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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Timothy, Newson A.

Abstract

Estimation of the bearing capacity and deformations of large shallow foundations under combined loadings can be of great significance. Some geotechnical design guidelines recommend the failure load envelope method for ultimate limit state analysis. Since many investigations of this method focus primarily on undrained failure envelopes using unlimited-tension interfaces, zero-tension interfaces have not been well investigated. In addition, less work is available on drained failure envelopes. For serviceability limit states, the majority of approaches used are based on uncoupled, isotropic assumptions. Further work needs to be done on more appropriate elastic solutions for combined loadings.

General VHMT failure envelopes for circular foundations under undrained and drained soil conditions have been investigated using finite element analysis. The effects of soil strength heterogeneity, foundation embedment and surficial crustal layer have been examined for undrained soil conditions. In addition, cohesive-frictional soils have been considered for drained soil conditions. In comparison, classical bearing capacity theories appear to be rather conservative for combined loadings.

To estimate the deformations of large shallow foundations under combined loadings, researchers have proposed many analytical isotropic elastic solutions based on idealized soil conditions. However, many natural soils will be anisotropic or at least transversely isotropic due to their complex deposition history. This research has investigated the coupled elastic stiffnesses for circular foundations founded in cross-anisotropic soils. Gibson and embedment correction factors have been derived to account for the effects of soil stiffness non-homogeneity and foundation embedment.

Summary for Lay Audience

Large onshore and offshore foundations are often subjected to complex environmental and climate loadings. To ensure the safety of structures, these foundations are designed to satisfy various criteria, but two of the most critical are: (1) the foundation capacity is larger than the possible maximum loading and (2) the foundation deformations under working loads are within given tolerances. This thesis has developed new methods for estimating the capacity and deformations of large circular foundations.

To estimate the capacity for circular foundations, the failure load envelope under combined vertical-horizontal-moment-torsional loadings have been investigated. Practical foundations are often designed to be embedded and many soils exhibit increasing strength with depth and have a thin layer of stiff crust; the effects of foundation embedment, increasing soil strength and surficial crust on the foundation capacity have been examined. This method should aid the capacity assessment for large shallow circular foundations and lead to cost savings.

To assess foundation deformations, the foundation stiffness, which is defined as the ratio of load and resulting displacement, is often adopted. This research has investigated the stiffness for circular foundations. Many natural soil deposits exhibit some degree of directionally dependent stiffness anisotropy due to their complex deposition history, where the soil has different mechanical properties in the vertical and horizontal planes. The developed approach should aid the deformation assessment for large shallow circular foundations and lead to more economic foundation construction.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Available for download on Thursday, September 01, 2022

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