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Problem soils — a West Australian perspective
Problems soils, defined in this paper as those soils reactive to moisture, are widespread across Western Australia. These soils have caused structural distress to numerous buildings and other structures and distortion and deterioration of many pavements.
This paper provides information on the distribution of these soils across Western Australia and the order of costs associated with remediation works required to correct distress and distortion caused by movements of the problem soils. Some methods used and local relationships developed to predict the movement of the soil associated with changes in moisture content are also discussed. Field indications of the presence of moisture sensitive soils are also mentioned.
The paper also includes some background on studies undertaken on sites containing problem soils in the south-eastern Wheatbelt and Pilbara regions of Western Australia.
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Numerical modelling of ground movement under covered areas
Rigorous design of interactive slab-ground foundation systems on expansive soils remains as a challenge for both practitioners and researchers. Attempts to solve these problems in a generalized way usually involve substantial approximations and have limited success. In this paper, a simple soil behaviour model is adapted into a new finite element formulation. The success of the approach is evaluated by comparing predicted foundation movements with those measured at the Maryland expansive soils field site. In particular, the computed vertical displacements are compared with field measurements. The numerical solutions are generally in good agreement with the observed data.
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Geotextile Specifications Made Simple
Geotextiles are currently specified in many different ways. Engineers specify geotextiles by brand name, mass, type, and/or any mix of properties including strength, puncture resistance, “G” Rating, EOS, and hydraulic properties. However, in Australia, no unified method exists of specifying geotextiles in a simple manner. Some specifications are unnecessarily biased against certain types of geotextiles, particularly wovens. Each State road authority has its’ own method. This causes unnecessary confusion and complexity to engineers, manufacturers and contractors alike. This paper presents a case for adopting a simple, unified method of specifying geotextiles. Current Australian specifications are reviewed against worlds best practice.
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Quality Management of Earthworks with Deflection-Based Devices
Soheil Nazarian and Andy Doe
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Marine geophysical investigations of palaeo-drainage systems in the Hawkesbury River Estuary, New South Wales, Australia
The Hawkesbury River is a key element in a major river system in eastern Australia. The river and its tributaries virtually encircle Sydney’s metropolitan area, extending northward to the Pittwater and Brisbane Water embayments and entering the Tasman Sea at Broken Bay, some 35 km north of Sydney Harbour. Since the 1960’s marine geophysical techniques, principally seismic reflection, supported by land gravity surveys have revealed extensive and deep palaeodrainage systems incising the underlying sedimentary rocks mainly beneath the River and its tributaries. These are masked by considerable thicknesses of recently deposited sandy sediments.
Case studies from three recent infrastructure and research projects, completed near the mouth of the Hawkesbury River system demonstrate the application of marine seismic and gravity technologies in the mapping parts of this palaeodrainage system. These projects are within the maritime zone of the Hawkesbury River. In this zone the Hawkesbury River estuary is a drowned river valley within steeply incised gorges surrounded by dissected plateaus. The terrain is dominated by the sandstone geology with an extensively dissected and generally rugged landscape.
Installation of a wastewater transfer main beneath the Hawkesbury River between the then unsewered Dangar Island and Brooklyn on the mainland was required. This involved a 1400 m long directional bore beneath tidal mud flats and a deep tidal channel. The marine geophysics mapped the bedrock profile, identified a fault and strong seismic reflectors within the bedrock near the centre of the palaeochannel at about 45 m depth. These were interpreted as regions of stress concentration in the Newport Formation created by valley bulging processes following rapid erosion. The geotechnical model inferred from these investigations was applied in the design of the directional drilling operation that was successfully completed in rock. This upgraded sewer system is now in operation and has removed a significant pollution source from the Hawkesbury River.
Upgrading of the electricity supply from Wagstaffe to Booker Bay required installation of an 11kV power cable across Brisbane Water, a distance of 630 m. Previous regional gravity surveys in this area had identified a deep palaeodrainage system beneath the Woy Woy and Ettalong peninsulas. A marine seismic reflection and refraction survey along the proposed crossing confirmed the presence of a palaeochannel margin extending to about 25 m below the seabed. The conduit was subsequently successfully installed by horizontal directional boring up to 30 m below sea bed.
