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Physical modelling of soft clay using Vacuum-Surcharge Method
In this paper, the large-scale consolidation apparatus was employed to investigate the performance of vacuum preloading with conventional surcharge loading. Several tests were conducted to examine the effect of a vacuum and associated parameters such as smear zone and soil permeability characteristics. The measured settlement and excess pore pressure indicates that the vacuum application has certain benefits to control excess pore pressure for a given total loading application. The analytical modelling for radial consolidation and vacuum preloading considering both variation of soil compressibility and permeability was employed to predict the soil consolidation behaviour. It is shown that the analytical model can reasonably capture the laboratory behaviour.
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Modelling and uncertainty analysis of softening behaviour of sand using disturbed state constitutive model
An evaluation of the predictive ability of a disturbed state constitutive model for softening behaviour of sand is performed. The model is shown to capture the softening behaviour of sand under both drained and undrained monotonic loading conditions. To use a particular constitutive model into a finite element or finite difference numerical procedure to solve real geotechnical problems, it is often desirable and necessary to include the variability in constitutive model parameters to capture the effects of model imperfections and inherent soil variability. Uncertainty in stress strain characteristics of sand due to inherent soil variability and model parametersโ variability are analysed. The sensitivity or the effects of disturbed state constitutive model parameters on stress dilatancy characteristics is stochastically analysed using first order second moment method to include the effects of model error and inherent soil variability. The most influencing parameters for both the deviatoric stress and volumetric strain are identified.
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In situ dissipation testing of soft soil under reclamation fills
In situ dissipation tests provide a means of evaluating the in situ coefficient of consolidation and hydraulic conductivity of soft clays due to horizontal flow. Dissipation tests using piezocone (CPTU), dilatometer (DMT), self-boring pressuremeter (SBPT) and BAT permeameter (BAT) were utilized in the characterization of the coefficient of horizontal consolidation and horizontal hydraulic conductivity of Singapore marine clay at Changi in a land reclamation project. Dissipation tests were carried out to compare the changes in the coefficient of consolidation and hydraulic conductivity due to horizontal flow prior to reclamation and after ground improvement. Tests were carried out in a Vertical Drain Area as well as in an adjacent untreated Control Area, 23 months after preloading for comparison purposes. The purpose of this research is to determine the consolidation parameters of soft soil before reclamation and after 23 months following preloading with and without vertical drains by means of in situ dissipation tests. This should indicate the change in these consolidation parameters associated with void ratio changes caused by the consolidation process under the same magnitude of load at different degrees of consolidation.
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Rock strength and the coring interface
Boreholes in rock usually involve drilling to practical tungsten carbide bit refusal and then changing the drilling technique to rock coring. Often this change is used as an indication of a change in rock weathering or strength. Before the commencement of rock coring the SPT result would typically be to refusal. After obtaining the rock core, point load index tests are used to classify the rock strength. This paper examines the validity of the usual assumption of a change in rock property at this drilling interface. The results show that a drilling interface does not necessarily translate into a geological interface or change in rock properties. The current definition of SPT refusal level in rock is shown to be inadequate, and the strength interpretation of such tests in rock is dependent on the rock type and defects of the rock.
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Woolgoolga to Ballina Pacific Highway Upgrade โ Embankment Performance Monitoring and Back-Analysis
The section of the Pacific Highway Upgrade project between Woolgoolga and Ballina (W2B) traverses areas having significant depths of soft soils. The presence of these soft soils causes significant risks in relation to timely project completion and potential cost blow out. To reduce these risks, soft ground treatment using wick drains and preloading was used in the form of Early Works to de-risk the project and to reduce the post construction settlement. During the embankment construction and preloading period, instruments were installed to monitor the embankment performance. Monitoring was carried out using an innovative web-based system to enable continuous and real time monitoring of embankment performance, technical review and modifications of construction sequence as required.
