Date of Award

2011

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

First Advisor

Christopher D.P. Baxter

Abstract

Movement of adjacent ground and support-of-excavation structures due to pile driving in non-plastic silts is a significant issue in urban areas of Rhode Island. There have been several cases in which such movements have damaged historic structures and transportation infrastructure. The objective of this research is to perform a finite element analysis of a particular case study involving movement of a sheetpile wall-supported excavation due to the excavation and pile driving activities.

The case study involved construction of a pile-supported gate and screening structure that is part of the combined sewer overflow project by the Narragansett Bay Commission. The structure was built by first driving sheetpiles around the site, then excavating in stages to the desired elevation, and then driving piles at the base of the excavation. Geotechnical instrumentation at the site included three inclinometers located behind the sheetpile walls and two piezometers in the excavation. Deformations of the wall were observed during each stage of excavation. Additional significant movements of the wall and elevated pore pressures were measured during pile driving.

A 2-Dimensional finite element analyses was performed to model the deformation of the sheetpile walls using the commercial software PLAXIS version 7. Soils at the site were modeled with either a linear elastic, perfectly plastic Mohr-Coulomb constitutive model or a non-linear hyperbolic model. The excavation sequence was taken from construction records and simulated directly by removing soil in the model. Properties of the soils (strength and stiffness) were varied around values from the literature until the predicted wall movements matched observations.

There was good agreement between the modeled displacements and observations for the first two stages of excavation using reasonable values of strength and stiffness for Rhode Island silts. These parameters would be a good place to start in future modeling efforts involving support of excavation projects in Rhode Island.

The only way to simulate the last stages of excavation and wall displacement was to use unreasonably low values of strength and stiffness. Possible explanations for this poor agreement include: a) loss of ground during pumping reduced the stability in the excavation and led to larger movements; b) the excavation caused significant disturbance (almost liquefaction) of the soil at the base of the excavation; and c) the soil surrounding the inclinometer tubes behind the wall moved or became disturbed and the measured movements are not representative of the actual wall movements.

Dynamic loading of the soil from pile driving could not be directly modeled within PLAXIS. Therefore, the effects of pile driving were modeled by reducing the strength and stiffness of the underlying silts to simulate disturbance and possible liquefaction. Again, the properties of the soil were reduced until the predicted movements match field observations. Although this ignores the fact that the actual process is at least partially undrained, the approach used in this thesis is a first step in understanding movement of adjacent structures in Rhode Island silts due to pile driving.

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