Economy in foundations is important. But reliability is even more important, as unreliable substructure is never cheap.
In a forthcoming paper, Achieving economy and reliability in piled foundation design for a building project (to be published in The Structural Engineer in June), my colleague Alain Marcetteau and I argue that geotechnical risk reduction deserves far greater emphasis in the overall structural design process.
We demonstrate that compared with the costs of normal geotechnical design, the costs to the client of something going wrong in the ground can be vast.For a suburban office block, the desk study might only cost £5,000 and the ground investigation a further £35,000.Yet the direct cost of delay would be about £300,000 a month before occupation and perhaps more than £2.5M a month if it prevented the occupants from working.
The costs of resolving disputes are often disproportionate to the amount in dispute.Arup's insurance broker, Griffiths and Armour Professional Risks, says 'the ratio of legal and forensic costs to damages payments in respect of professional indemnity insurance claims has been as high as 5:1 in the past'(but it notes that the Woolf reforms [of the way civil claims are handled] have tended to reduce the ratio in recent years).
Statistics indicate that about 20% of projects are delayed by at least a month due to unexpected ground conditions, so it is clear risks are not being adequately addressed.Interestingly, the average direct cost of ground-related delay for all projects is higher than the typical cost of investigating the risks in the first place.
Alarmingly, the incidence of ground-related delays shows no sign of abating, although the consequential costs stemming from them has risen sharply.
Too often, we preach geotechnical risk reduction to each other, rather than at the key decision makers.We need to explain the reasons behind our regular mantras, rather than just repeat them.Yet we all too regularly see examples of poor geotechnical design:
lincoherent overall design process lno or insignificant desk study lhit-and-miss ground investigations lfailure to develop coherent ground and groundwater models ldisjointed structural and geotechnical designs lappointment of specialist contractors solely on the basis of least cost.
Geotechnical risk reduction is not just about choosing a higher factor of safety in design to compensate for whatever might happen.Many measures can help reduce risk without increasing construction costs significantly.
These include the use of more robust superstructures that are not so critically dependent on limiting differential movements; use of multiple piles per column to mitigate the consequences of a minor defect in an individual pile; recognition of the special skills appropriate to the project that certain contractors may bring; allowing some float in programmes after activities more likely to be delayed;
or increased testing and supervision to detect problems before they have a major impact.
So, in an area where problems occur all too frequently, why do clients tolerate so little being spent when it is plainly in their best interests to do more?
Sadly, many of the people who buy geotechnical services consider them as a simple commodities, to be bought at least cost, and don't realise that it is one of the best opportunities to reduce risks in the entire construction process.
We need to educate the rest of the construction team about the consequences of ground risks.We need to use language that project managers, clients and other construction professionals understand.We need to quantify probabilities and costs of potential consequences.Clients usually value cost and programme certainty far more than minor savings.
Often the costs of avoiding the consequences, or providing suitable levels of reliability, are not very high.Good geotechnical design should provide an economical solution, but most of all a reliable one.