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This presentation describes geotechnical issues that should be considered, but are often forgotten, drawing from the work done in the European Commission’s COST C7 committee described in the book “Hidden Aspects of Urban Planning” co-authored by the presenter and published by Thomas Telford. Numerous examples of international best practice in geotechnical engineering and underground space development will be described, along with two case histories of foundation reuse in London.

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Golder Associates is the geotechnical engineer for the Nakheel Tower in Dubai. The tower was to form the centre piece of the Nakheel Harbour and Tower development in the heart of “new Dubai”. With a planned height of over 1 km and a dead weight in excess of 2,000,000 tonnes, the tower was set to become not only the worldÕs tallest building but also one of the heaviest. The bearing pressures applied to the ground coupled with the soft rock ground conditions present at the site provided a significant challenge to the design of the footing system. Chris Haberfield led the geotechnical design team for the design of the foundations of the tower. Chris will present an overview of the tower and provide details of the ground investigation and geotechnical engineering undertaken which led to the development of the footing solution finally adopted.

Due to the economic down turn the construction of tall tower has been put on hold with the foundations two thirds completed.

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Until a few decades ago, little thought was given to the environmental impacts of man-made underwater noise, but this has now changed, with a growing awareness of the importance of sound to marine animals resulting in a mounting concern about the possible detrimental effects of the underwater noise that we make on these animals. In Australia, this concern has considerable legislative backing in the form of the Environmental Protection and Biodiversity Conservation Act (EPBC Act), which makes it an offence to unduly interfere with endangered and migratory species, which includes whales, dolphins, dugong, and sea turtles.

It is unlikely to surprise anyone that pile-driving for the construction of ports and other marine facilities produces high levels of underwater noise. Regulators are certainly aware of this, and they now often require proponents of marine infrastructure projects that involve piling to include a consideration of the resulting underwater noise as part of their environmental impact assessments. The Centre for Marine Science and Technology (CMST) is often asked to assist in this process by carrying out measurement and/or modelling of underwater noise from pile driving, which turns out to be more complicated than one might imagine.

This talk will describe the characteristics of underwater noise from piling, and how the propagation of sound varies with such factors as water depth and seabed geology. Modelling methods will be described and there will be a brief overview of possible impacts on marine animals. Finally, there will be some discussion about the effectiveness of methods to reduce underwater noise from pile-driving.

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This presentation will discuss a number of challenging geotechnical issues relating to footing design that the speaker has observed over the course of his career.

These issues include:

• collapsing soils;
• residential type footing designs in arid areas;
• the special geotechnical considerations associated with jacked piling leading to the incorporation of several aspects of jacked piling into AS 2159-2009 “Piling- Design and installation” code; and
• the difficulties of Standard Penetration Testing (SPT) in saturated sand.

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The coastal limestones along the Mount Eliza Escarpment in Kings Park, Perth, have a history of landslides and ongoing slope instability. With a major arterial road along its base, the escarpment has been the subject of a number of geotechnical and quantitative risk assessments. A programme of landslide risk management has been implemented by the Botanic Gardens and Parks Authority (BGPA). Slope instability along the Escarpment has developed largely as a result of man-made interference with the slopes, in the form of road cuttings, quarrying, logging, excavations and development, which date back to the early 1800′s. The main types of landslides consist of rockfalls and debris slides, with the principle trigger mechanism being rainfall. The weak and variably cemented limestones or aeolianites are subject to ongoing deterioration and erosion, which results in dynamic and continually changing slopes and development of unstable conditions.

To control and mitigate against the effects of the landslides, the BGPA has undertaken extensive remedial measures in the form of scaling operations, revegetation of the slopes, drainage control, review and modification of public access, underwalling, erection of rock catch fences, drape meshing of potentially unstable boulders and regular monitoring and inspection of the slopes. Remedial measures are undertaken in accordance with strict cultural heritage requirements and environmental controls. The presentation will review landslide risk assessment at Kings Park and the management and mitigation measures used to control them.

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This annual event gives a platform for our younger (under 35) geo-professionals to present their work, gain feedback from experienced colleagues and become more involved in AGS activities. Please support them by coming along to what should be a varied, interesting and exciting evening. There will be four presentations, each 10 minutes long, with 5 minutes for questions after each talk. The Dr. Baden Clegg Award will be awarded to the best contribution.

