Award Date

12-1-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

First Committee Member

Moses Karakouzian

Second Committee Member

Douglas Rigby

Third Committee Member

Donald Hayes

Fourth Committee Member

Mohamed Kaseko

Fifth Committee Member

Ashok Singh

Number of Pages

142

Abstract

The current design methodology for a drilled shaft foundation in cohesionless soil is primarily based on ultimate skin friction values of drilled shafts. In order to obtain these values for each soil type, load tests such as Osterberg test are designed and performed. The Osterberg test layout is designed to estimate the capacity of drilled shaft by applying an upward load during the test and then calculating the downward capacity assuming the upward and downward capacity are the same. This method is appropriate for soils not containing caliche layers because caliche layers bond to the shaft and prevent skin friction to reach its ultimate capacity during the load test. As long as ultimate skin friction is achieved, the location of O-cell with respect to any of existing soil layers is not an effective. Osterberg test results in soils containing caliche indicate that the ultimate skin friction is not achieved and shaft/caliche interaction is mostly elastic. In these cases, the behavior of the shaft when it is loaded from the bottom is different from when it is loaded from the top.

This study will show that, the location of O-cell with respect to the caliche layers will influence the interpretation of test results. The study will investigate the current interpretation method when O-cell is installed at a location far from caliche and will compare the equivalent top-down load from test results to when the shaft is loaded from the top. The reason for discrepancies between the behavior of the shaft in these two loading scenario will be explained. Additionally, the interpretation for tests when O-cell is installed close to caliche will be investigated and the behavior of the shaft will be compared for upward and downward loading. The procedure is performed by collecting 30 Osterberg load tests in soils containing caliche. The test layouts with O-cell installed at identified locations are selected. The 2-D finite element software PLAXIS 8 is then used to simulate the Osterberg tests. The models are calibrated using field Osterberg tests and then loaded conventionally from the top. The behavior of the shaft during top-down loading is compared to interpreted test results from Osterberg test.

A test layout with O-cell at a location far from the caliche layers shown to have a higher capacity during conventional loading compared to interpreted test results from Osterberg load test. On the other hand when O-cell is installed close to caliche, the top-down loading shows a similar behavior to interpreted test results from Osterberg load test. In fact when O-cell and caliche layers are close to each other, the test layout is similar to the procedure performed to estimate rock socketed drilled shafts capacity.

The results of this study will help engineers to have better understanding of the drilled shafts behavior in soils containing caliche by introducing an appropriate test design and interpretation of the test results.

Keywords

Boring; Caliche; Desert soils; Drilled shafts; Load test; Loads (Mechanics); Osterberg test; PLAXIS; Shafts (Excavations)

Disciplines

Civil Engineering | Geotechnical Engineering

Language

English


Share

COinS