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AGS Townsville 2024 Symposium
Slope Stability
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Interface shear behaviour of coir geotextiles and sand using large-scale direct shear tests
The practice of soil reinforcement using natural products is attaining widespread acquiescence in developing countries due to its availability, cost effectiveness and also due to its ability in effectively replacing synthetic reinforcing elements. Coir is an eco-friendly, biodegradable, organic material which has high tearing strength, stiffness, and durability compared to other natural reinforcement materials. Frictional characteristics of geotextiles were analyzed through large-scale direct shear tests to knock out boundary effects and to reproduce the results in prototype scale. Detailed parametric study was conducted by varying the particle size, normal stress and type of geotextile. The improvement due to the provision of coir geotextiles is expressed in terms of a non-dimensional parameter called friction angle efficiency. The results indicate that coir geotextile inclusions enhance the peak shear strength, angle of internal friction and ductility of sand. Also, geotextiles with small mesh openings and higher tensile strength provide better performance in terms of increasing the shear strength of sand.
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Investigations of the Xiao Tou landslide stabilisation in the Three Gorges catchment area, Hubei Province, China
The Three Gorges project on the Yangtze River in China is the world’s largest hydro-electric power plant. Since the first impoundment of the 660 km long reservoir, more than 260 landslides have occurred along the reservoir banks.
The study summarised in this paper comprised geological and geotechnical mapping in the area of Xia Kou, located on the Xiang Xi River, approximately 40 km northwest of the Three Gorges Dam. The study aimed to determine whether the reservoir, with its significant water level changes, triggered the landslides or contributed to movement of existing landslides.
The study area is located at the Xiang Xi River, a tributary of the Yangtze River, flowing into the Yangtze River 28 km up-stream of the Three Gorges Dam. Most of the slides occur on the western banks of the Xiang Xi River, where sandstones and claystones strike parallel the flow direction of the river and dip into the slope. Landslides are triggered by a joint direction also striking north–south but dipping towards the river. The joint surfaces are filled with clay which decreases the shear strength of the rock mass.
Stability analyses of the Qiao Tou Landslide in the north of Xia Kou were carried out. The analyses took into account seasonal variations in the reservoir water levels and showed that the discharge from 175 m to 145 m AMSL is critical. Therefore, the drainage of the slope along the surface of rupture is a considerable factor triggering sliding.
Existing remedial measures consist of a large number of surface drains which have already been destroyed by continuous movement within the slopes. Furthermore, protecting shotcrete surfaces showed some wet areas and are also partially cracked as is the infrastructure around the area.
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Landslide susceptibility and landslide hazard zoning in Wollongong
This paper describes ‘knowledge-based’ data-mining techniques, developed for the assessment of landslide susceptibility and hazard with particular reference to its application in the Wollongong area. Large scale maps of geology and a comprehensive Landslide Inventory with regional coverage have been prepared. GIS-based derivatives of the digital elevation model including slope, geomorphology, curvature, flow accumulation and wetness index have been developed. Model performance has been assessed as part of a refined methodology for validation, including field inspections. Susceptibility zones outside known landslide areas have been classified as (a) high (b) moderate, (c) low and (d) very low susceptibility. Results show the high susceptibility zone covers 10% of the study area and contains 60% of known landslides, the moderate zone covers 12% of the study area and contains 32% of known landslides, the low zone covers 6.4% of area and contains 3.3% of known landslides and the very low zone covers 71% of the study area and contains 4% of the landslides. The susceptibility maps have been upgraded to hazard level maps with identification of individual zone landslide likelihoods, specific landslide frequency, volume and ‘profile’ angles. The paper concludes with a preliminary landslide susceptibility map for a segment of the Sydney Basin Region developed using the methodology described in this paper.
