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Stabilisation of a fill embankment using soil nails
Drilled and grouted soil nails have been successfully used to stabilise a section of fill embankment on the Warrego Highway near Toowoomba, Queensland that failed as a result of heavy rainfall in January 2011. Instability occurred due to groundwater rise within the fill materials resulting in a tension crack developing at the traffic lane edge and an approximate 300 mm displacement within the outer portion of the embankment. Reinforcement of the unstable fill slope with soil nails was the selected remediation method. The soil nails were installed through the existing granular fill materials and grouted into the fill and underlying basalt. Soil nail holes were drilled using equipment fitted to excavators to typical lengths of 10 m to 12 m with temporary casing used to prevent hole collapse during drilling. A reinforced shotcrete facing was constructed over the face of the embankment to provide the necessary nail head restraint and prevent erosion. Soil nails were installed in a prescribed sequence to manage the risk of construction plant trafficking the marginally stable fill embankment. Full time monitoring of construction was carried out in accordance with an action plan developed specifically for the site. The coordinated approach to the design and construction of the works resulted in a successful implementation of the remedial works.
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Engineering geology of Fremantle Harbour (An historical perspective)
Prior to the construction of the Inner Harbour in Fremantle in the closing years of the nineteenth century, the body of water that is now the Inner Harbour comprised a broad estuary, and the Swan River flowed in to the sea across a shallow rock bar between two rocky headlands.
The history of construction of the Inner Harbour is described to illustrate the complexity of the geology in this part of Fremantle. The geology includes a deep sediment filled palaeochannel associated with a former course of the Swan River during the Quaternary when the sea level was considerably lower than its current level. The construction of the harbour involved blasting of the rock bar, major dredging and land reclamation and extensive piling. The distribution and impact of the various geological units on the harbour and a number of the geotechnical issues that have lead to construction difficulties in the past are detailed.
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Illustrative sections depicting landslide susceptibility of the Illawarra Escarpment
The challenge that landslides pose to infrastructure and to domestic and commercial development in the Illawarra region has been recognised in land-use planning for decades. The seminal regional mapping undertaken by Bowman (1974) and, before him, the work reported by Shellshear (1890), set the technical benchmarks for others to follow. The challenges presented to development and road and rail infrastructure were recognised by both Wollongong City Council and major infrastructure providers in the NSW Roads and Maritime Services (previously NSW Roads and Traffic Authority) and NSW Rail Corp (previously State Rail Authority). Continuing support from this group has permitted research and work in the field throughout the Illawarra by Flentje since 1993, which has brought together the collectively faced issues into a composite landslide inventory.
The landslide issues are well recognised by those who are familiar with the Illawarra area – in particular, the typically steep terrain of the 45 km long Illawarra Escarpment, the presence of a colluvial mantle draped over the steep terrain, the presence of many sub-horizontal coal seams throughout the stratigraphy, past and present underground coal mining throughout the region and intense rainfall events generated by virtue of its location and also as a consequence of the escarpment’s influence upon local meteorology with the orographic rainfall.
One of the challenges of the application of Bowman (ibid) is the mapping base available at that time. Bowman reported mapping at 8 chains to the inch (1:6,336) but appears to have used a basemap enlarged from a much earlier base (possibly enlarged from 2 inch to the mile, viz: 1:31,680, or 1 inch to the mile, viz: 1:63,360). Bowman also noted the poor edge matching of his basemaps in his work. The NSW Central Mapping Authority 1:4,000 scale 2 m contours generated in the late 1970’s provided a significant enhancement to the mapping base, though draping of the Bowman mapping over this improved basemap faced obvious challenges. Recently, the availability of the contours in electronic format and the rise of both Geographic Information System (GIS) capability and expertise, have greatly facilitated mapping in the Illawarra. Subsequent Airborne Laser Scan (ALS) digital terrain mapping at high resolution has also recently become available, which together with Flentje’s detailed mapping (Flentje, 1998) and access to landslide records of Wollongong City Council (through council’s Geotechnical Engineer, Peter Tobin) has permitted enhanced understanding of the specific conditions and characteristics that influence the Illawarra landscape. This area is also the scene of the Engineering Geology course run biennially on behalf of the Australian Geomechanics Society (2010).
