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2006 Higher Degree Theses

The following postgraduate students were awarded higher degrees for their theses in 2006.

Doctor of Philosophy

Bosco Poon
Thesis title here
Supervisor: David Airey

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Manh Tran
Installation of Suction Caissons in Dense Sand and the Influence of Silt and Cemented Layers
Supervisor: David Airey

Suction caissons have been used in the offshore industry in the last two decades as both temporary mooring anchorages and permanent foundation systems. Although there have been more than 500 suction caissons installed in various locations around the world, understanding of this concept is still limited. This thesis investigates the installation aspect of suction caissons, focusing on the installation in dense sand and layered soils, where sand is inter-bedded by silt and weakly cemented layers. The research was mainly experimental, at both normal gravity and elevated acceleration levels in a geotechnical centrifuge, with some numerical simulations to complement the experimental observations.

This study firstly explored the suction caisson installation response in the laboratory at 1g. The influence and effect of different design parameters, which include caisson size and wall thickness, and operational parameters including pumping rate and the use of surcharge were investigated in dense silica sand. The sand heave inside the caisson formed during these installations was also recorded and compared between tests. The 1g study also investigated the possibility of installing suction caissons in layered sand-silt soil, where caissons were installed by both slow and rapid pumping. The heave formation in this case is also discussed. The mechanism of heave formation in dense sand and deformation of the silt layer was further investigated using a half-caisson model and the particle image velocimetry (PIV) technique.

The installation response at prototype soil stress conditions was then investigated in a geotechnical centrifuge. The effects of caisson size, wall thickness, as well as surcharge were investigated in various types of sand, including silica sand, calcareous sand dredged from the North Rankin site in the North West Shelf (Australia), and mixed soil where silica sand was mixed with different contents of silica flour. Comparison with the 1g results was also made. The general trend for the suction pressure during installation in homogenous sand was identified.

The installation in layered soil was also investigated in the centrifuge. The installation tests were performed in various sand-silt profiles, where the silt layers were on the surface and embedded within the sand. Comparison with the results in homogenous sand was made to explore the influence of the silt layer. Installations in calcareous sand with cemented layers were also conducted. The penetration mechanism through the cemented layer is discussed, and also compared with the penetration mechanism through the silt layer.

Finite element modelling was performed to simulate key installation behaviour. In particular, it was applied to simulate the sand deformation observed in the PIV tests. The likely loosening range of the internal sand plug during suction installation in silica sand was estimated. By investigating the development of hydraulic gradient along the inner wall, the principle underlying the suction response for different combinations of selfweight and wall thickness was identified. FE modelling was also performed to explore the influence of the hydraulic blockage by the silt layer.

This study found that the caissons could penetrate into all soils by suction installation. Among the key findings are the observations that the suction pressure increases with depth following a distinct pressure slope, corresponding to a critical hydraulic condition along the inner wall; and the installation was possible in both layered sand-silt and uncemented-cemented soils if sufficient pumping was available. While the caisson could penetrate the weakly cemented layers well with no notable adverse effects, problems were observed in the installation in layered sand-silt soil. These include piping failure in slow pumping rate installation at 1g, and the formation of extremely unstable soil heave during installation.

Download Manh Tran's thesis (6 MB PDF)

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Maura Lecce
Distortional Buckling Of Stainless Steel Sections
Supervisor: Kim Rasmussen

This thesis describes the experimental and numerical investigation of cold-formed, thin-walled stainless steel sections subject to distortional buckling under compression. Austenitic 304, ferritic 430 stainless steel and ferritic-like 3Cr12 chromium weldable steel sheets were brakepressed into simple-lipped channels and lipped channels with intermediate stiffeners. A full set of coupon tests reveal material nonlinearity with low proportionality stress and low n-parameter, anisotropy and yield strength enhancements up to 2.33 times due to cold-forming. A total of 19 experimental tests and more than 570 finite element tests covering a distortional buckling slenderness range 0.47 < lambda_d < 3.64 and corner to thickness ratios of 1 and 2.5 reveal material nonlinearity and enhanced corner properties have the most influence on the strength and behaviour of stainless steel sections and that material anisotropy can be ignored. Enhanced corner properties may become significant for stocky sections, where lambda_d <1, with a corner area of at least 10%. Current effective width approach and direct strength methods to design for distortional buckling of stainless steel and cold-formed carbon steel available in Australian, North American and European codes are generally inadequate. Modified resistance factors are recommended for the effective width approach of current design codes to meet limit states design criteria. Direct strength design curves are developed for austenitic and ferritic stainless steel alloys for inclusion in the Australian and American standards for stainless steel structures.


