Study On Fluid-solid Coupling Theroy Of Gravelly Soil Influenced By Internal Erosion And Its Engineering Application

This work was undertaken to solve the following scientific and technological problems in natural gravel soil affected by internal erosion: i)predictive methods for internal stability of natural gravel soil;ii)constitutive relation for erosion-transport of gravel soil;iii)fluid-solid coupling model and numerical method for gravel soil influenced by internal erosion;iv)effect on overall deformation and stability of gravel soil deposit due to internal erosion.By using the methods of theoretical analysis,equipment development,laboratory tests and numerical simulation together,the aforementioned four problems are investigated in this study.Based on this research,an internal stability criterion for gravel soil is suggested and a stress and strain controlled triaxial apparatus suitable for seepage and shearing tests of gravel soil is developed.A constitutive relation for erosion-transport of gravel soil under deformation is proposed,porosity equations for porous media considering both deformation and fine particle losses is derived,and a theoretical analysis model as well as numerical methods for gravel soil affected by internal erosion are demonstrated.In addition,the influence on the overall deformation and stability of a debris landslide from the Three Gorges Reservoir Region is quantified.Based on the results of the study,the following concluding remarks are presented:(1)Based on the analysis of the relationship between grain composition characteristics and internal stability,this paper presents a criterion applicable to predict the internal stability of natural gravel soil.By using the homemade test apparatus with rigid-wall permeameter,eight gravel soil specimens with different granular composition were tested in the laboratory under upward seepage condition.The internal stability of the eight specimens were assessed according to the judgement method that the permeability coefficient of the specimens increasing with the hydraulic gradient during erosion process.Four commonly used geometric criteria(i.e.,Kenney & Lau's criterion,Kezdi's criterion,Burenkova's criterion,and Wan & Fell's criterion)were adopted to assess the internal stability of the specimens.The limitations of such criteria were analyzed on the basis of the comparison between predictive results and testing results.Based on the hierarchical cluster analysis and rank correlation analysis,140 specimens collected in this paper were analyzed on the relationship between their grain composition characteristics and internal stability,and an internal stability criterion suitable for natural gravel soil is proposed.The criterion was then used to assess the internal stability of multiple broad-graded gravel soil specimens including the eight samples in the test,and the reasonability was verified according to predictive results.(2)A stress and strain controlled triaxials apparatus which can be used to simulate the internal erosion process of gravel soil with large particles and broad-graded granular composition was developed.Based on the test results,the influence on soil characters caused by internal erosion was discussed and a constitutive model describing the mass change of eroded gravel soil under deformation was suggested.By using the triaxials apparatus,a series of seepage erosion tests as well as sheer and compression tests under confining pressure were carried out on gravel soil samples with two different grain distributions.Depending upon the test results,the constitutive equations were constructed using strain,average velocity and seepage time as three independent variables.The multi-stage loading method was implemented in the shear and compression test for the specimens after internal erosion.Based on the test results,this paper demonstrates the influences on the mechanical properties of gravel soil due to internal erosion from three aspects: stress-strain characteristic,yield strength and peak strength.(3)A multi-field fluid-solid coupling model based on the coupling mechanism of bidirectional effect between eroded particle losses and volumetric strain was developed by theoretical analysis and deduction.Considering the bidirectional coupling feature between eroded particle losses and volumetric strain,a multi-field fluid-solid coupling model using the porosity of the gravel soil as the coupling variable was set up based on the porous medium seepage theory,continuum mechanics theory and solute transportation theory.The gravel soil was assumed as a four-phase porous medium.Based on the assumption,the stress field equation using displacement and pore pressure as independent variables was set up with the introduction of effective stress principle and stress equilibrium;the concentration field equation and the coupled equation were established according to the law of conservation of mass.Comsol Multiphysics was used as the numerical simulation platform to stimulate the loading procedure of indoor internal erosion experiments.The coupling model as well as the solving method was testified based on the comparison between model simulation results and experimental results.(4)Based on the numerical analysis method,a typical talus slide in Three Gorges Reservoir Region was taken as a mock object and its internal erosion process was simulated under long-term stable water-level condition and fluctuating water-level condition.According to the simulation,the influence on the overall deformation and stability of the debris landslide is quantified.Based on the type section geological information as well as the hydrologic condition of the typical talus,a finite element analytical model is built to study the deformation behavior as well as the effect on the overall stability of the debris landslide due to time accumulation effect of internal erosion and the change of hydraulic and boundary conditions.The test results from the laboratory internal erosion tests of gravel soil are taken as the material parameters of the numerical analysis model.Two simulated conditions are long-term stable water-level condition and fluctuating water-level condition.According to the numerical modeling results,the influence on the overall deformation and stability of the debris landslide is quantified in three aspects: characteristics and change rule of eroded-grain distribution,change and deformation rules of displacement extreme value,and change rule of overall safety factor.