Monitoring data were back analysed periodically to assess the embankment performance. Through this exercise, additional filling and stability measures were implemented over a number of embankment sections, where the monitored embankment settlements exceeded the design prediction. In addition to the innovative web-based monitoring system, advanced numerical modelling was used to predict future creep settlement using the back-analysed results. It was found that advanced numerical modelling of creep settlement must carefully consider the two-dimensional stress distribution beneath the embankment including toe berms in order to model accurately the stress history and over-consolidation effects on creep after surcharge removal. When the two-dimensional stress distribution is properly taken into account, the results of forward creep prediction matches well with one-dimensional analytical methods such as that proposed by Wong (2010).
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Increasing Consideration Of Sustainability And The Implications For The Resource Sector
The issue of sustainability and sustainable development is now one of the most hotly debated issues that faces the Resources Sector. It has the potential to affect every aspect of the industry, and its operations and rights to carry out exploitation of natural resources. This paper looks at the growing need for the West Australian Resources Sector to address sustainability and sustainable development requirements in light of increasing scrutiny and consideration by the community as well as regulatory authorities. In the last few decades there has been considerable improvement in environmental performance by the Resources Sector, which has included community involvement. However, scepticism remains as to whether the Resources Sector has truly embraced the concept of sustainability and sustainable development and their willingness to deliver on their words and commitments.
This paper endeavours to provide a brief background on sustainability, the concept and what direction industry, community, government and individual professions may pursue in dealing with this issue. It draws upon recent resource projects and various initiatives that have identified the need for change and the likely direction. In conclusion, it explores the need for individual professions and disciplines as well as industry to take a proactive approach in fostering the sustainability concept.
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Design Of A Piled Raft Foundation For Twin Gold Coast Towers
Golder Associates Pty Ltd (Golder Associates) Melbourne office was commissioned by Grocon Pty Ltd (Grocon) to assess the feasibility of engineering a piled raft foundation for the twin 50 storey and 40 storey towers of the Oracle.
The twin towers are to be joined by a multilevel basement and a podium structure. A three level basement excavation is to be up to 10 m deep and is to be supported by a driven and anchored sheet pile wall.
The 3 level podium structure and the towers were initially proposed to be supported either on bored or continuous flight auger (CFA) piles some of which were required to resist permanent uplift loads. The site investigation revealed the presence of a stiff clay layer up to 10 m thick which was present above the weathered siltstone which underlies the site at depth. Up to 15 m thickness of dense to very dense sand extends from basement excavation level to the top of the stiff clay.
Initial geotechnical calculations, carried out by others, indicated that a raft founded on the dense sands would settle between 280 mm and 550 mm under the centre of the 50 storey tower. It was therefore recommended that the towers be supported on bored piles founded into the lower siltstone.
A later analysis of the performance of a piled raft carried out by Golder Associates, indicated that acceptable piled raft settlement performance and bearing capacity could be obtained without the need to extend piles to the rock. A more rigorous analysis carried out for final design, including additional geotechnical investigations and laboratory testing of the clay material, confirmed that a piled raft would provide acceptable settlement performance providing the piles were extended to below the clay layer.
The piled raft analysis provided the structural engineers with estimated pile and ground stiffness values that were incorporated into the structural design models used to predict design actions for the raft and the piles.
The design performance of the piles was confirmed by high strain dynamic pile testing and initial on site monitoring of the CFA piles to confirm that the piles were being constructed to the design depth and with full penetration of the clay layer.
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Introducing Geotechnical Design to Shallow Geothermal Systems
2012 John Jaeger Lecture
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Geotechnical risk: Itโs not only the ground
A significant amount of geotechnical risk discussion relates to ground model development and the identification of geotechnical hazards. However, there is a considerable amount of risk hidden in the interpretation of parameters, design methods adopted, software used, and even the team members chosen for a project. This paper presents a discussion on these challenges and highlights the need for better communication of risk to other designers, contractors and clients. Technical and team management recommendations are made.