Mapping of marine and quick clays
Picrure: Surficial mapping in the field. Photo: Louise Hansen.
NGU’s activities focus on surficial geology mapping, which includes the mapping of marine deposits, including marine clay, and other related features such as marine limits and landslide deposits. The surficial geology maps and maps of the marine limit form a basis for the indication of quick clay slide susceptibility. These are used in the online map system Skrednett, and can be downloaded from NGU’s website. A new map service indicating ‘possible marine clay’ is also available from NGU. http://www.ngu.no/no/hm/Norges-geologi/Losmasser/Marin-grense/. A new database of subsurface engineering studies has been established at NGU; something that will eventually be helpful in obtaining an overview of quick clay deposits (NADAG).
Mapping of quick clays is based upon knowledge of the extent of ocean and fjord deposits from the surficial geology maps. Using topographic, geological, and simple geotechnical criteria, we can identify areas where it is possible that large quick clay slides can occur. Final confirmation of quick clay has to be done in a laboratory. Collection and testing of samples is expensive, sometimes prohibitively so. Alternative methods that give a good indication of the probablility of quick clays, including geophysical methods. An important aspect of preventative work is to highlight the problem and give guidelines for development in areas where quick clay is found or likely to be found.
In recent years, NGU has tested the use of 2D resistivity surveys to map quick clays, as a supplement to geotechnical surveys. The method is based upon the fact that quick clay has relatively low salt content, resulting in higher electrical resistivity than normal, salty marine clay.