Particle crushing, shear banding, interface abrasion and migration of crushing products all have the potential to influence the behaviour of displacement piles in sands. This paper considers these particulate processes, reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests. The findings offer new insights into the mechanics of displacement piles in sands.

Jardine, R. J. et al. Géotechnique [http://dx.doi.org/10.1680/geot.9.P.137] 1 Measurement of stresses around closed-ended displacement piles in sand R. J. JAR DINE Ã , B. T. ZHU †, P. FORAY ‡ and Z. X. YANG § Calibration chamber experiments are reported that investigate the evolution of stresses around closedended, 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.

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.KEYWORDS: model test; piles; sands; stress analysis; stress path INTRODUCTION Considerable uncertainty exists over the stress conditions developed around displacement piles, particularly when considering sands in which vertical tip stresses may exceed those acting on the shaft by orders of magnitude. Experiments are required to give key insights and test potential conjectures, assumptions and hypotheses regarding the stress regimes applying during and after installation. Establishing these stress conditions is critical to improving the understanding and modelling of features such as non-linear loaddisplacement behaviour, group installation effects, or base and shaft capacity variations with time.Jardine et al. (2013) report measurements made as 36 mm diameter steel instrumented piles and cone penetration test (CPT) probes were jacked into pressurised, air-pluviated Fontainebleau NE34 sand in the heavily instrumented INPG calibration chamber illustrated in Fig. 1. The fine sand (D 50 ¼ 0 . 21mm) was placed at an average initial void ratio e 0 ¼ 0 . 62 and relative density D r ¼ 72%; its mechanical properties are described by Yang et al. (2010), Jardine et al. (2013) and Altuhafi & Jardine (2011), who consider particle breakage, one-dimensional compression, triaxial and interface shear behaviour under a wide range of pressures.The pile experiments described included two 'pilot' tests made with a CPT probe, and three installations with a stainless steel Mini-Imperial College pile (ICP). A vertical surcharge of ,200 kPa was applied in the first test, CPT1, which was reduced to ,150 kPa for the following installations. As outlined in Fig. 2 and Tables 1-3, continuous measurements were made in the sand mass of local ó 9 r , ó 9

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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.