Impacts Of Cyclic Freeze-thaw On Strength Properties Of Coarse-grained Soils And Slope Stability Of Mine Dump

As a strong weathering process,cyclic freeze-thaw actions can lead to considerable changes in engineering properties of rock and soil.In high-altitude and cold regions,seasonal freeze-thaw of ground are very common,and a series of geological hazards related to cyclic freeze-thaw have a serious impact on the construction and maintenance of engineering projects and the social and economic production activities.For the mine dump in high-altitude and cold regions,due to its unique nature,serious geological disasters,including landslides and debris flows,are prone to occur under the adverse influence of cyclic freeze-thaw,which threate the safety of mine production and people s lives and property around.In this paper,variations in stregnth properties of coarse-grained soils with different particle size distribution taken from a mine dump under freeze-thaw actions are studied through systematic cyclic freeze-thaw test,uniaxial compression and triaxial compression tests.The strength deterioration of coarse-grained soil under freeze-thaw cycles is quantified with the test reulsts.Then,a large-scale direct shear test of frost-thaw interface of coarse-grained soil is carried out.The shear behaviors of melted soil and freeze-thaw interface are compared.Finally,the slope stability of mine dump under natural conditions and different freeze-thaw cycles are simulated with consideration of the adverse effects of cyclic freeze-thaw on the strength of coarse-graded soil.It is hoped that the result in this study can provide reference for the prevention of freeze-thaw disasters of slope engineering in cold regions.The main conclusions are as follows:(1)Uniaxial compressive test of coarse-grained soil:Under freeze-thaw cycles,the uniaxial compressive properties of coarse-grained soil changes significantly.After freeze-thaw cycles,the stress-strain curve of coarse-grained soil tends to change gradually from strain softening to strain hardening.With increasing freeze-thaw cycles,the uniaxial compressive strength and elasticity modulus of coarse-grained soil decrease exponentially.Before nine freeze-thaw cycles,the two parameters decrese quickly,while after that they remaine almost stable.The failure strain of coarse-grained soils increases slowly with increasing freeze-thaw cycles.Under the same freeze-thaw cycles,the compressive strength and elasticity modulus of coarse-grained soil decrease with increase in fine gravel P2-5 contents.In addition,with increase in fine gravel P2-5 contents,the failure strain of samples generally increases.(2)Triaxial compression test of coarse-grained soil:When the fine gravel P2-5 contents is 23%,the stress-strain curve changes from weak strain softening to strain hardening with the increase in freeze-thaw cycles.However,with increase in fine gravel P2-5 contents,the effect of freeze-thaw cycle on the stress-strain curve is no longer obvious.With increase in fine gravel P2-5 contents,the stress-strain curve of the soil changes from weak strain softening to strain hardening under the same freeze-thaw cycles.The shear strength and cohesion of coarse-grained soils decrease in varying degrees with increasing freeze-thaw cycles,and the shear strength of coarse-grained soils decreases exponentially firstly and then remains almost stable after 5-9 freeze-thaw cycles.The elasticity modulus decreases with freeze-thaw cycles increases.The internal friction angle fluctuate considrably under the action of freeze-thaw cycles,and there is no obvious change rule.Under the same freeze-thaw cycles,the shear strength,elastic modulus,cohesion and internal friction angle of coarse-grained soil decrease with increase in fine gravel P2-5 contents.In addition,with increase in fine gravel P2-5 content,the impacts of freeze-thaw on the shear strength,elastic modulus and cohesion decrease considerably.(3)Large direct shear test of freeze-thaw interface of coarse-grained soil:There are obvious differences between the shear stress-shear displacement curves of freeze-thaw interface and thawed soil.The curves of thawed soil show strain hardening characteristics without peak strength.However,the curves of freeze-thaw interface show strain softening characteristics,and the peak stress generally occurs with 1/100 shear displacement.Under the same gravel P2-20 content,the shear strength of the freeze-thaw interface is obviously greater than that of thawed soil,being about two times of the latter.This is mainly due to the cementation of ice developed at the freeze-thaw interface,which increases the cohesion of soil and contributes about 50-60 kPa.For the shear strength index,the cohesion of thawed soil decreases rapidly with increase in gravel P2-20 content,while the internal friction angle increases gradually.With increase in gravel P2-20 content,due to the contribution of ice cementation,the cohesion of freeze-thaw interface decreases significantly less than that of thawed soil.However,with increase in gravel P2-20 content,the internal friction angle of freeze-thaw interface increase greater than that of thawed soil.This shows the contribution of ice encapsulation on the surface of coarse particles to the particle occlusion friction.(4)Numerical simulating on slope stability of mine dump at high altitude and cold:The slope of mine dump under natural conditions is simulated and analyzed,and the results show that the slope is in a safe state at present.With the increase of slope angle and slope height,the safety factor of slope decreases linearly.The optimal dumping scheme under natural conditions is proposed through the simulated results.The temperature field of the dump slope shows that the effect of freeze-thaw on the shallow surface soil is remarkable.The maximum freezing depth is about 4m.The fluctuation range of soil temperature decreases gradually with the increase in the depth,and tsoil temperatures below 10m do not change with the change of the environment temperature.Considering the influence of freeze-thaw cycles on the strength parameters of coarse-grained soil,the slope which undergone different freeze-thaw cycles is simulated.The results show that the safety factor of the slope decreases exponentially with increase in freeze-thaw cycles,and the maximum decrease can be about 11%.