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TMI assessment and climate zones in Western Australia
Thornthwaite Moisture Index (TMI) is commonly used in climate zone classifications when designing residential slabs and footings on foundation soils with potential surface movements due to moisture changes. However in the current design guidelines – AS2870-2011, there is no specific guidance for the large land area of Western Australia. This paper has collected climate data in 39 town specific locations for 25 years (1988 ~ 2012). The TMI values were calculated using the method by Thornthwaite (1948). A climate zone map of Western Australia was produced based on the average TMI values of each town specific locations. The new climate zone map provided better predictions on the depth of soil suction change due to moisture change in Lake King, Ravensthorpe and Jerramungup with field data available.
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Moving on from soft soil modelling to engineering application
Prof. Minna Karstunen
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Assessment of the post-compaction characteristics of a silty sand
Conventional field compaction control methods are effective at the time of placement. However, the discrete nature of these measurements and a limited depth of investigation can render them unsuitable for post-construction compaction quality assessments of deeper fills or larger surface areas. In this situation, classical destructive geotechnical surveys (i.e. boreholes, cone penetration tests) are sought to evaluate the current fill conditions. Nevertheless, these methods often do not provide the required level of information because only certain locations are tested and they have tremendous implications in terms of cost. The use of available non-destructive methodologies, such as shear wave velocity surveys (i.e. SASW, spectral analysis of surface waves or HVSR, horizontal-to-vertical spectral ratio) together with electrical resistivity tomography surveys (e.g. evaluation of water content), offers a valuable alternative to efficiently control compaction over large areas during post-construction stages and locate areas within the existing formations where the soil was not sufficiently compacted.
This study explores the performance of a cost effective method for evaluating the characteristics of compacted fills by measuring the shear wave velocity and matric suction to evaluate the void ratio or dry density of compacted soil. Laboratory studies of compacted specimens were used to evaluate this method and their performance under different isotropic confining pressures. The results showed that the shear wave velocity and matric suction can effectively predict how the soil is compacted, but its success requires field measurements of both shear wave velocity and matric suction. The application of this relationship would enable practitioners to efficiently control compaction over large areas during post-construction stages, and locate areas within the existing formations where the soil was not sufficiently compacted.
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Changes in subsurface water quality during coastal canal estate constructions
Construction of coastal canal estates has increased in recent times in Australia. The lowering of the watertable is required during certain canal construction periods. Coastal low lands contain can contain pyrites and associated potential acid sulfate soils. Dewatering or lowering of the water table to allow for dry excavation, may permit oxidation of the pyritic soil causing acid generation. Saline intrusion may also occur via lowering of the watertable. A paucity of literature on coastal canal estates led to this study, which investigated the subsurface water quality during construction of a canal estate. Groundwater depth, pH, dissolved oxygen, titrated acidity and salinity were monitored on site at the Lake Kawana development in Queensland, Australia. Groundwater monitoring was carried out before, during and after the dewatering program. Data analysis showed that water depth increased during dewatering activities and decreased after re-flooding. The dissolved oxygen, titrated acidity and salinity levels rose during periods of increased groundwater depth, while pH levels fell. The increase in oxygen availability and the associated increase in acidity, may lead to dissolution of heavy metals, but monitoring data was lacking in this regard. Since salinity may increase due to other close by saline surface water bodies, further studies are required to understand subsurface flow patterns during dewatering operations. Computer simulation models could be developed to help determine the flow of contaminants during dewatering and re-flooding operations.
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3-Dimensional numerical modelling of sand bed reinforced with 3D grids of triangular form
In this paper, the bearing capacity improvement of a square footing resting on sand bed reinforced with 3D geogrids of triangular form is numerically studied with the help of Plaxis 3D software. The performance of 3D geogrid reinforced sand is also compared with planar geogrid reinforced sand to understand its effectiveness. In the numerical modelling, the soil behaviour is simulated by linear elastic-perfectly plastic Mohr-Coulomb model. The 3Dgeogrid and planar geogrid is modelled using geogrid structural elements available in the software. The model was validated with the experimental results and found to be in fairly good agreement with each other. The effect of various parameters on the behaviour of reinforced soil system was also investigated. It was found that the bearing capacity of multilayered planar geogrid reinforced sand bed improved by 3.68 times, while, the 3D geogrid reinforced sand bed shows 6.8 times improvement compared to the unreinforced sand bed.
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Field instrumentation monitoring of land reclamation projects on marine clay formations
In land reclamation on marine clay formations, ground improvement works are often necessary to be carried out in order to negate future settlement under the projected dead and live loads. In the case of thick deposits of marine clay, it is necessary to accelerate the consolidation process. The use of prefabricated vertical drains with preloading option is the most widely-used ground improvement method for such cases. The degree of improvement attained by the marine clay has to be ascertained to confirm whether the soil has achieved the required degree of consolidation to enable surcharge removal. This analysis can be carried out by means of observational methods for which continuous records of ground behavior can be monitored from the date of instrument installation. Field instruments installed, monitored and analyzed in such projects include settlement plates, deep settlement gauges, earth pressure cells, pneumatic piezometers, electric piezometers and water-standpipes. The type of field instrumentations installed and their method of analysis is discussed in this paper. Field instrumentation readings obtained at a Case Study Area comprising a Vertical Drain Area (1.5 m x 1.5 m) and an adjacent Control Area (No Drain) in the land reclamation project are also discussed in this paper.
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Geophysical survey as a part of a multi-tiered investigation in fault characterization and dam seismic hazard assessment – a case study from South Australia
Fault characterisation and definition of its activity is an important task in defining the seismic hazard for dams. This is especially the case when some evidence of fault activity exists. A multidisciplinary approach (i.e. geomorphology, structural mapping in the spillway, seismic reflection and refraction, paleoseismic trenching, downhole geophysics and geotechnical boreholes) including use of lidar and bathymetry investigation have been undertaken to inform the overall dam safety and seismic hazard assessment of a dam site in South Australia. This multi-disciplinary investigative approach has led to new active strands of an active fault being recognised, which pass near the main dam and close to a proposed saddle dam, presenting challenges to the dam upgrade design and ongoing dam safety. The geophysics segment of investigation (Seismic Reflection, Seismic Refraction, and Downhole geophysics) has improved understanding of seismic risk and assisted in developing the dam upgrade design.