Field and model investigations into the influence of age on axial capacity of displacement piles in silica sands

Abstract

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.

Evaluation of installation effects on set-up of field displacement piles in sand

“…As a result, Chow et al (1998) argue that hoop stresses develop by arching and shield the pile from a region of high radial stresses some distance from the pile wall, but that creep induced relaxation of the arching subsequently allows gains in radial effective stresses close to the pile. However, recent laboratory model tests reported by Jardine et al (2013) and Rimoy et al (2015) confirmed that while arching did occur, it did D r a f t not provide an explanation for the low levels of set-up observed in their series of tests on jacked piles. Mitchell (2008) discusses the 'enigma of sand ageing', providing many examples of its effects on stiffness, penetration resistance and liquefaction potential.…”

Joint Research into the Behaviour of Driven Piles

“…The novel features of the successful 'CPT' sand methods listed in Table 2 originated in the use of highly instrumented displacement piles to investigate the key factors that governed shaft and base failure in sands, with a focus on the local shear and radial stress distributions developed on the pile shafts during installation, equalisation and load testing to failure. Establishing reliable experimental observations of the stress regimes developed around displacement piles and understanding how these may change is vital to making any further improvements and to considering key features of field behaviour that remain poorly understood, including the marked effects of pile ageing on shaft capacity [19][20] and the impact of cyclic loading [21].…”

“…Their large calibration chamber was modified considerably to allow long-term, highly instrumented, model pile experiments to be conducted under closely controlled pressurized conditions. Jardine et al [22], Zhu et al [23], Yang et al [24], Tsuha et al [25], Jardine et al [26][27], and Rimoy et al [20] describe multiple static and cyclic experiments with a stainless steel cyclically jacked mini-ICP pile (with 36mm diameter, 1m length) that could monitor shaft shear and normal stresses at three levels. As indicated in Figure 2, dozens of soil stress sensors were also deployed within the masses of Fontainebleau sand (of both NE34 and GA39 grades) that were air-pluviated into a medium dense (75% relative density) state before being pre-loaded and aged under 150 kPa vertical stress to match the average sand state (void ratio and pressure) applying to the field and calibration chamber tests.…”

Characterisation of sand-steel interface shearing behaviour for the interpretation of driven pile behaviour in sands

“…Extensive studies into offshore piled foundations have advanced current understanding of pile behaviour under various loading conditions and led to the development of improved design guidelines, for example the CPT-based ICP design approaches for driven piles in sands and clays [1], and the PISA design methods tailored for offshore monopile foundations [2]. Significant ageing increases in driven pile shaft capacities over time in sands have been investigated in a series of field and model studies, see for examples [3][4][5][6][7]. The latter authors proposed that the following ageing mechanisms may govern the shaft capacity increases observed over weeks and months in the field: #1.…”

“…1. Normalised static tension capacity-ageing time trends established for industrial-scale steel piles [7][8] Further tension pile tests were carried out at these sites on 51 micro-piles (with 48-60 mm outer diameter, 1.75-2.4 m contact length) that were made of different types of steel (stainless, galvanised and mild steel) with a range of surface roughnesses (centre-line average roughness, RCLA ranging 1-10 μm). The latter were tested after ageing E3S Web of Conferences 92, 13001 (2019) https://doi.org/10.1051/e3sconf/20199213001 IS-Glasgow 2019 periods of 0.1 to 696 days, as described by [9].…”