Research On Geotechnical Problems Involving Extremely Large Deformation Using The Material Point Method

It is not uncommon for geotechnical engineers to encounter problems involving extremely large deformation,such as dynamic compaction,pile driving,landslides,progressive failure of retaining wall,tunnel collapse and land reclamation.As the development of computational mechanics,geotechnical researchers attempt to simulate these problems to explore the process,mechanisms and influence areas of the deformations.Nevertheless,it is of great challenge to model extremely large deformation problems in geotechnical engineering due to the complex physical natures,for instance,breakage,multi-field couplings,high velocity collision,phase transformation,strain localization,anisotropy and history dependent features,involved during the deformation process.The material point method(MPM)developed in last decades is a powerful method for handling extremely large deformation problems.The intent of this thesis is to research the large deformation problems in geotechnical engineering by the MPM.Special attention is to be paid to found new phenomena,put forward new calculation methods,uncover new mechanisms,validate proposed models and derive new formulas during(or after)the simulations of these problems.The main achievements of present study are listed as follows:1)A parallel dual domain material point method(DDMP)with modified weighting function of the gradient of shape function is developed.The parallel program is based on OpenMP framework and list data structure with no-need to partition the computational mesh.2)A density dependent soil model for describing the deformation characteristics of sand under wide range pressure condition based on the elastoplastic theory is proposed.It is applicable to analyze large deformation problems sensitive to the volumetric strain deformation,such as dynamic compaction,pile driving and impact rolling.The adaptive substepping Runge-Kutta method is developed to integrate the stress along a prescribed strain path with high accuracy and strong stability.3)A dynamic relaxation technique is developed by introducing kinetic damping into MPM.The kinetic damping without any parameter is efficient,stable and easy to use,especially for elastic problems and initial stress field generation problems.4)A new method for the calculation of active earth pressure based on the normal stress function along failure surface with soil obey nonlinear failure criterion is proposed,in which the failure surface is assumed as a parabolic curve found from the result of the simulation of progressive failure of retaining wall by DDMP.This method can yield the magnitude and action point of the active earth pressure,the failure surface and the stress distribution along it.5)A new model,named as static liquefaction model,to explain the Shenzhen long runout landslide is put forward.Based on this model,2D and 3D simulations are all performed to validate the theory.The simulation results agree well with the field measurements,which illustrate the high flowability and long transmission characteristics of the deposit,and indicate that the rapid construction model is correct and reasonable to explain the Shenzhen landslide.6)Two simple formulas to predict the crater depth and the densification depth of dynamic compaction are derived.A contact algorithm between rigid and deformable bodied in DDMP is proposed to simulate the dynamic compaction experiment carried out in Chengde airport.The improvement effects and energy efficiency,defined as the ration of plastic strain energy absorbed by the soil and the tamping energy,of single rammed compaction is analyzed.With the simulation results of the dynamic compaction experiment,the laws of pre-consolidation pressure distribution in space and plastic volumetric strain variation along depth are uncovered,namely,the iso-surface of pre-consolidation pressure is an ellipsoid surface and the plastic volumetric strain of soil beneath the center of the hammer is of exponential decay with depth.Based on the discoveries,the crater depth and the densification depth of dynamic compaction are derived from the conservation of energy.