Integrated scanning for quick clay with AEM and ground-based investigations

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 (< 0.5 kPa in Norway after NGF, 2011; and <0.4 kPa in Sweden after Rankka et al., 2004) 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 (Figure 1).