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Improvements To The Observational Method In New South Wales Road Tunnel Construction
The past five years has seen an unprecedented boom in tunnel construction in Sydney. Road tunnels, in particular, continue to push both design and construction to their limits, no less than when Sydney Harbour Tunnel was constructed 30 years ago. Integral to the safe and efficient construction of road tunnels has been the application of the Observational Method in design and construction. This paper describes some important “lessons learned” in implementing the Observational Method in New South Wales road tunnels since the construction of the Sydney Harbour Tunnel.
Construction of the major underground excavations relied on application of the principles of the Observational Method as described by Peck in 1969. This concentrated on validating design assumptions against detailed monitoring data. As confidence in the ability to predict the behaviour of rock masses in the Sydney Region increased, it could be argued that appreciation of the fundamentals of the Observational Method diminished to a process of collecting data for the sake of collecting data, rather than being a live tool to identify and manage geotechnical hazards. The tunnel collapses in the Cross City Tunnel (2004) and Lane Cove Tunnel (2005) led to the industry reassessing tunnel construction risk management. The Permit to Tunnel (PTT) process was born, and at its heart, it provides a means to manage geotechnical risk via a formalised process that includes reviewing of observations by both designers and constructors and agreement to continue construction, within the main principles of the Observational Method.
However, over the past 5 years, the author has observed waning deference to the Observational Method. Construction processes used on major projects, including the Observational Method, the PTT and associated processes have become opportunities for contractors and designers to modify certified designs in an ad hoc manner, often without design changes being adequately reviewed against key criteria such as safety, stability and durability. This paper aims to identify key concerns with the current implementation of the Observational Method through the PTT process. Recommendations are proposed to reset practice to ensure designs are constructed safely and owners are provided with assurances that construction delivers the intended design.
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‘Listening to the earth’: an unconventional approach to mapping basalt flows
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 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 therefore provides a non- invasive and cost-effective way of characterising ground conditions across entire sites, with data acquisition able to take place in areas where drill-rig access is restricted. This paper demonstrates the contribution ReMi shear-wave velocity assessments can make towards obtaining robust subsurface models to mitigate risk from unforeseen ground conditions in Victoria’s booming construction industry. While traditional seismic refraction methods are unable to detect softer soils underneath harder material, ReMi was able to “see through” the basalt intrusions and delineate the upper and lower surface of the flows. This is particularly advantageous in Victoria, where multiple basalt flows are commonly interlain by soil horizons.
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Suction caissons: A new offshore foundation concept
Suction caissons have been used increasingly as a new offshore foundation system in the last decade. This paper goes through a number of different practical uses of suction caissons to date, which demonstrate the advantages and benefits of suction caissons and explain the reasons behind their popularity in the offshore industry. Some results from an experimental study aiming to investigate the possibility of installing suction caissons in layered sand-silt soil conditions are also presented. In particular the soil deformation is discussed which, when suction is applied, can be traced using an image-based technique in a half-caisson model test.
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The Role Of Progressive Brittle Fracture In The 1931 Landslide At Dogface Rock, Katoomba
The 1931 Dogface Rock landslide in Katoomba NSW was a complex, progressive cliff collapse with a failure volume in the order of 100,000 m3 that was triggered by the extraction of remnant coal pillars from the Katoomba Colliery, about 200 m below the top of the escarpment. Although underground coal mining is generally accepted as a cause of the rockslide, previous studies have not explicitly investigated the role of progressive brittle fracture in the collapse. This paper presents an integrated study which incorporates remotely piloted aircraft photogrammetry with a discrete element method numerical investigation of the landslide, and thereby explores the role of progressive brittle fracture, and re- examines the failure mechanism and runout motion of this multi-stage landslide.
