Ground investigation is a crucial part of any development. It will establish the site setting, identify potential hazards, support pre-planning appraisals and inform detailed site design and any remediation. So what should housebuilders and developers be mindful of when it comes to groundworks? When should they be undertaken and what are the most effective methods? Chris Wall, Geo-environmental Associate at specialist technical consultancy, MEC Consulting Group shares his insight…
Good ground investigation begins with a desk-based assessment. This will research site history, environmental data, geology, and informs a complete picture of the site before any intrusive work is undertaken. It is advisable to commission desk studies pre-planning as this will provide the scope for further ground investigation works such as sources of contamination that need testing.
Next, physical ground investigation should be conducted. For residential development, common techniques are trial pitting, dynamic sampling, cable percussion boreholes and rotary boreholes, however, the exact scope should be based on the findings of the desk study.
Again, I advise undertaking on-site investigation as soon as possible. Not only is it needed to discharge planning conditions, but it will also help pick up anomalies or concerns. So, for example, if poor ground is found, there is an opportunity to refine and zone this, saving valuable time at foundation design stage.
Common outcomes
The most common outcomes from a physical ground investigation are where there are aspects of the site that need remediation. These will either be contamination or geotechnical challenges.
Contamination can be derived from a point source, such as a fuel tank, or could be more widespread and associated with previous industrial uses. This is particularly prevalent on brownfield sites. Geotechnical challenges often include weak or reworked ground, either naturally occurring or man-made, with the latter known as ‘Made Ground’.
Uncovering these contamination and geotechnical challenges as soon as possible allows developers to plan early and potentially mitigate these issues as part of the design process.
Remediation
The UK approach to assessing contamination is based on the ‘Source- Pathway-Receptor Concept’. To successfully mitigate a contamination risk, one of these links must be broken.
Source options can include excavation and disposal of soil at landfill, although this can represent a high-cost solution if the soil is classed as hazardous waste. The Source can also be treated, either insitu or ex-situ. For example, if the site has an area of hydrocarbon impacted soil, this needs to be broken down. Naturally, this could take 100+ years so treatment encourages and accelerates the natural breakdown process. The earlier Source treatment is started the better, as this can potentially allow previously contaminated soils to be reused on site.
Breaking the Pathway primarily occurs by preventing contact with contaminated soils. On residential development, this remediation is typically dealt with by including clean capping in garden and soft landscaping areas. Clean capping involves importing and installing clean soil, usually with a membrane beneath to restrict future residents from digging down and reaching the contamination.
Where weak soil or deep Made Ground has been encountered, traditional foundations become less feasible and further geotechnical consideration is required. Alternative solutions can include piling or ground improvement such as vibro or dynamic compaction to increase the soil strength.
Current legislation
The majority of ground investigation methods are tried and tested; however, contamination guidance is regularly reassessed with updated threshold values continually reviewed by industry experts.
Last year, NHBC also updated its earthworks guidance, requiring all residential developments to provide a verification report on completion of earthworks filing and testing. This is important for housebuilders and developers to bear in mind, as if site levels have had to be changed for drainage or visual reasons, then the NHBC will want to see evidence that the potential risks associated with earthworks, such as settlement or subsidence, have been fully addressed. If not managed or reported correctly, this could present cost and time delays to resolve.
One final word of advice relates to brownfield development sites. Here, it is strongly advised to pair ground investigation with underground utility mapping. In dense, urban areas, many utilities maps don’t show the necessary level of detail. Commissioning a utility survey, which can be done with ground penetrating radar or electro magnetic detection, gives us a clearer picture before we start any site digging.
Ultimately good ground investigation mitigates risks, allows for collaboration with other project disciplines, enables informed decision-making and keeps us all safe.
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