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Impact of spudcan footprints on a gravity base structure
In the offshore oil and gas industry, structures such as gravity base structures and jack-ups are commonly used as drilling and facilities platforms. Installation locations for these structures are usually chosen on flat lying, featureless, undisturbed seabed to reduce installation risks and avoid costly seabed preparation works. In some cases, these structures are installed in close proximity of each other and care is taken to ensure the foundations do not influence the stability of the adjacent structures. This was the case with the skirted gravity base structure discussed in this paper as it was designed by Arup to be installed next to an existing wellhead platform linked with a connecting bridge. However, a jack-up rig unintentionally installed at the designated site of the gravity base structure and therefore significantly changed the seabed profile as well as the strength properties of the underlying soils. The jack-up spudcan footings punched three 20 m diameter craters to about 3 m depth into the seabed. The craters and the disturbance of the soil beneath and around the craters affected the stability of the gravity base structure and increased the risk of installation refusal of the skirted foundation. The craters necessitated the reorientation of the gravity base structure to minimise foundation intersection with the spudcan footprints. In 2011 the gravity base structure was successfully installed and is currently in operation. This paper discusses the investigation process and the analyses that were conducted to assess the impact of the spudcan footprints on the performance of the gravity base structure.
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Response to Discussion provided by Philip Pells on the paper: Unsaturated Free-Standing Mainline Railway Embankments – Part 2
The authors sought from the profession discussion of the merits of suction in appraisal of instability of free-standing embankments, and especially those embankments supporting major infrastructure.
The authors illustrated that the incorporation of the beneficial influences of suction within analyses was beneficial in the assessment of the likelihood of instability, as expressed through determination of Factors of Safety. The authors asked of you, the reader, to “join in the discussion, with the intention to advance the science”. We noted that we “don’t pretend to have all, or indeed any, of the answers”, and looked forward to your contributions.
Philip Pells has responded to the authors’ request for that discussion. We are appreciative and thank him for his time and effort.
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Experience of Overcoring the ANZI Strain Cell in HQ Exploration Boreholes to Determine the Three Dimensional In Situ Stresses A Depths Approaching 1KM
This paper describes recent use of the ANZI (Australia, New Zealand Inflatable) strain cell and the overcoring method of stress relief in an HQ exploration borehole at depths approaching 1km to determine the three dimensional in situ stress field in a one-day operation. The stages of a routine overcoring operation are presented to illustrate each step of the process. The results from a European metalliferous mining site are presented to illustrate the process of characterising the three dimensional in situ stress environment when multiple high confidence measurements are achieved.
The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three dimensional in situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects but technical advances over the last decade have allowed the system to be deployed routinely in surface exploration boreholes. Recent development of a downhole electronic data logger, a wireline enabled drilling system and an instrument deployment system has simplified the process of obtaining three dimensional overcore measurements at previously inaccessible depths remote to any underground excavation.
The capability to deploy ANZI strain cells from surface exploration boreholes represents a significant breakthrough for the design of underground civil structures. High confidence characterisation of the in situ stresses at design stage provides the opportunity to design key infrastructure to take advantage of the in situ stress field from the outset before any excavation and construction activity even begins. Understanding the three dimensional in situ stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it is also provided insight into the mechanics of the all the various processes driving ground deformations.
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Overview of The Role of Testing and Monitoring in the Verification of Driven Pile Foundations
Static pile load tests have traditionally been considered the gold standard test and, if well executed, provide the reference load-movement response of the pile. Setting aside any difficulties with proper execution of static pile load tests, their primary deficiency is in the statistically insignificant rates at which they are undertaken — typically 0.5% to 2.0%. Furthermore, static pile load tests cannot be directly related to installation parameters and are therefore not well suited to development of driven pile acceptance criteria. If well executed, dynamic pile tests provide a rapid and generally reasonable estimate of pile load-movement response. The primary issue is that the static response is inferred from a dynamic response using simplistic models of complex dynamic pile-soil behaviour. However the advantages of dynamic testing are that it is generally performed on a statistically meaningful sample size – 5% to 15% in many cases – and it is concurrent with installation, which allows dynamic testing to be the basis for construction control and development of pile acceptance criteria. The remaining 85% to 95% of piles are necessarily installed using simple set criteria, or dynamic formula approaches which of themselves have significant deficiencies and represent project risk. Given that the foundation system will only be as good as the pile installed with the least confidence, improvements in foundation quality will be most effectively achieved by improvements in the monitoring and assessment of untested piles. This paper discusses a state-of-the art approach to reduction of overall foundation risk.
