Experimental Study On The Influence Of Freeze-Thaw Cycles On Physical And Mechanical Properties Of Sandstone

In perennially frozen soil region or seasonally frozen soil region,the freeze-thaw cycle deteriorates the physical and mechanical properties of rock mass,which will lead to many geological disasters such as landslide and rock mass denudation.Therefore,the variation law of physical properties,mechanical properties,and acoustic emission information of rock under different freeze-thaw cycles is studied.It can effectively evaluate and predict the occurrence of geological disasters and prevent heavy losses,which has essential engineering significance and practical value for many engineering construction and production in frozen soil areas of China.In this paper,two sandstone samples,green sandstone and yellow sandstone,were subjected to freeze-thaw cycle tests and supplemented by nuclear magnetic resonance detection technology to study the change of quality,porosity,bound fluid saturation,and T2 spectrum of rocks under different freeze-thaw cycles.Then,the uniaxial compression acoustic emission(AE)test is carried out to study the changes of mechanical properties such as stress-strain curve and freeze-thaw coefficient.The time-domain parameters(AE event rate,cumulative event number,AE energy rate,cumulative energy)and frequency domain parameters(AE dominant frequency,dominant frequency proportion,amplitude)in the acoustic emission signal are compared and analyzed to explore the deterioration law of rock under freeze-thaw action.The main results are as follows:(1)Except that the change of mass and bound fluid saturation is smaller than that of yellow sandstone,the deterioration degree of other physical indexes of green sandstone is much higher than yellow sandstone.From the influence of freeze-thaw on petrophysical properties,the frost resistance of yellow sandstone is more robust than that of green sandstone.(2)The discreteness of the stress-strain curve of green sandstone gradually increases with the increase of freeze-thaw cycles,the stress decreases significantly,the strain increases greatly,and the freeze-thaw coefficient decreases highly obviously.The freeze-thaw deterioration of yellow sandstone is reflected in the slight increase of strain and the slight decrease of freeze-thaw coefficient.(3)The change of the AE event rate of green sandstone is mainly in the elastic deformation stage and the stable crack propagation stage,while that of yellow sandstone is concentrated in the crack closure stage.During the loading process,the energy release trends of the two sandstones are similar.The 30 times freeze-thaw cycles of green sandstone are the dividing line of brittle-plastic transition,which is reflected in the high AE event rate in the whole process,the stage division is no longer obvious,the lack of low AE event rate almost disappears,the energy release in the "stationary period" increases,and the cumulative energy increases most obviously.(4)Under the action of freezing and thawing,the characteristic parameters of the acoustic emission frequency domain,such as the development mode and severity of cracks,change greatly when sandstone is damaged.Microcracks change from concentrated development to uniform development in the whole process.The stage division defined by the crack volume strain method is no longer apparent,and the crack development changes to complexity and diversification,and the severity decreases obviously.