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Recent advances in the application of vertical drains and vacuum preloading in soft soil stabilisation
Much of the world’s essential infrastructure is built along congested coastal belts that are composed of highly compressible and weak soils up to significant depths. Soft alluvial and marine clay deposits have very low bearing capacity and excessive settlement characteristics, with obvious design and maintenance implications on tall structures and large commercial buildings, as well as port and transport infrastructure. Stabilising these soft soils before commencing construction is essential for both long term and short term stability. Pre-construction consolidation of soft soils through the application of a surcharge load alone often takes too long, apart from which, the load required to achieve more than 90% consolidation of these mostly low lying, permeable, and very thick clay deposits can be excessively high over a prolonged period. A system of vertical drains combined with vacuum pressure and surcharge preloading has become an attractive ground improvement alternative in terms of both cost and effectiveness. This technique accelerates consolidation by promoting rapid radial flow which decreases the excess pore-pressure while increasing the effective stress.
Over the past 15 years, the Author and his co-workers have developed numerous experimental, analytical and numerical approaches that simulate the mechanics of prefabricated vertical drains (PVDs) and vacuum preloading, including two-dimensional and three-dimensional analyses, and more comprehensive design methods. These recent techniques have been applied to various real life projects in Australia and Southeast Asia. Some of the new design concepts include the role of overlapping smear zones due to PVD-mandrel penetration, pore pressure prediction based on the elliptical cavity expansion theory, and the rise and fall of pore pressure via PVD under cyclic loads. These recent advances enable greater accuracy in the prediction of excess pore water pressure, and lateral and vertical displacement of the stabilised ground.
This E.H. Davis Memorial Lecture presents an overview of the theoretical and practical developments and salient findings of soft ground improvement via PVDs and vacuum preloading, with applications to selected case studies in Australia, Thailand, and China.
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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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A Few Notes On Embedment Design With The ‘What You Design Is What You Get’ WYDIWYG Method For Propped Cantilever Walls
The WYDIWYG method for stability design of propped cantilever walls was recently published in the 2019 ANZ Geotechnical Conference. The new method has been shown to be consistent between total and effective stress designs, numerically friendly, stable, and also produces economical designs. The paper focussed the consideration on overturning stability, which is a critical design for this type of structures. In geotechnical engineering designers often treat passive earth pressures as soil resistances and active earth pressures as soil loads. Are active pressures really loads and passive pressures really resistances? It raises an interesting question and the proposition forms an important assumption in the formulation of the new method. Given the interests from the geotechnical design community a more general discussion on the model development will be given together with application of the method to design. Worked examples are also included to demonstrate simplicity of the design process.
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Temporary Support Of Deep Basement Excavations In Rock
Deep basements have become common in our modern cities. Whilst the analysis and design of retention systems for deep basements in soil are relatively well established, the same cannot be said for deep basement retention systems in rock. In many instances the design of retention systems in rock are based on a soil mechanics approach and as a result often ignore unique aspects of rock mass behaviour that can significantly impact the performance of these retention systems. It is important that the characteristics of the rock mass are well understood and are quantified during the ground investigation. Some of the aspects of retention system design and analysis that are set out in this paper only became apparent following unsatisfactory performance of a retention system. Due to the sensitivity surrounding such unsatisfactory performance, it has not been possible to include the specific case studies. As a result, this paper provides an overview of the many aspects involved in respect to ground investigations, analysis, design and construction of deep basement retention systems in rock that are required to mitigate against unsatisfactory performance. This paper is restricted to temporary embedded wall retention systems that are installed to allow the construction of the permanent basement structure.
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Scrap-tyre Soil Mixture For Seismic Protection
Scrap tyre stockpile has been a significant disposal problem around the world. Significant research attention has been devoted in recent years to find new beneficial ways to recycle and reuse the huge stockpile. This paper proposes a new method of utilizing scrap tyres for infrastructure protection. The method involves mixing scrap tyres with soil materials and placing the mixtures around foundations for vibration absorption. This method provides two major benefits: (i) the low-cost would make it accessible to developing countries and rural areas of developed countries where resources and technology are not adequate for earthquake mitigation with welldeveloped, expensive, techniques and (ii) potential to consume the huge stockpiles of scrap tyres all over the world. However, the success of the proposed method depends on the static and dynamic properties of scrap tyresoil mixtures. This paper presents results of recent experimental investigations on tyre (tyre crumbs)-soil mixtures carried out at University of Wollongong.
