Introduction
Near shore, high latitude lowlands throughout Canada, Norway, Sweden, Finland and Russia are prone to a particular geohazard – quick clay. The key geotechnical properties of quick clay, also called Leda clay in Canada, are a very small remoulded shear strength and consequent high sensitivity (= undisturbed/remoulded shear strength). These properties stem from the fact that this formerly marine clay has been leached by ground water owing to postglacial uplift, losing salt ions that stabilized the flocculated clay structure. When quick clay fails, it liquefies and leads to retrogressive landslides that have caused massive damage and claimed lives both in North America and Europe.
Quick clay can be found in sedimentary areas close to the coast and below the marine limit, areas that are attractive for human settlements. Two of Norway’s four most populated urban areas (Oslo and Trondheim) are located in quick clay areas. Detailed study and mapping of areas prone to quick clay slides relies on geomorphology, quaternary geology and point information from geotechnical boreholes and laboratory tests. This is a labour intensive and long lasting process that, in Norway, has been going on since the 1980s and only the most vulnerable parts of the country are mapped so far. The detailed hazard mapping continues, at a pace of a hand full of municipalities a year.
We present two case studies that illustrate the value of AEM in the early phase of a Norwegian railway planning project and a regional study along the Göta-river in Sweden. Both cases demonstrate that AEM can provide the necessary resolution to distinguish saline, marine clay from leached, potentially quick clay.
Conclusion
The article show that AEM provides the opportunity to rapidly delineate sediments with a probability to be quick clay. A limiting factor that must not be forgotten though, is that ‘higher’ resistivity (> 10 Ωm) may be quick clay but can also be a false positive owing to silt, weathering and smooth resistivity transitions to bedrock. Very low resistivity (< 5 – 8 Ωm) can, however, be taken as a comforting factor, indicating intact saline clay.
As with all geophysical surveys, a sound regional geological understanding and integration with geotechnical data leads to results that can be used in geotechnical design. The vertical resolution of our presented airborne resistivity models is almost as good as one would find with ground-based electrical resistivity tomography (ERT). The very first metres of depth would, however, still be better resolved with ERT, especially in terms of lateral resolution. Detailed ground follow up with ERT and finally drillings, based on the regional understanding gained from AEM, is an advisable next step.
Acknowledgement
NGI is grateful to NIFS and BaneNOR: The NIFS (Natural hazards, Infrastructure, Flood and land Slides) project funded the Norwegian nationwide assembly of geophysical and geotechnical data. The Norwegian National Rail Administration (BaneNOR), the owner of the Norwegian case study, gave permission to publish these data. SGU acknowledges MSB (Swedish Civil Contingencies Agency) for funding the presented feasibility study.
References:
The full paper can be requested through the download link above or found directly at www.researchgate.net.
Pfaffhuber, A. A., Persson, L., Lysdahl, A. O. K., Kåsin, K., Anschütz, H., Bastani, M., Bazin, S., and Löfroth, H. 2017. Integrated scanning for quick clay with AEM and ground-based investigations, First Break, 35 73-79.
