SITE INVESTIGATION contractor Fugro has developed a natural gamma cone for cone penetration testing.
Development was driven by the difficulty in determining chalk lithology when using only CPTs in chalk deposits. Without local knowledge or site specific borehole data, the signature of chalk is very similar to a deposit of sand with layers of clay, Fugro's Paul Jacobs explained.
'Natural gamma logging was developed decades ago as a tool for logging boreholes in rock and soil to enhance strata differentiation and is based on different soils (and rocks) emitting markedly different levels of naturally occurring gamma radiation, ' he said.
'Clays, for instance, have high levels of naturally occurring gamma radiation because they are rich in the minerals which give off natural gamma radiation during the disintegration of radioactive elements, mainly potassium, thorium and uranium.
'These are concentrated in feldspar, mica and glauconite, and the argillaceous rocks derived from these minerals such as clays, marls and shale.' Jacobs said a lower clay content corresponds with a reduction in natural gamma radiation.
'While the levels in sands and silts are still significant, the amount of the natural gamma radiation in chalk is negligible.
'This means that the additional measurement of natural gamma radiation gives precise delineation of chalk strata as well as solution features infilled by soil.' The technique can also be used to detect radioactively contaminated soil and groundwater.
While the firm's video cone can be used to look for colour change to white, the push is four times slower than the standard geotechnical cone, he added. Both methods can now be used to confirm results.