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A new lightweight dynamic cone penetrometer for laboratory and field applications
Developed by Scala (1959), the dynamic cone penetrometer (DCP) has been extensively used in recent decades for the design of flexible pavements due to its inexpensive and easy-to-operate features. This paper presents some of the results of a study concerning the effect of vertical confinement from the CBR mould on the dynamic cone penetrometer index, and the development of a lightweight DCP that can be used in the laboratory as well as in field conditions with similar results. The results show that the effect of vertical confinement is very significant, especially with a hammer mass larger than 4.6 kg. The results also indicate that the influence of the vertical confinement on the penetration index is not significant when the hammer mass is less than 2 kg. Based on these results, a new lightweight DCP is proposed with a hammer mass of 2.25 kg, which can be used in the laboratory in the CBR mould and also in field conditions with similar results for a similar soil.
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Geotechnical And Coastal Engineering Aspects Of Risk Assessment For Coastal Protective Works And Assets
There are numerous coastal assets in NSW that are located in areas subject to beach erosion and hence are at potential risk of damage in coastal storms. Two risk assessment procedures are presented herein, namely:
- a geotechnical risk assessment of foreshore stability and protective works that was applied by Jeffery and Katauskas on the north coast of NSW and
- a coastal engineering risk assessment of damage to assets along beaches in Warringah as applied by WorleyParsons for Warringah Council.
The geotechnical assessments of risk to property and risk to life were based on the AGS (2007) procedures. These risk assessments are seen to be useful in informing emergency services of the potential need for evacuation of some properties in coastal storms, and allowing a framework for monitoring of the foreshore to be developed and implemented.
The coastal engineering risk assessment comprised the development of:
- an inventory of individual property details relevant to consideration of risk,
- resistance ratings for existing protective works located along the beachfront (e.g. based on toe levels and rock size),
- procedures for assigning likelihood ratings for occurrence of damaging events (based on the position of the Immediate Coastline Hazard Line relative to the seaward face of an asset, assuming no protective works),
- procedures for assigning consequence ratings to expected property damage (including consideration of whether the asset was supported on piles and the likely effectiveness of protective works seaward of the asset) and
- an overall risk analysis matrix used to derive a risk rating from different combinations of likelihood and consequence ratings.
These coastal risk assessment procedures are noted as being useful in preparing in advance for coastal storm
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West Pilbara Iron Ore Project, Port off-loading facility, Anketell Point, Western Australia
API Management Pty Ltd (API), a 50/50 Joint Venture between Aquila Resources and AMCI propose to construct a multi-user port off-loading facility at Anketell Point in the Pilbara region of Western Australia, which will form part of the West Pilbara Iron Ore Project (WPIOP). The port off-loading facility will have an initial iron ore handling capacity of 30 million tonnes per annum.
This paper describes the nearshore geotechnical investigations that have been completed at Anketell Point. It outlines the purpose of the investigations and provides a geotechnical account of the planning, occupational health and safety and operational aspects of the investigations that led to the successful completion and factual geotechnical reporting of the investigations within program and budget. An overview of the logging, field testing, on site sub-sampling, laboratory testing and the ground conditions is provided.
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Problems with liquefaction criteria and their application in Australia
In many parts of the world, including Australia, the state of practice in assessing if liquefaction will occur is based on the recommendations of Youd et al. (2001) which arose from workshops convened in the United States by NCEER (now MCEER). In some regards, the final publication did not so much represent a consensus view as a compromise between differing opinions within the expert group. Since then, disagreements over key aspects of liquefaction assessment in North America have increased to the point of chaos (Youd, 2011). There is little awareness in Australia of this situation nor appreciation of the NCEER limitations in applying these recommendations. Poorly informed decisions are increasing costs and causing project delays. This paper presents no original research but is an attempt by a practising geotechnical engineer to point out some problematic aspect of the NCEER liquefaction criteria, and of current recommendations in the literature and in so doing to encourage other practitioners and regulators to consider reasonable adjustments or alternatives.
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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.