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AGS SA-NT Annual Seminar 2018
Design and construction of earthworks and pavements on expansive clay
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Confined and partially confined swelling pressure of basaltic clays
This paper looks at the swell pressures developed in basaltic clay under confined and partially confined conditions, including the pressure reduction with expansion of the soil. The paper makes a comparison of the measured values with those presented in literature for similar materials and looks at the suitability of empirical equations from literature for the estimation of confined swell pressure in Melbourne’s basaltic clays. The paper also considers the potential applications for the findings in design and relevant considerations with regard to design in accordance with Australian Standards.
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In ground concrete durability
Durability analysis is often a difficult part of the design for structures subject to severe exposures. Project specifications often quote national standards as a minimum requirement and require a specified design life be achieved. This paper shows how national standards can be conflicting and may be inadequate with regard to design for concrete in contact with ground. This places the responsibility on the contractor to ensure that state of the art durability designs will provide the serviceability required over the design life.
The paper specifically considers in-ground deterioration applicable to loaders, linings, railways, slabs and ports. Deterioration mechanisms considered include cracking, corrosion, and chemical attack (including acid sulphate soil issues). The rate of decay is dependent on the access and penetration of contaminants and their subsequent reaction with the concrete and steel. The paper reviews these processes to give a logical approach to design. The importance of cement systems to provide chemical resistance, mix design to provide impermeable paste and aggregate selection to avoid integral problems are discussed.
Even so, the best design will fail if construction is not adequate. The paper also outlines specific concrete properties (e.g. bleed, setting time, slump loss, heat of hydration, shrinkage) that the Contractor needs to understand and use in planning concrete pours so that construction defects do not compromise durability objectives and lead to expensive repairs.
Authors’ note: Throughout this paper, we employ the use of capitalised text to denote the various roles and elements that contribute to a major project’s success, in particular for durability management.
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Spoil piles – comparison of stability analysis methods
A study has been undertaken to confirm appropriate analysis methods applicable to design of granular waste materials and specifically that of “spoil piles” for open pit coal mining. The authors’ experience with spoil pile stability is that the critical failure mechanism is a two wedge mechanism in accord with that highlighted in the literature. Analysis requires careful consideration of the combination of spoil shear strengths and the angle of the “rear” scarp adopted. Moreover, the literature indicates analyses need to take account of the inclined “inter-slice” within the mechanism. To highlight these considerations an example case was assessed utilising five limit equilibrium stability packages, two “numerical based” packages and a spreadsheet developed by the author. Several analyses are based on vertical slices to pose the question: what is the magnitude of error not using an inclined “inter-slice”? The results are presented as overall spoil pile angle indicated by the use of different software, stability method and assumptions on inclination of “rear” scarp and inter-slice. The example case highlights that, provided a rigorous stability method is utilised, for a given rear scarp assumption, the results of the analyses are insensitive to the software utilised. Therefore with appropriate judgement assessment of spoil pile stability should not be predicated by use of one particular software or stability method.
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Some Design And Construction Issues In Deep Excavations And Shoring Design
This paper presents, firstly, a critical review of the current codes of practice on design and construction for retaining structures. The key factors impacting on the design and construction of shoring system is examined and discussed in detail. The importance of groundwater pressure on the design of a retaining structure is highlighted, which is as much as three times of the active soil pressure. Some design issues with respect to the SLS and ULS load combination and associated analysis are discussed through practical examples. A real case history in Sydney due to a burst water main behind the retaining wall is presented to demonstrate the significance of the groundwater pressure on deep excavation shoring design. A detail design approach is proposed as to how best the retaining wall structures could be designed to take account of the accidental water pressure induced by a burst water pipe. It is the Author’s opinion that this proposed approach will provide useful guidelines to the future retaining structure design.
