama mining area,Tibet,the physical and mechanical properties,microstructure and phase analysis tests are carried out on the basis of freeze-thawle tests,and the physical and mechanical properties and damage deterioration mechanism of freeze-thaw gray sandstone with different water content are systematically studied from macro and micro perspectives,combining with the theories of freeze-thaw mechanics and damage mechanics.The principal research findings are as follows:(1)Prior to the freeze-thaw cycling test,an increment in moisture content led to a corresponding rise in the longitudinal wave velocity of grey sandstone.Post the freeze-thaw cycling test,the attrition ratios of mass and longitudinal wave velocity of grey sandstone with varying water contents escalated as the number of freeze-thaw cycles increased.Furthermore,there was a conspicuous amplification in dissimilarity among the loss rate curves of grey sandstone with various water contents with an increase in the number of freeze-thaw cycles while the alterations became more gradual.Despite the freeze-thaw cycles’ impact,the dry grey sandstone’s porosity did not undergo significant changes.Conversely,the natural and saturated grey sandstones manifested a gradual augmentation in porosity as the number of freeze-thaw cycles upsurged,and the differences among them became more prominent.(2)The peak strength and elastic modulus etc of grey sandstone before and after freeze-thaw cycling tests exhibit an exponential function change with respect to the increase in water content or freeze-thaw cycles.Specifically,the peak strength and elastic modulus both decrease exponentially,while the peak strain increases exponentially with the increase in water content.Additionally,as the number of freezethaw cycles increases,the frost resistance coefficient of grey sandstone decreases for different water content states.The reduction in frost resistance coefficient is more significant for natural and saturated state rocks.Moreover,when the freeze-thaw cycle times are constant,the rate of reduction in frost resistance coefficient gradually increases with the increase in water content.(3)The results of scanning electron microscope show that under the action of freeze-thaw,the overall structure of the gray sandstone in the dry state has little change,and its surface was relatively flat and closely arranged,which was less affected by freeze-thaw action.In the natural state,the surface of gray sandstone is rough,producing some flaky particles,accompanied by microcracks.There are fewer large holes,mainly small holes.The saturated gray sandstone has more honeycomb areas and loose mineral particles,and there are obvious fracture traces,and the rock structure is significantly damaged.(4)According to the X-ray diffraction test,it is found that grey sandstone mainly contains six elements,such as oxygen,aluminum,silicon,calcium,potassium and iron.Shi Ying,mica,kaolinite and chlorite are the main materials.In the dry state without freeze-thaw,the content of clay minerals decreased from 22.5% to 16.3%,which was unstable and easily affected by water and freeze-thaw,leading to the deterioration and destruction of gray sandstone.(5)In order to explore the damage evolution law of freeze-thawray sandstone,the damage factors of elastic modulus,longitudinal wave velocity and uniaxial compressive strength of freeze-thaw rock were compared and analyzed.Based on the theory of damage mechanics and freeze-thawechanics,the freeze-thaw damage evolution model of gray sandstone with different water contents under uniaxial compression was established.On this basis,based on the SMP criterion,a damage softening model of freeze-thaw-loaded rock considering residual strength characteristic is established.This model has been verified.It is found that the model can well reflect the damage and degradation process of freeze-thaw rock under uniaxial or triaxial compression,and its damage mechanical characteristic curve conforms to the general law.