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Annualised Probabilities of Pit Slope Failures Using Two Methods
A novel framework for assessing the annualised probabilities of slope failures in open-pit mining is presented by integrating traditional geomechanical models with a comprehensive semi-quantitative assessment of multiple sources of information available for the pit design. The methodology is developed through a comparative analysis of two distinct methods applied to a real-world case study, allowing for a deeper understanding of slope failure probabilities and their implications for risk management. The paper emphasizes the importance of moving beyond conventional adimensional probability of failure (APoF) definitions to estimate annualised probability of failure (PoF) values. A novel quantitative methodology is introduced to estimate the annual PoF of slopes based on geomechanical values, incorporating considerations of slope lifespan and factor of safety (FoS). Additionally, ORE2_Slopes is presented as an alternative semi-quantitative probabilistic evaluation method, and its estimates are compared with literature benchmarks and real- life pit experiences. The study highlights the significance of estimating annualised PoFs for quantitative risk assessment (QRA), considering various parameters such as time dependency, human factors, and monitoring for a comprehensive risk evaluation. The methodology aims to provide a unified framework that balances geomechanical considerations with expert judgement assessments, offering practical tools for informed decision-making in open pit designs.
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Design And Construction Of A Geofoam Embankment Over Soft Ground
Development of the new Service Centre required construction of an off-ramp from the existing road together with an internal road network. Together these will provide access to the Service Centre for traffic from the existing roads. The off-ramp and road network were constructed on fill embankments, up to 4m in height, overlying a relatively uniform but very significant thickness of soft clay. In addition, the project site had been subject to a complex history of loading and unloading as a result of previous development on the site and ground improvements adjacent to the site, as part of new Service Centre development. To mitigate the effects of post-construction settlement to acceptable levels, safe and cost-effective ground treatment measures were developed along the preferred road alignment.
This paper provides some background to the project and presents in detail the design and construction methodology adopted for the lightweight fill treatment works using geofoam. The new geofoam embankment had to be constructed within the vicinity of existing and new infrastructures, i.e. pile-supported bridge, existing roads and a new drainage structure.
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Advances In Diaphragm Wall Construction
Diaphragm walls are reinforced concrete retaining wall structures that are constructed by excavating the ground using slurry supported trenches. Although diaphragm walls have numerous benefits, and are a favourite option of engineers who intend to retain the ground without massive preparatory excavations or who desire to create dry excavated work places, and even though this type of wall has been in use for approximately 40 years in Australia it is the experience of the authors that this technology has not received sufficient exposure, its benefits and its construction process still remains mysterious to the typical geotechnical engineer. In this paper the history of diaphragm walls and the benefits of this wall system will be briefly presented, which will then be followed by a discussion on the how diaphragm walls are constructed and how technology has evolved and advanced throughout the past several decades. Reference to projects will also be presented to further demonstrate local Australian and international achievements.
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Differentiating Fill And Natural Soft Clays — The Value Of Desktop Studies In Building A Geological Model
This paper presents the methodology and rationale used when differentiating fill and natural soft clays in a site on waterfront reclaimed land. The area of reclaimed land has a history of settlement impacting existing developments, which has motivated careful consideration of the ground conditions for future works. As the reclaimed land was dredged locally then loaded by fill and warehouses, it closely resembles the underlying estuarine and marine sediment lithology and consistency.
Two historical ground investigations and one Arup ground investigation have been conducted, including clusters boreholes, cone penetrometer tests, seismic dilatometer tests and lab testing. The four types of subsurface information were compared to build and verify a ground model with an emphasis on the extent of reclaimed land. Peripheral desktop study information including historical maps, imagery, sea level records, and the historical settlements observed at site were considered to assist in differentiating the fill and natural soft clays. The importance of understanding this site’s history reinforces the value of a thorough desktop study when developing geological models.
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Bridge Approach Treatment Works On The Coopernook To Herons Creek Section Of The Pacific Highway Upgrade
The Coopernook to Herons Creek Alliance was formed with the New South Wales Roads and Traffic Authority (RTA), Parsons Brinckerhoff and Thiess Contractors to design and construct the upgrade of 32.7 km of the Pacific Highway to dual carriageway between Coopernook and Herons Creek on the NSW Mid-North Coast. The upgrade works involved the construction of 15 new bridges, including two major river crossings over the Stewarts River and the Camden Haven River, which will duplicate the existing bridges. Both bridge approaches were constructed on embankments supported by a geotextile reinforced granular platform over soft ground adjacent to the existing bridges. Embankment movement, if significant, would not only affect the pavement performance but also impose additional loads on the piles supporting the adjacent bridge structures.
