Effective stress regime around a jacked steel pile during installation ageing and load testing in chalk

Abstract

This paper reports experiments with 102 mm diameter closed-ended instrumented Imperial College piles (ICPs) jacked into low- to medium-density chalk at a well-characterized UK test site. The “ICP” instruments allowed the effective stress regime surrounding the pile shaft to be tracked during pile installation, equalization periods of up to 2.5 months, and load testing under static tension and one-way axial cyclic loading. Installation resistances are shown to be dominated by the pile tip loads. Low installation shaft stresses and radial effective stresses were measured that correlated with local cone penetration test (CPT) tip resistances. Marked shaft total stress reductions and steep stress gradients are demonstrated in the vicinity of the pile tip. The local interface shaft effective stress paths developed during static and cyclic loading displayed trends that resemble those seen in comparable tests in sands. Shaft failure followed the Coulomb law and constrained interface dilation was apparent as the pile experienced drained loading to failure, although with a lesser degree of radial expansion than with sands. Radial effective stresses were also found to fall with time after installation, leading to reductions in shaft capacity as proven by subsequent static tension testing. The jacked, closed-ended, piles’ ageing trends contrast sharply with those found with open piles driven at the same site, indicating that ageing is affected by pile tip geometry and (or) installation method.

Behaviour of piles driven in chalk

“…Although two levels of instrumentation were deployed in the SNW tests, up to four clusters have been employed per pile in previous campaigns at sand and clay test sites. Buckley et al (2018c) identified trends from ICP installation at SNW that matched those seen in sands by Lehane et al (1993) and Chow (1997). In precis, installation in Chalk led to:…”

“…However, the far finer grain sizes of chalk lead to a different mechanism involving a shear band forming in the chalk with far smaller values of Δr. A radial dilation of just 0.5μm was interpreted from the ICP test results by Buckley et al (2018c), suggesting that the degree of radial expansion experienced is far smaller than 2R a , reflecting the higher relative roughness of the pile shaft in comparison to the chalk's mainly silt-sized particles.…”

“…Imperial College Pile configuration used in chalk tests;Buckley et al (2018c) Well structured, clean, low to medium density white…”

Laboratory investigation of interface shearing in chalk

“…Finding similar values of Δr at all roughnesses is consistent with shearing occurring within the chalk mass, where dilation is largely independent of the interface properties. However, the laboratory 'dilative' movements exceed those inferred from field measurements with instrumented piles; see [2]. Both δ′ peak and δ′ r appeared to increase with stress level, as shown for example by the 100 and 400kPa normal stress MS-STD and MS-RGH tests.…”

Joint Research into the Behaviour of Driven Piles

“…Jardine [1] summarised findings from several recent and current research projects involving the Imperial College Geotechnics group that have contributed to advancing the design of large driven piles. These included the PISA Joint Industry study for monopiles under lateral and moment loading in sands and clays [2], work with inter-national colleagues on a range of experimental, theoretical and database projects and large scale investigations of pile behaviour under axial loading in chalk [3][4][5]. The latter have involved the first large-scale offshore field tests of which we are aware where autonomous underwater pile tests have been conducted on the seabed.…”