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Numerical Modeling of the Characteristic Seismic Behavior of Retaining Walls
AdvisorSiddharthan, Rajaratnam V.
Civil and Environmental Engineering
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Since the damage caused to retaining walls during past earthquakes are common, the behavior of such earth retaining structures has attracted the attention of researchers and practicing engineers. In this thesis, dynamic response of retaining walls is being studied using numerical method. The numerical analysis was undertaken using FLAC3D (Fast Lagrangian Analysis of Continua) along with FISH functions, which use a programming language embedded within FLAC3D. The FLAC family of programs have found wide use and acceptance among the geotechnical community because of its capability of modeling important aspects such as stress-dependent constitutive model, hysteretic nature non-linear stress-strain behavior and soil damping under dynamic loading, separation and slippage of soil at interface between soil an structure (i.e., interface elements), and incorporation of quiet lateral boundaries. More importantly, the FISH functions extend FLAC3D's usefulness since geotechnical applications often require reset and modification of stresses, strength and modulus properties during the execution of the program. Such requirements are needed to model failure, reset of initial condition prior to dynamic loading etc. As a baseline case, modeling of active and passive earth pressure were conducted and computed results were compared with those available from classical methods. The active and passive cases were modeled using a rigid wall under displacement control. The numerical model predictions for the passive and active pressures for various soil-wall friction angles were in good agreement with the available classical solutions. Retaining structures considered include a fixed end cantilever wall, flexible diaphragm wall and a gravity wall supporting a dry medium dense cohesionless soil. The fixed end cantilever wall allows for a closer inspection of the mechanism of interaction between the wall and backfill. The static analyses consisted of the stage-by-stage incremental construction of the wall using elastic-plastic backfill material modeled using stress-dependent incrementally elastic stiffness properties. Dynamic analysis followed the static analyses where the soil was modeled using non-linear stress-strain relationship along the Masing criteria for unloading and reloading. Particular attention has been devoted to physical modeling issues, use of appropriate soil constitutive relations and selection of ground excitations. Under sinusoidal motion, the dynamic characteristic behavior of the fixed cantilever wall and the gravity wall were clearly captured. The dynamic displacement of the fixed cantilever wall was found to always be outward from the backfill. The bending moments increased steadily. In the case of flexible walls, the residual bending moments at the end of excitation were substantially higher than the initial values. The effect of the flexibility of the wall and the effect of the integration of the Finn model for permanent volumetric strain in the constitutive model on the dynamic behavior of the cantilever wall were investigated. For the gravity wall, the movement of the wall was progressively away from the backfill and the gravity wall ends up with a permanent outward lateral movement and tilt. The results obtained with FLAC3D in terms of displacements and bending moments (in the case of flexible wall) were reported for different levels of excitation from four different past earthquakes of magnitude between 6.5 and 7.