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Performance Of Anchored Pile Walls For A Deep Cut
The Banora Point Upgrade Project in NSW Australia comprised key features including two interchanges, a 300m long viaduct, a large cutting through Sexton Hill with an associated 100m wide Land Bridge and retaining walls reaching 490m in length and up to 22m high. The geology at Sexton Hill comprises a volcanic succession of variably weathered basalt flows and agglomerate deposits overlying Mesozoic sedimentary rocks at a depth of approximately 48m beneath the crest of the hill. Two types of retention system were selected to support the Sexton Hill cutting and were required to suit the ground conditions, satisfy the performance criteria and consider the narrow project corridor. These included cantilever/anchored piled retaining walls and soil nail walls. The paper focuses on the design approach and numerical modelling techniques and compares the predicted and actual performance of the anchored pile wall system based on the monitoring data collected during and post construction. Numerical analyses using computer programs PLAXIS, Phase2 and WALLAP have been undertaken to model the behaviour of the anchored pile wall including wall deflections and anchor loads. In addition, the analyses sought to determine whether neighbouring residential properties were adversely affected by the construction of the cutting. The predicted values of anchor loads and wall deflections have been compared with actual performance and are presented. A back analysis has been performed focusing on a portion of the retaining wall section where the predicted and actual performance differed significantly. The results from the back analysis revealed that the deflections displayed by the piles are particularly sensitive to small changes in stiffness of high strength rock just above the final excavation level.
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Study of the stability of the 90 year old Millswood Underpass protected by revetment walls in unsaturated Keswick Clay
In recent times, there has been much debate regarding the use of unsaturated soil mechanics for the design of earth retaining structures in Adelaide’s semi-arid conditions. The soil-water characteristic curve (SWCC) is the main design tool within this field as a means to incorporate the additional shear strength associated with total soil suction. However, the majority of research to date only incorporates matric suction. This paper is focused on adopting the more commonly measured total suction, and to more effectively capture the behaviour of Keswick Clay as osmotic suction is the dominant component of total suction. Using the Millswood Underpass as a case study, weather data over a 120 year period was used to examine the variations of the stability of slopes in unsaturated Keswick Clay. The reduction in stability due to precipitation-induced flooding, and water leakage due to buried services were also considered. This paper demonstrates the significance of total soil suction to the shear strength of Keswick Clay that is well-known to design consultants but rarely used in slope stability problems.
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Bulli Pass Landslide Risk Management Part 1 – Hazard Assessment
The Princes Highway along Bulli Pass is a narrow, heavily trafficked two lane section of the Princes Highway that traverses steep slopes on a grade of 9H:1V on the Illawarra Escarpment, about 11 km north of Wollongong, and 75 km south of Sydney in New South Wales (NSW), Australia. It is an important arterial road for the northern suburbs of Wollongong, connecting Mt Ousley Road (M1 Princes Motorway) at the crest of the escarpment to the suburb of Thirroul on the coastal plain at the base of the escarpment. Bulli Pass has a long history of landslide and rockfall events, some of which were reported as early as 1890. One of the most significant of these events occurred on 17 August 1998 during a 1 in 100 year rainfall event. The 1998 landslide event comprised approximately 38 debris flows and slides and numerous rockfalls which partially inundated a number of cars and trapped about 15 cars on the pass. More recently, in early 2015, a small rockfall penetrated the windscreen of a car travelling up the pass.
Transport for New South Wales (TfNSW) commissioned an investigation into slope instability hazards affecting the road in late 2011. This was followed in 2015 by a Risk Mitigation Options study and the detailed design of risk mitigation works in 2016. This paper provides an overview of the methods used to investigate hazards and assess risk at the site over a five year period. This has included research into the landslide history, geomorphological mapping, acquisition and review of airborne laser scanning (ALS) data, review of rainfall data and the development of a landslide volume frequency model. The development of this model allowed hazards to be readily communicated and risks to be assessed. The actual design and construction of the Shallow Landslide Barriers and the Debris Flow Barriers that followed on from these assessments will be discussed in a subsequent companion paper.
