Limit Analysis Of Three-dimentional Stability Of Unsaturated Soil Slopes

Stability assessments of earth slopes are classic problems in geotechnical engineering,which have aroused the interest of geotechnical engineers around the world.Hence,a question rising in both practice and theory is how to evaluate the slope safety more accurately.Up to now,stability analyses of earth slopes are commonly performed using two-dimensional(2D)methods and the three-dimensional(3D)effect on the slope stability is commonly ignored.Meanwhile,most regions in China are in arid or semi-arid regions and the soil in engineering practice is often unsaturated.The influence of suction in soils on the slope stability has been investigated by many researchers and fruitful achievements have been made.However,most of the achievements are accomplished by using two-dimensional limit equilibrium method or finite element method.Up to now,few works exists in 3D stability analyses of unsaturated earth slopes using the kinematic limit analysis method.Using the kinematic limit analysis approach,this paper performed a series of studies on the stability of unsaturated earth slopes.Based on the divergence theorem,a series of semi-analytical methods were proposed to account for the suction effect on the slope stability.The stability of saturated and unsaturated earth slopes under both 2D and 3D conditions was analyzed.The proposed semi-analytical methods are proved to be useful tools for the stability analyses of unsaturated earth slopes and for further studies.The main achievements of this research are listed as follows:1)3D stability of two-stage earth slopes with different slope shapes(i.e.,concave and convex shapes)was analyzed by assuming different failure modes(i.e.,toe failure,face failure and base failure).By reducing the two-stage earth slope to a single-stage slope,the feasibility of the optimization program was validated by comparing with published solutions.The impacts of failure mode,slope shape and depth coefficient on 3D stability of two-stage earth slopes are primarily studied through a series of parametric studies.The influences of slope shape and depth coefficient on the critical slip surfaces were also illustrated graphly.Stability charts with wider range of parameters were provided for practical use.2)The 2D stability of unsaturated earth slopes under steady flow conditions was analyzed by employing the log-spiral failure mechanism and incorporating the effect of suction on the capillary cohesion,unit soil weight and tension crack.Due to the highly nonlinear features of suction in soils,a simplified method was proposed to compute the power rate of unsaturated soil weight.The proposed approach was validated by comparing with the existing results.The effects of suction,infiltration condition,tension crack and filling water in the crack on the slope stability were primarily analyzed.3)3D stability of unsaturated steep and gentle earth slopes under one-dimensional steady infiltration condition was studied.Based on the divergence theorem,a layer-wise summation method(LSM)was developed to compute the energy dissipation rate due to the capillary cohesion of unsaturated soils.A simplified method based on the volume integral of rotating body was developed to compute the power rate of unsaturated soil weight.The influence of layer number on the usage of the LSM was systematically studied and the reasonability of the proposed method was verified by comparing with existing solutions.Through a number of parameter analyses,the influences of suction,soil types,unsaturated flow conditions,ground water variation and seismic loads on the slope stability were systematically studied.4)The proposed LSM and the simplified method were both extended to 3D unsaturated earth slopes with tension cracks and the slope stability was primarily studied.The influence of layer number on the usage of the LSM was systematically studied.The influences of steady infiltration conditions,crack depth and filling water in cracks on the slope stability and on the critical failure surface were systematically studied.5)In view of the 3D stability of partially unsaturated earth slopes after water rapid drawdown,a novel LSM was proposed incapable of dealing with the problem of water level variation in slopes.The LSM was utilized to compute the power rate of unsaturated soil weight and the energy dissipation rate of capillary cohesion.The influence of layer number of the application of the LSM was studied through the parameter analyses,indicating the validity of the proposed method.Effectiveness of the developed method was validated by comparing with several other solutions.The effects of pore-water pressure coefficient,soil type,hysteresis and 3D effect on the slope stability and on the critical slip surface were studied through the parameter analyses and case studies.6)The 3D stability of unsaturated earth slopes reinforced with a row of anti-slide piles was investigated.The objective is accomplished by extending Ito and Matsui's theory to unsaturated soils and by developing a local LSM to account for the effect of moisture content variation on the lateral forces acting on piles.By comparing with existing results,the rationality and effectiveness of the proposed method were illustrated.The influence of suction on the reinforcement of piles and the effect of pile design parameters(i.e.,pile location and pile spacing)and 3D effect on the stability of unsaturated earth slope were studied.7)The seismic loads are regard as uniformly distributed inertia force acting on sliding soil mass according to the quasi-static assumption.Employing the proposed LSM,the 3D stability of unsaturated gentle earth slopes subjected to seismic loads was analyzed.The stabilizing effect of anti-slide piles on the seismic stability of unsaturated earth slopes was studied.The comparisons with existing solutions also demonstrate that the proposed methods are valid in assessing the seismic stability of unsaturated earth slopes.8)The stability of partially unsaturated hillslopes was numerically investigated.To deal with this problem,an inclined LSM method was developed that can account for the influence water table variation,suction in soils and 3D effect on the hillslope stability.The proposed method was validated by applying the inclined LSM to spherical cap failure mechanism and comparing with benchmark solutions.The effects of suction,water table variation,pore-water pressure and seismic loads on 3D slope stability were numerically investigated through parametric studies.