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Behaviour of fiber reinforced soil
The increasing value of land and the limited availability of sites for construction are greatly encouraging engineers to considered in situ soil improvement of weak soil deposits. Geotechnical engineers often encounter problems in designing foundations of structures on soft clayey soil. There may be a need for ground treatment to improve the bearing capacity of the soil. In granular soils in situ the soil may be very loose and indicate potential large elastic settlement. Under these conditions soils need to be densified to increase the unit weight and shear strength. The soil at a construction site or part thereof is not always totally suitable for supporting structures. In practice admixtures with fly ash, lime and geogrids are used frequently to stabilize soils and improve their load carrying capacity. Polypropylene fibers have been extensively used in civil engineering applications for many years. These fibers are used in concrete as a three dimensional secondary reinforcement. The influence of randomly oriented polypropylene fiber on the engineering behaviour of soil has not been reported to the same extent. Ease of application and reduction in cost are making this treatment more popular. The purpose of this investigation is to identify and quantify the influence of fiber variables (content and length) on performance of fiber reinforced soil specimens. In this study fibers were mixed with soft clay in various proportions (0%, 0.5%, 1.0%, 1.5% and 2.0%) to investigate the relative strength gained in terms of compaction, CBR, unconfined compression, etc. This paper presents a review of existing experimental and analytical work in this field and identifies other areas needing attention.
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Responses Of Free-Standing Railway Embankments As A Consequence Of Mine Subsidence In The NSW Southern Coalfield
To support our civilisation’s requirement for carbon steel, longwall mining of the Bulli Seam at Appin and Tahmoor Collieries has occurred to recover high quality coking/metallurgical coal, being for steelmaking. The Main Southern Railway crosses the footprint of both mines, and it was therefore important to manage the risk to infrastructure and public safety during longwall retreat in this strain-driven environment. One technical issue is the response to subsidence-induced ‘valley closure’. This has occurred in both gently undulating Wianamatta Group Ashfield Shale and the steeper upper Hawkesbury Sandstone valleys.
This paper covers the observed responses of embankments on the Main Southern Railway and the heritage railway Picton to Mittagong Loop Line, and illustrates the responses of the embankments to valley closure that produced up to 11% strain, as well as illustrating displacement field development and derived principal strain vectors. The responses of four embankments are presented beneath which longwalls have been successfully extracted, and done so without adverse impact upon public safety. An understanding of the strain-driven responses of the embankments is presented.
This paper is adapted from and builds upon a submission to 11th Triennial Conference on Mine Subsidence – refer to Leventhal et al (2022).
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Pile Testing Verification – an Alternative Approach
Dynamic pile testing is undertaken for a number of reasons including: 1. To confirm that the pile meets serviceability and geotechnical capacity requirements; 2. To assess pile integrity, either during installation (driven piles) or after construction (cast-insitu piles) and 3. To verify that the piling hammer delivers the energy required to satisfy the design criteria and that stresses during testing are kept within acceptable limits. In addition, testing allows us to establish and calibrate acceptance criteria – relationships such as resistance vs set curves, and/or correlated pile driving formulas. These relationships are premised on the interrelationship of capacity (C), transferred energy (E) and pile movement (M) which is represented primarily by pile set. These ECM relationships allow capacity to be inferred from measurement of transferred energy and pile movement and are used to infer the capacity of untested piles. However, for a variety of reasons, transferred energy can vary significantly between piles which, being undetected, undermines the reliability of ECM relationships. An alternative approach to using ECM relationships is proposed based on pile set and pile force (F). We demonstrate through parametric studies and review of project data, that these FCM relationships are reliable alternatives which bypass the problems with variable energy transfer. Of course, impact force will also vary with hammer performance, but impact force can generally be accurately determined from the measured impact velocity as a proxy. Pile velocity can be measured by attaching a single accelerometer to the pile, or by using a high frequency displacement monitoring device. FCM-based acceptance criteria have the significant advantage that both the necessary force (F) and displacement (M) inputs can be verified by simple measurements on all untested piles.
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Frequency Of Landsliding As Part Of Risk Assessment
An integral part of systematic risk management for landslides is the assessment of landslide frequency. Qualitative approaches have frequently been used for landslide risk assessment but there is now an increasing trend towards quantitative assessments. In Australia, this has been highlighted by the publication of a comprehensive paper by the Australian Geomechanics Society in March, 2000. This paper replaced an earlier 1985 publication which provided guidelines and recommendations for qualitative risk assessment based primarily on site inspection, previous experience and engineering judgment.
The assessment of landslide risk requires assessment of hazard, elements at risk and the consequences of landsliding on those elements. Attention must be given to the mobility of a landslide as well as to the vulnerability of elements at risk before the consequences can be assessed reliably. A hazard-consequence matrix approach is often a convenient framework for an integrated approach to risk assessment on a qualitative or quantitative basis (AS/NZS, 1999; Flentje et al 2000; Walker et al., 2000).
