Micromechanical Simulations Of Sand Behavior Under Monotonic And Cyclic Loading

Soil is a multiphase particulate medium. In spite of the granular nature, soil has traditionally been modeled as a continuum. This continuum mechanics cannot get the precise solution while soil's nonlinear, discontinuous, large deformation, and strain localization behavior are considered. The discrete element method(DEM) is based on discontinuum theory, and is pioneered by Cundall. DEM treats each block or particle as an element, and the motion of element need not conform to displacement continuity and deformation compatibility conditions, so it is a perfect method to capture the discontinuous behavior of soil.It is an important application of particle discrete element method to simulate the laboratory tests in soil mechanics. On the one hand, through comparing the macromechanical results of lab tests and numerical tests, the validity of numerical model can be proved. On the other hand, numerical tests are the effective supplement to lab tests in microscope, because the wealth of micromechanical and statistical information can be generated by the simulations together with the macroscopic response.In this paper, one of the particle discrete element method named particle flow code in 2-dimension(PFC^2D) is used. Based on the results of lab tests, sand behavior under monotonic and cyclic loading is simulated from the aspect of association of macro and micro scope. Firstly, the calibration of numerical model is carried out by the results of conventional triaxial tests, and the micromechanical parameters for the simulation of Fujian standard sand are achieved. Secondly, the liquefaction behavior of sand under cycling loading is simulated by using Hertz-Mindlin contact model. Thirdly, to cover the shortage of circular element simulations, elliptical element and angular element are developed by using CLUMP logic in PFC^2D, and the effect of particle shape on sand properties is numerically analyzed. Finally, cluster element is developed by using contact-bond model in PFC^2D, and the particle breakage property of sand under high pressure 1D compression condition is investigated. It is found that compared with liner contact model, Hertz-Mindlin contact model is more suitable to use in the simulations of sand properties under the variable stress level. Numerical simulations by assemblies of circular panicles can qualitatively reflect the general characteristics of liquefaction behavior of sand. The interval effect of aspect ratio of elliptical panicle on the mechanical behavior of numerical sample is obtained, and the reason is related to the initial average co-ordination number. The regularity of evolution of fabric in the process of sand liquefaction under cyclic loading can be qualitatively achieved by using the numerical sample randomly mixed of panicles with different shapes. Numerical simulations can not only demonstrate the macro phenomenon of particle breakage, mostly importantly, it can also expose the evolution of particle breakage microscopically, and would contribute to further research of the micromechanics of particle breakage.