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Innovative design of reinforced soil wall on a steep slope subject to land slip risks
This paper presents a case study of an innovative reinforced soil wall (RSW) design on a steep slope using site-won material that is not in full compliance with Roads and Maritime Services (Roads and Maritime) QA specification R57. Firstly, the local geology and the landslips that occurred along the project corridor are briefly described. The key design and material requirements for RSW are reviewed with respect to the use of site-won material. A detailed design strategy is then presented on how to mitigate the potential risks of using the non-compliant site-won fill material. Comprehensive laboratory testing of the site-won material and large-scaled pullout tests of RSW reinforcement were undertaken. With the test results and engineering judgement, the design proceeded with the following key assumptions/factors: 1) An allowable fines (<75 microns) content of up to 25%; clay (<2.4 microns) content of up to 7%; 2) the use of reduced friction angle of 30 degrees; 3) a 10% reduction in the calculated pullout capacity of reinforcement; and 4) 1.5 times sacrificial allowance required by R57. These are to cater for long term corrosion on steel reinforcement and to ensure longevity and integrity of the RSW. Heavy rainfall in February 2010 resulted in two significant landslips within the project corridor, with the larger one being immediately down slope of the proposed reinforced soil wall (RW01). As a result an additional geotechnical investigation was carried out and landslip remedial works implemented to ensure the long term global stability of RW01. A three dimensional effect was considered in the assessment of the slope instability during the construction stage review.
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Implementing The AGS Landslide Risk Management Guidelines In A Municipal Planning Scheme – A Case Study In The Colac Otway Shire, Victoria
The Colac Otway Shire in south west Victoria are in the process of amending their Planning Scheme to include landslide risk assessment for new developments in landslide prone areas. The amendment is primarily intended to limit the Shire’s liability and will require consultants to assess risk using the Australian Geomechanics guidelines on landslide risk management. However data limitations, particularly the paucity of historical information on landslide events, have precluded landslide hazard mapping for all types of landslides and their likelihood of occurrence, at the site-scale required for planning controls. The solution adopted was to extend the Erosion Management Overlay to cover all areas of the Shire in which landslides are credible, and the implementation of a documented process to determine when a landslide risk assessment for any development is required. The process includes reference to existing information stored on a GIS database, a checklist for use in an initial site visit by the Shire, specific requirements for consultant’s reports, and information checking and mediation processes for the Shire. The limited landslide risk management experience of the Shire planners and some small and medium-scale consulting companies has highlighted a local desire for a more prescriptive code.
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Application of arch shaped deep cement mixed walls for excavation support
In this paper, a typical arrangement of compound DCM columns with bored piles at the toes for excavation support is investigated. The excavation is analysed considering the three-dimensional nature of the problem. A case study was modelled, where the wall design was carried out assuming a uniform lateral pressure distribution over the wall section in the horizontal plane. Both computed and measured deformations are similar and confirm that the wall design was conservative and hence the deformations are very small for this case. Also this study shows that it is too conservative to assume uniform pressure distributions for arch walls. An analytical computation presented in the paper shows that the flexural rigidity gained by an arch wall is substantially higher than that gained by a planar wall section between bored piles. Finally a parametric study was carried out varying the geometric parameters of the arch section (span and rise) to investigate the viability of DCM wall sections in supporting excavations. Results confirm that the even with a small curvature, tensile stress development within the wall sections can be completely eliminated.
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Assessment of the impact of climate change on expansive soil movements and site classification
There is overwhelming evidence that climate change leads to a wide range of climatic and weather changes that can affect the performance of built infrastructures. Climate change is likely to have significant impacts on the performance of residential buildings constructed on expansive soils. The Thornthwaite Moisture Index (TMI) as a useful climate parameter has been widely employed to estimate the depth of design soil suction change (Hs) which is needed for the determination of characteristic ground movement (ys). Precipitation and temperature are the primary weather parameters required for the TMI computation. By applying the projected rainfall reduction and temperature increase in 2030, 2050 and 2070 in the TMI calculations, the effects of climate changes on expansive soil movements and site classification can be quantified by the use of the predicted TMI. In this study, TMI values of various areas of Victoria were calculated under A1B and A1FI emission scenario using climate projections generated from 23 climate models. These predicted TMI indices were then used to delineate TMI isopleth lines on the map of Victoria to visualise and compare climate conditions in 2030, 2050 and 2070. A case study was also carried out to assess the effect of climate changes on the magnitude of ground surface movements in the top five most densely populated cities (i.e. Sydney, Melbourne, Brisbane, Perth and Adelaide) in Australia for three specific years (i.e. 1990, 2030 and 2070). The results show that both Hs and ys values are expected to increase significantly with climate change.
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New insight into the compressibility and structured nature of Coode Island Silt
As part of the Regional Rail Link (RRL) project in Melbourne, Australia, a number of geosynthetic reinforced (GR) embankments with ground improvement were constructed where the rail alignment passes over the Coode Island Silt (CIS), a well-known soft soil encountered around inner Melbourne. To better understand the behaviour and performance of the load transfer platform (LTP) at the base of these embankments, a field case study has been undertaken which has seen an extensive array of instrumentation installed within the North Dynon embankment. This paper presents and describes a significant amount of field and laboratory data gathered as part of the geotechnical site characterisation of the instrumented areas. Based on this data a description of the compressibility and permeability of the CIS is presented and insight into the structured nature of the CIS is described. In addition, it is shown that a far better characterisation of soft soil behaviour can be gained through the use of more sophisticated oedometer testing techniques.