As no doubt is often the case, a simple question triggers a line of action, and the raison d’etre for this paper fits that pattern. In this case, the question asked of themselves by the authors was “Is there a type-section of landslide issues within the Illawarra?” This paper, and the development of the illustrative sections herein, is a response to that somewhat rhetorical question.
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Residential development on the New Volcanics: A case study of suspected barium contamination and soil amendment
As Melbourne expands to the north and west in response to growing demand for residential accommodation, soils developed on the Lava Plains (Geological Survey of Victoria, 1967), otherwise known as the New Volcanics (Cochrane, Quick & Spencer Jones, 1999), will be exploited. These soils are commonly shallow, alkaline and duplex, whilst the interspersed grey cracking clays pose significant constraints on building foundations and urban infrastructure due to their high “shrink-swell” nature. Rock floaters, clay fissuring, calcareous layers, sodicity and high electrolyte levels can serve as additional impediments.
The aforementioned soil characteristics also constrain garden development, landscaping and revegetation due to the presence of relatively heavy B-horizons with poor internal drainage, a relatively flat landscape with poorly-defined surface drainage, shallow heavy finely-textured topsoils and shallow calcareous bands which limit root propagation. The range of native and exotic plant species that can be planted in this landscape is limited, despite the need to improve aesthetics in order to attract residents.
Whilst the geological characteristics of soils on the New Volcanics are well understood and their limited potential as plant growth media recognised, the impact of soil chemical properties on some foundations, plant growth and heavy metal availability is not.
This paper describes the outcome of an investigation into potential barium (Ba) contamination of a 15 ha site located 18 km west of the CBD which threatened to stop the proposed residential development. Barium, as a trace element, concentrates in intermediate and acid magmatic rocks and its occurrence is linked with alkali feldspar and biotite (Kabata-Pendias, 2001). In soils in the natural environment, barium will generally occur as barium sulphate (BaSO4) due to the ubiquitous presence of sulphate ions. Some paint pigments contain barium salts and thus barium levels can be elevated on derelict land (Bridges, 1987). Barium is also associated with radio-active fall-out and nuclear waste. Barium sulphate is widely used as a safe tracer in medical practice due to the insolubility of the compound.
In conducting a soil survey for residential development where barium levels are found to be high it is essential to determine if possible why the levels are high, how widespread is the area impacted and whether anthropogenic input is likely. Of particular interest are the distribution of barium concentrations with depth and the association of barium with appropriate anionic concentrations. Evidence of site disturbance or encapsulation is also important as is the prospect of pseudo-stratification.
Knowledge of the availability of heavy metals for plants can assist the assessment of risk to humans from food chain transfers and contact with soil barium; fortunately there are well-defined relationships between pH and soil electrolytes that dictate this availability.
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Hydrogeology of the Botany Basin
This paper seeks to review the current knowledge of the geology and hydrogeology of the Botany Basin, and focuses on how the latter impacts on its geomechanical behaviour. It will consider, briefly, the basin’s encapsulating bedrock foundation rocks, their intersection with the basin fill sediments, the latter’s structure, stratigraphy, lithology, distribution and how these impact on its hydrogeological behaviour. The discussion will then consider the basin’s economic and beneficial value, development within the basin and how this has historically impacted on the basin’s hydrogeology, hydrogeochemistry, water quality and economic value and how development projects need to consider their impact on the basin’s condition and the existing development it supports. Two case studies are presented to illustrate the latter.