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Founding piles on the Hallett Cove Sandstone in Adelaide: A case study
The majority of the more recently constructed buildings in the Adelaide CBD are founded within the very stiff to hard Keswick and Hindmarsh Clay, because these strata are relatively homogeneous. This Paper describes the work recently undertaken for a 21-storey office building founded on the underlying more variable Hallett Cove Sandstone formation. Founding piles within the Hallett Cove Sandstone meant fewer piles carrying higher loads could be used than for piles founded in the overlying clay. However the variable nature of the Hallett Cove Sandstone presented several challenges that needed to be overcome.
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Key Observations and Findings on Liquefaction Impacts in the 2010-2011 Christchurch Earthquakes
Professor Misko Cubrinovski
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Stabilisation of a fill embankment using soil nails
Drilled and grouted soil nails have been successfully used to stabilise a section of fill embankment on the Warrego Highway near Toowoomba, Queensland that failed as a result of heavy rainfall in January 2011. Instability occurred due to groundwater rise within the fill materials resulting in a tension crack developing at the traffic lane edge and an approximate 300 mm displacement within the outer portion of the embankment. Reinforcement of the unstable fill slope with soil nails was the selected remediation method. The soil nails were installed through the existing granular fill materials and grouted into the fill and underlying basalt. Soil nail holes were drilled using equipment fitted to excavators to typical lengths of 10 m to 12 m with temporary casing used to prevent hole collapse during drilling. A reinforced shotcrete facing was constructed over the face of the embankment to provide the necessary nail head restraint and prevent erosion. Soil nails were installed in a prescribed sequence to manage the risk of construction plant trafficking the marginally stable fill embankment. Full time monitoring of construction was carried out in accordance with an action plan developed specifically for the site. The coordinated approach to the design and construction of the works resulted in a successful implementation of the remedial works.
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Back analysis of the Cumbalum Trial Embankment
A trial embankment has been constructed on a deep soft clay deposit near Ballina. The monitoring data has been back analysed using one-dimensional consolidation theory. The results of the back analysis indicate that a more highly compressible layer exists beneath the embankment than was identified during site investigations. It is inferred that this layer might comprise a structured soil. Implications for design of embankments on such soils are discussed.
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Collaboration, A Journey From Isolation To Co-Creation “The 5 C’s Journey”
In the midst of major challenges in the construction industry, this paper outlines a model for collaboration and attempts to define important differences between “collaboration” and “coordination” in the context of large multidisciplinary infrastructure projects. The cultural transformation and the fundamental steps needed to move from “Isolation” (i.e. no collaboration) to “Full Collaboration” are presented in this paper, as a road map referred to as the “The 5 C’s Journey”. The barriers and opportunities to improve collaboration are also discussed with some suggestions and ideas to improve collaboration among geo-professionals across the industry. As geotechnical risks are frequently among the most significant risks for large infrastructure projects, there is a unique opportunity for geo-professionals who work within a wide range of disciplines, spanning full project life cycles, it is important that geo-professionals at all levels be front and centre, leading discussion across the industry to improve collaboration and productivity on the coming large projects across Australia.
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An assessment of the geotechnical strength reduction factors specified In the new Australian piling standard
The two most common forms of pile load testing currently used in practice are the Static Load Test (SLT) and High Strain Dynamic Load Test (DLT). The Australian Piling Standard AS2159 applies a geotechnical reduction factor (φg) to the load capacities obtained from these tests to derive the pile’s design ultimate geotechnical strength. However, recommendations for assessment of φg in the current (2009) edition of AS2159 differ significantly from those in the previous 1995 edition. This paper investigates differences in φg that are related to the method of pile testing by examining the reliability of load capacity predictions from DLTs with respect to the capacity measured in a SLT. It is shown that the increased φg value allowed by AS2159 (2009) for dynamic testing may be un-conservative compared to the corresponding increase allowed for static load testing.