Study On The Micromechanism Of Sand Liquefaction And Its Evluation

Granular materials, which are composed of grains and quite common in nature, are generally treated as continuum in classical framework of geomechanics. However, the static and dynamic behaviors are very complicated due to the complex interactions between particle and particle, particle and its surrounding liquid and/or gas. So even the fundamental mechanisms of some basic and important phenomenon, such as sand liquefaction induced by earchquake, has not been well established. With the recognition that granular material is discrete in nature, rather than continuous, the basic understanding can only be obtained from the particle scale. The complexity in granular materials behavior lies in the fact that the meso-scopic behavior of granular material is determined by not only the interactions operating at contacts, but also how the particles become arranged in space to form an internal structure. This research is aimed to microscopically investigate the mechanism of sand liquefaction and its evaluation, as well as the fundamental mechanism about the critical state. Research results obtained are as follows(1) For Discrete Element Method (DEM), the key to a successful simulation lies in proper micro-parameters. However, the current way of selecting micro-parameters is often accused of being subjective, unreliable and rendering different simulations incomparable. Controlling criterions on particle number and static loading for both Linear Contact Model and Hertz Model are proposed based on the dimensional analysis. A set of empirical formulas are presented to describe the correlation between macroscopic elastic constants of granular materials and microscopic elastic constants of particles by simulating tri-axial tests with PFC3D and through regression analysis of numerical results. It is found that the Linear Contact Model failed to catch the stress-dependent behavior of granular materials, while Hertz Model can well compensate this defect, e.g. the initial Young’s modulus approximately proportional to one third of the confining pressure.(2) Conventional triaxial tests under both drained and undrained conditions are implemented in DEM simulation. The evolutions of micro-structure of granular materials, including mechanical coordination, anisotropy of fabric, normal contact force, tangential contact force, as well as the different contributions of strong force and weak force to stress ratio are carefully analyzed. The micromechanism of critical state is deposed based on the previous analysis. The uniqueness of the critical state line is verified by performing triaxial tests under different effective stress paths and initial fabrics. A new critical state line is proposed on the basis of mechanical coordination number. (3) Numerical simulation of shear wave propagation using DEM is implemented by applying a velocity pulse to the transmitter in a certain direction and monitoring the corresponding average velocity of the receiver. The cross-correlation analysis is adopted due to its superiority of both determining the travel time and indentifying similarities between two signals. The shear wave velocity is calculated using the wave travel time and the distance of the travel path, in exactly the same way as in laboratory tests. The influencing factors including excitation frequency, excitation amplitude, size of transmitter and receiver as well as damp are carefully analyzed and reasonable values of the parameters for shear wave modeling are proposed. It is found that the appropriate excitation amplitude should be chosen on the basis of avoiding the generation of frictional work. It is also indicated that fruitful results would be obtained if both the radius of transmitter and receiver are chosen as one half of that of DEM specimen’s. The research results are verified through outcomes of even-particle assemblies.(4) A micromechanical study on the influence of cyclic loading history is attempted by conducting a series of undrained cyclic triaxial tests using Discrete Element Method (DEM). With the implementation of shear wave modeling, the value of small strain shear modulus Gmax of granular soils under the influences of cyclic loading history was investigated by measuring the shear wave velocity at a number of certain effective stresses, and that of Gmax without such effects was examined for comparison. Variations of coordination number, fabric, contact forces as well as contact stiffness are carefully examined to interpret the microscopic mechanism of both reduction and anisotropy of small-strain shear-modulus induced by the effects of cyclic loading.(5) Static liquefaction on very loose granular materials under undrained condition is simulated in DEM. The micromechanism of this special phenomenon is proclaimed by analyzing the evolution of micro-structures, including mean normal contact force, mechanical coordination number, percentage of rattlers, sliding fraction, anisotropy of contact normal, normal contact force as well as tangential contact force. It shows that the mechanical coordination number will reduce to4or even lower when static liquefaction happens. It seems that the mechanical coordination number will gain very similar situation for cyclic-induced liquefaction.(6) The influence of initial fabric on the behavior of granular material is revealed by conducting numbers of triaxial tests under undrained condition. In order to create different initial soil fabrics, specimens are subjected to undrained preshearing after isotropically consolidating, and then isotropically reconsolidating back to the initial confining pressure. It is indicated that, specimens with the same initial void ratio but different initial fabrics may follow quite different effective stress paths, and may even induce sharp changes of fabric during phase transformation.(7) The influences of micro-parameters on Cyclic Resistance Ratio (CRR), shear wave velocity, state parameter are detailedly discussed based on a number of dynamic triaxial tests under undrained condition during constant stress amplitude, as well as the measurement of shear wave velocity. Correlation between cyclic resistance ratio and shear wave velocity of granular materials are estabilished based on paticle-scale analysis, are verified by liquefaction and non-liquefaction data from centrifuge tests. It is indicated that the index number in CRR-Vsl correlation is about5for normal sand. The correlation between cyclic resistance ratio and state parameter is further raised.