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Rimoy S.,University of Dar es Salaam | Silva M.,University Grenoble Alpes | Jardine R.,Imperial College London | Yang Z.X.,Zhejiang University | And 2 more authors.
Geotechnique | Year: 2015

The axial capacities of piles driven in silica sands are known to grow over the months that follow installation, long after all driving-induced pore pressures have dissipated. However, there is uncertainty over the processes that govern the observed set-up and how they may vary from case to case. This paper evaluates three hypotheses against evidence from updated field test databases and laboratory investigations with highly instrumented and pressurised model piles. Potential influential factors are considered including: pile and sand particle sizes, installation style, access to free water, test conditions and external stress change cycles. Laboratory local stress measurements support the hypothesis that moderation, over time, of the extreme stress distributions developed during installation is a key contributor to capacity growth, while field tests confirm the action of enhanced dilation near the shaft. However, field and laboratory piles show paradoxically different ageing trends. The paper proposes that the fractured but compacted sand shear zone that forms around pile shafts during installation leads to set-up being far more significant with large field driven piles than in model tests. © 2015 Thomas Telford Services Ltd, All right resereved.

Li W.-C.,Tongji University | Yang M.,Tongji University | Zhu B.-T.,NOMA Consulting Pty Ltd.
Yantu Lixue/Rock and Soil Mechanics | Year: 2015

Winkler model based p-y curve method has been widely used in the design of laterally loaded pile. This semi-empirical method was originally proposed for the offshore oil/gas platform and developed from field lateral loading test results mainly on flexible piles with diameters not greater than 1.2 m and ratios of pile embedded length to outer diameter larger than 20. In the past decade, the boom in the wind energy industry, especially for offshore, has increased the dimensions of piles out of the range for derivation of current p-y models. To date, it is generally agreed that, for the design of a laterally loaded pile with a large diameter, such as diameter D=6 m, the reliability of current p-y models is not clear and should be further investigated. According to the test results of two laterally loaded piles, a detailed case study was conducted to investigate the p-y model recommended by the API code and other researchers' refinement. The results show that the negligible error is shown in bending moment predicted by different p-y models. The pile head deformation is mainly dependent on the initial stiffness of subgrade reaction and the expressions of p-y models. Not only the internal friction angle and relative density of sands, but also the geological history of ground should be considered for determining constant of subgrade reaction stiffness. Finally, future study is highlighted. ©, 2015, Science Press. All right reserved.

Jardine R.J.,Imperial College London | Zhu B.T.,NOMA Consulting Pty Ltd | Zhu B.T.,Imperial College London | Foray P.,CNRS Grenoble Laboratory for Soils, Solids, Structures, and Risks | And 2 more authors.
Geotechnique | Year: 2013

An interpretation is given of instrumented calibration chamber experiments involving comprehensive measurements of the stresses developed on and around closed-ended model displacement piles installed in pressurised silica sand. Conclusions are drawn regarding the mechanisms and stress regimes that apply during and after penetration, and how these compare with cavity expansion treatments and other analyses. The experimental arrangements and measurement details are described fully in a companion paper.

Jardine R.J.,Imperial College London | Zhu B.T.,NOMA Consulting Pty Ltd | Zhu B.T.,Imperial College London | Foray P.,CNRS Grenoble Laboratory for Soils, Solids, Structures, and Risks | And 2 more authors.
Geotechnique | Year: 2013

Calibration chamber experiments are reported that investigate the evolution of stresses around closed-ended, highly instrumented, model displacement piles during simulated driving into a heavily instrumented sand mass. The soil stresses are shown to vary spatially relative to the pile tip location. As well as showing considerable radial variation, the stresses developed at any given depth build sharply as the tip approaches, and reduce rapidly as it passes. Clear differences are evident between the behaviours seen close to the shaft during alternate penetration and pause periods. Load-cycling effects are most significant close to the shaft, where the local stress paths indicate a tendency for constrained 'dilatant' behaviour, with radial stresses increasing, during loading. In contrast, markedly 'contractant' radial stress reductions are evident on unloading.

Yang M.,Tongji University | Ge B.,Tongji University | Li W.,Tongji University | Zhu B.,NOMA Consulting Pty Ltd
Procedia Engineering | Year: 2016

Piles are usually used as mooring or berthing dolphins in harbor to resist lateral loads mainly induced from ships' impact, and as foundations for bridges and offshore structures (e.g. offshore wind turbines) to resist lateral and axial loads. During the design, the response of these piles to lateral loading should be analyzed, and in some circumstances, the lateral response governs the piles' design. P-y curve method is the most popular approach and also is recommended by the offshore design standards/guidelines, such as American Petroleum Institute and Det Norske Veritas. However, the reliability of current P-y models is questionable when these models are employed to design a pile with dimensions beyond their originated field tests. In this study, firstly, field lateral loading tests on a rigid pile and a number of finite element analyses are comparably investigated. Then, a detailed evaluation of current P-y models is performed and discussed. Finally, a refined design guideline is presented for laterally loaded piles with a wider range of dimensions. © 2016 The Authors. Published by Elsevier B.V.

Yang Z.X.,Zhejiang University | Jardine R.J.,Imperial College London | Zhu B.T.,NOMA Consulting Pty Ltd. | Rimoy S.,University of Dar es Salaam
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2014

Establishing the stress conditions developed around displacement piles in sands is crucial to improving the understanding and modeling of their behavior. High-quality experiments and theoretical analyses are providing new insights into the effects of penetration on stress conditions. This paper synthesizes the findings from three independent experimental studies on normally consolidated silica sands and a trio of numerical analyses that tackle the problem from different perspectives. The significant degrees of uncertainty in the measurements and predictions are recognized and significant differences between data sets are discussed and largely resolved. Applying a consistent normalized interpretive framework leads to clear common trends regarding how installation affects the stress regime. While the main emphasis is placed on the radial effective stresses developed around pile shafts, the circumferential and vertical stress states are also considered. © 2013 American Society of Civil Engineers.

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