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Geotechnical Designs Gone Wrong — Lessons Learnt
Since the occurrence of several apartment building problems in recent years, government agencies have taken steps to promote a greater level of building design and construction compliance. For example, the Design and Building Practitioners Regulation 2021 (NSW) that came into force on 1 July 2021 requires engineering reports associated with Class 2 buildings (i.e. residential or part-residential) to be signed by a registered Design Practitioner with relevant qualifications and skills, and a completion compliance certificates to be issued by the various design discipline practitioners of the project and submitted by a registered Professional Engineer.
Perhaps time will tell, but the author is sceptical that the introduction of additional government regulations would necessarily reduce the number of failures in geotechnical practice. The author believes that it may be helpful for the industry to publish more case studies on geotechnical failures so that lessons can be learnt from common mistakes.
This paper outlines some common geotechnical design problems encountered based on 50 or so expert witness cases that the author has been involved in during the last decade, and provides a generalized discussion on some of the most prevalent geotechnical design issues and the lessons learnt.
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Backfilled Quarry Development with Inbuilt Landfill Gas Solution
Backfilled Quarry Development with Inbuilt Landfill Gas SolutionA basalt quarry north east of Melbourne operated for over 30 years from the mid-1960s. Rock extraction occurred to a depth of approximately 28 m below ground level with dewatering. Since operations ceased in 1999, the quarry has been progressively backfilled with variably compacted ‘clean fill’ until 2017 and the groundwater level allowed to rebound close to its former level. Potential development of the land, for residential or mixed land use, triggered geotechnical and environmental investigations with the objective of identifying and managing below ground issues to support a planning rezoning process. Based on the investigation results, some of the key considerations for future development of the site were settlement of the backfilled quarry, foundation system options across the filled quarry extent and management of landfill gas. Development time, future ongoing management pressures and the need for large scale ground improvement, led to the consideration of a landfill gas control barrier integrated within the engineered fill ground solution, and an on-site trial. This paper provides the findings from the surcharge trial to date to combine the reduction in time to achieve an acceptable settlement of the fill and the management of entrapped landfill gas.
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The role of proof rolling in pavement construction
Proof rolling of pavement subgrades is a key construction operation for all pavements but despite this there appears to be confusion about the process, the plant used and the evaluation criteria which is leading to subgrades that are adequate being condemned and having to be removed and replaced by stronger materials. The paper looks at the process and whether the plant currently being used as the proof roller is appropriate. -
Strength and compressibility behaviour of lime-slag-treated Coode Island silt
This paper presents the strength, compressibility and microstructural behaviour of lime-slag treated Coode Island Silt (CIS). The microstructure was investigated using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), while strength and compressibility properties of the treated CIS were measured using Unconfined Compressive Strength (UCS) and Oedometer Consolidation tests. Bulk untreated CIS is sampled from an excavation site in Melbourne CBD, while hydrated lime and Ground Granulated Blast Furnace Slag (GGBFS) are used as binders. Results of UCS tests on CIS treated with fixed lime and varying slag contents showed an increase in unconfined compressive strength with increasing slag contents and curing periods. The role of lime in the activation of slag is also seen to have an upper limit whereby excess proportions fail to provide significant additional benefit. Oedometer consolidation test results of treated CIS showed an increase in apparent pre-consolidation pressure and decrease in void ratio with an increase in slag content. This is due to the formation of more crystalline cementitious products, which accelerate the cementation effect between lime-slag and clay particles. XRD analyses of lime-slag treated clay showed the existence of numerous reaction products such as Calcium Hydrate (CH) and Calcium Silicate Hydrate (C-S-H) which facilitates the strength increment. Slag was found to be very active in promoting the pozzolanic reaction with lime, as evidenced from the presence of crystalline reticular cementitious product (C-S-H) in the SEM images.
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New piling code
Gary Chapman
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The design and construction of the 16 hectare basement at Marina Bay Sands, Singapore
Jack Pappin, Arup Fellow
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Effect Of Clay Structure Degradation On Settlement Of Embankment
Due to the degradation of initial structures, soft clays can experience significant settlement without much change of pore pressure. Such a phenomenon has recently been observed at a fully instrumented trial highway embankment near the town of Ballina (New South Wales, Australia). In contrast, the displacements in saturated soils without structure (such as a fully remoulded clay) are always associated with the dissipation of excess pore pressure through the effective stress principle. This paper demonstrates the effects of clay structure degradation on the settlement of embankments, through numerical analysis of a trial embankment on soft clay. A constitutive model that accounts for destructuration of soils is used to characterise the behaviour of the soft clay. The material parameters are derived from conventional oedometer and triaxial tests for the structured constitutive model. It is shown that the settlement and the dissipation of excess pore pressure during construction of the embankment are closely related with the destructuration of the soft clay. With appropriate choice of the constitutive model and material parameters, the lag between the settlement and pore pressure dissipation can be well predicted by the coupled finite element method based on the effective stress principle and consolidation theory.