Study On The Flow Process Of Granular Materials With The Material Point Method Simulations And Its Engineering Applications

Granular medium is a disordered system composed of discrete solid particles,which is widely observed in the engineering and nature.Due to the characteristics of discreteness,motility and dissipation,the granular materials usually exhibit multi-state behaviors: solid-like(stable accumulation),liquid-like(slow flow like Bingham fluid),and gas-like(rapid dilute flow).The three flow regimes are found to coexist or transform mutually in a granular flow.In recent years,earthquake-induced or rain-induced landslides have frequently hit the southwest China,especially the mountainous areas,which account for 80% of the national geological disasters.In order to predict the agitation and the flow process of landslides,we conducted both the experimental and numerical works to reveal the dynamic mechanisms.In this thesis,the geometric,kinetic and mechanical quantities were analyzed based on the experiment of granular collapse.A uniform constitutive model was constructed to describe the multi-states of granular flow.For engineering applications,3D material point method(MPM)was developed to simulate the large deformation problems,including a sand pile collapse,the large-scale flow of Hongshiyan landslide,and the wheel rotation on the soil.The main contents of the thesis are summarized as follows:1.The 3D material point method was developed and extended with the Update Stress First(USF)scheme for large deformation problems.Based on the consistency between the eight-node background mesh and the raster terrain data,the coupling framework of ArcGIS and MPM was constructed.Furthermore,a constitutive model for multi-states granular flows was proposed and embedded into the continuum-based MPM,which was a viscoelast-plastic stress-strain relation derived in combination with the Drucker-Prager model and ?(I)rheological model of granular flow.2.In order to validate the developed MPM model and the constitutive model,an experiment of granular pile collapse was selected as a typical process of granular flows.The high-speed camera and Tekscan pressure sensor were used for the velocity and pressure measurements,respectively.The particle tracking velocimetry(PTV)module was embedded in the image processing software(ImageJ)for the velocity calculation.Then some physical quantities in different flow regimes,such as the flow patterns,velocity distribution,pressure distribution and volume fraction,were analyzed and discussed.Finally,the discrete element(DEM)simulation and MPM simulation were carried out for the comparison with the experimental results,after which the applicability of the model for the multi-states flowing process of granular media was verified.3.Three-dimensional MPM modelling of the runout behaviours of the realistic landslides or debris flows was conducted for the engineering application of the Hongshiyan landslide in complicated topography.According to the theoretical solution of a steady inclined flow,we derived the range of the dissipation coefficient ? for the three-dimensional landslide.The MPM simulation was mainly aimed at the post-failure behaviours of landslides,and it reproduced the runout and deposit process of Hongshiyan landslide.The evolution of the deposit thickness and velocity distribution were presented in the paper,and we discussed the influences of the dissipative parameter ? on the energy dissipation speed,the flow duration,and the final deposit shape.4.The numerical simulations of the wheel rotation on the soft soil were conducted with the 3D material point method.Based on the theory of terramechanics,a wheel rotation model was simplified and adopted to update the movement of the wheel.In the framework of the three-dimensional MPM,a wheel-soil contact model was proposed to solve the equations of wheel motion,and it was verified by the theoretical solutions of the one-dimensional and two-dimensional models.Numerical simulations of a typical wheel going over different terrains modelled in GiD platform were presented to demonstrate the capabilities of the new approach.Different cases of inputting a constant angular velocity or just an initial angular velocity were carried out to observe the different movements of the wheel.The finite element method(FEM)simulations using Abaqus were also used to compare and validate the MPM simulations,and their results were in good agreements.