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Prediction Of Groundwater Impacts For Excavations
Groundwater inflow and levels associated with excavations are important considerations for construction and operation of the completed development. Drawdown associated with excavation has potential to result in settlement, impacts on vegetation and ecosystems and the distribution of existing groundwater contamination. Inflows to excavations need to be dealt with including possible need for treatment prior to release to the surface water system.
The paper presents methods which can be used to address a range of groundwater issues associated with excavation drawing upon closed form analytical methods. The methods presented are illustrated with examples drawn from practice.
Topics discussed include:
- Prediction of maximum groundwater level over the design life of a project
- Prediction of the rate of groundwater inflow to an excavation
- Prediction of groundwater level drawdown associated with excavation.
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Problems in testing of carbonate sediments
Carbonate sediments are formed in marine environments, in the tropical and sub-tropical climate belts around the world, such as southern Africa, India, Indonesia, Brazil and Australia. These sediments are characterised by their high crushability potential and variability in composition, grain shape, fabric and mineralogy. The design of foundations for offshore structures to be installed in these areas requires engineering parameters, which are generally determined using offshore site investigation data combined with onshore laboratory tests. The reliability and robustness of the design criteria are heavily reliant on the accuracy of the field and laboratory data. The field testing methodologies are generally well understood and can be verified using the recovered samples. However, conventional laboratory testing procedures are generally inadequate for testing carbonate sediments. The results obtained using the standard testing procedures may result in the derivation of a wide range of engineering parameters, which could result in costly design of structures and in some cases may jeopardise the development of the field.
An audit was carried out by Advanced Geomechanics (AG) as part of their QA process. The audit was undertaken in two parts. The first part consisted of testing seven material types, three non-carbonate (Silica Sand, Silica Flour and Kaolin Clay) and four carbonate materials (one terrestrial and three offshore) at four different laboratories. The identity of the tested samples was kept from the laboratories (blind tests). The tests requested included classification, permeability and consolidation. The second part of the audit was to investigate the effect of the operator on the test results, which is currently being carried out at AG’s laboratory (agLAB) and will be reported in a separate paper.
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Suction-monitored direct shear apparatus: A simple device for unsaturated soil testing
Two very common devices used in unsaturated shear strength tests are suction-controlled direct shear apparatus and triaxial devices. During the tests, matric suction is applied to the specimen by controlling pore air and pore water pressure. Compared to using triaxial device, the suction-controlled direct shear apparatus is considered to be simpler to use due to the shorter drainage path of the specimen. A relatively new and a simpler technique for unsaturated shear strength is the suction-monitored direct shear test. The device is made by modifying the conventional direct shear apparatus with the attachment of low capacity tensiometer on its top cap; this is connected to an electronic readout monitor which assesses negative pore pressure during the test. The aim of the study is to find out the capability of the suction-monitored direct shear apparatus with regard to use in unsaturated soil. Conventional, as well as suction-monitored direct shear tests have been conducted on various compacted brown sand-kaolin clay mixtures of differing proportions. The first one was performed to obtain the effective shear strength parameters of the saturated specimen, with subsequent testing carried out on unsaturated specimens. The results indicates that in general, shear strength with respect to matric suction, exhibits a bilinear envelope with an initial value of φb higher than the effective internal
friction φʹ. This phenomenon was due to the effect of dilation on the strength development of sand. Matric suction of the specimen was generated indirectly by adjusting the specimen’s water content to the desired value, and for this purpose the Soil-Water Characteristic Curve (SWCC) can be a very useful tool for predicting the required water content. Despite the suction capacity of the tensiometer being relatively low, the suction-monitored direct shear apparatus was effective for soil with a low high entry value, such as sand or sand with relatively small portion of fine grained material. -
Decked Rockfill Dams In Tasmania
The construction of dams for the seasonal storage of water began with the arrival of European settlers in Tasmania during the early part of the 19th century. A variety of small water storages with dams <15m high were constructed to meet the needs of local communities. The construction of ‘large dams’ began in Tasmania with the Lower Reservoir during the 1850s. It was Hobart’s first seasonal water storage.
During the second half of the 19th century three water supply dams were completed to serve Hobart and Glenorchy. These dams, Lower Reservoir, Upper Reservoir, and Tolosa Reservoir, had a combined storage volume of 945,000m3 (ANCOLD, 1990). With developments in mining, agriculture and industry the need for reliable supplies of water increased notably. The result was a near exponential growth in the construction of large dams (as defined by ANCOLD) for water storage during the 20th century.
The majority of these dams were rockfill (36) and earthfill (34) embankments. Other types included concrete gravity (7), concrete buttress (2) and concrete arch (5). There are 21 decked rockfill dams, i.e., ~25% of all large dams. Most of these are concrete faced rockfill dams (CFRDs), two are bituminous concrete faced rockfill dams (BFRDs). The first CFRD was constructed in 1907-8; the last was completed in 1993. This paper covers the development and performance of decked rockfill dams in Tasmania.