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Case studies of typical landslide events in the Adelaide Hills
The Adelaide Hills experience a variety of landslide events involving rock, soil and a combination of materials. These events are described by Sanders and Moon (2007). This paper is intended to complement the work of these authors by describing three typical recent events. Case Studies 1 and 3 describe events involving less that 50 m3 of material, a volume range typical in the Hills. Larger events do however occasionally occur and Case Study 2 describes one such event that involved up to 250,000 m3 of material.
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Mt Eliza — Slope Stabilisation
Triggered by excessive water, a landslip occurred on the cliff face of this bayside suburb with the potential to repeatedly slip and undermine beachfront residences located at the top of the cliff. Stabilization was imperative.
The coastal cliff is almost 15 m high, at an angle of 40 degrees to 60 degrees to the horizontal. The landslip, which was of approximately 4 metres wide was located immediately above a foreshore area that is frequented by the public. The cliff geology comprises Tertiary age sedimentary sandy clays, clayey sands and gravels of the Baxter Sandstone formation. Unlike most of the landslip issues in the Frankston South / Mt Eliza area, which are associated with the Selwyn Fault or the unfavourable geological conditions of the Balcombe Clays, the landslip on this site was considered a direct result of human interaction with the slope.
For the remedial works ATC Williams considered several methods including staggered retaining walls, and retaining wall with soil nail combinations; but ultimately developed a soil-nail only solution. Design challenges included a high groundwater with preferential seepage zones within the cliff face. However, the prime challenges were during construction, with site access only available across the foreshore at low tide, and access up the slope for nail installation achievable by small plant only. The success of the project depended on developing an appropriate construction method which enabled design requirements to be achieved.
With no equipment access to the top of the cliff, various means of providing access from the foreshore were considered including earth bunds, platforms, scaffolding and long reach excavators.
It was concluded that the only viable option was to use a crane to support a drilling rig mast at each nail position.
This arrangement limited the drilling depth and soil nail installation length to a relatively shallow 6 m and required a design modification necessitating the re-design of the pattern of soil nails to match the depth capabilities of the rig, whilst still meeting the local and global slope stability requirements. In total 137 soil nails were installed and the cliff was adequately stabilised.
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The performance of steel struts in a braced excavation in Perth
A deep and wide braced excavation constructed for the new Esplanade Station in the Perth CBD provided a useful opportunity to assess appropriate parameters for retaining wall design by backanalysis of its performance. The section of the wall under consideration involved a staged excavation adjacent to sheet piles supported by three levels of props at the final excavation depth of ~13 m. Two adjacent sheet piles were strain gauged to allow assessment of the wall’s bending moment, while the lateral soil movements adjacent to the wall were monitored using inclinometers. The struts were also instrumented with strain gauges and the output from these gauges provides the main focus of this paper. The effects of temperature on the inferred strut loads are examined and it is shown that the data recorded during excavation pause periods could only be explained if the operational axial strut stiffness was about a quarter of its theoretical stiffness. Finite Element backanalyses of the measurements obtained during the excavation phases also show that a best fit between measured and predicted strut loads is achieved for a similar operational stiffness. It is concluded that for large braced excavations of this nature, imperfections/curvatures in long struts lead to strut loads that are significantly lower than expected.
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Pavement Materials and Design in WA
Geoff Cocks
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Deep soil mixing for Whiteman Bridge, Reid Highway Extension, Middle Swan, Perth
The Reid Highway Extension project from West Swan Road to Great Northern Highway in Perth, Australia was completed in February 2010. The project includes a new bridge over the Swan River, named the Whiteman Bridge, and associated bridge approach embankments of about 8 m height. The approach embankments are underlain by a relatively weak and compressible silty clay/clayey silt zone of up to 4 m thickness. This stratum was predicted to undergo relatively large time-dependent settlements unless ground improvement was carried out. To alleviate the effect of relatively large time-dependent differential settlements between the bridge abutment and bridge approach embankment, Deep Soil Mixing (DSM) ground improvement was carried out below the approach embankments and abutments, combined with surcharge preloading of the approach embankments. This ground treatment enabled the abutments to be supported on shallow spread footings.