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Innovative Design Of Reinforced Soil Wall On A Steep Slope Subject To Land Slip Risks
This paper presents a case study of an innovative reinforced soil wall (RSW) design on steep slope using site-won material that is not in full compliance with Roads and Maritime Services (RSM) QA specification R57. The steep slope was impacted by a landslip occurred in February 2010. Firstly the local geology and the landslips occurred along the project corridor are briefly described. The key design and material requirements for RSW have been reviewed with respect to the use of site-won material. A detailed strategy is presented on how to deal with the potential risks of using the non-compliance site-won fill material to the specification R57. A comprehensive laboratory testing regime of the site-won material and large-scaled pullout tests of RSW reinforcement were undertaken. With the test results and engineering judgement, the design was proceeded with the following key assumptions/factors: 1) the allowable fines (<75 microns) content of up to 25%, clay (<2.4 microns) content of up to 7%; 2) the use of reduced friction angle of 30 degrees; 3) a 10% reduction in the calculated pullout capacity of reinforcement; and 4) 1.5 times sacrificial allowance as per R57. These were to cater for long term corrosion on steel reinforcement and to ensure longevity and integrity of the RSW. A heavy rainfall in February 2010 resulted in two significant landslips within the project corridor, with the larger one being immediate at the down slope of the reinforced soil wall (RW01). This event required us to carry out an additional geotechnical investigation and landslip remedial works to ensure the long term global stability of RW01. Three dimensional effects were considered in the assessment of the slope instability in the construction stage review.
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Unsaturated free-standing mainline railway embankments – Part 1: Can you handle the awkward truth?
The presence of negative pore pressures within cuttings and embankments, and their benefit to the reduction of the likelihood of instability, have long been recognised by the Geotechnical profession. Negative pore pressures are usually a consequence of natural environmental influences upon soils, clay in particular, and are frequently (and perhaps misleadingly) termed “soil suction”.
The recognition of pore suctions in the assessment of potential instability of embankments by way of conventional stability analyses is not common. Tools are becoming available to conduct such analyses once the boundary conditions and parameters involved are understood. Measurement of suction in-situ is beginning to assist in the identification of reliable suction values suitable for use in such analyses.
In this paper, the authors develop a philosophy to include suction and present a defensible model for the analysis of free- standing embankments. The impetus for the work was the need to demonstrate via analysis that Main Line Railway embankment infrastructure within NSW was not in jeopardy, as was indicated by “the usual” saturated soil mechanics approach. Discussion includes the challenges posed to realistically implement and consider the benefits resulting from recognition of the “known” suctions.
The paper is intended to alert the profession to an assessment technique that includes the benefit of these suctions.
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Laying the foundation for the Sydney Light Rail
The Sydney Light Rail project alignment covers the typical geological and geomorphological settings for the Sydney and Botany Basin. This comprises three distinctive zones: (1) manmade fill; (2) recent Quaternary aeolian, alluvial and estuarine sediments, which overlie; (3) residual soil and weathered to fresh bedrock. When investigating subsurface conditions within a metropolitan area, various constraints can have impacts on the scope and extent of geotechnical investigations that can be safely and practically completed. For the SLR project, investigation of the subgrade conditions along the alignment was significantly constrained due to the route which follows existing roads through the CBD and eastern suburbs. Constraints for working in these areas included complex traffic management planning to avoid disruptions to the community and also an equally challenging network of underground utilities and other service tunnels. These constraints required thorough planning for traffic management and consultation with utility service providers, thus significantly reducing the amount of work that could be completed within the program. A robust geotechnical validation regime was developed so that the in-situ subgrade could be re-assessed prior to track slab construction, to mitigate the limited extent of geotechnical investigation undertaken. The objective of the site validation regime was to verify the inferred design subgrade California Bearing Ratio values along areas where geotechnical investigation was constrained. The site validation regime included nominally spaced plate load testing, dynamic cone penetrometer testing and laboratory CBR testing to assess the subgrade CBR value. This paper explains the challenges faced in validating in-situ CBR in different subgrade conditions, appropriateness of published DCP to CBR empirical methods and the approach using calibrated site specific DCP-CBR correlations.
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Rate effects in centrifuge piezocone penetration on dilatant silica silt and subsequent dissipation
Centrifuge testing on industrial silt-sized silica particles was performed to investigate the possibility of using variablerate piezocone tests to gain additional data in soils. In order to perform partially to fully drained piezocone tests in the saturated silica samples, a careful combination of penetration rates, probe diameter and coefficient of consolidation of the centrifuge samples was assigned. The piezocone tests were then performed at 50g with a 12-mm diameter piezocone at different penetration rates. Measurements of excess pore pressure ∆u in the samples allowed the assessment of the radial distribution of pore pressure during penetration, where the tendency for the silica flour to dilate was observed. In general, negative values of ∆u were measured near the piezocone, whilst positive ∆u values were measured in the far field. The data were then used to describe the behaviour of dilatant soils with increasing penetration rate of the probe, and its shortcomings in the soil penetration.