Analysis Of The Progressive Failure Of Slope Based On Shear Strength Character Of Slip Zone Soils

As a key component of landslide, slip zone soil is the major control factor for the form, growth, andevolution of landslides. The shear strength index of slip zone soil is an important parameter for the estimationof stability and design calculation for landslide, so the study of shear strength characteristic for slip zone soilhas important practical significance and mentoring function for the prevention and treatment for landslides.Slope formed by soils with strain-softening characteristic often generate a progressive failure duo to thestrength reduce of slip zone soil. How to establish a deformation control analysis method for progressivefailure based on the study of shear strength of slip zone soils and how to carry a correct analysis for theprogressive failure of slope are the issues of common concern for academic and engineering world, and alsoare research topic that worthy further study. Taking practical slope engineering as the research object,laborbary tests, theoretical derivation and numerical simulation were comprehensively used to study theprogressive failure of slopes. Ring shear test, triaxial test and microstructure observation were carried out tostudy the shear characteristic of slip zone soil, theoretical analysis were used to set up a strain localizationmodel, the model was applied to finite element software, and numerical analysis were used to study theprogressive failure of slope due to the local slide. The main research contents and achievement includes:(1) Residual strength characteristic study of slip zone soil. Ring shear tests were carried out to study theresidual shear characteristic with large shear displacement based on the spicemens taking in the engineeringfield. As the slip zone soil belongs to fine grained soil containing coarse which has a high coarse content thancommon clays, this study focuses on how the coarse content effect the shear characteristic of slip zone soil.The test shows that slip zone soil has a different strain-softening characteristic compared with clays underlarge displacement shear as the slip zone soils contant high coarse, the stable residual strength can commonlybe abtained after80-100mm shear displacement, and the relationship between residual strength index andcoarse content is nonlinear positive. In addition, the vertical settlement in the shear process was effected by thenormal stress, and was also effected by the coarse content. The tests also shows that shear rate has an effect onthe residual strength to some extent, the influence extent is relate to the normal stress, and the effect of shearrate is not obvious, within3%more or less. Based on the tests, calculation procedure was programmed tocalculate the shear strength index according to the least square method, and the results show that ring shear test is a good method to oatain the residual strength of slip zone soils.(2) Micromechanism study of shear strength of slip zone soils. Microstructure observation of shearsurface before and after ring shear test were carried out to study the changes of particle directional alignment,pore area, particle size distribution, and particle shape in the shear surface. The shear strength mechanism ofslip zone soil was proposed from micro perspective, and a microstructure model was set up based on the fractaltheory. The tests show that, as the soil particles was sheared for large displacement, soil particle got directionalalignment in the shear direction, the shape of soil particle got long and narrow, pore rate of shear surfaceincreased, and the moisture content of shear surface also increased as the water concentrated to the shearsurface. The microstructure changes of ring shear surface before and after ring shear test can reflect themacro-mechanical properties. Based on the perimeter-area method, the fractal analysis of ring shear surfacewas carried out. It was found that the shear surface has obvious fractal characteristic with different coarsecontent, and the residual strength has a good negative correlation with the fractal dimension. A microstructuremodel has been set up to connect the microstructure of shear surface and macro-mechanical properties throughthe fractal dimension.(3) The strain-softening characteristic study of slip zone soils during whole process. Triaxial test on precutspecimens was proposed to study the residual strength of slip zone soils under complex stress condition. Thepeak and residual strength of slip zone soils were obtained from the triaxial tests and trixial tests on precutspecimens, and the cohesive force drops sharply from peak to residual while the internal friction angle has aslightly reduction. A method was proposed to obtain the whole process of strain-softening based on the twotests, and the reduction law of slip zone soils was obtained under complex stress condition.(4) Strain localization model of slip zone soils. The Mohr-Coulomb yield criterion consideringgradient-dependent plasticity was deduced, the yield strength not only depends on softening parametermeasure, but also on the Laplacian thereof, and the finite element solver of the softening model was deducedbased on gradient-dependent plasticity theory. The parameters of the Strain localization model were obtainedfrom the stress-strain whole process curve. The finite element solver of a modified element with four nodeswas deduced based on gradient-dependent plasticity theory, and the solver was programmed into the UELsubroutine of Abaqus finite element software. The strain-softening model was used to simulate the trixaial test,and it can be seen from the results that the model can solve the problems that often met in the strainlocalization problem, such as mesh sensibility and lost of positive definiteness, and can reflect thestrain-softening characteristic of soil well. (5) Study of progressive failure of slope based on deformation control. Based on the Strain localizationmodel proposed, a calculation model was propoded and numerical simulation was carried out to study theprogressive failure of slope trigged by the local slide of toe area. The calculation model presents the inducingfactor of landslides through simulate the intial stress field of slope, pecks up the strain-softening elementthrough the control of threshold value of strain-softening, simulates the progressive failure process of slopethrough the regenerate of stress field, and proclaims the shear mechanism of soil strength during theprogressive failure process. The finite calculation shows that the plastic zone concentrates in the slip zonewhich gradually extends from the toe area to the top area, and finally the thorough of plastic zone caused thefailure of the landslide. The slip zone soil in different plastic zone can offer different shear strength in theprogressive failure process. When the failure of the historical landslide occurred finally, the soil in the top andintermediate region offer a shear strength approaching peak strength, the soil in the toe area offer a shearstrength approaching residual strength, while the soil in other regions can offer a shear strength between thepeak and residual strength. The results proclaim the failure mechanism of slopes, and provide the basis ofrslope strengthening.