Structural Engineer Pocket Book

Geotechnics 115
Site investigation and sampling
Once a desk study has been carried out and the most likely contaminants are known, an
assessment must be carried out to establish the risks associated with the contaminants
and the proposed land use. These two factors will determine the maximum concentrations
of contaminants which will be acceptable. These maximum concentrations are the
Soil Guideline Values published by the Department for the Environment, Food and Rural
Affairs (DEFRA) as part of the CLEA range of documents.
Once the soil guideline or trigger values have been selected, laboratory tests can be
commissioned to discover if the selected soil contaminants exist, as well as their concentration
and their distribution over the site. Reasonably accurate information can be
gathered about the site using a first stage of sampling and testing to get a broad picture
and a second stage to define the extents of localized areas of contamination.
Sampling on a rectangular grid with cores of 100 mm diameter, it is difficult to assess
how many samples might be required to get a representative picture of the site. British
Standards propose 25 samples per 10 000 m 2 which is only 0.002 per cent of the site
area. This would only give a 30 per cent confidence of finding a 100 m 2 area of
contamination on the site, while 110 samples would give 99 per cent confidence. It is
not easy to balance the cost and complexity of the site investigations and the cost of any
potential remedial work, without an appreciation of the extent of the contamination on
the site!