Mott MacDonald's Alan Willoner on contractor versus consultant pile design.

In the UK we commonly have contractor designed piled foundations. If asked why this is, the response might be “to get a more economical solution”. All clients have reason to look for the most economical solution, and some put consideration into risk management, where the apparent lowest price is not necessarily the best choice.

The Institution of Civil Engineers (ICE) Specification for Piling and Embedded Retaining Walls provides options to choose clause B1.4 option 1 – employer design of piles – or option 2 – contractor design of piles. To get guidance on what option to use the reader may refer to the ICE Manual of Geotechnical Engineering 55.11, which evaluates pile group design responsibility.

Contractor designed piles may provide efficiencies for typical mid height buildings with simple load combinations. But there can be difficulties. For example, on a complex site with multiple ground risks, the piling contractor may not have the time to fully appreciate and mitigate these risks. The lead consultant typically has a year to take a building project through the early Royal Institute of British Architects stages covering the geo-environmental desk study, site visit, specification, review and interpretation of ground investigation, outline foundation design to inform pile layout drawings, and typically a D30 piling performance specification. The piling contractor’s designer may look at many projects each week and produce a tender design in one day followed by a contract design with a second read of the desk study and ground investigation report within one week. Timescales dictate that the piling contractor’s designer may not fully appreciate, or have opportunity to, investigate an unusual ground risk.

Consultant designed piles are required when the pile design cannot be separated from the design of the superstructure, for example new integral bridges. The same logic may be true for structures where the stiffness of the foundation is pivotal for the performance of the superstructure, such as the core of a tall slender building.

There are situations where it is difficult for a consultant to produce a performance specification for “fair tendering” of a contractor designed piling contract and be prescriptive about certain risks, such as negative skin friction, heave or lateral squeezing of soft clays. The simplest solution to these situations is generally to opt for consultant designed piles.

Contractor designed piles are generally the best option when bespoke systems such as screw piles or displacement auger piles or designs controlled by drivability rely on “in-house” case histories.

A skilled designer is needed for an economical pile design. This person could be working for a lead consultant, a contractor or a specialist consultant, so in theory any organisation could produce an economical solution if there was incentive to do so.

The UK has a system that evolved by culture rather than planning. In other countries, piles are typically designed by geotechnical engineers within or teaming up with the consultant designing the superstructure, with piling contractors providing input in the design, when needed, on methodology or practicality issues.

This talking point is about which organisation in the supply chain is best placed to carry out and own the pile design for the circumstances of a particular project. Discussions should extend to structural engineers and developers who (in the UK) have got used to a culture of piles being contractor designed. This is so that projects with high geotechnical risks or complex pile loading have fair consideration of the advantages and disadvantages of opting for contractor or consultant designed piles.

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