Quick Clays

Exploration of potentially unstable underground zones in Scandinavia

Shear wave seismic in Trondheim harbour, Norway

Clay formations originally deposited in a submarine environment and were later raised above sea level into meteorological water zones by isostatic or tectonic movements. They are called 'quick clays' (or Leda clays). They primarily occur in the northern hemisphere, in areas previously covered by thick ice sheets, such as in Canada, Scandinavia and Russia, although formations of this kind are also known in the southern hemisphere as well.

The name reflects the property of liquefying spontaneously through a chain reaction that can lead to the "drifting away" of large sections of the underground and giving rise to major damage to infrastructure, buildings and commercially used areas. In these formations, two clay minerals are originally bound electrolytically by a cation (e.g. potassium, sodium). The cations are successively removed by exposure to meteorological water, which destroys the physical bond. Only a small external shock is then required, e.g. from construction activity, to instigate the liquefaction process. The formations are usually not visible on the surface because they are covered by younger Quaternary sediments. Detecting them, as well as mapping their extent and thickness, is therefore of extreme importance for establishing safe foundations.

Quick clays were assumed to be beneath the new Trondheim harbour that was artificially built on the estuary delta of the Nidelv River. Submarine slides on the delta slope, which also gave rise to small tsunamis in the fjord, are historically known. Detailed exploration of the largely paved-over surfaces was therefore ordered for the planned expansion of the harbour and the construction of new facilities. In co-operation with the International Center for Geohazards (ICG, Oslo), and the Norwegian Geological Survey (NGU, Trondheim), LIAG carried out shear-wave seismic investigations for the first time in Scandinavia. The findings revealed the underground structures in high-resolution detail in the sub-metre range down to the depth of the basement at approx. 180 m.

Contact person