How To … manage site investigations

Site investigations are an exercise in risk management. Increasingly, marginal building land and brownfield sites are being used for construction, particularly housing. Failure to carry out an appropriate site investigation increases the risk of ground movement, contamination, ground water or drainage issues such as soakaways, potentially leading to over-engineered foundation solutions. There may also be health and safety risks and/or third-party claims. Whatever the outcome, ultimately the client sees higher costs.

The problem
Skimping on a site investigation could give you a nasty shock or three. There may be delays as the full extent of the unexpected ground conditions are revealed, construction could be halted while foundations are redesigned to suit conditions as found, and there will be knock-on effects causing further delay to the rest of the building programme, especially if the workforce that has been mobilised for your job moves on because of the delay caused by the investigation.

On the flipside, though, spending too much time and money on investigations when not required is an unnecessary cost.

Key issues
The site investigation should be staged and methodical. Define the objectives of the site investigation before starting any work: what is the purpose of the investigation, what information is expected from the investigation and what should the extent of the ground investigations be?

The site should be viewed as a whole, including its surroundings,so as to consider the effects of neighbouring factors that may influence the site. Typically, an assessment of the risk of flooding, nearby trees or inclined strata would be considered.

Be prepared
Carry out the desk study and site reconnaissance before the ground investigation. Use suitably qualified and experienced people to carry out the site investigations. The desk study is an essential stage at which to evaluate the ground conditions based on existing information, for example, by referring to old maps, records of local and statutory authorities, mining reports, register of contaminated sites, utilities and water authorities for underground services and drainage and previous site investigations. Question people with local knowledge to confirm recorded information.

Do a risk assessment exercise early on in the site investigation programme. This should identify and rank hazards and risks and should examine the likelihood of exposure to those hazards while on site.

Question people with local knowledge to confirm information

On the ground
The ground report should be in accordance with the British Standard recommendations and signed off by a suitably qualified professional, such as a chartered civil engineer or geotechnical engineer.

Further investigations may be recommended if there is a risk of ground contamination. Contamination or gas surveys should include recommendations for remediation and further monitoring, if necessary, and should also be signed off by a scientist or engineer.

Later ground investigations enable assessment of the technical and environmental properties of the ground. Refer to British Standards BS 8103–1, BS 5930 and BS 10175. This stage involves trial pits and boreholes to collect soil samples and allow testing to determine soil properties, such as bearing capacity, shrinkability (if clay), and sulfate content.

Act on what you find
Where contamination is suspected, chemical tests should be carried out on samples to determine the most suitable course of action; remediation options or further long- or short-term monitoring.

Even after site investigations have been carried out, feedback needs to be considered. There is still a possibility that something unexpected may turn up.