Effect Of Coarse Grain Gradation And Particle Size On Shear Properties In Southeast Tibet

The coarse-grained soil in southeast Tibet is abundant and the rainfall is abundant.With the development of economy,a large number of traffic and water conservancy projects have been built in this region.The prevention and control of geological disasters,the construction of roads and water conservancy facilities are inseparable from the performance and parameters of coarse-grained soil as a reference basis.To study the mechanical properties of coarse-grained soil is of great significance for engineering safety and economy.Based on these reasons,this paper takes the coarse-grained soil in southeast Tibet as the research object to study the shear performance and shear failure mechanism of the coarse-grained soil.The main work and research conclusions are as follows:(1)The Yigong landslide soil in southeast Tibet was collected,and the basic physical properties of the coarse-grained soil samples were tested and analyzed.Particle analysis tests,specific gravity tests and electron microscope scanning tests were carried out to study the basic physical properties of the coarse-grained soil.(2)Direct shear tests were carried out on 24 groups of samples with six single particle sizes under four vertical loads.The results showed that the shear strength of samples increased with the increase of particle size.With the increase of particle size,the Angle of internal friction and cohesion of coarse-grained soil generally increase.The shear curve of coarse grained soil under different vertical loads is strain softening type,and the strain softening phenomenon of coarse grained soil with larger grain size is more obvious.The shear stress-shear displacement curves of coarse-grained soil with different particle sizes all have certain "fluctuation" phenomenon,and the fluctuation phenomenon is more obvious with the increase of particle size.(3)Direct shear tests were carried out on 104 groups of coarse-grained soil samples with 6 different gradations and different dry densities under 4 different vertical loads.Through theoretical analysis of the test data,the results show that the shear process of coarse-grained soil with different gradations can be roughly divided into three stages: elastic deformation,strain hardening and plastic deformation.With the increase of stone content,the shear strength of coarse-grained soil increases,and the shear displacement corresponding to the shear stress peak also increases,and the strain softening phenomenon is more obvious.The change of grain composition of coarse-grained soil affects the shear strength of coarse-grained soil by affecting the skeleton structure and pore structure.The deformation of coarse grained soil is mainly affected by the stone content and vertical load.Under the same vertical load,the dilatancy of the sample increases with the increase of the stone content.For the sample with the same stone content,the higher the vertical load is,the larger the shear shrinkage is.(4)Direct shear tests were conducted on 15 groups of soil-rock mixtures under 5 different freeze-thaw conditions under different vertical loads.According to the analysis of the test data,the shear strength of coarse grained soil generally decreased with the increase of the number of freeze-thaw cycles,the internal friction Angle continued to decrease with the number of freeze-thaw cycles,the cohesion(biting force)reached the maximum at the fifth freeze-thaw cycle,and then continued to decrease with the increase of the number of freeze-thaw cycles.(5)Using the numerical simulation software PFC2 D,a two-dimensional direct shear test model was established.The numerical simulation tests were carried out on 47 groups of coarse soil samples under different working conditions.The results show that: the shear stress curves all show strain hardening characteristics,and the higher the stone content,the more obvious the strain hardening phenomenon.The sample size ratio with the highest shear strength is 10 times.The shear stress curve fluctuates due to the sliding,overturning and crushing among the particles in the shear process,and the distribution of force chains in each shear stage is quite different.