At the outset, it is appropriate to define what the term ‘Botany Basin’ constitutes. Rickwood (1998) notes that there “…are those geologists who regard it as a tectonically formed bedrock depression that is the result of post Triassic uplift and warping, and is the smaller part of the larger Sydney Basin” (referencing Roy, 1983), which contrasts with the general view held by hydrogeologists that tend to apply ‘… the name Botany Basin to the topographic depression that is covered by the unconsolidated sediments that form the Botany Sands aquifer’ (referencing Griffin, 1963). Rickwood (1998) further develops the interpretation of the Botany Basin as being “… an easily verifiable bedrock basin … centred on Botany Bay and approximates to the catchment area, but excludes(ing) the extensive drainages of the major rivers entering the basin.” Rickwood then considers the relevance of the Pleistocene basin area and that of the modern basin, settling on the latter as the general basis for outlining the extent of the Botany Basin boundaries.
This paper broadly adopts the general extent of the modern basin as the definition of the Botany Basin, and focuses on the Quaternary sediments contained within that basin and how this aquifer interacts with older bedrock formations which comprise the Pleistocene bedrock paleochannel/paleobasin. The hydrogeological aspects of the bedrock formations, primarily the Hawkesbury Sandstone and Ashfield Shales, are discussed in detail in papers elsewhere in this volume.
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Homebush Bay Dioxin Remediation Project
The paper presents an overview of the Homebush Bay Dioxin Remediation Project. The project represents the most expensive and challenging site clean-up project undertaken in Australia to-date. Information is provided on the history of the areas to be remediated and the site investigations and assessments that have been conducted. Details on the extent of contamination at the site are provided, followed by a description of the clean-up assessments. The remediation objectives are then described, with an assessment of potential remediation technologies then provided. Summary details of the current remediation strategy and project status are then provided.
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Why Are There Seashells In My Alluvial Valley? – The coastal geologist’s perspective of valley-fill sequences.
For engineers tasked with placing infrastructure across alluvial valleys in eastern Australia the mélange of unconsolidated sand and silt/clay deposits encountered beneath modern floodplains must often appear baffling, particularly the juxtaposition of river derived and marine influenced deposits. However, it is this apparent anomaly that immediately alerts coastal geologists, for whom modelling these valley-fill sequences is their stock and trade, that they are dealing with an ancient ‘drowned-valley estuarine’ depositional environment.
Valleys that ultimately connect to the coast have experienced numerous cycles of erosion and deposition as a consequence of major sea level fluctuations through geological time. The last major phase of fill has occurred in response to marine inundation of coastal valleys by rising sea level which stabilized at approximately its present level some 6,500 years ago.
With the exception of the alluvial capping layer that supports human habitation of these valleys, the greater proportion of silt/clay sediments deposited during this last phase have remained beyond the influence of pedogenic processes and are therefore ‘unripe’. This immaturity provides a number of challenging geomechanical and environmental aspects to working with these sediments.
Although local complexity may occur, stratigraphic models of ‘drowned-valley estuarine’ deposition provide a good general framework for understanding the distribution of both geomechanical and environmental properties of the valley- fill sequences.
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Use of carbon fibre reinforced polymer (CFRP) as an alternative material in permanent ground anchors
Steel tendon ground anchors are an integral construction technique for numerous civil engineering applications ranging from deep excavation support to resistance of structural uplift and overturning of superstructures. Failures of steel strand ground anchor systems are rare but, when they occur, corrosion and human error are the primary reason. Several methods of minimising anchor system corrosion have been adopted over time to minimise ingress of corrosive substances. However, anchors are still failing due to corrosion. Advancement in the development of corrosion resistant materials has been at the forefront of materials research. In this respect, research and development of FRP materials is enabling the progress of providing the industry with a more potentially robust anchor system aimed at eliminating current limitations encountered with steel strand ground anchors.
This paper provides an overview of current best practices for the application of permanent ground anchors and investigates the current developments in FRP materials for ground anchor applications as an alternative to conventional steel tendon ground anchors. The paper also provides insight into known areas where further research is required to assist the introduction of FRP ground anchors into standards.