PFC Numerical Simulation Of Dynamic Properties Of Liquefiable Silt

Silt exists widely in the middle and lower reaches of the Yangtze river region,where silt with high permeability, low compressive strength, is easy to liquefy. Scholars study macroscopic mechanics response of silt through many laboratory tests, but with the deepening of soil mechanics research, people come to realize that silt engineering properties improvement is caused by microstructure changes. DEM(Discrete Element Code) can analyze macro-mechanical response of soil, and study the evolution of microscopic fabric parameters under loading.In recent years, DEM has become an effective and prospective numerical simulation method to solve problems of non-continuous media, such as seepage damage, liquefaction deformation and shear band and so on.In this paper, one of the particle discrete element method named particle flow code in 3-dimension(PFC3D) was used. Based on the results of lab tests of silt and silt modified by microbes, silt behavior under monotonic and cyclic loading was simulated from the aspect of macro and micro scope. The main work and achievements are as follows:(1)Based on the conventional triaxial test results, microscopic parameters of three-dimensional simulation were obtained, and discrete element method numerical model under monotonic biaxially loading was established. The simulation results showed that the impact of granular material mainly depended on shear forces to transfer between particles, and macroscopic properties changed with the coefficient of friction.With the increase of the friction coefficient between the particles, the initial elastic modulus and peak intensity became larger, but pattern of stress-strain curve shifted gradually from the strain hardening pattern to strain softening pattern. In volume strain-axial strain curve, the corresponding dilatancy phenomenon became more obvious with the increse of the friction coefficient,(2) Based on cyclic triaxial test results of silt and silt modified by microbes, Two loading patterns of constant-amplitude stress cycle and constant-amplitude strain cycle were used to simulate the laboratory tests. And silt dynamic characteristic change regularity was discussed under different fines content and strain amplitude.The numerical results showed that PFC simulation through constant-amplitude stress cycle loading could reproduce silt characteristics, such as pore pressure accumulation, strength degradation, stiffness recession and initial liquefaction, also can qualitatively simulate the general rule of silt liquefaction damage under cycle loading. Through using linear contact model between the fine particles and adjusting the side wall displacement rate, could reproduce both of strain and liquefaction damage in lab tests. Besides, the dynamic stress-strain relationship and dynamic pore pressure law were consistent with the experimental results by constant-amplitude stress biaxial cycle loading pattern.(3) Laboratory test results showed that fine particles content significantly affected the liquefaction characteristics of silt. Comparing the results of the loading patterns of constant-amplitude stress cycle with that of constant-amplitude strain cycle by using bond model between the fine particles, the laws of dynamic strength with fine particles filling content was similar. Numerical analysis shows that values of silt dynamic strength did not increase with fine particles content, but there was a turning point when fine particles content below this point, dynamic strength showed a decreasing trend under constant-amplitude strain cycle loading pattern or little change under constant-amplitude stress cycle loading pattern, otherwise dynamic strength increased, which is similar to previous experimental law. The numerical analysis results further illustrated that soil macroscopic mechanical properties corresponded to microstructure changes.(4)Macroscopic liquefaction process of silt filled fine particles corresponded to microscopic structure variation. Before initial liquefaction, the average number of contacts between the fine particles continued to decrease with the increase of vibration times. After that, with alternating loading dilatancy and unloading shear contraction, the average number of contacts oscillated, and initial contacts number and fine particles content relation curve were consistent with damage vibration times.When filling less fine particles, the average number of contacts were relatively few, so as damage vibration times. When fine particles content reached a certain value, the curve of damage vibration times with initial average contacts number or fines particles content had a transition, the initial average contacts number increased with fine particles content, and damage vibration times gradually increased.