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An improved approach for characterisation and design of chemically stabilised pavement bases
This paper aims to develop measures to minimise the initial fatigue damage of prematurely opened cement treated bases (CTBs) due to repeated application of heavy traffic loads. A laboratory investigation was undertaken to characterize the early-age flexural fatigue performance of cement-stabilised pavement materials (CSPMs) under repetitive loading. The flexural fatigue test results evinced the existence of an endurance limit in CSPMs, even at seven days curing age. A stress- based flexural fatigue performance model was developed for predicting the early-age flexural fatigue performance of CSPMs in service. In parallel with the laboratory tests, mechanistic analyses were performed using the CIRCLY program to assess the early-age response of CTBs to heavy traffic loading. The computed critical pavement responses and the flexural fatigue performance model developed in this study were then used to estimate the early-age fatigue damage of CTBs in terms of seven days curing. It was found that the asphalt cover over CTB required to prevent the occurrence of initial fatigue damage to CTB decreases with increasing modulus and thickness of CTB and subgrade strength.
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Soil nailed retention – A practical approach to field verification
In Melbourne, soil nailed retention has been used on a number of major road projects for about 20 years and since that time the walls appear to have performed satisfactorily. During the intervening period to the present, there has been much debate about how the system of soil and embedded nails actually works and world wide a number of design approaches have been developed. These design approaches fall into three main categories:
- bi-linear slip surfaces and a limiting equilibrium analysis method (UK and USA – NailSolver, 1990; SnailWin)
- circular slip surfaces and limiting equilibrium analysis method (Australia – STARES)
- numerical analysis methods (FLAC and PLAXIS)
The nails form passive resistance elements that become stressed as excavation proceeds and the nails strain as there is load redistribution between the soil and the nails. It is now generally accepted that the contribution of the shear (bending) resistance of the nails is minor and most current design methods ignore the nail shear resistance. However, there is much debate about various aspects of design and it is still unclear what distribution of nail forces exists in practice, with various researchers finding a large variation in actual nail forces compared with the forces predicted according to the design methods (FHWA, 1999). The design for head force can be a particularly complicated issue, with the FHWA design method recognizing the contribution arising from both the flexural strength of the wall facing and punching shear modes of failure.
If the nail force distribution is not well predicted by the various design methods it would seem reasonable to adopt a simplified type of analysis in which the nail head force is limited to a nominated value and the wall facing pressures are then determined according to the limiting nail head forces established, providing that near-face failures cannot occur after assessing possible shallow face failure modes. The computer program STARES developed by the University of Sydney is one design approach that allows ready analysis of soil nailed walls using this approach.
A major factor in soil nail analysis is the bond between the nails and the ground and this is one of the most critical aspects of soil nailed retention design because if the bond fails, large volume wall failures can potentially occur. It is therefore important to carefully consider soil-to-nail bond in design and this is one aspect that can be checked by a careful program of field verification and testing. Unfortunately, in many cases the testing and field verification procedures do not relate explicitly to particular soil nail designs and are often derivatives of test specifications for stressed anchor systems. For production soil nails, it is often impossible to carry out testing to the loads required to validate design bond values without nail yield so proof load testing of completed nails may be of little practical use in confirming design bond values.
A carefully designed program of field testing can however be used to verify design bond values and provide confidence in this critical aspect of soil nail performance.
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2021 Tasmania International Symposium
Offshore Geotechnical Engineering: Challenges in Wind, Wave and Tidal Renewable Energy
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Listening To The Earth: An Unconventional Scientific Approach To Understanding Sub-surface Ground Conditions
Digitally recorded background ambient noise can be used to extract details regarding subsurface soil conditions. This unique methodology has been implemented to provide comprehensive assessments of geotechnical site conditions. Ambient noise is the persistent vibration of the ground in response to anthropogenic and natural causes. In many contexts, these background vibrations are classified as noise, and efforts are made to remove these signals from recorded data. However, these background vibrations also contain valuable information regarding the materials they travel through. The refraction microtremor (ReMi) technique separates these waves from noise recordings to determine soil shear-wave velocities. Interpolation of the closely spaced one-dimensional velocity-depth profiles along linear arrays allow two or three-dimensional velocity-versus-depth representations to be produced, thereby mapping lateral variations and extending subsurface characterisations between more expensive spot borehole measurements. ReMi technique provides a non- invasive and cost-effective way of estimating vertical soil/rock shear-wave versus depth profiles. This paper examines the contribution ReMi shear-wave velocity assessments can make towards enhancing subsurface geological and geotechnical models to mitigate risk from unforeseen ground conditions.
