2001 Higher Degree Theses
The following postgraduate students were awarded higher degrees for their theses in 2001
Click on the title to read the summary of the thesis
Doctor of Philosophy
Weimin Deng, The Inclined Uplift Capacity of Suction Caissons
Master of Engineering (Research)
Information to come
Suction caissons, also known as skirted piles and bucket foundations, are being used increasingly as foundation systems for offshore structures. Their applications include skirted gravity base structures, anchorage systems (as alternatives to drag anchors and tension piles) for catenary moorings, and foundations for taut leg moorings and jacket structures. This thesis describes the research that has been carried out to analyse the inclined uplift capacity of suction caissons. The main thrust of this research is three-dimensional finite element modelling.
The three-dimensional, semi-analytical finite element approach used in this study provides a rigorous and efficient numerical tool for the analysis of the problem of a suction caisson subjected to three-dimensional loading. The efficiency of this numerical tool makes it possible to tackle this problem for a wide range of cases, which would otherwise need enormous computing time and thus would be impractical.
The response of a suction caisson to vertical uplift loading for cases where the soil is undrained, drained and partially drained is firstly investigated. The behaviour and failure mechanisms under this form of loading are examined. Simplified methods for estimation of the pullout capacity under undrained, drained and partially drained conditions are proposed. The developed methods take into account the influence of the aspect ratio of the caisson (L/d), the soil strength parameters, the loading rate (v), the soil stiffness and soil permeability (k) and the effect of the initial stress state K0.
Detailed theoretical investigations of the behaviour of suction caissons subjected to inclined uplift loading, for cases where the soil strength increases linearly with depth (undrained), or the soil is homogenous (undrained) or the soil is sandy (drained), are then carried out using the three-dimensional semi-analytical finite element approach. For the undrained cases, the upper bound techniques from the theory of plasticity are also used to validate the finite element predictions. Simplified methods for estimation of the uplift capacity are suggested. The developed solutions take into account the influence of the aspect ratio of the caisson (L/d), the load application point (D/L), the load inclination angle (qa), the soil strength parameters, and the effect of the initial stress state, as expressed by the parameter K0.
All the simplified solutions developed in this research are examined and validated in the light of available experimental data, including 109 individual tests from 18 independent studies. Two design examples exploring the use of suction caissons as anchorage systems are examined. Both examples illustrate application of the solutions developed in this research. A general design procedure is also presented.
In order to examine possible changes of the ultimate pullout load over time, the research has been extended to include a study of the effects of loading history on the ultimate pullout load. A typical case is considered which shows that the ultimate pullout load may decrease overtime due to the loss of soil strength at the bottom and backside of the caisson.
The purpose of this thesis is to provide experimental data on the strength of cold-formed lipped channel and Z-beams to verify theoretical predictions, and to develop recommendations for the design strengths of such beams.
The design buckling strengths of cold-formed beams are currently calculated using theory and experimental data for hot-rolled I-section beams. Other loading conditions such as combined bending and torsion of channels or biaxial bending of Z-beams have few design guidelines, and those available are largely based on methods of designing hot-rolled I-section columns. The design code formulations based on those for hot-rolled I-sections may be inappropriate for cold-formed sections because of the different behaviour and properties of cold-formed beams. To satisfy the need for test data on cold-formed unbraced beams, an experimental program was established to measure the strengths of cold-formed channels and Z-beams and to gather a series of subsidiary test data.
Ten lateral buckling tests were conducted on simply supported unbraced cold-formed lipped channel beams of two different cross-sections, and six tests on lipped Z-beams made from one type of cross-section. The beams were loaded with central concentrated loads at different heights with respect to the shear centre. The test results are compared with the theoretical predictions and capacities calculated using existing Australian design codes AS 4100 and AS/NZS 4600. Improvements are suggested for future design code formulations for the lateral buckling capacities of cold-formed channel and Z-beams.
Thirty four bending and torsion tests were conducted on simply supported channel beams of two different cross-sections loaded eccentrically at midspan and ten biaxial bending tests on Z-beams made from one type of cross-section. The concentrated loads were applied with different eccentricities for the channel beams, and with different angles of inclination for the Z-beams. The results from both series of tests are compared with the analytical predictions and used to develop simple interaction equations that can be used in the design of eccentrically loaded channels and Z-beams under inclined loading.
The subsidiary tests included tension tests, a stub column test, section moment capacity tests, torsion tests, residual stress measurements, and measurements of the initial crookednesses and twists.
The research carried out in this thesis is two-fold, covering the application of large deformation analysis in soil mechanics, and the problem of tensile failure of soil. The work may be conveniently divided into four sections.
(1) Four large deformation analysis approaches have been implemented in the finite element package AFENA. Discussions of the advantages and limitations of the four approaches are given, based on the formulations adopted in these approaches and the results of analyses of some test problems. The applicability of the four approaches for some selected soil mechanics problems were investigated, by comparing the load-displacement curves predicted by these four approaches.
(2) Large deformation analyses of strip and circular footings penetrating into layered clay, and of strip anchors uplifted in homogeneous and non-homogeneous clay, were carried out. The bearing or uplift capacity factors, load-displacement curves, failure zones, and stress distributions in the soil were investigated in the analyses. A wide range of soil parameters was adopted in the analysis, and the results presented cover a large variety of circumstances. The necessity of large deformation analysis of these problems is discussed, based on the comparisons of the results predicted by both small and large deformation analyses.
(3) Soil tensile failure models were established for the problem of strip anchors uplifted in unsaturated and saturated soil, in terms of both total stress and effective stress.
(4) The analyses adopting the soil tensile failure models were carried out for the problem of strip anchors uplifted in homogeneous or non-homogeneous soil. The significance of soil tensile failure on the uplift behaviour of anchors was investigated in a wide range of circumstances.
It was found that the large deformation formulations adopted in the four approaches have little effect on the elasto-plastic soil mechanics problems considered in this work. However, a distorted finite element mesh may affect the accuracy of the numerical predictions significantly after a relatively large deformation occurs. The large deformation analysis can capture the softening and hardening behaviour of the soil-structure system, while the small deformation analysis can not predict this type of response. Whether a large deformation analysis is important for engineering practice depends on many factors, which include the magnitude of the geometry change, the soil strength profile, the initial position of the structure, the stiffness of soil and the soil self-weight.
The analysis considering soil tensile failure for a strip
anchor in homogeneous and non-homogeneous soil showed that the tensile
failure reduces the uplift capacity from slightly to very significantly,
depending on the particular conditions. The significance of tensile failure
depends on the embedment depth of the anchor, the ratio of overburden
pressure to shear strength, the soil strength gradient and the capacity of
the pore fluid to sustain suction.