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Enhancing Coastal Geotechnics With Integrated Marine Seismic Reflection And Refraction Geophysics: Case Studies
Strong world demand for energy, mineral and agricultural products and the advent of larger transport vessels is underpinning new construction and upgrades at many Asia-Pacific and Australian ports. Overwater geotechnical investigations are required at the feasibility and design stages of these projects, directed mainly at entrance channels, pipeline routes and supporting land-based facilities. These are costly and difficult when overwater drilling is involved due to the costs of jack-up rigs and barges and restricted drilling sites within busy waterways. Consequently, there is increasing reliance on marine geophysics to provide the necessary subsurface information, typically in water depths of less than 20 metres.
Since water is acoustically transparent, continuous seismic reflection profiling (CSP) using boomer, sparker or airgun sources has been applied to these projects for many years, despite its limitations in certain conditions. From a geotechnical perspective a more important problem is that it is very difficult to determine engineering properties from single channel marine CSP data as Australia’s near shore marine environment is essentially a drowned continental land mass that has experienced a wide range of both terrestrial and coastal weathering and depositional processes over an extended geological time scale. These have created a wide range of materials with very different geotechnical properties and behaviours that are not easily quantified with marine seismic reflection alone. Recently, single-ended, continuous underwater seismic refraction (CUSR) with near-bottom towed equipment and air-gun sources and static USR (SUSR) systems have been developed. These provide subbottom seismic P-wave velocities that can be correlated with engineering properties.
We present a series of case studies to demonstrate the application and integration of CSP, CUSR and static SUSR methods using advanced geophysical analysis processes to port infrastructure and near shore construction. In Victoria, combining conventional boomer CSP and CUSR improved the definition of a submerged, buried basalt flow and assisted dredging design along the Geelong Ports navigation channels. In East Malaysia, the same technologies assisted assessment of the viability of HDD as a pipeline installation option in variably weathered granites. In Western Australia, SUSR imaged the granitic regolith beneath sediments and indurated layers at a proposed new berthing where deep piling was required. This allowed preferred piling sites to be identified that minimised pile lengths.
USR technologies supported by advanced processing and analysis methods have demonstrated an ability to improve marine geotechnics in a diverse range of applications and will be increasingly applied in Australia’s coastal waters.
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SA-NT Symposium 2020
Geotechnical aspects of renewable energy
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The Sensitivity Framework: Behaviour Of Richmond River Estuarine Clays
The stability and long-term settlement behaviour of NSW estuarine clays under load has become increasingly significant as a result of the large-scale infrastructure development currently occurring in coastal NSW.
The Structured Clay Framework (SCF) developed by a number of authors over the past 20 yrs (Burland et al., 1996; Chandler, 2000; Cotecchia and Chandler, 2000) and based on the work of Burland (1990), provides a general framework for understanding the behaviour of natural structured clays. This is analogous to the framework for remoulded soils provided by Critical State Soil Mechanics (CSSM).
The basis of the SCF is a normalised void ratio vs. effective stress relationship generated from oedometer tests on remoulded clay. The parameters used in the normalisation procedure are the intrinsic properties of e*o, e*100 and e*1000 the void ratios of the remoulded clay at the liquid limit, (eo), σ’vo=100kPa, (σ’100, e*100), and at σ’vo=1000kPa (σ’1000, e*1000). The “intrinsic” properties represent the values of clay with no microstructure i.e. its baseline properties.
Unlike standard empirical correlations between geotechnical properties and Atterberg limits there is a rigorous analytical basis (i.e. CSSM) for these correlations (Burland, 1990). The behaviour of high quality undisturbed tests can then be compared and classified according to this intrinsic baseline. The SCF provides a quantitative measure of the structure component of the clays consolidation and shear behaviour above that predicted based purely on void ratio stress relationships (Burland, 1990).
The work presented in this paper describes the implementation of the SCF on Holocene estuarine clays from the Richmond River in Northern NSW. These geologically normally consolidated clays form a significant component of the foundations of the proposed Ballina Bypass and it has been noted, anecdotally, that a number of these clay deposits suffer from large (3 m under a 7 m embankment) settlements and have relatively high sensitivities (4-10, shear vane). Both these factors suggest micro-structure may play a significant role in the overall behaviour.
The initial results indicate that the SCF method provides a useful tool for relating mechanical behaviour, in terms of oedometer and shear vane results, to periods in the geological evolution of the estuarine deposits. Significantly, zones of high sensitivity clays, the deposition of which relates to a period of rapid flooding within the estuary, have been identified. They have much higher intrinsic values that can be accounted for under normal sedimentation conditions, suggesting that they are highly structured. This fact is not apparent when using the standard Atterberg correlations.
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Characteristics and Variability of the Martin Place Joint Swarm — A Sydney Metro Case Study
The Martin Place Joint Swarm (MPJS) is a well-known, but poorly defined, regional geological structural zone extending through inner Sydney. The recently completed Sydney Metro City & Southwest (Sydney Metro) Tunnels and Station Excavation (TSE) project provided a unique opportunity to study the MPJS across large continuous excavations within both the new Pitt Street and Martin Place stations. Both stations involved complex geometries of running tunnels, station caverns, interconnecting adits and station shafts. These excavations allowed the MPJS to be observed across significant geospatial extents thereby exposing the structure’s inherent variability with respect to its character, lateral and vertical continuity, and stratigraphic dependency. The encountered geological features further advanced our understanding of the major vertically-persistent structures comprising the MPJS as well as strata-bound structures associated with the regional MPJS domain. Observations of displacements across significant fault structures are discussed, including ratios of strike- slip to vertical displacement magnitudes being up to 7H:1V.
