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Separating influences of yield stress ratio (YSR) and partial drainage on piezocone response
Calculation of failure loads is typically a part of most geotechnical designs. For evaluation of the potential for failure, soil behaviour is essentially governed by the initial yield stress ratio (state), the stress level at failure, and the degree of consolidation during loading. These facets of behaviour will not only influence geotechnical design, but also strongly influence the soil response measured during a cone penetration test (CPT) with pore pressure measurements, or piezocone penetration test (CPTU). This study evaluates a method to separate the influence of yield stress ratio (YSR) from partial drainage during a CPTU, with particular application to soil classification by piezocone. Increases in YSR and degree of consolidation during loading tend to result in an increase in normalized cone tip resistance (Q=qcnet/σ’v0) and decrease in pore pressure parameter (Bq=∆u2/qcnet), which are typically used for soil classification by piezocone. Using theoretical studies, centrifuge experiments, field experiments, and databases of CPTU measurements, this study illustrates that for many cases the influence of YSR and partial consolidation have opposite effects when plotting data as Q against ∆u2/σ’v0 (=Bq⋅Q). Therefore, charts of Q plotted against ∆u2/σ’v0 are more useful for evaluation of soil type than conventional plots of Q against Bq.
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Connecting geotechnical investigations with project risk
It is often stated by geotechnical engineers that a project pays for a site investigation one way or another as a means of justifying a scope of work aimed at managing a project’s risk. This proposition is critically assessed through reviewing three case histories covering site investigations performed for design only, design and construct, public private partnership (PPP) and alliance contract models. An extensive site investigation was performed for the Ballina Bypass Alliance project. Most of the project risks were identified and managed. However, some risks were realised relating to detecting palaeochannels, variable subsurface topography, quantifying material parameters and coping with corestones. An extensive site investigation has been performed for the Snowy 2.0 project, but its scope has been limited by time and access constraints. Risk is managed through adoption of a geotechnical baseline report. Site investigations for Inland Rail for design only, design and construct and PPP contract models have been scoped and partially delivered. In addition, Inland Rail has developed an earthworks materials specification that can be varied to suit site characteristics. Integration of site investigations with the specification and design is shown to be key to controlling the major materials risk. Some thoughts about scoping an investigation to inform geotechnical risk when procuring a PPP are presented. Overall it is concluded that the geotechnical industry generally scopes investigations to adequately manage risk. Quality is shown to be as or more important than quantity. The critical importance of engineering geology for identification of potential risks is demonstrated thus allowing a focussed drilling and geophysics scope to be delivered. Challenges remain when communicating residual risk to stakeholders.
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Alliance Enhances Geotechnical Outcomes Port Of Brisbane Seawall
One of the largest infrastructure projects handled by Port of Brisbane Pty Ltd (formerly Port of Brisbane Corporation) in the last two decades is the construction of a 4.6km long seawall extending 1.8km into Moreton Bay. The perimeter seawall constructed encloses an area of 230 hectares immediately north-east of the current Port area. The area contained by the wall is being reclaimed in a progressive manner and currently 40% has been fully reclaimed and a further 20% partially reclaimed.
The Future Port Expansion (FPE) seawall was designed and constructed by an Alliance comprising Coffey Geosciences (Geotechnical Consultant), Leighton Contractors (Contractor), Parsons Brinckerhoff Australia (Civil Consultant) and WBM Oceanics (Hydraulic Consultant) along with the Client, Port of Brisbane Corporation (PBC), a semi-government organisation. After studying several delivery methods, the Alliance delivery method was adopted by PBC to best meet the potential risks associated with the construction of the seawall on a site underlain by more than 30m of deep soft compressible soils and concerns over impacts of construction on the sensitive environment of the adjacent Moreton Bay Marine Park which surrounds the site. The complexity associated with the project also involved the variable water depth to the seabed, significant tidal variations over 24 hours, weather conditions, timelines and turbidity issues related to construction. From Day 1, the Alliance committed to risk management related to geotechnical and environmental issues which led to several innovative solutions and outcomes. The positive outcomes achieved could be directly related to the collaboration between a semi-government organisation, a major contractor and several consultants through the Alliance delivery mechanism which helped to streamline the design and construction work and removed the inflexibility usually associated with other contracted forms of project delivery.
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The design of displacement piles in siliceous sands using the CPT
This paper provides recommendations for the assessment of the shaft and base capacity of driven and jacked (or ‘pressed-in’) displacement piles in siliceous sand using CPT data. The recommendations related to open and closedended driven piles are those proposed by the ‘UWA-05’ method and the rationale behind adoption of the selected formulations for this method is provided. The paper also provides guidance on the assessment of the base stiffness of piles in sand.
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Foundation Design For Underground Metro Stations In Dubai
This paper is based upon the investigation and design stages for two of the 30 metre deep underground stations constructed in water-bearing sands and very weak rock as part of the initial development for the Red line of the Dubai Metro, United Arab Emirates (UAE). The paper firstly outlines the geological and hydro-geological conditions encountered at the City Centre and Burjuman underground stations designed by Hyder, with specific details of the geotechnical design parameters and groundwater data. The design criteria and constraints for the structures are subsequently discussed from a geotechnical perspective, in particular the issues of dewatering during construction and the impact of the long term uplift pressure after completion on the design solution. Two critical geotechnical design issues are the use of the tension piles/barrettes against uplift for the station box and load bearing barrettes for the viaducts immediately above the underground station structures. This paper also describes the progression from concept to detail designs and how the uplift issues were resolved and the lessons learnt.
Both empirical design methods and numerical modelling for the design of tension piles/barrettes are presented in the paper to emphasize the complexity, of what at face value appears straightforward. The load transfer mechanism of the viaduct load through the barrette panels and their displacement compatibility with the station box slabs were analysed using both 2D analysis and 3 dimensional methods.
