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Foundation Design For The Marina Barrage, Singapore
Singapore’s Marina Barrage is a major government project to provide a multi-purpose urban reservoir integrating flood control, water supply and lifestyle attractions in the heart of the city of Singapore. The SGD$226 million project includes the construction of a 305m long barrage at the mouth of the Marina Channel to form a freshwater reservoir and will play a pivotal role in enhancing Singapore’s water supply.
This paper describes how the engineering challenges for the geotechnical aspects of this project were addressed, including the detailed geotechnical analyses and design of the Barrage piled foundation, comprising over 275 bored piles of diameters ranging from 1 m to 1.5 m. With such a large number of piles of different diameters, the challenge was to simulate the pile group interaction effects under both lateral and vertical loading (including negative skin friction due to the consolidating soft Marine Clay deposits) in order to optimise the pile group design.
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A correlation for the shrink swell index
Site classification to AS2870-2011 requires the soil shrinkage index, Ips. A common assessment method is to carry out a shrink swell laboratory test to calculate the shrink swell index (Iss), which is then used to develop the soil shrinkage index. To undertake a shrink swell index test, an intact thin wall tube sample of cohesive material is required. In hotter and drier areas of Australia, including northern Victoria, it can be difficult to recover intact cohesive samples. The geotechnical practitioner is often required to consider other methods to estimate Iss in order to provide the required site classification. This paper considers a variety of laboratory tests results including Atterberg Limit, Particle Size Distribution, Hydrometer and Iss results from clay samples within Victoria (including Melbourne) to provide an improved correlation to estimate Iss when thin wall tube sampling is not practicable. The data presented to support the correlation includes published test results by others and unpublished test results from projects the author has been involved in.
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Analysis of installation forces for helical piles in clay
Installation forces play a central role in the design and performance of helical piles, especially since the installation torque is often used as an indicator of the pile’s ultimate capacity. This paper presents an analytical model for predicting the installation torque for single-helix piles in clay. As an extension of a recent study by the authors, the proposed model considers not only the forces occurring on the helical plates but also the shear stresses generated along the shaft, both of which impact the installation forces. The model yields a straightforward expression that relates installation torque to the undrained shear strength of the soil, embedment depth, helix diameter and pitch, shaft diameter, crowd (axial) force, and adhesion coefficient along the shaft. The influence of these factors on the installation torque, as well as the “capacity-to-torque ratio” used to infer capacity from the installation, is assessed through a sensitivity analysis. Some level of validation is provided through a comparison with empirical capacity-to-torque ratios, and the sensitivity analysis reveals factors that are neglected in empirical models but nevertheless have a significant influence.
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Ingenuity And Intelligent Risk Assessment For Resilient Geotechnics
Geotechnical engineering is a risky business and there is much that can, and does, go wrong. It is often said that the single most common cause of failure in construction (including delays and additional costs) is in the ground. This would indicate that the logical path to design more resilient infrastructure would be the adoption of over-conservative designs. However, the in-ground structures that collapse often have a number of fundamental and basic design flaws. In reality, most in-ground structures move considerably less than predicted at design stage, suggesting that they were, in fact, over-designed. Over-design can also be considered a form of failure as it can add cost and delays in construction. It is generally accepted that a resilient piece of infrastructure is not necessarily one that does not fail upon a catastrophic event, i.e. one that is overdesigned to withstand such event. Otherwise the concepts of sustainability and resiliency would be conflicting. A resilient design is one that does not cause significant disruption to the community and can function effectively as quickly as possible after the catastrophic event.
So, how can geotechnical engineers achieve resilient infrastructure designs? The best approach seems to be associated with intelligent risk assessment that is based on an in-depth understanding of how such a design will perform before, during and after a catastrophic event. This approach requires good knowledge of the fundamental principles of geotechnical engineering such as solid mechanics, geology, failure mechanisms and so on. This paper will discuss some of the requirements for intelligent risk assessment and presents a practical example of an approach that could be adopted for the design of resilient infrastructures. Its primary focus is on the anticipated performance during a potential failure and the intelligent risk assessment forming the basis of the entire design.
