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The geomorphology of the Perth region, Western Australia
An overview of the geomorphology of the wider Perth region is presented in the context of issues relevant to geotechnical engineering. The region is divided into the erosional terrains of the Darling Scarp and the Cenozoic depositional landforms of the coastal plain. Attention is drawn to claims of possible neotectonic activity associated with the Darling Fault, and the likely geomorphological expression of recent events. The soil, geomorphological and stratigraphic divisions of the Cenozoic coastal deposits are discussed, and the importance of the Tamala Limestone is stressed. The stratigraphic and geomorphological details of the coastal sequences are provided and evaluated in the context of global Late Cenozoic sea-level events. The alluvial deposits of the coastal plain are described and an overview of the controls on deposition and the resultant alluvial architecture is provided. The stratigraphic architecture of both the coastal and alluvial deposits has important implications for hydrogeology and contaminant movement. In closing, the paper draws attention to specific geomorphological events and processes of direct relevance to geotechnical engineering, which may not have received sufficient attention.
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Impact of spudcan footprints on a gravity base structure
In the offshore oil and gas industry, structures such as gravity base structures and jack-ups are commonly used as drilling and facilities platforms. Installation locations for these structures are usually chosen on flat lying, featureless, undisturbed seabed to reduce installation risks and avoid costly seabed preparation works. In some cases, these structures are installed in close proximity of each other and care is taken to ensure the foundations do not influence the stability of the adjacent structures. This was the case with the skirted gravity base structure discussed in this paper as it was designed by Arup to be installed next to an existing wellhead platform linked with a connecting bridge. However, a jack-up rig unintentionally installed at the designated site of the gravity base structure and therefore significantly changed the seabed profile as well as the strength properties of the underlying soils. The jack-up spudcan footings punched three 20 m diameter craters to about 3 m depth into the seabed. The craters and the disturbance of the soil beneath and around the craters affected the stability of the gravity base structure and increased the risk of installation refusal of the skirted foundation. The craters necessitated the reorientation of the gravity base structure to minimise foundation intersection with the spudcan footprints. In 2011 the gravity base structure was successfully installed and is currently in operation. This paper discusses the investigation process and the analyses that were conducted to assess the impact of the spudcan footprints on the performance of the gravity base structure.
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Reliability-based geotechnical design in an Australian context
The design philosophy adopted by the international geo-engineering community over the last few decades has largely evolved from working stress design (WSD), in which a single or lumped factor of safety (FoS) is adopted, to a load and resistance factor design (LRFD) approach. In LRFD, partial factors are applied to actions (i.e. loads), soil parameters and / or resistances. These partial factors vary in magnitude depending on the relative uncertainty of each parameter to which they are applied. The approach adopted in Australia has differed, with both WSD and LRFD approaches being widely used. In the authors’ experience, there is limited awareness in the Australian geo-engineering community of the relationship between the concept of reliability and the partial factors adopted in LRFD, and therefore of the potential benefits of undertaking reliability-based design (RBD). The outcome of this is that RBD, in which the uncertainty of the variables which may affect the design is individually assessed, is rarely undertaken. This paper discusses the concept of RBD and its place within the framework of Australian Standards and presents practical means of adopting RBD with accompanying examples from the literature. The intention of the paper is to encourage practitioners to consider uncertainty in geotechnical design more rigorously, whilst acknowledging the importance of maintaining engineering judgement in design.
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Ground improvement case studies – Chemical lime piles and dynamic replacement
This paper presents two case studies of soft ground improvement, one involving the use of Chemical Lime Piles for a project in Penang, Malaysia and the other involving the use of Dynamic Replacement in Alexandria in Egypt.
The background and the design approach of each of these ground improvement techniques are presented together with a description of their application in two recent projects. In both cases, field performances are compared with the original designs, and are found to exceed design predictions with respect to strength increase.
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The Engineering Geology of the Sydney Region – Revisited
In keeping with annual tradition, on 13 October 2004 the Sydney Chapter of the Australian Geomechanics Society will present its seventh Mini-Symposium, the Engineering Geology of the Sydney Region – Revisited. This volume of Australian Geomechanics Journal is dedicated to the papers that are to be presented at the mini-symposium.
This topic was previously addressed in the well-known 1985 publication on the Engineering Geology of the Sydney Region. Since 1985 the Sydney region has seen significant infrastructure development, including housing and transportation. This development has provided new opportunities for geotechnical practitioners to advance their understanding of the engineering geology and hydrogeology of the region.
The mini-symposium is intended to provide a forum for the compilation of this knowledge. Topics presented in 1985 have been supplemented with recent data and several new topics have been addressed.
Topics include a review of the engineering (Pells) and hydrogeological (Tammetta and Hewitt) properties of the Hawkesbury Sandstone and case histories demonstrating the engineering implications of variability in the Hawkesbury sandstone (Speechley, Walker and Scholey). Och, Pells and Braybrooke have used recent data to compile a map illustrating geological faults and dykes in Sydney CBD. This map will be handed out at the Mini-Symposium and it is hoped that it will be published in an later issue of this journal.
The engineering properties of the Ashfield Shale have been well documented, but the paper by William and Airey provides similar data for the Bringelly Shale, which has received greater attention as the urban development of Western Sydney progresses. Salinisation in the shales has significant impact on the urbanisation of Western Sydney and is discussed by McNally. Hatley addresses the hydrogeology of the Botany Sands.
The characteristics and engineering implications of the regional stress field are discussed in three papers (Pells, McQueen and Walker). Clearly there are differences in opinion as to the best method for making in situ stress measurements and in the equations that best model the stress field. Interested readers are encouraged to study each of these papers to understand current limitations of stress measurements and predictions.
The convenors acknowledge the contributions made by the authors and the great efforts that have been made to present information that reasonably represents the current state of knowledge. In addition, thanks are extended to peer reviewers, nominated by the authors, who have provided valuable critique.
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Application of Volumetric Runout Methods for Structurally Controlled Highwall Failures
Highwall failures in open cut coal mines are often structurally controlled. Such failures are often brittle in nature which elevates the safety risk for operations below, requiring accurate assessment of exclusion or standoff distances to prevent near misses or injury to plant and personnel. Runout estimation in the open cut coal industry is typically based on empirical methods. However, volumetric methods for runout estimation are used extensively in the hard rock mining industry. This paper reviews the applicability of these methods for structurally controlled highwall failures using case studies from BHP coal mine sites and literature. The findings indicate that volumetric methods are typically more reliable than existing empirical correlations and provide a method to quantitatively assess the failure consequence during wall optimisation.
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Outline of the hydrogeology of the Perth region
Groundwater is important to the city of Perth for public water supply, irrigation of parks and gardens and horticulture and about one in four houses has its own bore for garden watering. The superficial aquifer receives stormwater drainage and is easily contaminated from accidental spills or waste disposal. It supports perennial wetlands, and has been drained in areas of urban development with shallow water tables. The confined aquifers are used mainly for public water supply and are used conjunctively with the unconfined groundwater and surface water. The Gnangara Mound, which is the main resource in the superficial aquifer and also recharges the confined aquifers, is protected from urban development and is covered by native woodland or pine forest. Groundwater in the crystalline rocks of the Darling Range is limited to low yielding bores and wells for gardens or orchards.