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Pavement Materials and Design in WA
Geoff Cocks
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Underpinning the Concord Road bridge under traffic – Westconnex M4 East Project
WestConnex is the largest transport infrastructure project in Australia. It is part of the Australian Federal Government and New South Wales Government’s efforts to ease congestion on Sydney’s roads by widening existing motorways and constructing new tunnels and bridges. M4 East is the section of WestConnex which extends from Haberfield to Homebush and includes the new Concord Road Interchange (currently under construction). The Concord Road Interchange is a complex junction of new bridges, cut-and-cover tunnel portals, retaining walls, widening and altering the alignment of the existing M4 Motorway lanes. Part of the works involves altering the alignment of the eastbound and westbound lanes under the existing Concord Road Bridge. To facilitate these works, the bridge needs to be underpinned with permanent support. The existing bridge abutments are founded on piles which are to be supported on a rock ledge permanently supported by rock bolts and prestressed ground anchors. Further, it is a requirement of the project that the Concord Road Bridge remain open to traffic for the duration of the works. Tight deflection criteria were imposed due to structural requirements at expansion joints. Additionally, the existing Concord Road Bridge was designed and constructed in the 1980s and there was limited information on the ground conditions and as-built founding levels of the bridge abutment piles. These factors, in addition to the requirements for working with low headroom under the bridge, were some of the key challenges during the detailed design and construction support of the works. This paper focuses on the methodology that was adopted to address these challenges during the design and construction phases of the project.
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The case of piles wholly embedded in a settling soil mass – What’s with all the negativity?
Negative skin friction forces (NSF), or drag load, would need to be considered in the geotechnical design of piles where the soil surrounding a pile undergoes settlement or consolidation after the pile has been installed. Typically, in the Melbourne region, piles that are prone to NSF forces generally terminate in the ‘stable zone’ below the consolidating layer (e.g. bedrock) such that there’s sufficient geotechnical resistance to sustain the combined serviceability load and NSF forces, and to satisfy the serviceability requirement. For the new road bridge over the Murray River at Echuca-Moama (the ‘Dhungala Bridge’), the geotechnical design considered the development of NSF forces in the abutment piles resulting from potential long-term consolidation of the ground due to the construction of fill embankments at the abutments. With the piles wholly embedded in the settling ground, the traditional method of determining a sufficient ‘stable zone’ beneath the consolidating layer to resist the drag load could not be applied and an alternative approach, the ‘Unified Design Method’, was adopted to allow for the drag load in the design.
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Debris Flow Hazard In Tasmania, Australia
Debris flow hazard in Tasmania has been largely under-appreciated until recently, and accordingly, emergency management plans are in the early stage of formulation. In this study the geological, historical, and contemporary evidence for debris flow processes in Tasmania is reviewed. From a landslide inventory, a geomorphic process rate with a derived magnitude-frequency power-law relationship was calculated in order to estimate event volumes, which are then used to derive input for numerical simulations. Specialised debris flow runout software was employed to predict at-risk areas in a similar fashion to standard design-event flood modelling practice. The rheological parameters chosen for each model were based on calibration with a limited number of past events. However, this small calibration dataset revealed contrasting properties, and our simulations represent a ‘worst case scenario’ within a spectrum of possible flow behaviours. Finally, the communications processes being used to share these results with Tasmania’s emergency management community is discussed.