Study On Engineering Characteristics Of Seasonal Coarse-grained Frozen Soil In Bangda Scetion Of Sichuan-tibet Railway

The Changdu Bangda section of Sichuan-Tibet Railway passes through a large number of seasonal coarse-grained permafrost areas with water-rich slopes,riverbeds and ditches in alpine regions.Seasonal frozen soil with conventional coarse particles and fine particles of coarse grained soil in permafrost,has its soil structure,freezing,moisture migration in different ways,Seasonal coarse-grained soil coarse particles due to freezing and thawing permafrost,its physical properties,water physical properties and mechanical properties change.Therefore,under the action of freeze-thaw,the slope formed by seasonal coarse-grained frozen soil is prone to sliding,especially when the snow melts and the water content increases greatly after snowfall,which is prone to surface sliding,sliding,scouring,hot melt sliding and other disasters.At present,there is no systematic research on coarse-grained soil in the western Sichuan-Tibet line.The task of this paper is to conduct on-site investigation in Changdu Bangda area,to study the engineering characteristics of coarse-grained frozen soil through field and laboratory tests,and to select representative slopes for stability analysis.The contents and achievements of this paper are as follows:(1)Investigation of coarse-grained frozen soil in Bangda area has been carried out,and the basic characteristics of the slope have been identified.In-situ density test,the field density and particle composition of coarse-grained frozen soil were determined,and the indoor particle analysis test was carried out to determine the breccia soil.Under repeated freezing and thawing,the density of soil sample decreased and the moisture content remained basically unchanged.(2)In the study of the hydraulic properties of coarse-grained soil,the water content,water capacity,water holding capacity and permeability of soil samples were tested.Under the action of repeated freeze-thaw,the void ratio,volumetric water content and water holding capacity of soil samples will continue to decrease,gradually increase with the increase of freeze-thaw times,and eventually stabilize,meanwhile the permeability coefficient decreases first and then increases with the increase of freeze-thaw times.(3)In the study of mechanical properties of coarse-grained soil,direct shear test,compression test and frost heave force test are carried out.Under the action of repeated freezing and thawing,the cohesion and friction angle of soil sample decrease continuously,and eventually tend to be stable;with the increase of density,the friction angle and cohesion increase;when pebbles or gravels increase and fine-grained soil decreases,the friction angle increases and the cohesion decreases.The compressibility of soil is obviously related to the content of coarse and fine particles in soil samples.Comparing the compressibility under different water content and freeze-thaw state,the compressibility from high to low is in the order of saturated state,saturated freeze-thaw three times and natural state.Under saturated condition,the frost heave force increases with the increase of the maximum particle size of soil.(4)Statistical analysis of the slope in Bangda area is carried out,and numerical simulation of the slope is carried out to analyze the deformation characteristics of the slope.The results of numerical analysis can be compared with the field slope investigation,which can provide a reference for slope treatment.