Three-dimensional Discrete Element Analysis Of Mechanical Behavior Of Unsaturated Soils

Unsaturated soils exist widely in nature,especially in arid and semi-arid areas.Influenced by climate conditions,there are several kinds of soils with special properties on the earth's surface,such as expansive soils,loess and residual soils,which all have the basic characteristics of unsaturated soils.In engineering practice,such as in the construction of embankments and earth dams,deformation analysis of expansive soil and collapsible soil,slope stability and landslide are usually involved.However,the basic engineering mechanical properties of unsaturated soils are very complicated due to the coexistence of pore water and pore gas.Previous studies mostly used direct shear tests and triaxial tests to obtain the strength parameters and stress-strain curves of unsaturated soils,or through discrete element method,to analyze the movement of soil particles and pore wat er forces from a mesoscopic point of view.The advantage of triaxial shear test is that it can control confining pressure,measure stress state and strain conveniently and reliably,and can reflect the whole process of specimen deformation until failure.H owever,indoor triaxial shear test has high cost,complex sample preparation,high technical requirements and time-consuming.By comparison,three-dimensional discrete element numerical simulation can not only generate numerical samples by controlling grain composition distribution and porosity,consider water content and shear strength by calibrating meso-parameters.It also has obvious advantages such as low cost,high efficiency and easy modeling.Based on the existing research results and triaxial shear tests of unsaturated remolded loess in laboratory,by using particle flow theory and discrete element numerical simulation tool PFC,taking into account the pore water pressure and water content of unsaturated soil,and basing on Hill contact model and li near parallel bond model,the particle flow model of unsaturated loess is established,the mechanical properties of unsaturated soils are studied.These all are in order to effectively solve the problems of deformation and strength in practical engineering.The main contents of this paper are as follows:(1)The basic ideas and principles of discrete element method and PFC particle flow process are introduced.At the same time,the basic principles of Hill contact model and linear parallel bond model in PFC3 D are elaborated in detail from three aspects: defining material,force-displacement law and model attributes,which provide theoretical basis for the discrete element model of triaxial shear test of unsaturated soil.(2)Based on Hill contact model,a discrete element model for triaxial shear tests of unsaturated soils is established.Based on the laboratory test results,the meso-parameters of the model are calibrated,and the discrete element numerical simulation of consolidation and drainage triaxial shear tests is carried out.The variation laws of the stress-strain relationship and strength characteristics of the model specimens under different densities,confining pressures and suctions are revealed.According to Mohr-Coulomb failure envelope under different suction,the relationship between suctio n and shear strength parameters is established.In addition,the relationship between suction and water content in Hill model was established by calibrating the stress-strain curves under different water content in laboratory test results.The results show that Hill contact model is in a good agreement with simulation of the mechanical behavior of unsaturated soils,and can provide a reference for the discrete element simulation of unsaturated soils.(3)A discrete element model for triaxial shear tests of unsaturated soils is established based on linear parallel bond model.According to the stress-strain curves under the same confining pressure and different water content in laboratory tests,the variation law of bond strength between particles under differ ent water content was calibrated,and the relationship between macro-and micro-parameters of soil was established.By means of discrete element simulation of triaxial shear test,the variation laws of macro-mechanical properties such as stress-strain relationship,volume-strainaxial strain relationship and strength characteristics of model specimens under different confining pressures and moisture content are revealed.Meanwhile,the micro-information of model specimens such as average coo rdination number is obtained.According to the strength envelope of discrete element specimens with different water content,the relationship between water content and shear strength parameters is established,and the comparison between the simulation results and the labo ratory test results,which further proves the accuracy of the conclusions.(4)The difference and relationship between two different contact models are systematically expounded from three aspects: the difference of model meso-parameters,the relationship between soil macro-meso-parameters and the numerical simulation results.The results show that both models can be used to simulate unsaturated soil samples,but the Hill contact model is more accurate,reasonable and applicable for numerical simulation of unsaturated soil.