The 2011 Dr. Baden Clegg award was made to Pauline Truong.

Simon Hope, Golder Associates

An assessment of an effective dynamic shear modulus value for dynamically loaded ore-processing infrastructure

With the continuation of Western Australia’s resources boom, the construction of specialised mining infrastructure in remote locations provides interesting and unique challenges to geotechnical engineers. Large ore-processing structures, such as screening plants and crushing facilities, are designed to vibrate as part of their routine operations. These vibrations force the footing to undergo small-strain dynamic loading that can potentially result in large differential settlements or bearing capacity failures if not founded on suitable material.

A site investigation comprising geotechnical drilling and geophysical MASW testing was performed for a series of dynamically loaded ore-processing structures for a potential mine site in the Pilbara region of Western Australia. The results of the investigation were used to assess the dynamic shear modulus of the in situ soil at each structure location.

To assist the structural and mechanical design of these dynamically loaded structures, an effective dynamic shear modulus value was required to simulate the soil-structure interaction. This effective dynamic shear modulus was calculated using a weighted average of the stress distribution below the footing and by finite layer elastic analysis. For instances where the dynamic shear modulus of the in situ soil was insufficient for design purposes, an assessment of the required depth of ground improvement was also performed as part of the analysis.

Mira Lee, Arup

Effective Stress versus Total Stress Analysis of undrained problems in geotechnical engineering

Stability and strength analyses in geotechnical problems can be carried out in terms of either effective stresses or total stresses. Given fundamental knowledge of soil mechanics principles and clear understandings of numerical modelling and its limitations, the numerical simulations should result in consistent outcomes from both approaches. Undrained excavations were modelled in ABAQUS, a software application for finite element analysis. The extended Modified Cam Clay was used to model the soil behaviour in the Effective Stress Analysis (ESA) and the Tresca model was used in the Total Stress Analysis (TSA).

For the comparison between ESA and TSA to be valid, it is critical for both analyses to represent identical soil conditions and characteristics. Therefore, the fundamental part of the analytical procedures was to derive the values of total stress parameters from the effective stress parameters and numerical outputs from ESA. In order to confirm the rigorous match of soil conditions between ESA and TSA, initial stress distributions and the initial values of K0 were compared.

The shape of yield surface in ESA was modified to overcome the differences in the yield surface for ESA and TSA. The values of su were also adjusted to reflect the shear strength in the plane strain problem. While those modifications improved results, most of the numerical outputs showed inconsistencies between ESA and TSA. By comparing the maximum values of forces and moments of structural elements, neither method produced results that are consistently greater than the other method throughout all excavation scenarios. It was justified that the differences in the structural forces and moments were mainly due to the differences in the passive stress on the retaining wall between ESA and TSA. The observations from stress paths at those passive soil elements revealed that the passive soil for all ESA did not reach the critical failure state, and for TSA, the soil reached the failure for cantilever problems, but not for propped excavations.

Mark Orr, Worley Parsons

Geotechnical Investigations for a large mining project in West Africa – A case study discussing the challenges faced and lessons learnt

Geotechnical investigations have recently been completed for a mining project in West Africa.

The proposed new iron ore mine is located some 200km inland and will be linked to a new port by approximately 220km of railway.

The presentation provides a summary of the project and discusses the geotechnical investigations that were conducted at the port and along the rail corridor. It also summarises the difficult ground conditions that were encountered, particularly at the port where superficial deposits were found to be underlain by a thick sequence of highly variable marine, estuarine and fluvial sediments.

Some of the obstacles faced during the investigations included challenging site and weather conditions, a lack of available equipment and skilled labour, poor existing infrastructure and a tight project schedule. The talk will discuss how these challenges were overcome without compromising the quality of data obtained or significantly impacting upon the project programme.

It will also reveal how a large amount of effort was spent ensuring that the local community were agreeable with the investigations proceeding in their backyards (literally).

Finally, a number of valuable lessons learnt during the course of the project are presented – with the objective of assisting the planning and execution of future investigations in similar parts of the world.

