Three-Dimensional Upper-Bound Limit Analysis Method On Stability And Reinforcement Design Of Soil Slopes

Earthquakes and "water" disasters are the main external factors inducing the collapse or settlement of soil slopes along transportation routes in China.The spatial variability of soil parameters is the main internal factor affecting the stability of soil slopes.In practical engineering,active or passive reinforcement methods are generally adopted for disaster prevention and disaster reduction of soil slopes under complex internal and external factors.In this engineering background,it is of great significance to scientifically understand the ultimate state behavior of slopes subjected to potential disasters and develop effective evaluation methods for reinforced slopes in the ultimate failure state.Aim at narrowing the aforementioned research gaps,this thesis carries out the study on the upper bound analysis method for the three-dimensional stability analyses of rotational earth slopes subjected to earthquakes,"water" disaster and reinforcement based on the upper bound limit analysis method of geotechnical engineering,global optimization theory,finite difference theory,random field theory.This work is supported by the National Key Research and Development Project named as "Key Technology for Rapid Assessment of Slope Collapse and Subsidence in Dangerous Areas".The following research results are obtained in this work:(1)A three-dimensional discretized kinematically admissible failure mechanism of rotational slopes is establishedA universal “point-by-point” generation technique is proposed to solve the mesh “distortion” phenomenon.A three-dimensional discreetized kinematically admissible velocity field of rotational earth slopes is constructed.A “projection-connection-separation” strategy is developed to discrete the computational domain.A “discretization-sum” method is developed to establish the three-dimensional virtual power balance equation of rotational earth slope.The strength reduction method and the bisection method are combined to derive the implicit object function of safety factors,and a hybrid optimization algorithm of rough global search(Genetic Algorithm)and a local fine search(Nelder-Mead Simplex Method)are adopted to determine the most unfavorable state of slopes.The irregular characteristics of the three-dimensional failure surface of rotational slopes with spatially varying strength parameters are elucidated from the perspective of the energy method.The comparative results show that the proposed model can improve the computational efficiency of finite difference numerical simulations by 10 times,which is expected to overcome the shortcoming of being time-consuming existing in the traditional reliability analyses of soil slopes with small failure probability(2)A three-dimensional finite difference-discrete upper limit analysis method for rotational earth slopes subjected to earthquakes is proposedThe finite-difference numerical method is used to obtain the timehistory of the three-dimensional acceleration field of rotational earth slopes subjected to complex nonlinear seismic excitations.By using the discretization-mapping technique,the three-dimensional acceleration field is discretized on a time scale and then are mapped to the three-dimensional kinematically admissible velocity field of rotational slopes sequentially.A model for calculating work rates done by spatiotemporal nonlinear varying seismic forces is established,which successfully considers the spatiotemporal distribution characteristics of seismic accelerations,satisfies the zero shear stress boundary condition,and avoids the problem of excessive acceleration amplification,thus expanding the application range of existing upper bound analysis methods.Through comparative analysis,the differences,advantages and disadvantages among the traditional pseudo static method,pseudo dynamic method,modified pseudo dynamic method and the proposed model are explained.The application of the proposed model to earthquake-induced slope instability is carried out,revealing the deep rotational failure mechanism with a“water droplet” shape of Long-Nan slope.(3)A three-dimensional finite difference-discrete upper limit analysis method for rotational earth slopes under complex transient seepage conditions is proposed.A three-dimensional finite difference method for determining steady or transient pore water pressure fields generated within slopes under the conditions of drawdown,water level rise and rainfalls is developed.A mapping-interpolation technique is developed to map the steady or transient pore water pressure fields into the kinematically admissible velocity field of rotational slopes.A unified discretization-based computational model of external working rates and energy dissipation rates is established for rotational slopes subjected to transient seepage conditions.The shortcomings of the traditional three-dimensional upper-bound limit analysis of slopes subjected seepages are discussed.Based on the ChenJia-Wan landslide,Xie-Jia-Ling landslide and Ai-Dong landslide,the influences of the change rates of water levels,rainfall duration,rainfall intensity and rainfall forms on the failure mechanisms of slopes are explored,and the shallow rotational failure mechanism with a “circular chair” shape of Xie-Jia-Ling slope and the deep rotational failure mechanism with retrogressive characteristic of Ai-Dong slope are revealed.The irregular characteristics of the three-dimensional failure surface and the time-varying evolution of limit state behavior of soil slopes with spatial variation of hydraulic parameters under water level fluctuation and rainfall conditions are clarified from the perspective of the energy method.(4)New velocity compatibility mechanisms and energy dissipation rate calculation models for soil-geosynthetic,soil-bolt,soil-prestressed anchor cable are established.A three-dimensional “quasi-plasticity” upper bound analysis method for slopes reinforced with components is proposed.New velocity compatibility mechanisms and energy dissipation rate calculation models for soil-geosynthetic,soil-bolt,soil-prestressed anchor cable are proposed.A three-dimensional “quasi-plasticity” upper bound analysis method for slopes reinforced with components is proposed.The differences,advantages and disadvantages,and inherent connection between the "strict plasticity" and the "quasi-plasticity" upper-bound limit analysis models are explained.A new criterion for the instability of soil slopes strengthened with prestressed members is proposed.The importance of considering the deformation coordination of components,the metal yield criteria,the contribution of flexural energy consumption,and the pullout of reinforcing elements from unstable bodies in the energy dissipation analyses is emphasized.