Study On The Evolution Law Of Physical And Mechanical Parameters Of Gravel-soil Accumulation Body Along Elevation Under The Action Of Rainfall

Quaternary loose deposits formed by accumulation are widely distributed in the Three Gorges Reservoir area.They have the characteristics of large scale,complex mechanism,strong suddenness,great harm,and high difficulty in prevention and control,which are extremely harmful to the reservoir area.Due to the complex rock-soil composition of the accumulation body itself and the joint action of external environmental factors,its soil parameters have a high degree of complexity and uncertainty.Therefore,the study of the degradation effect of the accumulation body parameter will have a significant impact on the reservoir area.The long-term safe operation of the project has important scientific and application value.This paper takes the Shuping landslide as the object and uses physical model tests to study the shear strength parameters and permeability coefficients of the rock and soil mass along the elevation direction under the action of rainfall.This law is used to calculate and analyze the Shuping landslide at 145m and 175m.Stability at the water storage level.The main research results are as follows:1.Determine the rainfall erosion model test materials according to the soil gradation of the Shuping landslide.The test gravel soil is made of silty clay,river sand（1mm～2mm）and gravel（5mm～3cm）in a ratio of 1:1:2..2.After the rainfall is over,carry out particle grading test,X-ray fluorescence analysis test（XRF）and nuclear magnetic resonance test（NMR）on the gravel soil at different elevations to obtain the particle gradation,element and mineral content of the gravel soil.Characteristics of porosity changes along elevation.From the test results,the effective particle size(9₁₀,continuous particle size(9₃₀,and restricted particle size(9₆₀of the gravel soil on the top of the accumulation are larger than the original gradation;with the increase in elevation,the porosity of the gravel soil gradually increases,reflecting the Si content of the fine-grained soil The gradual decrease of Al element,Si O₂ and Al₂O₃ content verifies the migration process of fine particle components such as clay and river sand in the accumulation body to the toe of the slope under the action of rainfall.3.After continuous rainfall,the permeability coefficient of the accumulation gradually increases with the increase in elevation.The mechanism for the above changes is:On the one hand,rainfall erosion and seepage effect change the particle size distribution of the surface gravel soil,and with the increase in elevation The larger the effective particle size(9₁₀of the gravel soil,the stronger the water seepage ability;on the other hand,the rainfall infiltration promotes the migration of fine particles into the slope,which causes the porosity of the gravel soil to gradually decrease with the increase of the depth of the accumulation body,and the deep layer of the slope is broken.The permeability coefficient of rock and soil is smaller than that of the surface layer.4.After continuous rainfall,there are obvious differences in the shear strength parameters of accumulations at different elevations.As the elevation increases,the cohesion gradually decreases,and the internal friction angle increases slightly;the mechanism for the above differences is:water scour and seepage As a result,the higher the elevation,the higher the relative rock content of the deposits（the lower the soil-rock ratio）,and the higher the elevation,the lower the clay content and the more obvious the solidification and occlusion between the gravel,which is affected by the soil-rock ratio.The friction angleand cohesive force(8 of the gravel soil showed increasing and decreasing trends respectively.5.According to the variation law of the mechanical parameters of the accumulation body along the elevation revealed by the indoor test,the calculation of the seepage stability of the Shuping landslide under the action of the reservoir water is carried out.The calculation results show that:After the volume parameters change along the elevation,the maximum deformation of the accumulation at the water level of 145m and 175m is 14.2cm and 16.9cm,respectively,an increase of 11.8%and 7%;The stability coefficient of the accumulation is1.08 and 1.15,respectively,a decrease of 2.7%and 1.29%,which is closer to the stability coefficient of Shuping landslide during thenormal operation period of the reservoir area.Therefore,considering the variability of the sliding body parameters of the accumulation body along the elevation in the numerical calculation,a smaller estimate of the stability of the accumulation body will be obtained,and a more conservative support for the subsequent engineering support design.