This paper presents a summary of key observations from the Pitt Street Station and Martin Place Station excavations in Sydney’s CBD. The intent is to knowledge share and contribute to existing publications and the current understanding of the MPJS within the engineering industry. Some practical construction considerations are also recommended based upon experiences from Sydney Metro. As future city developments will demand more extensive and deeper underground spaces, it is important to document findings from completed projects so that others can consider potential impacts on their planning, design, and construction phases.
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Modelling the Performance of a Permeable Reactive Barrier utilised to Reduce the Risk of Acidic Groundwater in the Shoalhaven Floodplain
Acidic groundwater generated from acid sulfate soil (ASS) usually carries high concentrations of aluminium (Al) and iron (Fe), which create unfavourable conditions to living habitat. The ASS research team at the University of Wollongong, Australia implemented an innovative geotechnical engineering technique for the remediation of acidic groundwater through a permeable reactive barrier (PRB) using recycled concrete aggregates as the reactive material. This PRB was installed at the Shoalhaven Floodplain, southeast New South Wales (NSW), Australia in October 2006 and has proved effective in neutralisation of groundwater by increasing the pH from 3.6 to 7 and removing 99% of Al and Fe from groundwater to date. Dissolved Al and Fe were removed through continuous precipitation which would clog the pore spaces of the reactive medium by secondary mineral precipitation. This paper provides a complete evaluation of the performance of the PRB through field work and groundwater flow modelling coupled with geochemistry. The developed model (using finite difference codes: MODFLOW and RT3D) describes the chemical clogging due to mineral precipitates and the associated reductions in porosity and hydraulic conductivity of the reactive medium. The results obtained from numerical modelling, groundwater samples analysis and mineralogical analysis of barrier specimens confirm that the current PRB has performed well during the last seven years. Only a smaller amount of clogging was evident at the entrance of PRB with only a 3% reduction of hydraulic conductivity. This model would be beneficial for the environmental scientists and geotechnical engineers who have to deal with the ASS problems, especially in coastal Australia.
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Significance of geosynthetic reinforcement in embankment construction
Geosynthetic reinforced pile-supported (GRPS) embankments emerge as a promising ground improvement technology when construction needs to be undertaken over soft soil deposits. This method has the potential to overcome many problems that arise due to undesirable characteristics of soft soil during embankment construction. There are many advantages in this method compared to conventional consolidation based ground improvement methods such as higher reliability, less time consumption and the ability to use even in very adverse soil conditions. This study concentrates on the significance of geosynthetic reinforcement in embankment construction. The effect of the geosynthetic reinforcement in a GRPS embankment is discussed in detail using three different analysis cases. Case 1 has no pile supports or geosynthetic reinforcement, Case 2 has only pile supports and Case 3 has both pile supports and geosynthetic reinforcement. An in depth analysis was carried out in order to investigate the influence of geosynthetic stiffness, interface friction coefficient of the soil-geosynthetic interface, height to the geosynthetic layer from the pile heads and the number of geosynthetic layers on the overall behaviour of GRPS embankment systems.
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The development of a fatigue transfer function for in situ foamed bitumen stabilised pavements
This report reviews the results of research undertaken into the stiffness and fatigue performance of in situ foamed bitumen stabilised pavement materials at various sites in the Cities of Canning and Gosnells in Western Australia. The aim of the research was to assess if a design relationship could be developed to predict the fatigue life of in situ foamed bitumen stabilised pavements and if the visco elastic properties of the bitumen binder were reflected in the stiffness and fatigue performance.
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The applicability of index tests on weathered rock: Interrelationships and experience from The Pilbara, Western Australia
Index tests can provide useful correlations with the mechanical properties of rock, but must be used with caution in lower strength and weathered materials. The authors summarize the results from laboratory testing and visual examination of rock from 1300m of core taken from 69 nearshore boreholes on the coast of the Pilbara in Western Australia. Interrelationships between weathering and index properties are discussed; correlations between the parameters are proposed and the limitations of index tests are considered.
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Effects Of Fines Content On The Strength And Stiffness Of Biopolymer Treated Low-Plasticity Soils
The use of biopolymers to enhance the engineering properties of soil has received increasing attention in recent years, however, the interactive role that biopolymers and the fines content of the soil play in governing the geotechnical parameters still requires insightful investigation, in relation to chemical soil treatment that can be ecologically detrimental. This paper examines the combined effects of Xanthan Gum (XG) derived from specific bacterial strains and the presence of clay fines content (kaolin) on the strength and stiffness of low plasticity soils, with special reference of cyclic traffic (road and rail) loading. In this study, fine sand is mixed with different contents of kaolin, whereby laboratory compression and tensile tests were conducted on natural (untreated) and XG-treated soil specimens. The results indicate that soil strength can be enhanced significantly when XG is added, however the effectiveness is a function of the kaolin content (KC). At an optimum XG content of 2% and a fines content increasing from 5% to 30%, split tensile strength increases from 230 to 750 kPa,while the unconfined compressive strength rises from 1.4 to 7.9 MPa, respectively. For XG content between 0.5% and 2%, the small strain stiffness of treated soil increases fourfold from 206 to 854 MPa.