To mitigate settlement effects, the bridge approach treatment works consisted of displacement columns to support the bridge approach embankments and a geotextile reinforced load transfer mat constructed on top of the displacement columns to assist in distributing embankment load. The main challenges were construction on soft ground, requirements to maintain design life of the existing bridges and the need for an accelerated construction technology for timely delivery of the project. This paper outlines the project, presents the case history of the bridge approach treatment works, and discusses analysis, design and construction monitoring.
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Effect of anisotropy on the bearing capacity and deformation of sand
Although the effect of anisotropy on soil behavior due to the parallel alignment of particles remains a subject of great interest, studying its effects on load deformation behavior of stabilized sand has yet to be undertaken. Therefore, this paper presents a study of the behaviour of a laboratory footing model, supported on anisotropic cemented sand, considering two relative densities and three cement contents. The initial anisotropy was induced by preparing samples with different orientations of the bedding plane α (from 0° to 90°). The results show a strong effect of anisotropy, wherein the ultimate pressure of untreated sand took place when α = 0° while the minimum value was achieved at α=60° & 75° for loose and dense sand respectively. Cementation results in clear elimination of variation in bearing capacities, and this significant reduction in anisotropy is accompanied by much improvement in ultimate bearing capacity and stiffness. However, soils with α in the range of 60°–90° still exhibit some little anisotropic behaviour.
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Bridge approach treatment works on the Coopernook to Herons Creek section of the Pacific Highway Upgrade
The Coopernook to Herons Creek Alliance was formed with the New South Wales Roads and Traffic Authority (RTA), Parsons Brinckerhoff and Thiess Contractors to design and construct the upgrade of 32.7 km of the Pacific Highway to dual carriageway between Coopernook and Herons Creek on the NSW Mid-North Coast. The upgrade works involved the construction of 15 new bridges, including two major river crossings over the Stewarts River and the Camden Haven River, which will duplicate the existing bridges. Both bridge approaches were constructed on embankments supported by a geotextile reinforced granular platform over soft ground adjacent to the existing bridges. Embankment movement, if significant, would not only affect the pavement performance but also impose additional loads on the piles supporting the adjacent bridge structures.
To mitigate settlement effects, the bridge approach treatment works consisted of displacement columns to support the bridge approach embankments and a geotextile reinforced load transfer mat constructed on top of the displacement columns to assist in distributing embankment load. The main challenges were construction on soft ground, requirements to maintain design life of the existing bridges and the need for an accelerated construction technology for timely delivery of the project. This paper outlines the project, presents the case history of the bridge approach treatment works, and discusses analysis, design and construction monitoring.
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2021 AGS Victorian Symposium
Innovations in Geotechnical Design
Dr Scott Taylor, Professor Jinsong Huang, Dr Julian Seidel and Chris Lyons
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Deep Excavations In Soft Ground Using Temporary Structural Steelwork
In many city areas, ground anchors and soil nails cannot be used for deep excavation retention in soft ground for various reasons. The solution is often braced excavations involving temporary structural steelwork, serving multiple functions, like strutting, waling, lagging, traffic and construction decking. Based on the author’s design and supervision experience on underground metro station, rail, expressway and commercial mall projects, this paper discusses the design, construction and monitoring of steelwork applications in deep excavation environments.
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Recent advances in practical design of rock socketed piles in Victoria
Piles socketed into rock are a common foundation solution to carry large loads from buildings and bridges. They have experienced widespread use in Melbourne for many years and have been installed over a wide range of subsurface conditions. The load applied to the pile is primarily carried by the bottom part of the pile which is “socketed” into the rock. Rock sockets in Melbourne comprise siltstone/sandstone and Newer and Older Volcanics (basalt). Throughout the world, rock socketed piles have been and are still designed by many engineers by assessing estimates of allowable shaft and allowable base resistance, with little if any assessment of the displacement performance of the pile. Settlements are quoted based on experience rather than by calculation. The estimates of allowable shaft and base resistance are usually based on local ordinances, experience or empirical correlations with the uniaxial strength of the intact rock (e.g. see Figure 1).
The performance of most rock socketed piles at serviceability loads is dependent predominantly on the shear resistance developed at the interface between the concrete shaft and the surrounding rock, i.e. shaft resistance. The shaft resistance developed depends on many factors including the shaft diameter, the type, stiffness and strength of the rock and construction effects such as socket roughness, the thickness of smear zone or residual drilling fluid coating the socket walls and the pressure imposed onto the socket rock due to fluid concrete placement (e.g. Johnston, 1977; Williams and Pells, 1981; O’Neill and Hassan, 1994). Shaft resistance is very sensitive to a number of these parameters and as a result, socket performance can vary significantly from one site to another, even in the same rock type. This provides an explanation for the large scatter observed in published correlations between shaft resistance and rock strength; for example Williams, Johnston and Donald (1980) or Rowe and Armitage (1984). The use of such empirical correlations requires the designer to choose a design shaft resistance from within a relatively wide scatter and as a result a conservative assessment is usually made.