Date of Award

Winter 2006

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Jean Benoit

Abstract

An in situ testing program and finite element modeling of Presumpscot Formation glaciomarine clays in South Portland, Maine was undertaken as part of a landfill expansion feasibility study. The very soft sensitive silty clay foundation soils have been monitored during the 20 year lifespan of the landfill facility by an array of settlement platforms, inclinometers, piezometer clusters and periodic field shear vane tests. During this time large horizontal displacements have been observed at a discrete elevation corresponding to a zone of reduced undrained shear strength. Proposals call for a vertical expansion at the site above existing landfill cells. While this proposal will require field corroboration that sufficient strength gain of the foundation soils has occurred to maintain minimum safety factors, a preliminary estimate of future strength conditions will aid the planning and design processes.

An in situ testing program consisting of piezocone, field shear vane and dilatometer profiles was performed along with laboratory testing of undisturbed samples to determine the geotechnical properties of the landfill foundation soils. These properties were then used with historical data to create two-dimensional finite element models of the landfill, and to simulate waste loading rates over the lifespan of the facility. The models were then used to project the strength properties and behavior of the soil as additional waste is placed in the proposed vertical expansion.

A comparison between historical data from the site and the modeling results demonstrated that the finite element models provide a good indication of the current behavior of the foundation soils with regards to the changes in pore pressure and magnitude of settlement due to loading. While the finite element soil behavior model did not work well in the zones of large displacement; results demonstrated that the calculated increase in undrained shear strength based on the dissipation of excess pore pressure closely matches the changes observed in field shear vane tests during the past ten years throughout the remainder of the profile. The conclusions drawn in this research program is hoped to provide useful information for the site engineer, as well as a strong basis for continuing research.*.

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