A recent pile predictions competition has shown practical foundation design procedures to be far less reliable than geotechnical engineers and their clients would like to think.
Sixteen entries were received from seven countries, mostly from well established practitioners, and the standard deviations between predicted and measured capacities were within 60%.
Participants in the Imperial College-organised event were asked to predict the performance of a single driven steel pile, an identical steel pile with a jet grouted bulb at its base, and as an optional component, the behaviour of the jet grouted pile under cyclic dynamic loading.
Predictions were judged against Imperial's extensive field testing at a site in Dunkirk, northern France, carried out under the EU-funded Gopal project.
Predictions varied greatly, although mean trends were significantly biased, and no one design method consistently out- performed the others. The wide scatter was not just for the novel technique of jet grouted pile, in which there is no previous design experience, but also for the control pile, essentially a common problem in foundation engineering. Very few competitors even attempted to predict the piles' cyclic response.
Too often it is assumed that the behaviour of a single pile is an area of geotechnics that is well understood, well documented and can be taken from the basic textbooks with confidence. But for those in the know, the scatter was expected. 'We should not be surprised,' says Richard Jardine, Imperial's professor of geomechanics. 'Geotechnics remains a difficult, partially researched subject and simple design rules do not constitute accurate models.
'The three best predictions involved quite different methods,' he continues. 'Their common thread was a philosophy of calibrating simplified calculations against more analytical methods and field data.'
The competition demonstrates the importance of a realistic awareness of the limitations of geotechnical design procedures. Good predictions, Jardine believes, come from a combination of skill, knowledge and analytical ability.
In geotechnics and foundation engineering, a structure's behaviour is governed by the mechanical properties of the ground. The geology is often complicated and the geotechnical engineer has to design with variable and complicated material, whose properties usually vary with time.
Soil is also highly strain dependent and its behaviour is controlled by groundwater pressure which can also change. In contrast engineers in other disciplines specify the properties of the materials with which they design.
Jardine believes geotechnical engineering is gradually becoming more reliable. 'It is only recently that we could afford to test our methods without risking truly awful results. Improvements have been, and will be made through a mix of pure and applied research; we need to recognise our limitations and acknowledge the value of true expertise backed by field monitoring - and we have to educate honestly and thoroughly to cope with uncertainty and change.'
Read this if
you want to know more about the importance of good foundation design
you are interested in predicting piles behaviour