The effects of time on the capacity of pipe piles in dense marine sand
Authors: Chow F.C. , Jardina R.J. , Brucy F. , Nauroy J.F. 
Published: October 2008

Investigations into pile behaviour in dense marine sand have been performed by lFP and [C at Dunkirk, North France. In the most recent series of tests, strain-gauged, open-ended pipe piles, driven and statically load tested in 1989, were retested in 1994. Surprisingly large increases in capacity were measured. The possible causes are evaluated in relation to previous case histories, laboratory soil tests, pile corrosion and new effective stress analyses developed using smaller, more intensively instrumented piles. The shaft capacities predicted by existing design methods are also assessed.

Introduction

The effects of time on the capacity of displacement piles in clay are well known and are usually explained through consolidation theory.1,2 In granular materials complete Pore pressure equalisation is expected within a few hours of driving, after which load tests are often assumed to measure long-term pile capacity. However, a growing number of reports of longer-term changes in capacity are emerging. Much uncertainty surrounds the factors responsible for these changes.

This paper describes static and dynamic load tests on four open-ended piles, 0.324m in diameter, 11 and 22m long, driven in dense marine sand at Dunkirk, North France. The piles were initially installed and tested in 1989 by Brucy et al.3,4,5 as part of the 'CLAROM' research programme. In 1994 the site was revisited by imperial College, becoming the sixth location for tests using the highly instrumented Imperial College Pile (ICP)2,6,7,8 as shown on Fig. 1. In addition to single lCP test9,10 and interaction experiments,11 1 two of the original CLAROM piles were loaded statically in tension. The shaft capacity of an 11m pile was found to have increased by up to 85% during the intervening five years. Strain-gauge measurements and corrosion inspection of the extracted piles provide information on the possible causes.

The paper is divided into three parts:a review of previous studiesa description of the Dunkirk testsa discussion which includes a critical appraisal of the possible causes of set-up in sands.

The Appendix examines shafl capacity predictions made for Dunkirk using the existing API recommendations and other recently proposed approaches,

Part 1: Previous Studies

A small handful of cases have been reported where the capacities of piles in sand reduced with time. These are limited to (a) short-term relaxation due to the recovery of pore pressures following dilation during driving,12,13,14(b) piles founded on rock15,16 or (c) closely spaced pile groups.17,18 Long-term reductions would not be expected on most offshore piles in sand under normal loading conditions.

Short-term(&lt;24 hours) increases in pile capacity after or during intermissions in driving due to pore pressure dissipation have been widely documented.19,20 The first account of medium-term (1 to = 100 day) increases in sand was given by Tavenas & Audy21 who measured an average =70% rise in capacity between static load tests conducted 12 hours and 20 days after driving, with no further gains appearing over the following 30 days.