The first important stage of any investigation concerns the assessment of landslide hazard which is often influenced by a range of factors. It is important to consider the basic causes and mechanisms of slope instability as well as the triggering factors. For example, the most common natural triggering factor in Australia is rainfall. Therefore, frequency of landsliding is often closely related to the intensity and duration of different rainstorms.
This paper will discuss different aspects of a landslide risk assessment task with particular reference to quantitative assessment of the frequency based on historical and observational data. Reference will be made to the monitoring of surface and subsurface movements and piezometric levels as well as to the detailed historical and spatial analyses of rainfall records.
The procedures and methods proposed in this paper will be illustrated with reference to two case studies. Both of these case studies are defined as ‘slide’ category (Cruden and Varnes, 1996) landslides and it is important to note that this paper is directed at the assessment of this type of failure mechanism.
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In situ rock stress and its effect in tunnels and deep excavations in Sydney
The behaviour of rock surrounding an underground opening depends primarily on the strength of the rock mass and the level of stress existing in proximity to the excavation. As the excavation is made, a redistribution of the virgin stress
field occurs, leading generally to a concentration of stress parallel to the wall, and a relief of stress normal to the wall. If the concentrated stress magnitude is high enough relative to rock strength the rock mass can dilate or fracture.
Likewise, the relief of stress normal to the wall can lead to loosening and unravelling of blocks of rock bounded by discontinuities in the wall.The role of the virgin in situ stress field has become well recognised in the Sydney area over recent years. Most geotechnical investigations for mining and civil engineering projects in the Sydney Basin will include a consideration of this factor, often involving direct stress measurements. This paper gives a brief summary of some of the factors controlling in situ stress, the historic evidence for the role of stress in the Sydney area, a discussion of the results of a large number of stress measurement campaigns, and a description of a number of projects undertaken in recent years that illustrate the importance of stress in the Sydney Basin.
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Green Square — Enabling Urban Renewal Through Smart Retaining Wall Design And Trenchless Construction
Green Square is one of the City of Sydney’s key urban renewal precincts, which is being transformed from old industrial land into a major new residential, retail and cultural hub. The Green Square Stormwater Drain (GSSD) is the culmination of a strategic alignment between City of Sydney and Sydney Water to provide flood protection in the Green Square area. Through a process of optioneering and hydraulic analysis, a new 2.5 km long underground drain consisting of multiple 1800 mm diameter pipes was installed by microtunnelling, and an open trench box culvert was replaced with channel widening via an anchored retaining wall in the final 300 m from Maddox St to Alexandra Canal. The new drain augments the existing trunk drain system and reduces flood hazard, allowing Australia’s largest urban renewal project to proceed.
The channel widening section of the GSSD was originally intended to be constructed into the bank of the existing open channel. A constructability assessment for installation of the box culvert within the narrow corridor between the existing open channel and adjacent buildings indicated that open trench box culvert construction would not be cost effective. This paper describes an innovative solution, where the existing channel was widened using an anchored retaining wall, replacing the proposed box culverts.
The trenchless (microtunnel) solution offered an alternative, value for money approach with significantly reduced environmental impact and achieved comparatively minimal community disruptions.
This paper also describes the ground engineering challenges and solutions employed on the site which included difficult ground conditions, landfill and addressing impacts of wall construction on adjacent infrastructure such as roads, bridges and buildings. Ground engineering risks were successfully managed through detailed scoping of investigations, numerical modelling of designs and adoption of observational methods during construction. The specification requirements, design, installation, monitoring and performance of the successful microtunnel drain and anchored wall system are discussed.
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Welcome To Geotechnical Engineering In South-East Queensland “Soft Clay One Day, Hard Rock The Next”
South-East Queensland is loosely defined as the area from the NSW border in the south to Gympie in the north and inland to the Great Dividing Range. The main population centres include Brisbane, Ipswich, and the Gold and Sunshine Coasts. It is a geologically complex area and consequently presents the geotechnical newcomer with a wide range of geological environments and their associated challenges.
In South-East Queensland we are fortunate to be well supplied with detailed geological maps, notes and texts from the Queensland Division of the Geological Survey of Australia, and this is the best place to start in order to understand the local geotechnical conditions. The publications available not only describe the detailed superficial geology of the area, but also, through a series of notes and texts, provide a clear outline of the geological development, rock types and stratigraphy. A simple text, which provides a very thorough geological introduction for the newcomer, is “Rocks and Landscapes of Brisbane and Ipswich” by W Willmott and N Stevens (Geological Society of Australia, 1992). Similar texts are available for other areas through Southern and Eastern Queensland. The other essential geological information available for the area is the1:100,000 and 1:250,000 geological maps. A very good series of maps, now no longer available, but still held and jealously guarded by many practitioners, is the 1965 1:31,680 series. Maps, memoirs and books can be obtained from the Queensland Division of the Geological Society by contacting them on [email protected].