Remotely piloted aircraft photography is used to build a georeferenced 3D model of the site with Structure-from-Motion photogrammetry software. A digital geotechnical mapping workflow is demonstrated to investigate the morphology of the landslide scar, extract statistics on discontinuity orientation, persistence, and spacing, and undertake trace mapping of newer brittle fractures that interacted with pre-existing high persistence joints as the landslide rupture surface developed. A series of discrete element method numerical laboratory tests are used to calibrate bonded block contact properties that reproduce laboratory scale intact rock index parameters including UCS and tensile strength. Upscaled rock block contact parameters are then applied to a cliff-scale model that investigates the progressive development of rock mass damage induced by mining. Following extraction of the remnant pillars, rock mass damage develops mostly by extensile strains that produce tension cracks. Brittle fractures propagate upwards from the mine level and eventually initiate toppling of massive sandstone slabs defined by high persistence pre-existing subvertical joints. The investigation illustrates how the integration of photogrammetry with discrete element numerical methods can be used to characterise progressive brittle failure and runout of large rock slope failures.
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Recent developments in continuous flight auger (CFA) and cast-in-situ displacement screw piling in Melbourne
This paper addresses the significant advances which have been made in the fields of CFA and cast-in-situ displacement screw piling over the past ten years. Whilst the majority of these advancements are ubiquitous, this paper deals with the effect of such advances with respect to their application in the Melbourne metropolitan area.
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SA-NT AGM, followed by the 55th Rankine Lecture:
Hazard, Risk and Reliability in Geotechnical PracticeDr Suzanne Lacasse, Norwegian Geotechnical Institute (NGI)
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A Study Of The Accuracy And Precision Of Some Landslide Risk Analyses
The results of about 600 landslide risk analyses carried out by the Roads and Traffic Authority of NSW, Australia were reviewed. The analyses relate to a variety of cuts, fills, retaining structures and natural slopes adjacent to the NSW road network and had been assessed using a new procedure based on quantitative landslide risk methodology, but using qualitative rules to assign ratings and matrices to combine the ratings. Measurements of the precision, accuracy and applicability of the procedure are presented. It was found that whilst the results of the landslide risk analyses are suited to prioritising slopes for maintenance programmes, the review demonstrates that it is difficult for experienced practitioners to carry out reliable landslide risk analyses without adequate training.
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Groundwater Inflow Assessment For Deep Basement Excavations: A Case Study
Approvals for proposed building developments in Sydney are granted by the consent authority with input on groundwater-related matters from NSW Office of Water. Developments with deep basements require approval for construction dewatering. NSW Office of Water input considers the requirements of the NSW Aquifer Interference Policy, including assessment of the excavation’s “groundwater take” and potential impacts associated with dewatering.
In order to adequately assess the potential impacts associated with construction dewatering, and to design appropriate construction dewatering systems, it is important to accurately estimate groundwater inflow rates to deep basement excavations during construction.
This paper discusses and compares established methods to assess groundwater inflows to deep basement excavations, including analytical, analogue and numerical approaches. A case study for a proposed development in Sydney is used to demonstrate differences in estimated inflow based on these approaches, and highlight the benefits and disadvantages of each approach. Consideration is given to geological structures, basement design, and uncertainty in conceptual models and aquifer parameters that can complicate accurate assessment.
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Designing For Brownfield Sites – A Young Geotechnical Engineer’s Perspective
Upgrading existing road infrastructure is becoming increasingly common in Sydney. The growing population places increasing demands on existing road infrastructure, requiring upgrades to cope with growing traffic volumes. Some of these upgrades require structures such as retaining walls and bridges. Upgrades sometimes require modifications to existing structures, which further complicates design and construction. Working on ‘brownfield’ sites adds further complexity to the design of both new and existing structures. Site constraints such as limited space, low headroom and both buried and overhead utilities are some of the factors that contribute to the complexity of designing on brownfield sites. Furthermore, sites are usually required to remain open to traffic during investigations and construction. This paper focuses on the methodologies the Author has learnt and adopted to address these challenges to geotechnical work during the site investigation, design and construction phases. Case studies are presented to illustrate some of the challenges and solutions.