U. El Shamy

Laminar Box System for 1-g Physical Modeling of Liquefaction and Lateral Spreading

Details of a large scale modular 1-g laminar box system capable of simulating seismic induced liquefaction and lateral spreading response of level or gently sloping loose deposits of up to 6 m depth are presented. The internal dimensions of the largest module are 5 m in length and 2.75 m in width. The system includes a two dimensional laminar box made of 24 laminates stacked on top of each other supported by ball bearings, a base shaker resting on a strong floor, two computer controlled high speed actuators mounted on a strong wall, a dense array advanced instrumentation, and a novel system for laboratory hydraulic placement of loose sand deposit, which mimics underwater deposition in a narrow density range. The stacks of laminates slide on each other using a low-friction high-load capacity ball bearing system placed between each laminate. It could also be reconfigured into two smaller modules that are 2.5 m wide, 2.75 m long, and up to 3 m high. The maximum shear strain achievable in this system is 15 %. A limited set of instrumentation data is presented to highlight the capabilities of this equipment system. The reliability of the dense array sensor data is illustrated using cross comparison of accelerations and displacements measured by different types of sensors.

Micromechanical Aspects of Liquefaction-Induced Lateral Spreading

This paper reports the results of model-based simulations of 1-g shake table tests of level and sloping saturated granular soils subject to seismic excitations. The simulations utilize a transient fully coupled continuum-fluid discrete-particle model of water-saturated soils. The fluid (water) phase is idealized at a mesoscale using an averaged form of Navier-Stokes equations. The solid particles are modeled at the microscale as an assemblage of discrete spheres using the discrete element method (DEM). The interphase momentum transfer is accounted for using an established relationship. The employed model reproduced a number of response patterns observed in the 1-g experiments. In addition, the simulation results provided valuable information on the mechanics of liquefaction initiation and subsequent occurrence of lateral spreading in sloping ground. Specifically, the simulations captured sliding block failure instances at different depth locations. The DEM simulation also quantified the impact of void redistribution during shaking on the developed water pressure and lateral spreading. Near the surface, the particles dilated and produced an increase in volume, while the particles at deeper depth locations experienced a decrease in volume during shaking.

A MICRO-MECHANICAL STUDY OF THE SEISMIC RESPONSE OF SATURATED CEMENTED DEPOSITS

A coupled continuum-discrete hydromechanical model was employed to analyse the effects of cementation on the dynamic response of liquefiable deposits of granular soils. The discrete element method was used to idealise the solid phase and parallel bonds were utilised to model the inter-particle cementations. The pore fluid flow was addressed using averaged Navier-Stokes equations. The conducted simulations revealed a number of salient response patterns and mechanisms. Cemented. granular soils were found to be generally highly resistant to liquefaction. However, full cementation of a shallow site may lead to a significant amplification of ground accelerations. A base isolation mechanism develops when a site is partially cemented and mitigates ground shaking hazard. The employed modeling approach provides an effective tool to assess the intricate micro-mechanical response mechanisms of saturated cemented soils.

Multiscale Modeling of Flood-Induced Scour in a Particle Bed

A three-dimensional transient fully coupled fluid-particle model is presented to simulate fluid flow-induced scour of a particle bed. The interaction of the liquid and solid phases is the key mechanism related to flood-induced failures of geotechnical systems. In this model, the mix of soil particles and water is idealized as two interpenetrating phases, each of which is modeled at a different scale. The fluid phase is idealized as a continuum using Eulerian formulation of averaged NavierStokes equations that account for the presence of the solid particles and solved using the finite element method. The particles are modeled at a microscale using the discrete element method. Computational simulations are conducted to investigate the response of a particle bed to Poiseuille flow conditions. The simulations provided information at the microscale level for the solid phase and the macroscale level for the fluid phase.

AN EFFICIENT COMBINED DEM/FEM TECHNIQUE FOR SOIL-STRUCTURE INTERACTION PROBLEMS

Computational simulation of geotechnical systems is essentially multi-scale in nature. The particulate nature of soil can be accurately simulated at a micro-mechanical level. However, due to the huge spatial extent of such systems, a model fully constructed at such scale is almost impossible with current computing technologies. Hence, continuum-based approaches are considered as the practical scale for modeling the majority of problems. Combining both scales enables benefiting from the advantages of both techniques while trying to overcome their drawbacks. Although a significant number of publications have addressed coupling both scales, only a few provide information regarding implementing the proposed procedures. In this study, an efficient framework for conducting multi-scale analysis is introduced. The framework is based on integrating continuum and micro-mechanical modeling packages and therefore benefitting from already existing codes. Computational simulations of a pile in contact with granular soil were conducted to demonstrate the capabilities of such technique.

MICROSCALE MODELING OF THE SEISMIC RESPONSE OF SHALLOW FOUNDATIONS

This paper presents a microscale approach to analyze the seismic response of a spread footing system founded on a liquefiable granular soil deposit. The approach utilizes a three-dimensional transient fully-coupled hydromechanical model while taking into account the effects of soil-foundation interaction. The porous soil medium is modeled as a mixture of two interpenetrating phases, namely the fluid phase (water) and the particulate solid phase. The fluid is idealized as a continuum by using averaged Navier-Stokes equations that account for the presence of the solid particles. The Discrete Element Method (DEM) is employed to model the assemblage of these particles. The interphase momentum transfer is modeled using an established relationship that accounts for the dynamic change in porosity. The spread footing is idealized as a rigid block and its motion is described by the resultant forces and moments acting upon it. A computational simulation is conducted to investigate the response of spread-footings on a saturated granular deposit when subjected to a dynamic excitation. Results of the conducted simulation showed that the foundation sustained excessive settlement as the ground shaking progressed. The conducted simulation appears to capture essential dynamic response patterns typically observed in such systems.

DEM Modeling of the Effect of Hydraulic Hysteresis on the Shear Strength of Unsaturated Granular Soils

A micromechanical model for capturing the effect of hydraulic hysteresis on the behavior of unsaturated granular soils at low saturation (below 30%) is presented. The discrete element method is employed to model the solid particles. The capillary water is assumed to be in a pendular state and thus exist in the form of liquid bridges at the particle-to-particle contacts. The resulting interparticle adhesion is accounted for using the toroidal approximation of the bridge. Hydraulic hysteresis is accounted for based on the possible mechanism of the formation and breakage of the liquid bridges during wetting and drying phases. Shear test computational simulations were conducted at different water contents under relatively low net normal stresses. The results of these simulations suggest that capillary-induced attractive forces and hydraulic hysteresis play an important role on affecting the shear strength of the soil. These attractive forces produce a tensile stress that contributes to the apparent cohesion of the soil and increases its stiffness.

A Micro-Mechanical Study of Soil-Pile Interaction during Dynamic Excitations

A fully coupled continuum-discrete hydromechanical model is employed to analyze the dynamic response of saturated granular soils. The fluid motion was idealized using averaged Navier-Stokes equations and the discrete element method was employed to model the solid particles. Well established semi-empirical relationships were used to quantify the fluid-particle interactions. Numerical simulations were conducted to investigate the liquefaction mechanisms of soil-pile systems when subjected to a dynamic base excitation. The outcome of these simulations was consistent with experimental observations and revealed valuable information on the micro-mechanical characteristics of soil liquefaction and associated loss of stiffness and strength.