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Slope instability – Managing the risk A regulator’s perspective
The Wollongong City Local Government Area has records of slope instability dating back to early settlement. Following very rapid growth of the city in the post war years there was little good developable land left and marginal land became progressively more attractive to developers. In 1974 and 1975 periods of prolonged as well as very intense rainfall resulted in extensive hillside instability. Many houses were lost and litigation followed. As consent authority and without clear guidelines on the development of hillside land Council fared poorly in court.
In response, Council quickly sought and received legal advice with respect to what it needed to do to fulfil its legal responsibilities and duties as the consent authority. The legal advice made it very clear that Council is exposed to actionable negligence where it fails to consider whether the land to be developed has potential slope instability. The legal advice also stated that Council may seek a review by its engineers of the basic facts of the submitted geotechnical information and to set appropriate geotechnical conditions to be applied to the development. On this basis Council is entitled to rely on the submitted geotechnical information and any claim would be unlikely to succeed. It is pointed out that in undertaking this review Council does not have a responsibility to provide professional advice but at the same time must avoid being negligent. The success of Wollongong City Council’s geotechnical review process has been demonstrated by Council’s minimal legal costs on geotechnical aspects of development since the 1970s.
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Probabilistic analysis of a spatially variable c’-ϕ’ slope
In practice, inherent soil variability is not commonly considered in routine slope stability analysis. This is due mainly to the fact that the effect of soil variability is complex and difficult to quantify. Furthermore, the majority of available slope stability analysis computer programs used in practice, which adopt conventional limit equilibrium methods, are unable to consider this aspect explicitly. To predict the stability of a slope more accurately, especially the marginally stable ones, the effect of soil variability needs to be accounted for. In this paper, an advanced probabilistic analysis method called the random finite element method (RFEM), developed by Griffiths and Fenton in the 1990s, is used to investigate the effect of soil variability on the reliability of a c’-ϕ’ soil slope. The results from the probabilistic study demonstrate that soil variability has a significant effect on the reliability of a slope. It is concluded that the deterministic factor of safety (FOS) is not a reliable measure of the true safety of a slope with spatially variable soils.
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Geotechnical design of transition structures for the Port Botany Expansion
The Port Botany Expansion (PBE) project involves the construction of an extension to the existing port in Sydney, Australia. The transition between the new structures and the existing Brotherson Dock (EBD) structures is a critical aspect of the geotechnical design. The Client, Sydney Ports Corporation (SPC), specified tight differential movement and settlement limitations for the transition between the new and old structures, including a stringent 5mm differential movement limit (horizontal and vertical) up to 20 years after handover of the new terminal. The subsequent geotechnical and structural design of transition structures included measures to comply with these movement limits. The Main Contractor, Baulderstone Hornibrook – Jan de Nul (BHJDN) are carrying out construction trials, in situ testing and movement monitoring to assess performance against Golder Associates’ (Golder) design predictions. This paper describes the key design issues, design approach and verification processes established to confirm the predicted behaviour of the structures and surface infrastructure in order to satisfy criteria extending up to 50 years following handover.
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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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Study On Shear Response Of Biopolymer-MICP Treated Sand-Steel Interfaces
This paper explores the influence of introducing a natural biopolymer, gum of Prunus scoparia (P. scoparia), to Microbially Induced Calcite Precipitation (MICP) treated soil-steel interfaces. The conventional MICP method, involving low-rate injection of cementation solutions into the soil, faces limitations in terms of cost and practical applicability at a field scale. To address this, the natural biopolymer is incorporated into the MICP process, enabling simultaneous application of the cementation solution and gum without controlled injection rates. Through a series of modified direct shear tests, the study investigates the impact of the biopolymer addition to the cementation solution and its potential to reduce the dependency of shear strength parameters on the cementation solution injection rate in treated sand-steel interfaces. The results demonstrate a significant enhancement in shear strength when the biopolymer is introduced into the MICP-treated soil-steel interfaces, independent of the cementation solution’s application rate. This innovative approach holds promise for achieving more efficient soil stabilization compared to the traditional MICP method.
