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Design of large span tunnels and caverns: back to basics
Increased demand to future-proof tunnel projects with respect to traffic has led to the proposal of some very large spans in recent road tunnel projects in Australia. For example, four lane tunnels are currently under construction in Sydney with mined spans of approximately 20 m and Y-junction caverns of unprecedented spans for road tunnels in Australia, all with a requirement for 100-year design life. As these spans are unprecedented in Australian civil tunnels, a direct comparison with local past experience is not possible and simple extrapolation of precedent designs, although potentially solving the problem, often result in uneconomical solutions that do not necessarily target the actual failure mechanisms involved in the excavation of such large spans. International experience could certainly be used but adequate design justification would still have to be provided. Although there is certainly room for cutting edge innovation, robust solutions can also be achieved by simply going back to basics. As a result, this paper intends to present and discuss how designs that focus on first principles and the basic objectives of rock reinforcement may allow for a better understanding of the design requirements and how to satisfy codes and standards but also provide savings with respect to ground support. The key to the design involves understanding the failure mechanism that needs to be addressed, its relationship with the different actions of rock bolting, i.e. suspension/anchorage and/or rock reinforcement and what could be acceptable. -
3.2 Blends of recycled materials as sustainable alternatives for backfilling sewer trenches
In this project, the suitability of four blends of recycled materials comprising different proportions of recycled glass, plastic, and tyre aggregates as alternative backfilling materials for deep excavated trenches was investigated. This paper presents results of an extensive testing program carried out for selecting two most appropriate blends for backfilling of trenches located in non-trafficable areas. These blends will be used for construction and instrumentation of trial sites for deformation monitoring over 12 months. Physical properties such as particle size distribution, maximum and minimum density, compaction properties, and field capacity of the blends were determined. Further, an application-specific geotechnical testing methodology was developed. This included determination of the dry density achieved using a proposed sand-raining technique (SRT) to simulate the real-life trench backfilling procedure and determination of the compressibility of the blends using a modified oedometer test. The SRT results showed that the obtained dry density (DD) increased as moisture content (MC) and height of drop increased. Based on the relative density achieved through the SRT compaction and the compressibility properties, Blends 2 and 4 showed the most advantageous characteristics. This paper presents the developed testing procedures and discussions on the results leading to the selection of the two most suitable blends for the proposed application. The paper also makes discussions on the stress-strain conditions expected on site and potential downfalls of the proposed application. The outcomes of this research aim to promote sustainable geotechnical design and construction by improving the industry’s confidence in utilising recycled materials.
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Design And Construction Of An Impermeable Silt Curtain In A Tidal Zone
A new design and construction solution was needed for a sediment remediation project in Homebush Bay, NSW. When faced with the requirement to remove the top 0.5 m thickness of deep very soft sediments containing persistent organic pollutants (including dioxins) and replace this with inert material laid on a geofabric, the ‘traditional’ earth bund coffer dam and/or sheet pile approach had an elevated risk of contaminant migration, and was a significantly more expensive methodology than the Impermeable Silt Curtain (ISC) designed by GHD and constructed/managed by Thiess Services. This is understood to be the first occasion on which an ISC was used for such purposes within a tidal zone (tidal range approximately 2 m). The final constructed length of the ISC was approximately 1100 m. The environmental advantages of this system over the ‘hard’ forms of construction originally proposed included: much less disturbance to the contaminated soft sediments, control of odours by conducting all works beneath water cover and an effective (impermeable) barrier against sediment migration into the bay (not able to be obtained by conventional silt curtains). The design utilised a tough impermeable geotextile, mounted on single piles at regular spacings and bottom weighted by chain link into the underlying ‘muds’. It also included ‘windows’ with flap covers in the top 300 mm, opened at a calculated distance from the dredging/placement work, which allowed tidal water transfer while ensuring that the sediment had settled to a level below the window. The work was a resounding success, albeit it required a high degree of management as part of the design.
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Embankment design and construction for a major rail upgrade project in Western Australia
Australia is one of the largest iron ore producing countries in the world. As a result of increased international demand for iron ore, development of new open pit mines with associated infrastructure has been on the rise in Western Australia. Design and construction of new rail lines and duplication along existing rail lines have been one of the key issues for timely delivery of iron ore from mine to port. This paper presents geotechnical issues associated with the design and construction of a major rail duplication project in the Pilbara Region in Western Australia that include cuts and embankments up to 20 m in height. The geotechnical issues include geotechnical investigation, sourcing borrow materials for construction of embankments including sub ballast capping, slope stability, settlement and construction methods. A variety of rock formations were encountered along the alignment comprising igneous, sedimentary and volcanics with variable degree of weathering. Slope stability assessment was undertaken using limit equilibrium method, kinematic analysis concept and visual assessment. Deformation analyses were undertaken using PLAXIS computer program. Geotechnical investigation, selection of parameters for engineering analysis and method of design during the construction stage are discussed in this paper.
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Pavement materials and surfacing aggregates used in road construction in Perth
The principal materials used for base, sub base and surfacing in the construction of roads in Perth are bitumen stabilised limestone, crushed limestone, crushed granite, crushed dolerite, hydrated cement treated crushed rock base, lateritic gravel and crushed massive laterite (ferricrete). The selection of the appropriate materials for a particular project depends on factors such as the drainage environment, traffic loading, traffic speed and cost. This paper presents typical specifications, properties and applications of the materials used in road construction in Perth.
