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Undrained shear strength of variably pre-loaded Launceston Silt
Earth levees were constructed in the 1960’s as flood protection around the suburb of Invermay, located immediately north of Launceston’s CBD. In 2007 an upgrade program was begun by Launceston City Council (LCC) to provide 1 in 200 year flood protection, meaning that about 5km of levee have to be re-aligned and raised, with the new levee alignment partially over virgin ground and partially over pre-consolidated ground. This paper presents the results of testing and analysis to determine the spatially variable undrained shear strength profiles of the compressible Launceston Silt foundation. Relationships between undrained and drained shear strength were used to predict strength increase caused by preconsolidation, with good agreement with in situ measurements. The analysis allowed development of an innovative stability model that has vertical interfaces between foundation zones of different strengths.
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The Thornthwaite Moisture Index and seasonal soil movement in Adelaide
In regions with highly expansive clay soil profiles, climate is a critical determinant of the behaviour and magnitude of seasonal surface soil heave. In this paper, the correlation between soil heave and soil moisture availability, as determined using the C.W. Thornthwaite method of estimating potential evapotranspiration, is investigated for two exposed vegetated sites near Adelaide. The study sites were characterised by highly reactive soil profiles, and were influenced by extremely variable moisture conditions due to the semi-arid climate of the Adelaide region. The effects of vegetation on soil moisture availability are demonstrated, with soil shrinkage observed to occur quite rapidly during the late spring when moisture uptake by maturing annual grasses is high. Maintaining vegetation ground cover as mulch was shown to reduce noticeably soil shrinkage during the dry summer months, thus limiting the seasonal variation in soil volume. The study results also provided good experimental justification for assuming a soil moisture capacity of 100 mm of rainfall, as recommended by Thornthwaite. Climate change predictions for Adelaide have been used to estimate future trends in soil moisture availability and Thornthwaite Moisture Index, and indicate that Adelaide’s climate will become increasingly arid. This may have significant implications with respect to footing design requirements in regions characterised by highly expansive soils.
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Thermal properties of some Melbourne soils and rocks
The thermal conductivity of soils and rocks is an important property for the design of thermally active ground structures such as geothermal energy foundations and borehole heat exchange systems. This paper presents the results of a laboratory study on the thermal conductivity and volumetric heat capacity of soils and rocks from around Melbourne, Australia. The thermal conductivity and volumetric heat capacity of six soils were experimentally measured using a thermal needle probe and the thermal conductivity of three rock types were measured using a divided bar apparatus. Soil samples were tested at a wide range of moisture contents and densities. The results demonstrated that the thermal conductivity varied with soil moisture content, density, mineralogical composition and particle size and that volumetric heat capacity was strongly dependent on the moisture content of the soils. Rock samples were tested dry and water saturated. Rock samples demonstrated an improvement in thermal conductivity with an increase in density when dry. However, when water saturated, siltstone and sandstone rocks showed no significant correlation between density and thermal conductivity. This was attributed to both variations in mineralogy and anisotropy. The thermal conductivity and volumetric heat capacity data obtained from this study provides an initial dataset of soils and rocks thermal conductivities for the design of thermally active ground structures installed throughout Melbourne, Australia.
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Assessing the Geometry of Defect Waviness from Borehole Data
Within the large, open cut, iron ore mines of the Pilbara region of Western Australia, defect shear strengths often control the slope design where bedding dips shallowly to moderately out of the pit slope. The presence of metre to decametre scale open folding or waviness in these units can contribute to the friction angle of bedding shear strengths, potentially allowing for steeper slope angles and improved economics for the deposit.
Traditionally, waviness affecting defect shear strength is assessed from surface mapping, bench mapping or qualitatively from observations in core. Surface mapping of bedrock is often not possible due to detrital cover or a lack of suitable outcrop, while bench mapping is inherently conducted perpendicular to the direction of sliding risk. The use of downhole data from boreholes drilled into the slopes circumvents these issues.
The method presented here involves assessing characteristic downhole wavelength, inter-limb angle and amplitude of folding from defect orientation data interpreted from borehole televiewer imaging. The downhole wavelength and defect orientations are transformed to a true down-dip wavelength, dilation angle, and estimated amplitude in the direction of sliding risk. The calculation of down-dip wavelength is critical for assessing the applicability of the associated dilation angle to the scale of the slope and failure mechanism in question. The adoption of defect shear strengths that include a waviness contribution to the friction angle allows for implementation of steeper slope angles in structurally controlled slopes.
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“Granular Pavements — Geomechanics’ Lost Child”
The central theme of this paper is that pavement technology is without a behavioural model for unbound granular pavements. Some very weighty questions about transport efficiency, load limits and pavement asset protection cannot be addressed because of this. It is argued that the basis for a suitable model is likely to be found within geomechanics. The paper gives a brief summary of current design methods and of the conventional understanding of pavement behaviour and as well provides insights into alternative understandings of material behaviour and appropriate directions for pavement research.
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Use of probabilistic methods in geotechnical engineering
Due to the intrinsic inhomogeneous nature of soils and rocks, the minimal site investigations, and the need to extrapolate available information over a large domain, geotechnical designs have inevitable uncertainties. To be conservative, geotechnical engineers traditionally use a safety factor to account for uncertainties. A more rigorous way of considering uncertainties is to use probabilistic methods. To promote the use of probabilistic methods in geotechnical engineering, this paper tries to address the following commonly encountered questions:
- Why do we need to use probabilistic methods?
- How can we use probabilistic methods if we don’t have enough test data?
- How much field/test data do we need?
- How can we use multiple sources of information?
- How can we use monitoring data to predict future performance?
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Sinkhole threats management in urban developments
Subsidence and sinkholes in urban areas pose catastrophic consequences if not detected and addressed effectively. Such phenomena have been observed in recent new urban developments in Madrid (Spain), highlighting the urgency of soil treatment to mitigate these risks. Upon discovering significant underground voids, traffic was halted until viable solutions could be identified. Preliminary studies utilizing microgravimetry, ground-penetrating radar, and boreholes revealed the presence of large cavities due to karstification. Subsequent research proposed testing plans, inspection programs, and intervention methods to repair existing sinkholes and prevent future occurrences. Throughout this article, the most suitable investigations for such terrains are explained, following a detailed study in a test area that has served as the basis for establishing a methodology for future work. This research underscores the critical need for proactive measures, emphasizing the necessity of soil treatments within urbanized areas to efficiently address situations and mitigate the potentially catastrophic impacts of subsidence and sinkholes while fostering a sense of security and stability within the community.
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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.