This thesis also describes the experimental and theoretical investigation of two commercially available wide-flange, ferritic 445 stainless steel roof sections under pure bending. A total of nine tests exhibited distortional buckling or local buckling behaviour with the interaction of flange curling. The effects of flange curling and material nonlinearity were evident as shown by the considerable loss of stiffness in the test results before reaching the ultimate load. The theoretical evaluation of flange-curling deformations shows that a true representation of the wide-flange support condition provided by the web is essential to obtain reasonable agreement with experimental tests. The iterative theoretical flange curling model which represents the changing geometric and loading conditions shows that an average of 6%- 17% reduction in yield moment due to reduced section modulus can occur and this is otherwise ignored by the simplified flange curling models. Stresses in the wide-flange can vary considerably, with nonlinearity increasing with increasing moment. The most drastic nonlinear behaviour was exhibited by the more slender section, and, at maximum moment, the centrewide flange stress was only 9% that at web/flange junction. Modified geometries due to flange curling can cause an increase of critical buckling stress ranging from 1.10 to 3.41 times that based on original geometries. Flange curling can produce a net increase of 10.6% for the distortional buckling design capacity but a net decrease of 12.2% for local buckling capacity. Given the current data, it would be prudent to ignore the effects of flange curling for distortional buckling but it would be necessary to consider them for local buckling. The proposed DSM design curve developed in this thesis for distortional buckling of ferritic stainless steel sections and the Winter curve for local buckling are generally conservative and suitable given the present data.

Download Maura Lecce's thesis (4 MB PDF)
Download Maura Lecce's thesis Volume 2 appendices (14 MB PDF)

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Ezzat William
Engineering Performance of Bringelly Shale
Supervisor: David Airey

This thesis is concerned with the general and fundamental engineering characterisation of a geological formation within Wianamatta group, known as Bringelly shale. Bringelly shale is the neighbouring member of Ashfield shale; both are soft rocks of Triassic age within a geological structure known as the Sydney basin in the state of New South Wales, Australia.

Bringelly shale rock and its residual material cover an approximate area of 700km2. It is found inland, to the west of the city of Sydney, where most of the new residential, commercial and industrial development is taking place. There is a limited amount of existing experimental data in part due to the technological difficulties in obtaining specimens and this has contributed to the uncertainties surrounding the engineering behaviour of the rock.

In this research, efforts have been made to identify index property tests useful for determining the engineering characteristics of the material. Further aims were to explore the reasons for the difficulty of obtaining core specimens using standard water flush drilling techniques and, to ascertain and explain why Bringelly and Ashfield shales behave differently in many aspects of their engineering performance, even though they are members of the same geological group.

Qualitative and semi-quantitative analysis by the X-ray diffraction technique was used to evaluate the clay mineralogy of the Bringelly shale materials at different degrees of weathering. Thin sections were examined by optical microscopy to study the nature of cementation and bonding. Polished sections of natural and reconstituted specimens were examined by electron microscopy to investigate the internal structure of each material and its mineral composition. It has been found that the presence of a significant amount of swelling clay and microcracks in the plane of laminations are responsible for increasing the swelling potential of the Bringelly shale. There is little evidence of induration and only apparently weak bonding due to re-crystallisation of mica at particle contacts. Changes in particle alignment following failure were also observed.

Because of the difficulty of obtaining specimens suitable for UCS testing, correlations were established between the point load strength index and the measured values of uniaxial compressive strength in the direction perpendicular to laminations. The strength anisotropy from the point load index was also determined. In this research, it was found that due to the limited number of specimens tested for UCS, the determined correlation factor could over-predict the strength of the shale.

Durability and swelling of the shale were also investigated. The durability of Bringelly shale was found to vary from medium for fresh intact material to very low for extremely weathered material. To further investigate the mechanisms responsible for the durability of the shale, unconfined and confined swelling tests were performed. Volumetric strains of 6-8% were measured for cube specimens with a volume of 27000 mm3, however, the material has shown an inverse relationship between its volumetric swelling and specimen dimensions. The chemical composition of the fluid into which the specimen was immersed was found to have a major influence on volume changes in the intact material. The results of the investigation confirmed that potassium chloride solution can be used to reduce swelling potential, and further, to improve core recovery during drilling.

An extensive experimental program to investigate the engineering performance of the shale has involved the use of conventional and specialised high pressure triaxial equipment. The program investigated the volumetric compression and shearing behaviour of three different forms of specimen. These were natural core specimens, and reconstituted specimens created from crushed shale by either pressing dry powder in a mould or by compression of a slurry.

Isotropic consolidation tests over a wide range of stresses were performed. The program has also involved a series of drained and undrained triaxial strength tests on the three different forms. The series has covered a wide range of confining effective stresses from 20 to 60,000 kPa, degree of saturation from 65-100%, and porosities from 10% to 60%. These tests have provided an extensive set of data to investigate the influence of stress, saturation, suction and internal structure on the compression behaviour of the reconstituted and natural rock. Analysis of these data has been conducted in terms of cementation, swelling, saturation, confining stresses, and frictional resistance.

A series of standard direct and ring shear tests has been carried out on the reconstituted Bringelly shale at normal stresses in the range from 50 to 200 kPa, and a residual friction angle was determined. It was found that this value has not been affected by the reorientation of clay particle despite the high clay fraction content of the material.

The results of this research indicate that the general pattern of behaviour for reconstituted material that has experienced a maximum effective stress of less than 6 MPa is consistent with the assumptions of critical state soil mechanics and similar to many other reconstituted materials. This pattern of behaviour shows a significant deviation from the framework of critical state when the same material (slurry or core form) is subjected to a maximum effective stress of 60 MPa. The significance of bonding and structure of the intact shale could be detected from investigating the same material at reconstituted state. However, further development of the critical state framework is required to take into account the reduction in strength caused by the high degree of alignment of clay platelets. The OCR seems to have minor effect on the strength of the material.

Download Ezzat William's thesis (19.2 MB PDF)