An Alternative Interpretation of the West Delta 58A Tension-Pile Research Results
Authors: Jardine R.J. , Saldivar E. 
Published: October 2008

This paper discusses pile experiments performed by others1–6 at the West Delta (WD) 58A site in the Gulf of Mexico. An alternative interpretation is offered to that published previously which focuses on:the slickensided shear-surfaces observed around the pile shaft,hydraulic conductivity in the shear zone,hypothetical explanations for anomalously low long term radial effective stress measurements,applying the Imperial College pile design method to under-consolidated clay profiles,a re-analysis of the WD 58A pile capacities andaspects of the cyclic loading tests.

It is shown that almost all aspects of the WD 58A pile tests can be explained in terms of an 'effective stress - interface shear' framework.

Introduction

A remarkable spread of papers was published at OTC 1998 which reported the full scale offshore pile tests conducted between 1983 and 1986 at the West Delta (WD) 58A platform site1–6. The project team who conducted, analysed and wrote up the experiments are to be congratulated on their many achievements. These included developing a high quality instrumentation system that could operate in the field for 2.5 years, conducting full scale tests to failure under static and cyclic conditions, and carrying out a detailed analysis of the soil fabric developed around the pile shaft.

The spread of papers has had a significant impact on offshore engineering, raising important questions which deserve careful debate. This paper addresses some of these topics, offering agreement on some issues, and alternative interpretations on others. The Authors' starting point is the long-term programme of piling research conducted by Imperial College London which led (in 1996) to new design procedures7 that are now being used widely in the North Sea and other offshore provinces.

The main issues that we discuss are:The significance of the system of discontinuous slickensided shear-surfaces that was developed concentrically around the pile shaft.Effects on pore water flow rates of the intensivelysheared soil zone.Possible reasons for anomalously low long term radial effective stresses observed around the pile shaft.The application of the Imperial College pile design method to underconsolidated soil profiles in general.The method's predictions for the WD 58A case history.Aspects of the WD 58A cyclic loading tests.Conclusions regarding soil models for analysing shaftcapacity.

Slickensided shear surfaces

One of the major conclusions from the Imperial College research into displacement pile behaviour in clays was that the local shaft capacity is controlled by (i) the radial effective stress regime developed as a result of pile installation, equalisation and loading and (ii) the effective-stress frictional characteristics of the (often slickensided) shear surfaces formed around the pile. Multiple, direct, on-pile measurements made at four clay sites confirmed that the simple Coulomb law represented the most realistic local shaft failure criterion.

Visual observation, Thin Section (TS), Scanning Electron Microscope (SEM), X-Ray and other techniques have been applied to samples excavated around piles at three sites8–11, proving that highly oriented shear surfaces are generally developed during pile installation, although they may be further modified during pile loading11–17.