On the use of CPTs in stratigraphy: recent observations and some illustrative cases

During processing and interpretation of about 200 Cone Penetration Tests (CPT) with electrical cones in a limited area of 60 km(2) in NE Belgium, a close analogy was observed between the vertical trend and signature of two CPT derived parameters (normalized soil behavior index I-c and hydraulic conductivity k(SBTn)) and borehole gamma ray (GR) measurements. This close analogy could be repeatably observed on Ave locations with a CPT executed along a cored and logged borehole. Using this feature proves to be a considerably help for stratigraphic interpretation, especially for sandy units displaying low lithological contrast. Lithostratigraphic units and their boundaries are determined by the specific log response patterns of these CPT derived parameters (k(SBTn) and/or I-c). These log responses prove to be more consistent and better to correlate than for the classical parameters (cone resistance qc and friction ratio Rf). This paper describes the case of a Neogene setting in the NE of Belgium where this feature was first observed. An informal stratigraphy based on CPT log signal is proposed for the geological units for the research area. Some other cases arc briefly described to illustrate the usability of this feature for other geological settings or issues.