Identifier

etd-03082016-111401

Degree

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Document Type

Restricted Dissertation

Abstract

During pile installation, the stresses and void ratios in the surrounding soils change significantly, creating large displacements, soil disturbance and the development of excess porewater pressures. The disturbed soils, especially fine grain soils, tend to regain their strength over time due to both the consolidation of the excess porewater pressure and thixotropic behavior of soil particles. In this research, the pile installation process and the subsequent consolidation, the thixotropy and load tests for several test piles were modeled using finite element (FE) model. A new elastoplastic constitutive model, which was developed based on the disturbed state concept (DSC) and critical state (CS) theory, was implemented to describe the clayey soil behavior. The developed model is referred as critical state and disturbed state concept (CSDSC). Pile installation was modeled by applying prescribed radial and vertical displacements on the nodes at the soil-pile interface (volumetric cavity explanation), followed by vertical deformation to activate the soil-pile interface friction. The soil thixotropic effect was incorporated in the proposed model by applying a time-dependent reduction parameter, which affects both the interface friction and the soil shear strength parameter. The results obtained from the FE numerical simulation included the development of excess porewater pressure during pile installation and its dissipation with time, the increase in effective normal stress at the pile-soil interface, and the setup attributed to both soil consolidation and thixotropy effects at different times after end of driving. The FE simulation results using the developed model were compared with the measured values obtained from the full-scale instrumented pile load tests to verify the proposed FE model. The results obtained from verification indicated that simulating soil response using the proposed CSDSC elastoplastic constitutive model and incorporating soil thixotropic behavior in the FE model can accurately predict the pile shaft resistance. A parametric study was then conducted by varying the main soil properties, which have significant contribution in setup phenomenon. The obtained data were analyzed using existing statistical techniques and applying non-linear regression analysis. Several nonlinear regression models were developed under different sets of variables, and finally three sets of regression model were proposed to correlate the soil setup behavior to the contributing soil properties.

Date

2016

Document Availability at the Time of Submission

Student has submitted appropriate documentation to restrict access to LSU for 365 days after which the document will be released for worldwide access.

Committee Chair

Abu-Farsakh, Murad

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