Minimum Requirements for Site Investigations for Self-Build Projects
In November 2021 LABC Warranty published a technical note on the site investigation work it expects before covering a self-build project, noting that such investigation is often seen as an unnecessary cost despite being one of the most important parts of a new build.
The warranty provider explained that its requirements go further than the Building Regulations. Those regulations set minimum standards for safety, health and welfare, largely drawn from past building failures, and building control mainly looks at compliance on the day of inspection or at completion. A warranty surveyor, by contrast, has to consider how the building performs over the whole life of the cover and beyond, using lessons from claims history.
As a minimum for self-build, LABC Warranty asks for a trial hole 3m deep, dug and witnessed by the project's structural or geotechnical engineer, who should comment on the ground conditions and their suitability for the proposals. The engineer should also design the foundations, ground floor and drainage to suit. A Phase One Geoenvironmental Assessment, a desk study of online data and records identifying geotechnical and contamination hazards, is also expected, with the engineer commenting on it. If it raises concerns, such as solution features in chalk, further investigation may follow.
Piled foundations need a detailed, site-specific Phase Two Geotechnical Site Investigation following BS 5930 and BS EN 1997-2. It should reach below the pile toe by the greatest of three pile diameters, 5m or the smallest plan dimension of the pile group, with reference to the Federation of Piling Specialists' 2013 minimum requirements. Boreholes should generally be 10m to 30m apart, at least three in number, and closer where there are hazards such as soluble soils or mining features. If the data are inadequate, the pile designer should call for more boreholes and testing.
The note applies to self-build projects only and was accurate as at September 2021; readers were told to check the current LABC Warranty technical manual with their risk management surveyor.
