Progressive Failure Of Unsaturated Expansive Soil Slope Based On Multi-field Coupling Model

Expansive soil is a kind of widely distributed special unsaturated clay in nature.Because of its swelling and shrinkage,it has great influence on slope engineering exposed to natural environment.Establishing effective analysis method,analyzing influence of expansion on slope engineering and defining progressive failure mechanism of expansive soil slope are the key to solve the safety problems caused by its particularity,which have important practical significance.Therefore,the unsaturated expansive soil slope is taken as the research object.The expansion model,parameter calibration,engineering characteristics,rainfall infiltration boundary and rainfall-induced progressive failure of unsaturated expansive soils are systematically studied by means of theoretical analysis,program development,laboratory tests and numerical simulation.The main work and achievements are as follows:(1)Based on the incremental plasticity theory framework,an improved nonlinear practical model(DM)was developed by introducing the concept of variable tangent modulus with two parameters.The non-linear stress-strain characteristics of DM model under compression and tension,cyclic loading/unloading,compression to reverse tension and strain softening of soil are verified by laboratory tests.Compared with the common theoretical model,the model adjusts the transformation of stress-strain curve from linear to hyperbolic by a single parameter adjustment,overcomes the limitation of single form of stress-strain curve,and has wider applicability.(2)Based on the nonlinear practical model(DM),the consolidation theory of two-phase flow and the linear assumption of expansion strain and matrix suction,a numerical program(DMUS)coupling unsaturated seepage field,stress field and expansion strain field was developed.The results of classical sand column drainage test and three-dimensional free expansion test show that DMUS model can effectively reflect the coupling characteristics of unsaturated seepage and consolidation of expansive soil,and the engineering characteristics of each stage in the process of swelling.It has certain applicability for predicting and analyzing expansive soil engineering.(3)According to the commonly used indoor test methods,the physical properties,strength,shrinkage and expansion tests of expansive soils in Guangxi were carried out.The engineering properties of expansive soils and the related factors were analyzed.The effects of initial dry density,initial water content and dry-wet cycle on the strength,shrinkage characteristics,expansion characteristics and expansive force of expansive soils are obtained.(4)Based on Swarm Intelligence Differential Evolution algorithm,the back analysis program of multi-field coupled numerical calculation program(DMUS)was built.And a prediction method for swelling modulus of expansive soil was proposed.Furthermore,based on the sensitivity analysis of expansive factors of expansive soil,the quantitative relationship and sensitivity between expansibility and Poisson's ratio,expansive modulus,initial saturation,overlying load,friction angle,initial modulus,cohesion,dry density and tensile strength parameters were obtained.(5)Based on the overland flow theory,a rainfall infiltration-runoff boundary suitable for multi field coupling model(DMUS)was established.The applicability of the boundary was verified by comparison of indoor rainfall infiltration test.Besides,the influence of slope saturation,rainfall intensity and slope ratio on slope infiltration and runoff is obtained by systematically studying the law of slope infiltration.By comparing with the conventional simplified rainfall boundary,the application scope of all kinds of boundary is obtained,and the rainfall infiltration-runoff boundary has a wider application scope.(6)The progressive failure of expansive soil slope induced by single rainfall was studied by using the developed multi-field coupled numerical calculation program(DMUS)and rainfall infiltration-runoff boundary.The multi-layer and step-by-step retrogressive tractive landslide characteristics of unsaturated expansive soil slope induced by single rainfall were clearly presented.Simultaneously,the failure characteristics,stress,displacement and its influencing factors in the process of progressive failure induced by rainfall were analyzed in detail,and the evolution law of progressive-failure of slope was clarified.Finally,the mechanism of multistage progressive failure of expansive soil slope induced by rainfall was summarized.