At this time the permanent box structures have been completed and the design assumptions validated through the construction stage using an observational approach/monitoring, reflecting the effective and successful application of the design and decision making process.
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Reactive Walls: An Overview
Conventional remediation methods such as pump-and-treat have been used for decades to clean up contaminated sites. However, these technologies have limitations both in terms of cost and cleanup efficiency. Partly in response to perceived shortcomings of conventional remediation technologies and partly for their advantages, a group of alternative remediation technologies called ‘reactive walls’ are under investigation.
In its simplest form, a permeable material is placed in a trench downgradient of the contaminant plume and the contaminant is transported through the soil by the natural hydraulic gradient into the reactive wall where it is modified in some way. Beneficial modification of the groundwater may be achieved by the correct selection of a biological or chemical process in the reactive wall. Possible treatment options include sorption, biodegradation, precipitation, metal-enhanced abiotic dechlorination, oxidation and photoremediation. Because of the range of treatment methods available, reactive walls can be helpful in managing a whole range of contaminants; these include heavy metals, inorganics, chlorinated solvents, hydrocarbons and other organic contaminants.
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Standpipe Piezometer Installations – Lessons Learnt
Standpipe piezometer installations are frequently commissioned as part of site investigations to monitor groundwater levels and chemistry. On a recent site investigation for a large tunnel infrastructure project in Sydney, Australia, 15 standpipe piezometers were installed and developed. Some of the well screen depths exceeded 100 m below ground surface. Several of these wells, especially the deeper ones, returned unexpectedly high pH values after development and sampling. The values were potentially misleading for the assessment of infrastructure durability and environmental impacts.
A hypothesis for these high pH readings was the potential ingress of the cement-bentonite grout in the annulus to the standpipe piezometer via the bentonite seal and/or the casing threads. The latter was confirmed by borehole imaging. Following this observation, a literature review and trials were carried out to investigate the impacts of typical well construction methodology and materials on the pH of the groundwater sampled. In particular, threads from several PN18 nominal pressure rated casing were tested, with their elastomeric joints, at different confining pressures. The effectiveness of the bentonite seal above the screened section was also tested by varying curing time and seal thickness for different overburden pressures. This paper discusses the results of these trials and describes measures to reduce the risk of groundwater contamination induced by cement-bentonite grout leakage.
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Foundation design for underground Metro stations in Dubai
This paper is based upon the investigation and design stages for two of the 30 metre deep underground stations constructed in water-bearing sands and very weak rock as part of the initial development for the Red line of the Dubai Metro, United Arab Emirates (UAE). The paper firstly outlines the geological and hydro-geological conditions encountered at the City Centre and Burjuman underground stations designed by Hyder, with specific details of the geotechnical design parameters and groundwater data. The design criteria and constraints for the structures are subsequently discussed from a geotechnical perspective, in particular the issues of dewatering during construction and the impact of the long term uplift pressure after completion on the design solution. Two critical geotechnical design issues are the use of the tension piles/barrettes against uplift for the station box and load bearing barrettes for the viaducts immediately above the underground station structures. This paper also describes the progression from concept to detail designs and how the uplift issues were resolved and the lessons learnt.
Both empirical design methods and numerical modelling for the design of tension piles/barrettes are presented in the paper to emphasize the complexity of what at face value appears straightforward. The load transfer mechanism of the viaduct load through the barrette panels and their displacement compatibility with the station box slabs were analysed using both 2D analysis and 3 dimensional methods.
The permanent underground station box structures have now been completed and are in operation. The design assumptions were validated through the construction stage using an observational approach/monitoring, reflecting the effective and successful application of the design and decision making process.
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Excessive Post-construction Settlement Of Improved Ground — Case Histories
This paper presents a few case histories where excessive settlements were reported following ground improvement of a number of soft soil sites. The case histories involved different ground improvement techniques such as preloading and surcharging with and without prefabricated vertical drains (PVD), deep soil mixing, concrete injected columns and vacuum consolidation. The ground conditions and the adopted ground improvement designs are discussed. The observed post-construction settlements for the various cases are also presented. Further, the back-analysis works are detailed in order to provide some insight into the possible contributing factors to the measured excessive settlements.
It is clear from these case histories that observational approach by monitoring the ground behaviour during and after ground treatment and construction should be adopted to ensure that the post-construction performance is consistent with design expectation. In addition, the paper demonstrates that selection and design of the ground improvement techniques should be conducted with clear understanding of the theoretical background and limitation of the improvement techniques, regardless of the system adopted. Consideration of construction activities and staging is also important in order to capture the impact of various construction loading on soft soil consolidation and settlement.
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Who needs constitutive models?
Constitutive models are essential in any rational theoretical modelling in geotechnics. How the stress-strain response of soil or rock is represented in these models is usually the key to successful prediction of the behaviour of geotechnical structures. However, the important details of these models, particularly the idealizations that are made, are often poorly understood or ignored, sometimes at significant cost to the unwary analyst. Indeed, the capabilities and the shortcomings of these models, especially the more advanced models, are not always easy to ascertain. In some cases determination of the input parameters is not straightforward. Consequently, it may be difficult to determine which model to select for a particular task. This lecture charts the development of constitutive models used to represent the mechanical behaviour of soils and provides an overview of the principles and the main features and components of existing, widely used constitutive models for soil. The intention is to emphasise the physical basis of these models, rather than their mathematical complexity. Some of the constitutive models encoded in the software packages used routinely in geotechnical practice are reviewed and discussion is also provided on their specific limitations. Examples of practical applications are used to illustrate the both the advantages and some of the pitfalls of the commonly used models. A brief description of recent developments in this area of geotechnical research is also included.