Mapping of quick clay by ERT and CPT-R in the Göta Älv river valley

Paper in proceeding, 2013

Within clay prone areas it is important to know if and to what extent quick clay is present. In Sweden undisturbed sampling and laboratory investigations are used to determine clay sensitivity and identify quick clay. Recent research for mapping of quick clay areas with the resistivity method is based on the fact that low pore-water salinity constitutes a criterion for quick clay formation. Electrical resistivity tomography (ERT) is an established geophysical method that provides an overview of the resistivity in large volumes, but resolution decreases rapidly with depth. CPT-R probes (Cone Penetration Test with Resistivity module) on the other hand can register the resistivity with very high resolution continuously along the probed depth. In a known quick clay area along the Göta river in Gothenburg, Sweden, both ERT and CPT-R were used to evaluate possible synergy effects when the results were combined. Field investigations comprised 11 CPT-R soundings and five parallel ERT profiles, each 400 m long. The results gave good agreement between the methods down to depths of 10–15 m. Below that the CPT-R probe generally registered lower values than the ERT inverted resistivity models. The results from ERT comply with the value of 6 Ωm as a lower threshold value for possible quick clay formation in Sweden. From CPT-R measurements this value was found to be approximately 3 Ωm. The discrepancy appears to be related to sulphide content and may have measurement technical explanations or be due to the differences in chemical composition. The results show that a combination of ERT and CPT-R provides an efficient approach for mapping of possible quick clay volumes. It can be an integral part of an optimized detailed geotechnical investigation program, and form a decision basis for where to do undisturbed sampling for laboratory analyses. It also opens possibilities for correlation between resistivity and mechanical parameters via the CPT-R results.