“The size of stones for a road has been described in contracts in several different ways, sometimes as the size of a hen’s egg, sometimes as at half a pound weight.” John Louden McAdam 1811.
“It is well known that the more clean and free from dirt the broken stone laid on roads is, the better”. Major-Gen Sir J Burgoyne 1844.
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Construction Methods And Associated Risks For Rigid And Flexible Retaining Walls
This paper describes two types of retaining wall systems, one is a stiff secant piled wall (usually resulting in small wall deflections) the second one is a less rigid and more flexible sheet piling wall (usually having larger allowable wall deformations). Furthermore the water retention capability of both wall types will be discussed as well as the requirement to laterally support the walls with props or anchors.
It is important to design the retaining wall system according to the medium which needs to be retained (usually soil or water); the surrounding ground conditions, the allowable movements of the retained soil behind the wall after excavation and the intended purpose of the retaining structure. Allowable movement is of particular importance to minimize the settlements of adjacent buildings and structures.
The different construction methodologies of the retaining wall types will be described and the main advantages and disadvantages of each system will be highlighted. Furthermore the major construction risks will be discussed.
The paper will also include a case study of a recently completed retaining wall project in NSW. Piling Contractors successfully installed a sheet piling wall for the construction of a cofferdam in the middle of a watercourse. Additionally a hard/hard secant pile retaining wall was installed inside the cofferdam with sockets into high strength rock using 1180 mm and 1300 mm piles. The requirements for both, water tightness and tight vertical wall tolerances of 1:200 were successfully achieved and the paper points out the major construction activities and monitoring techniques of this challenging project.
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Laying the foundation for the Sydney Light Rail
The Sydney Light Rail project alignment covers the typical geological and geomorphological settings for the Sydney and Botany Basin. This comprises three distinctive zones: (1) manmade fill; (2) recent Quaternary aeolian, alluvial and estuarine sediments, which overlie; (3) residual soil and weathered to fresh bedrock. When investigating subsurface conditions within a metropolitan area, various constraints can have impacts on the scope and extent of geotechnical investigations that can be safely and practically completed. For the SLR project, investigation of the subgrade conditions along the alignment was significantly constrained due to the route which follows existing roads through the CBD and eastern suburbs. Constraints for working in these areas included complex traffic management planning to avoid disruptions to the community and also an equally challenging network of underground utilities and other service tunnels. These constraints required thorough planning for traffic management and consultation with utility service providers, thus significantly reducing the amount of work that could be completed within the program. A robust geotechnical validation regime was developed so that the in-situ subgrade could be re-assessed prior to track slab construction, to mitigate the limited extent of geotechnical investigation undertaken. The objective of the site validation regime was to verify the inferred design subgrade California Bearing Ratio values along areas where geotechnical investigation was constrained. The site validation regime included nominally spaced plate load testing, dynamic cone penetrometer testing and laboratory CBR testing to assess the subgrade CBR value. This paper explains the challenges faced in validating in-situ CBR in different subgrade conditions, appropriateness of published DCP to CBR empirical methods and the approach using calibrated site specific DCP-CBR correlations.
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Quantitative Risk Assessment Of The Thredbo Landslide
A quantitative assessment of the risks associated with landslides from the Alpine Way was completed in 1998 as part of the advice provided on behalf of the NSW Coroner. The objectives of this risk assessment were to provide a basis for understanding the Thredbo Landslide and to compare the estimated risks with those generally accepted or understood in society. The assessment reported here considered the situation at the time of the landslide; considerable works have been completed since the landslide and it is considered that this assessment would no longer apply.
The assessment focused on the Alpine Way and did not include assessment of the natural slopes well away from the road because:
- the natural slopes along the Thredbo Valley showed no evidence of significant mobility of natural slides in the recent past, and
- over the last 43 years the only known mobile landslides in the area of interest had been associated with the Alpine Way. It is possible that mobile landslides from above the Alpine Way could occur in the future but these have not been considered further in this paper.
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Resilient modulus of some Victorian fine-grained soils at OMC, wet-of-OMC and soaked conditions
A typical flexible pavement structure consists of an asphalt wearing course and the underlying base and sub-base courses. The subgrade soil is the foundation of the pavement. In recent years, the resilient modulus has been recommended by pavement design guides as an important indicator for characterising the resilient behaviour of these pavement materials under dynamic traffic loading. The resilient modulus can be obtained from the repeated-load triaxial test in the laboratory. This paper reports and discusses some of the results from a study on the evaluation of the resilient modulus for eight Victorian fine-grained soils at different moisture contents and stress levels. The effects of the deviator stress, confining stress, moisture content and plasticity index of soils on the resilient modulus have been investigated, and a relationship has been established and discussed for the soils used in this study.
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A rational procedure for the evaluation of soil design parameters for use in Adelaide
This paper is concerned with site investigation practices that are used as part of the design of foundations for civil engineering infrastructure in Adelaide. The semi-arid conditions in Adelaide mean that the results of any soil testing need to be viewed within the context of soil moisture deficiency, and the soil parameters adopted for design need to take into account the critical moisture profile likely to be encountered over the service life of the structure. It is argued that guidelines should be developed based on the soil moisture conditions in order to ensure that realistic soil parameters are used in analyses and designs. The paper presents a procedure for selecting shear strength and compressibility design values. The variations in total soil suction profiles due to construction activities and over the service life of the foundation are highlighted.