The Benefit of Proper Site Investigations

by Andrew Lees, on 02-Dec-2021 04:22:50

'You pay for a site investigation whether you have one or not'

This quotation from Prof. Stuart Littlejohn dates from 1991 and it is still often cited, remaining equally true and insightful today. His words are generally used to emphasise the inevitable cost and consequences to a Client from an inadequate site investigation. He was, however, actually making the point that the costs of ‘unforeseen circumstances’ were initially being paid by insurance companies, which then resulted in higher premiums for all. Thus everyone paid for those who had made the decision not to have a thorough site investigation.

Adequate site investigations

Littlejohn was commenting in a report by the Institution of Civil Engineers titled ‘Inadequate Site Investigation’. In the thirty years that have followed, much has been written about the need to mitigate risks and associated costs by conducting appropriate site investigations in a timely manner. In this blog we focus on the flip side of that argument: the considerable economic and environmental benefits of adequate site investigation. 

A basic site investigation

Let us consider a project on a site deemed to be low risk, geotechnically: an access road to enable the construction and future servicing of a wind farm. The loads are well defined, and it’s a greenfield site with no previous development, so no complications would be expected. A site investigation could be carried out, focused on the turbine locations with just one or two trial pits along the road alignment. Subgrade would probably be variable, let’s say we obtained strengths of 1.5% and 5.0% CBR. It would be possible to design an access road using this information, but with this basic detail, the designer would need to take a conservative view on the subgrade. The resulting design might be a 900mm thick granular layer, or perhaps 550mm layer with stabilisation geogrid incorporated.

In this episode of "Ask Andrew", Andrew Lees drops by our office in Blackburn to tell us why site investigations are needed. 

New and improved - and adequate

But what if a more detailed site investigation had been carried out, which defined the locations of the weaker soil areas? The designer would be able to vary the design profile according to the actual subgrade strength. The stabilised layer thickness would then vary from as little as 250mm, and with only limited areas at up to 550mm. By reducing the stabilised layer thickness, the volume of soil excavated and removed, the volume of quarried aggregate, and the volume of asphalt would all be reduced. The savings in imported aggregate would be considerable, reducing construction cost and time, and easily covering the additional site investigation costs. Additionally, the reduction in imported aggregate has other cost and environmental benefits: the transportation costs are reduced, disruption and damage to local roads is reduced, and energy costs and construction carbon costs are also significantly reduced.

Sustainability advantages

With detailed and reliable information on ground conditions, engineers can be more nuanced with their designs, eliminating any unnecessary conservatism that would inflate construction cost and be wasteful of energy and natural resources. The road to Net Zero and other sustainability imperatives mean that politically, it is essential to consider these elements, and – even more important than the political implications - the importance of developing the sector to minimise the global impact of construction. With the onus on every engineer to address the environmental - as well as the economic - costs of construction, designs need to be smarter, more refined, and for that: we need to start with adequate site investigation.

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