ADVERSE PHYSICAL CONDITIONS ON SHEET PILING PROJECTS

In the construction industry sheet piling is a major part of the construction typically one may see sheet piles as a retaining structure. Sheet piles may be driven into the ground with a number of techniques the most common being vibro, impact drive or press-in piling.

The Piling Handbook, 9th Edition (2016) available for download here is a recognized industry guide. Chapter 4 deals with ground conditions, section 4.1 Introduction states “installation method and equipment is itself a function of the properties of the ground”.

So the determination of the right sheet piling methodology is key based on a proper assessment of the likely soil conditions which may be encountered is vital. If the soil conditions are not properly investigated this can lead to significant delays and claims. The Handbook states :

“For piling work, the number of boreholes, or other form of investigation, should be adequate to establish the ground conditions along the length of the proposed piling and to ascertain the variability of those conditions”.

Changing piling technique is a major event on a project and should be avoided where at all possible or where varying conditions are anticipated then a contingency plan for a change in method from one to another should be provided as a back up. It is rare to have a site where the ground is classified as wholly cohesionless or wholly cohesive and hence the selection of sheet piling equipment will always be a compromise, choosing equipment suited to the dominant soil type but also considering the ability of the equipment to penetrate the other soil type. The final selection of the sheet pile driving equipment and section size for economical pile installation is often based on previous experience of the contractor.

The Handbook states “Soil parameters for use in design calculation should be obtained, wherever possible, by sampling and testing material from the Site”, often a Driveability analysis is done by the designer to ensure that the pile depth are adequate for the design.

The soil parameters for the design of the pile may be made available to the piling contractor however this may lead to issues if the piling contractor solely relies on these. Under the terms of most construction contracts, the contractor or is obliged to make its own assessment of and may have to deal with varying sub-surface conditions.

The Handbook says “site investigation of the soils together with field and laboratory tests provided in a Ground Investigation Report (GIR) is necessary for installation assessment” and “Different types of soil demand different installation techniques. The most efficient driving method for a job can be determined by in-situ testing”. The most common form of in-situ testing being the SPT (standard penetration test) with USS (Undrained Shear Strength) being a common field or laboratory test.

The Handbook includes some excellent Tables as a guide to the appropriate methodology based on the soil conditions. Table 4.4, 11.2 and 11.3 are provided below

Piling unlike other forms of civil engineering like excavation will not reveal the nature of the ground it is entering into as no material is recovered during the process as the pile is driven into the ground. Rather the nature of the ground reveals itself in how difficult or easy it is for the pile to be driven. If there is more resistance then the pile will reach refusal at a depth earlier than the designer anticipated and if less resistance is encountered then the pile will have to be spliced and an additional pile length added. Both have cost and time implications either by the need to switch piling equipment to overcome the ground resistance or additional lengths of steel pile.

This gives issues with many standard adverse physical conditions clauses as the contractor generally has to demonstrate that it is a materially different condition has been encountered. The piling records kept by the contractor while the piling progresses being the only manner that the piling contractor can do this.

The takeaway is for Piling contractors to take note of this limitation in many of the standard industry construction contracts and propose amendments to the standard wording dealing with adverse physical conditions where appropriate.