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Back analysis of Lower River Murray riverbank collapses
Riverbank collapses in the Lower River Murray threaten public infrastructure, private property and the safety of river users, and also provide significant challenges for environmental and river management. According to the inventory of the South Australian Department of Environment, Water and Natural Resources (DEWNR), between 2007 and 2010, 50 riverbank collapse-related incidents were reported at four very high risk sites: East Front Road, Mannum; Woodlane Reserve; River Front Road, Murray Bridge and White Sands. The objectives of this paper are to: (i) model four known and representative riverbank collapses at these four sites and (ii) determine the soil shear strength properties by undertaking back analyses. Adopting a GIS framework incorporating light detecting and ranging (LIDAR) digital elevation models (DEMs) and high-resolution aerial images, four cross-sectional models have been accurately established based on the examined historical collapses. Slope geometries have been determined using topographic information obtained from the DEMs. Finite element analyses based on a transient water model have been adopted to simulate the response of pore water pressure under dynamic variations of rainfall, evaporation and river level fluctuations. The limit equilibrium method has been used to undertake the slope stability calculations. The model results, which agree closely with the findings of historical incidents, demonstrate the efficacy of the framework and the accuracy of the predictions.
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Frontiers in deepwater geotechnics: Optimising geotechnical design of subsea foundations
This paper outlines a toolbox of methods for optimising the geotechnical design of subsea foundations. Subsea foundations are becoming increasingly widespread as offshore development moves away from the conventional template of a fixed platform over a set of wells to subsea development of multiple wells and fields tied back to a single facility. Subsea developments comprise a network of infield flowlines and assorted pipeline and wellhead infrastructure, which is typically supported on shallow, mat foundations. The optimisation methods presented cover (i) capacity assessment methodology, (ii) foundation configuration, (iii) geotechnical input and (iv) mode of operation. The research results derive from a combination of physical model testing in a geotechnical centrifuge, numerical analysis and theoretical modelling. Many of the research results have been immediately adopted in engineering practice in Australia and overseas, demonstrating the relevance of the methods to the national and international offshore hydrocarbon industries.
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Characterisation of ground conditions in the Christchurch central business district
The magnitude Mw 6.2 earthquake of February 22nd 2011 that struck beneath the city of Christchurch, New Zealand, caused widespread damage and was particularly destructive to the Central Business District (CBD). The shaking caused major damage, including collapses of structures, and initiated ground failure in the form of soil liquefaction and consequent effects such as sand boils, surface flooding, large differential settlements of buildings and lateral spreading of ground towards rivers were observed. A research project underway at the University of Canterbury to characterise the engineering behaviour of the soils in the region was influenced by this event to focus on the performance of the highly variable ground conditions in the CBD. This paper outlines the methodology of this research to characterise the key soil horizons that underlie the CBD that influenced the performance of important structures during the recent earthquakes, and will influence the performance of the rebuilt city centre under future events. The methodology follows post-earthquake reconnaissance in the central city, a desk study on ground conditions, site selection, mobilisation of a post-earthquake ground investigation incorporating the cone penetration test (CPT), borehole drilling, shear wave velocity profiling and Gel-push sampling followed by a programme of laboratory testing including monotonic and cyclic testing of the soils obtained in the investigation. The research is timely and aims to inform the impending rebuild, with appropriate information on the soils response to dynamic loading, and the influence this has on the performance of structures with various foundation forms.
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Risky business: The development and implementation of a national landslide risk management system for Australia
The Australian Geomechanics Society (AGS) published a benchmark technical paper “Landslide Risk Management Concepts and Guidelines” in the year 2000. This was a continued recognition by AGS of the benefits of the concept of risk in potential landslide situations. The following paper discusses the subsequent strategies adopted for implementation of the principles of AGS (2000) into the legislative framework of Australian governments at National, State and Local levels.
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Kaikōura Earthquake Recovery, Design of a 13m high geogrid reinforced, no fines concrete gravity seawall in a high seismicity environment
The North Canterbury Transport Infrastructure Recovery (NCTIR) Alliance was formed to deliver the repairs to the national road and rail transportation corridors after the Mw 7.8, Kaikōura earthquake which occurred on 14 November 2016. At Ōhau Point, located approximately 26 km north east of the Kaikōura CBD via road, approximately 240,000 m3 of landslide debris buried the rail, road and adjacent coastline.
Between February 2017 and July 2018, a new 900m long seawall was designed and constructed as part of the coastal realignment of State Highway 1 (SH1) around Ōhau Point. This structure incorporates mechanically stabilised earth comprising mainly cement stabilised backfill with geogrid reinforcing, and five-ton concrete block facing.
The most complex section of this seawall is around a rock outcrop known locally as Shag Rock where the seawall is up to 13m high. The constraints and challenges in this area include maintaining access along the existing SH1 above the wall and ecological constraints. A special complex no fines concrete gravity wall (NFC G-Wall) was designed and constructed to buttress the slope and older seawall. This structure, and the wider fill and earth platform which supports the widened roadway, is designed to slide as a block under 0.76g peak horizontal ground acceleration.
