European demands for improved site investigation should give geotechnical contractors little excuse for not accepting ground risk, argues David Puller.
Consider for a moment the interrelated issues of site investigation and ground risk allocation in foundation contracts.
Many of our geotechnical contractor colleagues in mainland Europe think we are bonkers for taking on ground risk in piling contracts. And looking at the highly variable quality of ground investigation frequently available to us at tender stage, they could have a point.
We all seem too prepared, under the pressures of a competitive market place, to accept ground risk based on site investigation information that we suspect is inadequate. However, I would argue that we, as specialist geotechnical contractors, are best equipped to understand, quantify and accept the risk of unexpected ground conditions provided the source data is sound.
We should know the capabilities of our plant to overcome adverse soils.
In putting together tender costings and programmes for foundation works, we have assessed different construction methods, associated production rates and material consumption such as concrete overbreak. In design and construct contracts we should be able to take into account the possibility that the ground is less competent than we envisage and build in appropriate contingencies into our tender sum and programme to cover ground risk.
Surely the client, who is usually at best only mildly acquainted with geotechnical processes, cannot be expected to take on the risk.
The construction manager or main contractor will usually understand the consequences of construction delay or cost overrun; and indeed will appreciate the bigger picture - like achieving overall project delivery - much better than the specialist.
But general contractors are not in the best position to quantify the probability that a problem will occur. The client's engineer or advisor may or may not have past experience to draw on, but such know-ledge is inevitably gained through looking in from the outside.
A recent survey involving a large sample of building projects concluded that over a third of contracts suffered from at least one-month delay (GE Talking Point, June 2004). In over 50% of these contracts, the delay was attributable to difficult ground conditions. Despite this stark reality, clients still remain in many cases reluctant to invest appropriate funds in adequate ground investigation.
It represents, of course, an upfront expense for which there is no tangible return. However, it is a small price to pay to minimise the risk of costly project delay - often less than 0.1% of the final building cost will cover a generous site investigation.
It is timely that the scourge of the UK construction industry will soon be upon us with the advent of Euro-codes and specifically EC7 and its associated National Annex. Correlation factors, which are used in the codes to obtain factors of safety and hence derive safe pile load capacity, are defined in the current BS EN 1997-1 Geotechnical Design & General Rules as being inversely proportional to the number of soil profiles available.
As a result there will be a direct design benefit from carrying out a comprehensive site investigation. Put crudely, more boreholes will mean lower factors of safety. This approach is similar to the reduction in the required factor of safety resulting from undertaking multiple pile load tests as outlined in the National Annex.
A further European initiative should also be welcomed. The European Federation of Foundation Contractors EFFC has recently drawn up a working paper based on Dutch, Austrian and German standards, which sets out to define minimum standards for ground investigation in foundation projects.
It categorises projects according to their geotechnical complexity and, as well as describing general good practice in the scoping of ground investigation, it refers to the need for some form of investigation on a maximum 25m grid such as cone penetration tests. The guide also suggests a minimum investigation depth below founding level for piled foundations of five metres or ten times the pile diameter, whichever is greater.
If we can try to embrace - or at least engage with - European thinking, the benefits are at least twofold. Leaner design could become a reality and ground risk could be more easily quantified and allowed for. Ownership of ground risk should become less of a problem; though whether assessment of that risk becomes less contentious remains highly debatable.
David Puller is chief engineer for Bachy Soletanche.