Development of an airborne electromagnetic system for bathymetric mapping and sea-floor characterisation required independent calibration using marine geophysics within Broken Bay. A broad and deep channel representing a high energy palaeo-fluvial drainage system in the Hawkesbury River outreaches was identified. This extended to approximately 80 m depth below river level and was somewhat shallower than indicated by previous studies suggesting that there may be some uncertainty in seismic bedrock depth possibly due to the dense basal sediments. Also in another nearby area a dendritic fluvial pattern extending to approximately 70 m depth was observed. A moderately narrow palaeochannel extending to 90 m depth either side of the Palm Beach tombolo was also clearly identified.
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Geotechnical investigation, analysis and monitoring – BHP Billiton Western Stockyard, Finucane Island, Port Hedland
The BHP Billiton PACE Project comprises the construction of a new stockyard, shipping berth and ore handling facilities for BHP Billiton Iron Ore on Finucane Island near Port Hedland in Western Australia. This presentation focuses on the geotechnical work for the investigation and design of the new ore stockyard area on Finucane Island, known as the Western Stockyard.
Soil & Rock Engineering carried out the geotechnical investigation and design work on behalf of the BHP Billiton appointed project managers ‘Mine and Port Developments Joint Venture’ (MPD JV), a joint venture between Sinclair Knight Merz Pty Ltd and Fluor Australia Pty Ltd.
At the time of preparation of this presentation, the Western Stockyard facility is under construction. Bulk earthworks were completed between August 2002 and March 2003 and structural and rail construction is currently ongoing.
The new stockyard has a design capacity of 1,300,000 t of iron ore. Ore will be stored in stockpiles that are 19 m high and impart a ground pressure of around 500 kPa on to the underlying strata.
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Characterising compacted soil using shear wave velocity and matric suction
The manner in which soil compacts governs the practical and reliable criteria in controlling compaction in the field. A nuclear density meter, based on radioactive isotopes, is the method most commonly used for field compaction, and while it performs well for controlling placement, its localised nature is not suitable for deeper fills or for assessing larger surface areas. In those types of conditions, alternative non-destructive methods should be considered. Numerous research studies have focused on the characteristics of compacted soil at its optimum moisture content under saturated conditions, but only a few have evaluated compacted soil under unsaturated conditions using surface wave and shear wave velocity surveys.
This study explores the performance of a cost effective method for evaluating the characteristics of compacted fills by measuring the shear wave velocity and matric suction to evaluate the void ratio or dry density of compacted soil. Laboratory studies of compacted specimens were used to evaluate this method and their performance under different isotropic confining pressures. The results showed that the shear wave velocity and matric suction can effectively predict how the soil will compact, but its success requires field measurements of both shear wave velocity and matric suction. The application of this relationship would enable practitioners to efficiently control compaction over large areas during post-construction stages, and locate areas within the existing formations where the soil was not sufficiently compacted.
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Study of the stability of the 90 year old Millswood Underpass protected by revetment walls in unsaturated Keswick Clay
In recent times, there has been much debate regarding the use of unsaturated soil mechanics for the design of earth retaining structures in Adelaide’s semi-arid conditions. The soil-water characteristic curve (SWCC) is the main design tool within this field as a means to incorporate the additional shear strength associated with total soil suction. However, the majority of research to date only incorporates matric suction. This paper is focused on adopting the more commonly measured total suction, and to more effectively capture the behaviour of Keswick Clay as osmotic suction is the dominant component of total suction. Using the Millswood Underpass as a case study, weather data over a 120 year period was used to examine the variations of the stability of slopes in unsaturated Keswick Clay. The reduction in stability due to precipitation-induced flooding, and water leakage due to buried services were also considered. This paper demonstrates the significance of total soil suction to the shear strength of Keswick Clay that is well-known to design consultants but rarely used in slope stability problems.
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Engineering the Mansion’s heritage
A development at 83 Queens Road in Melbourne, Australia, required construction of a three level basement. The normal geotechnical constraints of excavating adjacent to an existing relatively new building were challenging enough for the complicated soils profile occurring at the site. An additional significant challenge faced by the designers was the requirement to excavate underneath the entire footprint of an existing heritage listed building, known locally as “the Mansion”, in the centre of the site. Maintaining the structural integrity of that building was afforded the highest priority during the 10 m excavation. This paper describes geotechnical aspects of the analysis, design and construction of the secant retention system adjacent to the new building and support of the Mansion enabling the 10 m excavation to proceed without causing damage to the structure. The secant wall comprised hard-soft interlocking piles constructed using CFA methods. CFA piles along the length of the Mansion provided short term support with the building load transferred to the piles by large steel beams jacked under the building. Survey results confirmed that movements of the retaining wall and also the load bearing piles were minimal and well within design expectations.