This paper presents the results of the two-dimensional FLAC modelling which was completed to analyse and design the 13m high geogrid reinforced NFC G-Wall at NCTIR site 6. It also describes the pragmatic observational approach which was taken for the seawall design, highlighting the seismic sliding mechanism and issues that arose during design and construction of the seawall.
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Monitoring Slope Instability During Reinstatement of State Highway 11 at Lemon’s Hill(35˚S) Northland, New Zealand
Slope failures within weathered rock are characteristic of road cuttings in the humid subtropics, where weathering profiles can extend 10s of meters into the subsurface. Typically, the high rainfall intensities provided by the passing of tropical cyclones, can generate widespread slopes failures along road transport corridors, requiring engineering responses such as hazard assessment and road reinstatement.
Here, we report on some engineering geological aspects of the 2018 slope failure and subsequent slope monitoring response along State Highway 11 (SH11) on the southern side of Lemon’s Hill (35°S), in Northland, New Zealand. In much of New Zealand, landslides along transport corridors are often initiated by seismic activity, but in the subtropical north, >500 km west of the active plate boundary, rainfall is the key landslide trigger. The 13 February 2018 landslide at Lemon’s Hill adjacent to SH11 followed prolonged rainfall from the passing of Tropical Cyclone Fehi. SH11 is a popular tourist route to the Bay of Islands region of Northland, and the landslide occurred as an ‘overslip’, shallow (2 m deep) translational failure, within completely (CW) to highly weathered (HW) greywacke.
The response included the construction of engineered batters, resulting in six months of traffic disruptions. The engineering geological investigation of the site including characterisation of the materials, and identification of probable failure modes. Monitoring via multi-temporal Unmanned Aerial Vehicle (UAV) photogrammetry and AccuMM GPS nodes installed on the slopes was also part of the engineering response, while recently available LiDAR provides future opportunities for an enhanced engineering geological understanding of slopes instability in the area.
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Dams in the Darling Range
Geotechnical factors play a major role in the design and construction of dams in the Darling Range east of Perth. These factors are described in relation to case histories for a number of water supply dams that have been constructed over the last one hundred years.
The Darling Range is underlain by the Archaean Yilgarn Craton, which consists of granitic and gneissic rocks that have been intruded in places by dolerites. Conglomerates of suspected Tertiary age occur at North Dandalup and Harvey Dams. The variable weathering in the granitic terrain requires special foundation and permeability control measures. In addition, weathering and ground water movements related to faults, shear zones and contacts contribute to adverse geotechnical conditions commonly found in the Darling Range. The effects of jointing, in particular the pervasive sheet joints, and the presence of corestones in foundation and cut-off excavations, are also described. Problems associated with natural construction materials used for dams in the Darling Range are outlined.
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Modelling ploughing and cutting processes in soils
Economic growth in Australia and the rest of the world is linked to the scale of construction and mining, and the amount of earth moved each year in these operations is difficult to fathom. When distributed evenly across the world’s population, each individual moves several tonnes of earth each year. This paper highlights current and future research initiatives within the ARC Centre of Excellence for Geotechnical Science and Engineering (CGSE) aimed at developing rigorous, mechanics-based models for fundamental ploughing and cutting processes in soils. State-of-the-art physical modelling is integrated with the development of new techniques for analytical and numerical modelling to elucidate and predict the full progression of forces and deformations in both two-dimensional and three-dimensional processes. A new analytical model for cutting in dry sand is presented, and preliminary results from numerical and physical modelling are described. The analyses reveal effects that available models fail to consider and illustrate how the development of rigorous models may facilitate improvements in production and efficiency in earthmoving operations.
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Brown gold: redevelopment of former quarry and landfill sites
This paper presents a discussion of key geotechnical considerations and challenges associated with the rehabilitation and redevelopment of former quarry and landfill sites, including open quarry pits to be backfilled with imported engineered fill and former quarry pits that are already backfilled with uncontrolled fill materials where it is not practical to remove the landfill/uncontrolled fill materials as part of the rehabilitation/redevelopment. Geotechnical considerations and challenges that are discussed include the importance of communication with regulatory authorities throughout planning, design and handover, the challenges associated with geotechnical investigation of former landfill sites, the availability of materials from onsite sources or importation for use as engineered fill, requirements for assessing and monitoring ground settlement and the importance of quality assurance (e.g. Level 1 inspection and testing) during rehabilitation/redevelopment earthworks. Development outcomes ranging from public open space to residential or commercial development are also discussed.
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Substance and mass properties for the design of engineering structures in the Hawkesbury Sandstone
The Hawkesbury Sandstone dominates the Sydney region, both from the viewpoint of engineering structures and the natural topography. This Formation thickens from its western and southern outcrop margins in the Blue Mountains and Illawarra to about 290 m near the Hawkesbury River.
This article is an expansion of one published in 1985 in the volume “Engineering Geology of the Sydney Region”. The original document concentrated on the engineering properties of the intact sandstone (i.e. the ‘substance’) whereas this article covers both substance and rock mass parameters. Much of the original information on substance parameters is reproduced here with additions from papers published in the volume “Sandstone City” published in 2000 by the Geological Society of Australia. The rock mass data are taken from papers published since 1985.