Experimental Study On The Mechanical Properties Of Sandstone After Freeze-thaw Damage

Because of changes in temperature between day and night,the water conservancy project,bridge tunnel,road foundation in cold regions will be subjected to freeze-thaw damage,thus affecting the safety and stability of engineering.At the same time,the freeze-thaw damage will not only deteriorate the mechanical properties of rock mass,but also lead to the change of the seepage characteristics.Study on the mechanical properties,permeability variation law and the progressive failure rule of rocks after the freeze-thaw damage,is the basis research topics for evaluation of the deformation and safety stability on cold geotechnical engineering.Therefore,in this paper,the sandstone specimens with large porosity are taken as the research object.The mechanical test and permeability test of sandstones after freeze-thaw damage are designed.On one hand,the physical and mechanical properties,permeability variation,deformation and acoustic emission characteristics,failure mode and failure process of sandstone have been analyzed.On the other hand,the sandstone damage model based on acoustic emission parameters under uniaxial compression and freeze-thaw damage attenuation model under triaxial compression have aslo been established.The main conclusions of this paper are as follows:(1)The normalized characteristic stress has been reduced from the dry state of 0.232 to the 30 times freeze-thaw of 0.154.And in the elastic stage and crack propagation stage,?ci/?p and ?cd/?p show increasing trend with the increase of freeze-thaw cycles.The shear strength index of c from 0 cycles to 30 cycles decreased 28.2%,24.7%,15.9%respectively,and the quantitative relationship between c and freeze-thaw cycles can be fitted by negative exponential model.The internal friction angle are not influenced by the effect of freeze-thaw damage.(2)In the compression phase,the number of acoustic emission impacts in the dry condition is 4 to 10 times that of the frozen rock sample,and the proportion of AE location points under the condition of freeze-thaw in this stage is become less.As a part of rock particles after the freezing and thawing has been destroyed,the particles will be reduced when the load is applied.So in later period of the progressive damage,the AE hits,ringing counts and locating points are weaker compared with freeze-thaw samples.(4)The failure mode of the rock is not the same under different confining pressure after freeze-thaw damage.Under uniaxial compression condition,the failure mode of sandstone is splitting failure.Under low confining pressure(2.5MPa,5MPa),the failure mode is single shear failure.While under high confining pressures(10MPa),the failure mode is double shear failure.(5)The freeze-thaw damage can increase the permeability of sandstone,and the permeability becomes larger with the increasing number of freeze-thaw cycles.At the same time,it is show that the larger the number of freeze-thaw cycles,the smaller the decrease of permeability in the process of triaxial compression.The permeability evolution of sandstone under the condition of freeze-thaw damage can be divided into 4 stages,namely the first stage is the descending stage,the second stage is the slow descent stage,the third stage is the stable ascending stage,and the fourth stage is the sharp increase stage.(6)In this paper,the complete stress-strain model for the sandstone caused by freeze-thaw damage under uniaxial compression is established.The model can not only take advantage of the real-time non-destructive testing of acoustic emission,but also consider the effect of freeze-thaw damage on the closure of sandstone.The quantitative relationship between the triaxial peak intensity or the residual strength of the sandstone and the number of freeze-thaw cycles can be expressed as ? = exp(-mN)?0,of which the model parameter m becomes smaller as the confining pressure increases,and the specific quantitative relationship between them deserves further research.