Pauline Truong, Arup

Impact of spudcan footprints on an offshore gravity base structure

A riser collector offshore steel gravity base structure (GBS) was installed early this year for a gas development project. The foundation is a square annulus (external dimensions 54x54m, internal dimension 34x34m) with 5m deep internal and external skirts forming 12 compartments to allow for suction installation. Arup (including the author) has been involved in the structural and geotechnical detailed design of the GBS since 2007 and have been providing technical advice throughout the construction and installation phase. In 2009, after completion of detailed design, a jack-up rig servicing the adjacent drilling platform was accidentally installed within the GBS designated site. The jack-up left footprints of three holes in the seabed each with a diameter of about 20m and a depth of about 3m. The client commissioned an additional scope of work for Arup to assess the impact of the spudcan footprints on the GBS foundation and to consider remedial measures.

The spudcan impact investigation was conducted over a period of one year. This included a preliminary assessment phase where a literature study, a centrifuge test, 3D and 2D finite element analyses for stability and settlement, and DNV based installation analyses were conducted. It was found that the GBS could no longer be installed in its proposed location and required reorientation to avoid intersection of the spudcan holes. A geophysical survey, site investigation and further geotechnical analyses were then conducted to assess the foundation at its new location.

Due to time constraints and lack of data, the geotechnical analyses were simplified to assess (i) squashing of heave mounds created by the spudcan which intersect portions the foundation, (ii) additional skin friction on the skirts from the heave mounds, (iii) stability issues due to weakened soil zones, decrease in overburden and lateral resistance, and (iv) differential settlement on the now uneven seabed. Final results indicated that the structure could be safely installed and would be stable over its design life.

This presentation includes an overview of the above events, the findings of the subsequent assessments and concludes with a successful platform installation.

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The population of Christmas Island has trebled in recent years following the arrival of asylum seekers arriving in boats from Indonesia. Designing a WWTP for a largely itinerant population has presented a serious challenge to Water Corporation engineers. The project comprises upgrade of the existing WWTP at Smith Point, a new pumping station at North West Point, and a new pipeline from NW Point to Smith Point.

The geotechnical design issues included:

• the presence of deep vertical fissures in karstic limestone formation;
• rearrangement of the plant layout due to a large blowhole fissure at the clarifier tank;
• cutting into the vertical limestone cliff to accommodate the sludge drying beds;
• foundation design to span a narrow fissure intersecting with clarifier tank no. 2;
• discharge of the treated wastewater into a narrow fissure within the site; – rockfall risk assessment for WWTP and pipe rack;
• pump station and overflow storage tanks founded on fill and pinnacle limestone; and
• surface drainage for pump station facilities into a local doling.

The talk will cover the geotechnical investigation techniques used by the Water Corporation and others as well as presenting photographs of the geotechnical features.

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The talk will outline the different factors to be considered in the design of flexible pavements for Ports, Mine Haul Roads, Airports and low volume Highways and Local Roads. The special case of pavements over highly expansive clay (Gilgai) will be discussed. On solar salt crystallizers, the crystallized salt forms the pavement supporting harvesting and haulage equipment. An approach to assessing the load carrying capacity of crystallized salt pavements will be presented.

The origins of the current Austroads pavement design procedure and its limitations will be examined.

Extensive use is made in Western Australia of low cost marginal quality natural gravels for construction of pavements on low volume roads. An explanation will be given about the origin of WA selection criteria for natural gravels. The reasons why the criteria are different for laterite, ferricrete and calcrete gravel compared to crushed igneous rock, will be outlined. The relationship (where one exists) between the various tests and pavement performance will be discussed. Have you ever wondered whether it really matters if the material being used has a plasticity index or maximum dry compressive strength that is outside the specified range?

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The Western Australian Chapter of the Australian Geomechanics Society (AGS) is pleased to provide notice of a Seminar on Geotechnical Near-Misses and Failures. This seminar will present local and international case studies dealing with near-misses and failures in the geotechnical field. It is aimed at all geotechnical professionals, ranging form students and recent graduates to experienced practitioners (consultants and contractors). The objective of this seminar is to provide an open and impartial forum to share experiences and discuss lessons learned.

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The somewhat dismaying typical failure rates for projects with a heavy geotechnical component, and the reasons for those failure rates, will be discussed. The sources of the geotechnical risks that often lead to failure will be described and related to factors as diverse as the ground conditions, technical and analytical approaches, management structures and project procurement strategies.

Case histories involving major civil, mining and oil and gas projects will be presented (anonymously) to illustrate the authors views, some lessons learnt and the techniques that can be used to try to avoid future failures on major projects.

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