South Australia & NT Chapter

Improving the Reliability and Effectiveness of Site Investigations

Dr. Mark Jaksa

Several studies have been published over the last 30 years or so that clearly demonstrate that, in civil engineering and building projects, the largest element of financial and technical risk usually lies in the ground. Indeed, structural foundation failure, construction over-runs and delays can often be attributed to inadequate and/or inappropriate site investigations. Unfortunately, geotechnical engineers have at their disposal limited guidance when planning the scope of such investigations.

Almost exclusively, the scope of geotechnical investigations is not governed by what is needed to characterise appropriately the subsurface conditions but, rather, how much the client and project manager are willing to spend on a site investigation. This has prompted the Institution of Civil Engineers to conclude that “You pay for a site investigation whether you have one or not.” The geotechnical engineering profession is in urgent need of a series of quantifiable guidelines that associate the scope of a site investigation with the reliability of the proposed foundation system. Such guidelines need to incorporate, not only under-design, where an inadequate investigation results in foundation failure and unforeseen ground conditions, but also over-design, where inadequate geotechnical investigations result in over-conservatism and, hence, more funds are spent on the foundation than would have otherwise been the case had a more appropriate site investigation been carried out. The presentation developed and proposed a series of guidelines that enable geotechnical engineers to scope site investigations based on the expected spatial variability of the site, as well as the probabilities of failure of the foundation system and over-design. It is this latter aspect that is often ignored and rarely quantified in civil engineering projects.

The guidelines were developed using Monte Carlo simulation based on many thousands of 3D realisations of soil properties, as well as 3D finite element analyses. The 3D soil simulations were generated using random field theory, which incorporates the observed soil behaviour that properties at adjacent locations are more correlated than those that are separated by greater distances. The objective of this work is to enable geotechnical engineers to compare and discuss, with the client, the ramifications and cost-effectiveness of several geotechnical investigation scenarios. In this way the client will be better informed in relation to the risk of foundation failure and over-design associated with the adopted site investigation.

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