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Design solution to a heritage piled rail bridge foundation using numerical modelling technique
This paper presents a case study of replacement works for a more than 100-year old rail bridge over Guess Avenue, Wolli Creek, Sydney. Firstly, the as-built drawings for the existing railway bridge and brick abutments were reviewed and a summary of findings is presented. A geotechnical investigation program was devised to assess the existing timber pile and pile cap conditions below the brick abutments, which is heritage listed, and the subsurface geological profile for the bridge site. The preferred option using filler beam units enabled the clearance of the new bridge above the existing road to be increased from original 4.13 m to 4.6 m, which is in compliant with the current Australian bridge standard AS5100. The ultimate capacity of a single pile was assessed as the lesser of its structural capacity and geotechnical capacity using the obtained investigation results; the geotechnical capacity of the pile governs. A factor of safety of 2.5 was considered appropriate to evaluate the adequacy of the pile capacity under the existing bridge and future bridge loading conditions. The pile loading was initially assessed using program Piglet based on the as-built drawings and the findings of the geotechnical investigation. Due to much higher loads calculated for the edge piles using the Piglet program which exceeds the assessed capacity of a single pile, Plaxis 3D modelling was carried out for both existing and the future bridge loading conditions. The results of Plaxis 3D modelling indicate that the piles are unlikely to be over-loaded beyond the assessed capacity of a single pile. Based on this comprehensive assessment of the pile load and the use of the piled raft concept we recommended to Sydney Trains that the existing pile foundations and brick abutments be used for the future bridge without any underpinning or foundation strengthening. At the time of writing this paper the bridge replacement has been completed, with the monitored movement of abutments being within the prediction.
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Settlement Prediction – How Much Is It A Matter Of Luck?
Accurate prediction of settlement remains one of the most difficult tasks geotechnical engineers have to contend with. Typically, settlement prediction is undertaken using a deterministic approach. This involves the idealisation of a soil profile as one or several uniform layers each associated with a set of model parameters. This idealisation glosses over the spatial variations of parameters within each layer. Much of the hit and miss in settlement prediction is tied to the uncertainty or inadequate knowledge regarding the spatial variation and how to reasonably model these variations.
This paper attempts to quantify the amount of luck associated with a prediction. It is shown that whether the design uses the mean, or some other percentile value of the soil parameter, spatial soil variability renders the deterministic prediction incapable of providing any information about the likely error of the prediction. Accordingly, it is a question of luck should the prediction coincide with the observed settlement. In order to quantify the probability (or luck), the maximum settlement of a flexible circular footing was evaluated using elastic theory and Monte Carlo simulation of randomly variable Young’s modulus with depth.
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Sugarloaf pipeline landslide risk management and planning approvals
The Sugarloaf Pipeline Project was a major infrastructure project completed in early 2010. The project required planning approval at all three levels of government including a Landslide Risk Management Plan. The Project Landslide Risk Management Plan was completed in accordance with AGS 2007. This paper discusses how the AGS guidelines were applied on this project including discussion on the risk assessment process, mitigation measures adopted and lessons learnt.
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Hydrogeological Assessment For A Land Reclamation Dewatering Operation
The Kingston Foreshore Redevelopment was an urban renewal project in the suburb of Kingston on the southern shore of Lake Burley Griffin in Canberra. It involved reconfiguration of the foreshore by the excavation of part of the existing foreshore and the reclamation from the lakebed to form a harbour. Conventionally, reclamation process for “wet conditions” involves end dumping of granular fill, with subsequent vibro-compaction or dynamic compaction stabilisation. However, this method would not be possible since local sources of granular fill were not economically viable. Moreover, while the excavated materials from the existing foreshore were expected to be firm to stiff clays, the dumping of these excavated “lumpy” clays into the ponded reclamation areas without compaction, and the reliance upon subsequent preloading for stabilisation, would be difficult and problematic. The more feasible option in the geotechnical context was the “dry reclamation”, where the reclamation areas was dewatered and fill placed in the dry. This approach required that the reclamation areas to be enclosed and water be pumped out and discharged into the lake. The selection of a dewatering system would in turn be a function of the hydrogeological model and the level of drawdown required.
This paper focuses on the hydrogeological model and properties of the site that were considered to be critical for the assessment and design of the construction options. The assessed hydrogeological model in the foreshore areas comprises an upper fine-grained alluvium, which acts as an aquitard to restrict the flow of the water from the lake to the underlying gravel/sand aquifer. The gravel/sand aquifer is therefore likely to be semi-confined. Laboratory tests and full scale pumping/recovery tests were undertaken to estimate the permeability and storativity of the gravel/sand aquifer. Back-analysis of the pumping test results indicated that the permeability values derived from various analytical methods such as the distance drawdown analysis and the recovery analysis compared reasonably well with those estimated using the more simplified Hazen’s (1911) empirical method, which was related to the particle sizes of the gravel/sand materials. No in-situ permeability test was conducted in the upper alluvial aquitard. The hydrological conductivity of this layer was instead inferred from the dissipation test results obtained from CPTU soundings. It had been shown that the inferred permeability of the upper alluvial aquitard compared reasonably well with published correlations.
A number of dewatering schemes were assessed by undertaking 2D Finite Element Analysis (FEA) to check the viability of various options and sensitivity to changes to soil permeability properties. A summary of the various FEA results is presented.