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Design of cantilever retaining walls for minimum tilting tendency
Ian Hooley and Dr Safat Al-Deen
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Seismic-Liquefaction Assessment Methods In Conjunction With Pore Pressure Development And Dissipation
The assessment of potential liquefaction on the performance of a given structure requires assessment of sitespecific factors with a certain degree of accuracy. In sensitive or high-hazard structures, such as dams, extreme seismic events are to be considered (ANCOLD, 1998), such as those with a return period in excess of 30,000 years compared with typical return period of 452 years for buildings.
Although site-specific earthquake parameters may be assessed by a seismologist for bedrock conditions, the seismic wave may be either attenuated or amplified by the overlying soil profile to a dam foundation. Liquefaction may be generated by transient pore pressures as the soil particles try to re-arrange themselves in a denser packing. Given the characteristics of the foundation hydrogeology and nature of the earthquake, concurrent dissipation of the generated pore pressures may occur, thus mitigating the consequences. Despite the recognition of potential benefits, in most cases undrained conditions are assumed (ANCOLD, 1998).
The authors present a methodology suitable for practical applications. A seismic ground response analysis, based on total stress approach, will be carried out in conjunction with a transient seepage analysis in the time domain. The analysed soil profile consists of an 11 m thick layered deposit of saturated clay, silt and sand overlaying a siltstone. Effects of liquefaction in the upper sand layer, as well as pore pressure development and dissipation, will be assessed.
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Technical Presentations and AGM
Associate Professors Jun-ichi Kodama and Atsushi Sainoki
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Earthworks For Waterside Green, Penrith, NSW, Australia
A residential and commercial subdivision, known as Waterside Green Development, in Penrith NSW, is situated in low- lying land affected by high groundwater table and the presence of saline and sodic soils. The subdivision comprises a commercial area, a residential area, internal roads and several lakes and laterals. The Development Control Plan (Penrith City Council, 2004) states that the drainage and stormwater management system across the site should be improved and future dwellings and roads should be constructed on ground higher than the 1 in 100 year flood level, without the necessity to import fill material. Therefore, earthworks for the subdivision comprised excavation of a series of lakes and laterals to improve drainage and stormwater management systems and use the materials gained from the excavations to construct fill platforms in order to raise ground levels above the 1 in 100 year flood level. This paper presents earthworks specifications and procedures for construction of fill platforms and clay liner.
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Roller compacted concrete at the Savage River Mine, Tasmania
Several civil engineering structures have been constructed from Roller Compacted Concrete (RCC) at the Savage River Iron Ore Mine on Tasmania’s west coast, including river crossings, spillways and hardstand areas. This paper discusses the design, construction and performance of one of these, a river crossing structure built to replace a previous structure, which had collapsed during flooding. The conditions leading up to this collapse are discussed. As there was a need to urgently construct the new crossing an RCC structure was proposed; to allow for winter construction and to provide long term overtopping capabilities. During the construction phase further flooding in the Savage River necessitated the adoption of a modified design, which incorporated the innovative use of a shipping container adopted to act as both a diversion culvert and formwork within the main RCC embankment. The river crossing was completed in time and the structure has been overtopped since final completion with little damage. Brief explanations of other RCC structures are included in the paper.
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Construction planning of microtunnelling works in urban areas
Microtunnelling a new sewerage pipeline or tunnel underneath densely built-up urban areas is more reliant on early planning than many other civil works. The nature and constraints of urban microtunnelling effectively fix key project details considerably before construction commencement, with little subsequent flexibility. These early constraints arise because of the high degree of interconnected and repetitive construction activity sequencing, and the high capital outlays for the precast production and microtunnelling machine. Based on the author’s work on eight microtunnelling projects in Singapore, this paper proposes and discusses 11 steps of construction planning elements which have been observed as critical to expeditious cost-effective project outcomes.
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Ground engineering – technology, common sense and good value
In 1985 it took the author a month to run thirty Bishop’s Simplified limit equilibrium slope stability analyses on a personal computer to support the design of remedial works on a medium size embankment dam. Today such an effort would probably be criticised as grossly inadequate (verging on the negligent) since it is not uncommon to execute three hundred, or more, similar calculations using far more sophisticated and accurate algorithms in less than a minute. The paper considers the development and application of technology and advanced analytical tools over the last twenty five years and whether they are being put to good use by adding value to our clients, improving our levels of service or reducing geotechnical risk.
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Simulating a tsunami event in Pine Rivers Shire (Brisbane) Australia
On December 26th 2004, a catastrophic tsunami devastated many countries of South East Asia. The inland saltwater surge impacted upon the coastal soil and groundwater quality in low lying areas. The groundwater resources along the coastal aquifers were affected through contamination and destruction to existing freshwater wells. Although most of the saline flood water eventually drained back to the sea and the period of flooding appeared short the sea water left a considerable salt load. For example, Pondicherry in India and Bande Aceh in Indonesia have both been severely impacted by salt contamination. These and many others around the world rely on groundwater for their domestic and agricultural needs. This paper investigates possible effects of a Tsunami type event on the Pine River’s coastal aquifer system. To simulate surface saltwater inundation a 3D density dependent flow and solute transport model was developed using finite element methods. The simulations show not only longer term influence on the aquifer water quality but also that a little change in soil water permeability could significantly alter the scenario.
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Strength improvement of Coode Island Silt by the soil mixing method
This paper investigates the effectiveness of using soil mixing technology via cement stabilisation of locally found soft silty clay, Coode Island Silt (CIS), by studying the improvement of the strength of the treated soils. Tests were carried out in two stages, a laboratory based stabilization of the silty clay and a small scale model mixing. The improvement in terms of strength was assessed using the commonly accepted unconfined compressive strength (UCS) test. A smallscaled model mixing was set up to mimic the actual mixing scenario on sites. Although it may not be perfect, it gives an indication on the effectiveness of the cement slurry when combining with the silty clay.