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Simplified finite element modelling of unsaturated soil behaviour for design of retaining structures for the Torrens Rail Junction project, Adelaide
The Torrens Rail Junction (TRJ) is where the interstate freight railway crosses the Outer Harbor passenger railway, located in the Park Lands, Adelaide. The original rail junction posed a productivity constraint to the strategically important Perth to Melbourne rail freight line, with freight trains forced to give way to Outer Harbor passenger trains at the junction. The TRJ project comprised a grade separation of the rail lines by lowering the Outer Harbor rail line below both the interstate rail line and the adjacent Inner Ring Route (Park Terrace). The lowered Outer Harbor rail line extends for a length of approximately 1.5 kilometres supported by a cantilever pile wall approximately 8 m deep.
The project is underlain by River Torrens alluvium. The River Torrens alluvial deposits transition to alluvial fan sediments of the Adelaide Lower Outwash Plain. These materials typically comprise stiff to hard clay soils with local interbedded sandier horizons. These clays are typically unsaturated above the groundwater table and are slightly to highly reactive.
The technical solution for the earth-retaining systems proposed the use of an economical, practical and compliant design. To achieve this, the retaining wall design approach which was proposed differed from normal design practice in that the beneficial effects of suction and unsaturated soil mechanics principles have been incorporated.
A simplified approach based on design standard from the Department of Planning, Transport and Infrastructure (DPTI), Government of South Australia, was adopted. The numerical methods which had been implemented via finite element modelling were employed to analyse soil-structure interaction. When assessing lateral earth-pressures against retaining structures and local/global instability, the analysis considered: staged construction; effects of volume change in clay soils due to unsaturated condition including variations from an initial equilibrium condition to a new equilibrium suction (wetting); effects of swell pressures; and the beneficial effects of suction in the shear strength of clays.
As this design approach was novel, the proposed methodology was compared to a full-scale pile wall trial, undertaken by DPTI, to verify the design approach. The results using the proposed design methodology indicate the numerical models could reliably predict both deflection and structural reaction at all construction stages.
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Geotechnical characteristics of coal mine spoil
This paper presents data on the geotechnical characteristics of the overburden, or spoil, produced during open-cut mining operations in the Hunter Valley, specifically in relation to the stability of spoil piles and the hydraulic characteristics of spoil and post-mining landscapes. A value of 33.4° is reported for the peak shear strength of “typical” spoil. This value is shown to reduce slightly at large shear strains and further when spoil materials are sheared after breaking down on exposure to water. Data on the permeability of different spoil types is presented, showing that permeability decreases with mudrock content. The presence of coal in the spoil is shown to have a significant effect on its hydraulic properties. Due to its hydrophobic tendencies, coal tends to resist the infiltration of water. However, spoils containing coal also tend to resist the storage of water that does infiltrate, causing coal-rich materials to drain quickly. This is demonstrated by considering the water retention properties of washed coal. The spoils are shown to have significant quantities of total leachable salt, but that leaching by a significant volume of water is needed for its total removal. The results show that a significant amount of salt is released upon first leaching, but that the rate of leaching decreases significantly with continued rainfall percolation. The paper concludes with a brief discussion of the potential for salt leaching from mine-spoil landscapes and its consequences for water quality in post-mining environments.
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Comparing the embodied carbon and local environmental impact of common geotechnical foundation solutions for the Australian market
In the Australian geotechnical market, foundation solutions for construction projects are commonly assessed against three key criteria; cost, program and quality. Safety and environmental aspects are often not assessed in the process until a geotechnical foundation technique has been selected based on the aforementioned criteria. As climate change has progressed to become one of the largest and most pertinent environmental issues in society today, it has in turn brought sustainability to the forefront of post construction assessments and is now a greater focus for government bodies, private developers and wider stakeholders. Using Keller’s in-house carbon calculator and a series of environmental metrics, this paper aims to inform decision makers of the carbon emission and social impacts of various geotechnical foundation solutions prior to their selection.Two theoretical projects are used to compare a range of geotechnical solutions: a 60-storey high-rise building requiring heavy foundations, and a low load warehouse building where ground improvement solutions can be used. Each geotechnical solution designed for the two Australian projects is then assessed using a standard cradle-to-gate carbon calculator. In addition, the impact on the local community is assessed according to the noise, traffic and emissions generated by each solution.
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Practical Approach For Piled Raft Stiffness Estimation
This paper makes use of the expressions of Randolph (1994) to derive simple expression for the proportion of raft stiffness that can be added to the pile group stiffness in order to estimate the overall stiffness of a piled raft foundation system. Simple methods of estimating raft and piled group stiffness values are summarized, and an example illustrating the application of the approach is presented. Finally, the approach is used to estimate the settlement of a piled raft foundation supporting a high-rise tower in the city of Frankfurt.