Experimental Study On The Damage Mechanism Of Gneiss Under The Effect Of Freezing And Thawing Cycle

The cold regions are widely distributed in China,there is a lot of frozen soil in the cold area,which including frozen soil and frozen rocks.Cold regions construction is facing complicated problems of freezing damage,for example Instability of slope,differential settlement of subgrade,surrounding rock instability of tunnel caused by freezing-thawing cyclic effect and so on,The study of freezing-thawing cyclic damage of rock in cold region is becoming more and more urgent.Gneiss was chosen as the experimental material for freeze-thaw cycles,which was frozen at-20 ? for 4h,then thawed in water at 20 ? for 4h,and a freezing-thawing cycle was completed,A total of 180 times were conducted in this freeze-thaw cycle test.The quality of gneiss,longitudinal wave velocity,ultimate compressive strength,elastic modulus,Poisson ratio,rock porosity,Pore volume ratio of different pore sizes was tested after the 20 freeze-thaw cycle.In this paper,the changes of physical and mechanical damage of gneiss were studied after freeze-thaw cycles,and the knowledge of damage mechanics is used to deduce the constitutive relation of rock damaged by freeze-thaw stress,The main conclusions are as follows:(1)During the whole freeze-thaw cycle,the gneisses' mass and longitudinal wave's velocity decreases with the increase of cycle times.The final quality loss rate is about-0.12%,the loss rate shows a trend of first increasing,then decreasing,and finally increasing gradually.The attenuating range of the longitudinal wave velocity of rock is-3.24%,the rate of wave velocity attenuation increases first,then decreases,and finally tends to stability.It can be seen that with the increase of the number of freeze-thaw cycles,the density and integrity of gneiss are continuously declining.(2)Gneiss strength tests show that with the increase of freeze-thaw cycles,the ultimate compressive strength,elastic modulus,freeze-thaw coefficient decrease gradually,and the Poisson's ratio?peak strain of longitudinal?peak strain of transverse increase gradually.After 180 cycles of freezing and thawing,the loss rate of strength is-23.05%.The loss of modulus of elastic modulus is close to 50%,According to the freeze-thaw coefficient,the gneiss is in the state of intermediary weathered sandstone.The growth rate of the Poisson's ratio,peak strain of longitudinal,peak strain of transverse is 141.67%,101.26%and 388.42%.From the uniaxial compressive stress-strain curves,with the increase of the freezing and thawing cycles,the compression phase gradually becomes longer.(3)The results of nuclear magnetic resonance(NMR)showed that the rock porosity increased with the increase of freeze-thaw cycles times.The porosity increased from 0.287%to 1.402%,and increased by 388.5%.According to the pore size and the proportion of gneiss,the pores in gneiss are defined as:small pore diameter(0.01?0.1 diameter m),middle pore diameter(diameter 0.1?100 0.1?100 m),large pore diameter(diameter 100?400 m).During the whole process of freezing and thawing.The proportion of small pores and medium pore is gradually increased,and the proportion of medium pores is gradually decreased.(4)Damage variable of freeze-thaw Dn is expressed by the micro-fracture propagation factor Kn Based on the extension of the equivalent strain principle,the constitutive relation of the damage for the rock under the frozen thawing is derived.Example calculation found that the damage variable of freeze-thaw Dn is 0.958 after the 180 times freezing and thawing cycle...