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Simplified finite element modelling of unsaturated soil behaviour for design of retaining structures for the Torrens Rail Junction project, Adelaide
The Torrens Rail Junction (TRJ) is where the interstate freight railway crosses the Outer Harbor passenger railway, located in the Park Lands, Adelaide. The original rail junction posed a productivity constraint to the strategically important Perth to Melbourne rail freight line, with freight trains forced to give way to Outer Harbor passenger trains at the junction. The TRJ project comprised a grade separation of the rail lines by lowering the Outer Harbor rail line below both the interstate rail line and the adjacent Inner Ring Route (Park Terrace). The lowered Outer Harbor rail line extends for a length of approximately 1.5 kilometres supported by a cantilever pile wall approximately 8 m deep.
The project is underlain by River Torrens alluvium. The River Torrens alluvial deposits transition to alluvial fan sediments of the Adelaide Lower Outwash Plain. These materials typically comprise stiff to hard clay soils with local interbedded sandier horizons. These clays are typically unsaturated above the groundwater table and are slightly to highly reactive.
The technical solution for the earth-retaining systems proposed the use of an economical, practical and compliant design. To achieve this, the retaining wall design approach which was proposed differed from normal design practice in that the beneficial effects of suction and unsaturated soil mechanics principles have been incorporated.
A simplified approach based on design standard from the Department of Planning, Transport and Infrastructure (DPTI), Government of South Australia, was adopted. The numerical methods which had been implemented via finite element modelling were employed to analyse soil-structure interaction. When assessing lateral earth-pressures against retaining structures and local/global instability, the analysis considered: staged construction; effects of volume change in clay soils due to unsaturated condition including variations from an initial equilibrium condition to a new equilibrium suction (wetting); effects of swell pressures; and the beneficial effects of suction in the shear strength of clays.
As this design approach was novel, the proposed methodology was compared to a full-scale pile wall trial, undertaken by DPTI, to verify the design approach. The results using the proposed design methodology indicate the numerical models could reliably predict both deflection and structural reaction at all construction stages.
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Estimating in situ compression parameters from remoulded and field test data
Conventional sampling and testing procedures cause disturbance of soft clays during sampling, transport and testing resulting in inaccurate measurement of in situ compression parameters. A pilot study has been performed to assess the feasibility of a method for estimating in situ parameters from the results of routine laboratory tests on remoulded soil and in-situ measurements of sensitivity. The computed parameters are compared with data back-analysed from embankments that settled more than anticipated. The results of the study were in reasonable agreement with the back analysed values and were greater than design values adopted from the results of conventional laboratory tests. Piezocone data can be used to develop a continuous profile of coefficients of compression with depth once a profile of plasticity index with depth for the soil is known. Further work is required to increase confidence in the method.
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Pavement design and construction in reactive clays
One of the key parameters considered for pavement design in Australia is the subgrade CBR. In reactive clays the soaked CBR value is low and influenced by many factors such as moisture conditioning of the sample, variability within the sample, surcharge during soaking and duration of soaking. These clays are also prone to volume change with changing moisture condition, which is typically reflected as shape loss and cracking in overlying pavements. Measures such as provision of a low permeability capping layer, provision of minimum cover over the subgrade and moisture conditioning of the subgrade are provided to improve the long term performance of the pavements constructed on reactive clay subgrades. Preparation of reactive clay subgrades also generally encounter difficulties. This paper presents a review and discussion on the above issues. A method of assessment of the potential long term surface movements in pavements constructed in reactive clays is also presented and discussed.
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Some Design And Construction Issues In Deep Excavations And Shoring Design
This paper presents, firstly, a critical review of the current codes of practice on design and construction for retaining structures. The key factors impacting on the design and construction of shoring system is examined and discussed in detail. The importance of groundwater pressure on the design of a retaining structure is highlighted, which is as much as three times of the active soil pressure. Some design issues with respect to the SLS and ULS load combination and associated analysis are discussed through practical examples. A real case history in Sydney due to a burst water main behind the retaining wall is presented to demonstrate the significance of the groundwater pressure on deep excavation shoring design. A detail design approach is proposed as to how best the retaining wall structures could be designed to take account of the accidental water pressure induced by a burst water pipe. It is the Author’s opinion that this proposed approach will provide useful guidelines to the future retaining structure design.
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Unbound Granular Pavements; Design With Understanding : A Revision And Extension
The original paper, Rallings (2018), presented a rudimentary behavioural model of the response of unbound granular pavements (UGPs), ones that are founded on compacted clay subgrades, to passages of single wheel loads. The following amends and expands the original model and includes new terms and concepts. The proposed model is based on the assumption that elevated subgrade shear strains in conjunction with a strain-controlled mechanism disturb the fabric of the overlying granular materials (OGM) causing reductions in their stiffness and in the UGP’s load carrying capacity, essentially its ability to maintain its surface shape. It is proposed that short term falls in the subgrade shear strength and/or increases in the frequency of the heaviest loads within the wheel load spectrum are the common major contributors to the degradation of a UGP’s load carrying capacity. The proposed model provides a direct and simple means to predict the response of a UGP to load passages, allowing designers and asset managers alike to distinguish between those wheel loads that pose a potential threat to the UGP from those that do not.