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Relative compaction: In search of a rational method for specification
The primary purpose of compacting engineered fills is to expel water and air from the soil matrix to achieve an increase in stiffness, thereby reducing the likelihood of post construction settlement.
Engineers and earthwork contractors are accustomed to adopting ‘relative compaction’ as a means of compliance testing. Specifications relating to earthwork construction control are commonly expressed as:
γd field / γd max x 100 > R (%) (1)
where γd field is the material density of the material measured in the field , γd max is the maximum dry density obtained from a known energy input and R(%) is the required relative compaction. Despite being widely accepted, relative compaction does not have any direct correlation with the known properties of a material. Hence, there is no rational method for selecting an acceptable percentage R(%) for a particular purpose (Gue and Liew, 2001).
The intention of this paper is to the investigate the rationale behind the perceived need within the construction industry to specify a minimum density ratio R(%) and consider what rational recourse a civil engineer or technician has when a compaction specification cannot be met.
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A Few Notes On Embedment Design With The ‘What You Design Is What You Get’ WYDIWYG Method For Propped Cantilever Walls
The WYDIWYG method for stability design of propped cantilever walls was recently published in the 2019 ANZ Geotechnical Conference. The new method has been shown to be consistent between total and effective stress designs, numerically friendly, stable, and also produces economical designs. The paper focussed the consideration on overturning stability, which is a critical design for this type of structures. In geotechnical engineering designers often treat passive earth pressures as soil resistances and active earth pressures as soil loads. Are active pressures really loads and passive pressures really resistances? It raises an interesting question and the proposition forms an important assumption in the formulation of the new method. Given the interests from the geotechnical design community a more general discussion on the model development will be given together with application of the method to design. Worked examples are also included to demonstrate simplicity of the design process.
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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. -
The Role Of Tunnel Engineers In Implementing The Observational Method On Site
The observational method is widely used in tunnelling works. For successful implementation of the observational method in tunnelling, proper understanding of support system behaviour, an efficient monitoring regime, data evaluation and proper interpretation by an experienced site engineer is of prime importance. Site engineers play a key role in successful implementation of the observational method. Hence, it is very important for site engineers to understand the key behaviours to be observed, issues that could be resolved at the site and the systematic approach to be implemented to understand the tell-tale signs.
This paper describes the concept of observational method as described by Peck in 1969 and the development of its definition into CIRIA R185 and Eurocode 7. These serve as a basis for considering observational approach in the design and highlight the conditions under which the observational methods must not be used. To explain this, this paper discusses a case study on observational method implemented for a shotcrete-lined tunnel and observational “approach” implemented to adjust TBM compressed air pressures during TBM cutter head intervention to maximize the working man-hours in the cutter head chamber while ensuring face stability. For a shotcrete-lined tunnel, a practical procedure for predicting and comparing displacement behind the face in relation to face advance and time is presented along with spatial visualization tool for tunnel lining displacements. For TBM compressed pressure adjustment, an iterative procedure based on groundwater pressure monitoring and water ingress is presented in the paper.
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Compaction And Compactability Assessment Of Difficult Soils – An Alternative Approach
Typical Australian industry processes for verifying compliance of earthworks compaction can result in production delays and cost impacts when difficult soils are encountered. Difficult soils can comprise high plasticity clays and halloysitic clays. Insufficient curing of high plasticity clay samples during rapid compaction testing can result in unrepresentative measurement of maximum dry density and optimum moisture content. This can cause apparent nonconformances, trigger the requirements for proof rolling and incur delays to construction. Verification of lot conformity is often delayed until the following working day due to the period of time required for measurement of moisture contents. Halloysitic clays present a problem because their structure irreversibly changes when they are oven dried. Oven dried soil tested in the laboratory does not represent field conditions because the soils are physically changed. This can cause apparent non-conformances, unnecessary rework and consequential delays. An alternative approach to Australian industry accepted compaction testing has been developed by the UK based Transport Research Laboratory (TRL). The Moisture Condition Value (MCV) test is used to assess whether a soil is suitable for use prior to compaction. End product verification is achieved through on-going calibration to standard Proctor density tests and field certification using the nuclear densometer. The MCV apparatus and theory are introduced and an example of how the process is implanted from site investigation to construction verification is presented.
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Spatial variability of pile founding layers: a case study
This paper explores the design and construction of a single span road bridge, highlighting the impact of geotechnical spatial variability on deep foundation projects. Due to various project constraints, the geotechnical boreholes used for initial design were located a considerable distance from the proposed bridge abutments. As the project progressed, and further geotechnical investigations were carried out, it was observed that the proposed pile founding layer showed signs of spatial variability in both thickness and strength. To address potential risks, the design was revised to extend the piles into a deeper layer of very dense sand, and proof bores were planned at the abutment locations to verify ground conditions before construction. This study incorporates 3D geological modelling, adopting the previous investigations and the latest proof bore data and provides a clearer representation of the variable properties of the pile founding layer, supporting the pre-construction design changes. The findings underscore the critical nature of geotechnical spatial variability and the need for strategic placement of investigative efforts. The paper also details observation and monitoring activities undertaken during construction to ensure that design intent and local government compliance criteria are met, and steps taken to manage potential risks during pile excavation in dense sands below the groundwater table, which are envisioned to be useful for deep foundation projects under similar conditions.
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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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Design and verification of compression capacity of Continuous Flight Auger (CFA) Piles in Batesford Limestone for Surf Coast Highway Bridge
The Surf Coast Highway (SCH) bridge is a vital rail bridge constructed as part of the South Geelong to Waurn Ponds Duplication (SGWP) project in Victoria, aimed at eliminating the at-grade crossing. The bridge is 105m long and four spans, with the abutment and piers supported on CFA piles. This paper presents a comprehensive discussion on the geotechnical model and interpreted design parameters for the underlying Alluvial Terrace and Batesford Limestone formation. The author examines the conventional methods of assessing pile capacity in accordance with the Australian Piling Code AS2159-2009, FHWA, and VicRoads standard specification Section 607. The approach encompasses a detailed analysis of stability, serviceability, and estimation of structural actions for design, considering uncertainties in ground conditions, constraints with varying pile spacings, and potential reductions in design parameters. A comparative analysis is also presented, contrasting the measured load capacity with the predicted pile capacity, providing valuable insights into the efficacy of the design and construction methodology employed.
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Probabilistic methods for slope analysis and design
Probabilistic methods combined with risk assessment are a better way to assess slope design in open pit mines compared to deterministic methods. These methods are suitable for use on evaluation of risk or when there is uncertainty in the input parameters.
Probabilistic analyses require more computer power than deterministic analysis. In many case a probabilistic analysis requires ten to thousands more computer resources than an equivalent deterministic analysis. Methods like Monte Carlo simulation (MC) may require thousands of analyses depending on the number of variables considered in the model. Other methods like First Order Second Moment (FOSM) or Point Estimate Method (PEM) and may require tens to hundreds of analyses.
Monte Carlo simulation is applied routinely today on simple analyses like wedge stability or limit equilibrium analysis; current computers can carry thousand of analyses in a relatively short period of time. This is not the case when more complex models are built like 3D models at mine scale including complex mining sequences, or dynamic analysis of a 3D model. Large scale models can run for hours even in fast computers. Where the Monte Carlo method is not an option other alternative methods should be used.
This paper compares four different methods and presents the equations required to use a Modified Point Estimate Method (mPEM) presented by Harr (1989). The methods are compared using simple examples in the paper. Recommended probabilities of failure for open pit design are also presented.