An integrated approach to quick-clay mapping based on resistivity measurements and geotechnical investigations

Abstract

Quick clay is highly sensitive, marine clay with an unstable mineral structure due to post-glacial heaving and subsequent leaching of saline pore fluids by surface- and groundwater. Quick-clay layers pose a serious geo-hazard in Scandinavia and North America and need to be delineated in detail. Geophysical methods, especially resistivity methods, have been tested for quick-clay mapping at several sites across Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and integrating them with geotechnical borehole data including Resistivity Cone Penetrometer Testing (RCPT), we confirm the value of an integrated study for quick-clay hazard zonation. ERT is an ideal tool to interpolate limited borehole results and thus to provide a more cost efficient and detailed result than with borehole data alone. Our resistivity data from ERT, RCPT and lab measurements are generally consistent and appear isotropic. Geochemical analysis confirms that changes in resistivity are directly related to changes in clay salt content and secondarily related to sensitivity. It’s a challenge to resolve small contrast in resistivity (to distinguish unleached from leached clay), in close vicinity to drastic changes in earth resistivity, for example the transition from clay to bedrock. To cope with this we improve results by means of constrained ERT inversion approaches based on drilldata and/or seismic refraction bedrock data. Though ERT is no silver bullet solution to detailed quick-clay mapping, it can provide a significant contribution to improve the risk assessment at comparably lower costs than extensive drilling campaigns. Remaining methodological ambiguities need to be handled by integration with further data.