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The Design And Construction Of Very Deep Excavations – Recent Developments
Technical advancements in construction plant, materials and numerical analysis tools have made possible a step change in the achievable depth of excavations required for infrastructure, building and mining projects. This has been in response to an increased complexity in such projects particularly in connection with rail, water and power infrastructure sectors around the globe. Such advances do not come without some risks and a clear understanding of the limitations of the techniques, capabilities of construction monitoring and the benefits of practical design details are key to successful execution. In addition, a sound knowledge of the behaviour and testing of materials particularly fresh concrete and support fluids is essential in the minimisation of defects in deep earth retaining structures, which can be extremely costly to remediate.
This paper considers the state of the art in the construction of very deep and complicated excavations by making reference to a number of recent case histories, where records have been broken and new technologies have been deployed. The construction of diaphragm walls to depths well in excess of 100 m and with wall thickness of 1800 mm and using concrete with a 28-day cube strength in excess of 60 MPa are now possible, provided great care is taken. Improved verticality tolerances of better than 1 in 400, coupled with precise monitoring and advanced design techniques, means that the structural capacity of earth retaining walls in shaft construction have increased significantly which has led to the realisation of deeper excavations, together with deep openings which may be necessary for associated tunnels.
The author will also include the presentation of recent improvements in safety both in cage lifting, handling and splicing as well as around open diaphragm wall excavations. A better understanding of the causation of defects in concrete which has been placed under support fluid via a tremie, has been gained through painful experience and has greatly benefitted from the recent publication of useful guidance in Australia, UK and by the EFFC (European Federation of Foundation Contractors). This has led to a number of new site tests on fresh concrete for mix stability and bleed potential which are gaining increasing traction in the industry. In addition, the introduction of more stringent testing on support fluid such as bentonite during excavation means that instances of defects including leaks, inclusions and areas of poor concrete cover can be reduced. However, despite the availability of extensive guidance on good reinforcement cage detailing for diaphragm cages, examples of poor practice still remain, with great potential to lead to extensive defects such as mattressing which may compromise the durability of permanent works. The author will highlight examples of good and bad practice.
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Ground surface deformation adjacent to a deep excavation in shale
This paper presents a case study of a deep basement excavation in Sydney’s Ashfield Shale. The intention of the paper is to present the high quality movement monitoring data that was gathered and analysed, and provide some accompanying interpretive commentary. These data may be of use when designing similar excavations.
Three mechanisms observed to cause ground surface deformation adjacent to the excavation are compared and contrasted:
1 – Lateral soil pressure acting on the shoring system, causing deflection of the shoring system.
2 – Excavation induced reduction in confining stress, leading to inwards movement of the rock mass.
3 – Drilling holes for ground anchor construction.
Mechanisms (1) and (2) above are those that geotechnical engineers normally consider in the design of deep excavations. Geotechnical engineers can usually predict these movements with reasonable accuracy. Lateral soil pressures and resulting deflections of shoring systems can be predicted or modelled using a range of basic and advanced techniques. These predicted deflections of the shoring system can be minimised if necessary by adjusting the design and the construction sequence. Rock mass relaxation resulting from a reduction in confining stress is more or less independent of the shoring design and as far as the authors are concerned cannot be controlled by any practical means.
Proven rules of thumb for common geological conditions in Sydney enable these deformations to be estimated with adequate accuracy for the majority of purposes.
Conversely we cannot usually predict ground deformations caused by anchor drilling using theory, modelling, or rules of thumb, with any real accuracy. Such deformations are highly dependent on the volume of soil removed by drilling, which in turn depends on the combination of geotechnical conditions, anchor hole drilling and construction methodology, and anchor design details including spacing, depth, hole diameter and declination. The volume of soil removed by drilling usually exceeds the theoretical volume, thereby meaning theoretical approaches are not usually particularly useful (at least for predictive purposes). These movements (and other movements due to shoring construction, e.g. pile excavation) are not always considered in a deep excavation design.
The case study highlights the significance of ground movements caused by shoring construction relative to total ground movements. Some conclusions are also drawn with respect to predicting and controlling anchor drilling induced deformations and appropriate types of monitoring for similar excavations.