Jet Grouting To Improve Offshore Pile Capacity: GOPAL Project
Authors: Parker Eric J. , Jardine Richard J. , Standing Jamie R. , Jullian Xavier 
Published: October 2008

The paper describes the GOPAL (Grouted Offshore Piles for Alternating Loading) Joint Industry Project. Innovative technology is being used to increase end bearing capacity substantially. Essentially, jet grout bulbs are installed beneath the tips of open tubular steel piles after driving. Field tests are being performed at Dunkirk, France on 457 mm (18 inch) diameter piles in North Sea sands to prove the effectiveness of the technique. In parallel with this research, tension tests are being carried out on six plain driven piles to measure variations in shaft resistance with time, and also to examine cyclic loading effects.

Introduction

The GOPAL project is investigating a new technology to increase pile capacity in sands. Jet grout bulbs are created below the toe of conventional driven steel piles. The jet grouted "soil-cement" would classify as a sandstone of moderate strength and provides a substantial increase in pile end bearing. In parallel research, the project is also studying effects of pile ageing and cyclic loading in dense sands.

Motivation.

Despite the industry thrust to ever deeper water and more extreme conditions, a large number of conventional platforms are still installed in shallow water. Many of these structures are standard steel jackets founded on driven pipe piles. In design, gravity and environmental forces act on the foundation pile giving both compression and tension loading. For typical designs compression loads are usually significantly greater than tension loads.

Piles resist applied loads by a combination of shaft friction and end bearing. Both components contribute to compression capacity while only shaft friction is developed in tension. As a result compression capacity is larger than tension capacity. Fig.1 shows a typical example, the design pile capacity curve for a platform recently installed in the Nile Delta.

Fig. 1--Typical pile capacity curve for platform in Nile Delta. Insufficient end bearing makes balanced design impossible. (Available in full paper)

To minimise pile length a balanced design would resist tension loads by shaft resistance and make up the difference between compression and tension loads through end bearing. Unfortunately, in most cases end bearing is not sufficient to meet this difference. Thus a significant portion of compression load must be taken by shaft resistance, requiring longer piles. For the case in Fig.1, the pile length required for tension capacity would have been 44 m, but compression loading increased pile length to 83 m. Furthermore, recent research at Imperial College and elsewhere has shown that the current API/ISO recommendations for calculating base resistance can be nonconservative, particularly with larger diameter piled driven in sands 1,2.

Solution.

One solution to this problem would be to have stronger soils below the pile tip. Onshore, jet grouting is a proven soil improvement technique that is used to mitigate difficult foundation conditions. The main objective of this project is to investigate the potential of using this process to create a strong durable bulb of soil-cement below the pile tip, increasing end bearing and allowing a reduction in pile length.