Improvement Of Transfer Coefficient Method For Stability Coefficient Calculation Of Wading Slope During Reservoir Operation

Since the Three Gorges Reservoir started operation in 2003,due to the influence of reservoir water level fluctuations,the particles inside the wading bank slope soil will be eroded and migrated along with the seepage action.At the same time,the physical parameters such as porosity and permeability coefficient of the soil body of the bank slope will change correspondingly,which will cause the bank slope to deform greatly,and even lead to the instability and damage of the bank slope.Previous studies have shown that the migration of particles in the soil is also considered to be one of the main causes of bank slope instability.The transfer coefficient method is widely used by water conservancy and transportation departments to analyze the stability of wading bank slope in China.The traditional transfer coefficient method does not consider the influence of particle migration in the calculation of stability coefficient of bank slope.In view of this,this paper simulates the seepage process of silty clay with gravel during the rapid drop of reservoir water level by using the self-improvement seepage device,and theoretically verifies the existence of hydrodynamic pressure.According to the existing research on particle migration mechanism in soil,a set of finite element program for bank slope seepage calculation under the condition of particle migration is compiled by Fortran,based on which the traditional transfer coefficient method is improved to consider the influence of particle migration on the stability of bank slope.Finally,a program for the improved transfer coefficient method is compiled,which can be used to analyze the seepage and calculate the stability coefficient of bank slope at the same time.Taking Shuping landslide in the Three Gorges Reservoir Area as an example,the program is used to analyze Shuping landslide The saturation line and stability are calculated and analyzed.The main research results are as follows:(1)Based on the concept of hydrodynamic pressure,the permeability test of silty clay with gravel was carried out.The test results show that under the same water head,with the decrease of the permeability coefficient of the soil sample,the piezometer head of the soil sample increases obviously,and the total stress in the soil also increases.Therefore,it is proved that if the permeability coefficient of soil is small,the decline of groundwater level in the soil will lag behind the decline of water level in the reservoir,thus generating hydrodynamic pressure.(2)Based on the theory of particle migration in the soil under the action of seepage,a finite element program for the calculation and analysis of seepage field in the soil of wading bank slope is compiled by using FORTRAN language.The correctness of the program is verified by comparing the seepage test results of the gravel soil of the accumulation.The results show that the particle migration calculated by the finite element method is increasing in the process of reservoir water level rising,and the cumulative rate of particle migration is increasing;in the process of reservoir water level falling,the cumulative rate of particle migration begins to decrease,and finally tends to be gentle.The results show that the finite element program can be applied to the seepage analysis of bank slope under the condition of considering particle migration.(3)Based on the above seepage finite element program and the theoretical analysis of the concept of hydrodynamic pressure,the traditional transfer coefficient method is improved and the subprogram for calculating the stability coefficient is developed.By combining with the finite element seepage calculation program,the integration of seepage analysis and stability coefficient calculation of bank slope is realized.(4)Taking Shuping landslide in the Three Gorges Reservoir Area as an example,the steady-state saturation line of Shuping landslide is calculated considering the influence of particle migration during the process of water level decreasing from 175 m to 145m(rate 1.2m / d).The saturation line in the calculation results is lower as a whole than that of the soil without considering the influence of particle migration.Based on the above seepage calculation saturation line results,the stability coefficient of Shuping landslide is calculated,the stability coefficient is 0.71,and the traditional transfer coefficient method is 1.21.The stability coefficient of the Shuping landslide considering the influence of particle migration is lower than 1.0 when the reservoir water level is reduced to 160 m,which proves that the landslide has begun to lose stability at this time.This calculation result is basically consistent with the results obtained by analyzing the deformation monitoring data of the Shuping landslide.