To complement the geological maps, there are also soils maps available. The Department of Natural Resources and Mining, Qld (DNR) published two Acid Sulfate Soils Maps at 1:100 000 scale, which show the distribution of potentially acid sulfate soils along the coast between Tweed Heads and Noosa. The DNR also publishes several soils, vegetation and terrain maps which can be of use. These can be obtained via their website at www.dnr.qld.gov.au.
In hard rock engineering jobs, such as deep excavations in rock, rock slope stability or road cuttings etc, the rock type generally dictates the investigation and design approach. The strong, massive and widely jointed Brisbane Tuffs lend themselves to deep excavations and steep slopes, with dominant jointing patterns dictating failure modes. Support measures such as bolting and anchoring are typically adopted when necessary. The highly foliated metasediments of the Neranleigh-Fernvale beds and the related Bunya Phyllite pose different failure mechanisms and support approaches. Foliation directions and the extent of weathering are critical, and minor block and wedge failures are much more prevalent. Rockfall mesh, flatter slope angles and surface protection are typical support approaches, in addition to bolts and anchors. The Tuff–Phyllite interface is of particular importance, and clay seams, carbonaceous material and paleo- soils have all been encountered at these locations. Further afield, the Mesozoic sedimentary beds, and also the younger Tertiary sedimentary and volcanic rocks cover a large area of the south-east. These require a different geotechnical approach, with generally deeper weathering profiles and larger variations in jointing, bedding and strength.
The stability of natural slopes throughout the south-east is of concern in many areas. Consequently, engineering works routinely require special attention. The steep slopes of the Main Range from the Gold Coast Hinterland, through to Toowoomba and the D’Aguilar ranges, and behind the Sunshine Coast, have always been prone to stability problems. The south-east’s sub-tropical rainfall, characterised by intense summer storms, contribute to instability in these areas. Other areas of inherent instability occur, typically but not always due to localised steep terrain and concentrated storm runoff. In the Brisbane suburb of Oxley for example, the weathering of the underlying rocks of the Oxley Group has resulted in an area that has experienced numerous large landslips, although the natural slopes are no steeper than many other stable areas within Brisbane.
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Preload Design, Part 1 — Review of soil compressibility behaviour in relation to the design of preloads
The method of treating soft soils by preloading has been used for over a century, and is still widely used today as one of most common form of ground improvement technique. Yet, every now and again, post-construction settlements have been observed to be more than those predicted after preloading. The author believes, in most cases, the poor preload performance is probably associated with lack of understanding of the time-dependent compressibility behaviour of the soft soils. And in particular, the behaviour of secondary consolidation (or creep) is still not well understood despite extensive research and numerous constitutive models that have been developed. The availability of powerful commercial computer programs does not help if they are used indiscriminately when the fundamental principles are not well understood.
Part 1 of this paper provides a review of the factors that influence creep. The dependency of creep on stress level and stress history expressed in terms of the over-consolidation ratio (OCR) is discussed, followed by a discussion on the commencement of creep. A brief overview of time-dependent consolidation and creep settlement analysis methods is provided, followed by a summary of the preload design approach given by Mesri (1991) that illustrates the possibility of the occurrence of higher creep rate some time following preloading.
In Part 2 of this paper, an analytical approach based on Bjerrum’s (1967) time line model, or principle of “artificial aging” will be presented for preload design to limit post construction settlement, and a preload design example is discussed to illustrate the importance of geological and stress history on post-preload settlement behaviour.
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Design Of Slope Stabilizing Piles For Reinforced Soil Walls On Hunter Expressway
The Hunter Expressway will provide a 40 km long four-lane divided carriageway motorway between the F3 Interchange at Newcastle and the New England Highway at Branxton, New South Wales Australia. The project is due to be opened by the end of 2013. The Hunter Expressway Alliance (HEA), comprising Roads and Maritime Services (RMS), Thiess Pty Ltd, Parsons Brinckerhoff and Hyder Consulting, is responsible for the design and construction of the 13 km eastern section of new freeway and local road adjustments. There are 28 bridges and major culvert structures and 29 Reinforced Soil Walls (RSWs). This paper discusses the design challenges faced by the RSW designers and the innovative engineering solution developed for RW17, a 120 m long RSW up to 10m in height on sloping ground with foundations containing bands of low strength tuffaceous claystone. To achieve the minimum design factor of safety (FOS) of 1.35 for the overall slope stability of the RSW as stipulated in RMS Specification R57, three rows of 450 mm/750 mm diameter and one row of 1500 mm diameter bored piles were designed and adopted at various selected sections along the 120 m long slope. Both the limit equilibrium program Slope/W and the finite element program PLAXIS were used to assess the FOS for the global stability of the RSW, ground movements during and after RSW construction and forces in the piles. Two inclinometers were installed to monitor the field lateral ground movements during and after construction to verify the design assumptions. This paper describes the challenging ground conditions, the development of the stabilising pile design, the analytical models used and the results of the construction phase monitoring of the completed RSW.