The UCD community has made this article openly available. Please share how this access benefits you. Your story matters! (@ucd_oa) Some rights reserved. For more information, please see the item record link above. ABSTRACT Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. The purpose of this paper is to assess the use of a number of geophysical techniques for identifying quick clay. The investigated area, Smørgrav, located in southern Norway has a history of quick clay sliding, the most recent event occurring in 1984. Geophysical techniques that are used include electromagnetic conductivity mapping, electrical resistivity tomography, seismic refraction and multichannel analysis of surface waves. These results are compared to geotechnical data from bore samples, rotary pressure soundings and cone penetration testing. A number of these approaches have proved promising for identifying quick clay, in particular electrical resistivity tomography and electromagnetics, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and sampling. Seismic refraction was useful for determining the sediment distribution as well as for indicating the presence of shallow bedrock whereas the multichannel analysis of surface-waves approach suggested differences between the intact stiffness of quick and unleached clay. It is observed that quick clay investigations using discrete rotary pressure soundings can be significantly enhanced by using, in particular, electrical resistivity tomography profiles to link together the information between test locations, perhaps significantly reducing the need for large numbers of soundings.

Highly sensitive, low-saline clays, termed quick clays, represent a risk for large landslides. An in-situ experiment was conducted installing salt wells filled with potassium chloride (KCl) in order to study how salt wells can be used to reduce landslide risk. The salt-plume migration and the clay volume surrounding the salt wells were investigated by resistivity cone penetration tests (RCPTU), piston samples, conductivity divers and groundwater samples. Correlating the geotechnical properties to the occurring pore-water compositions, the remolded shear strength (cur) was improved beyond 1 kPa when the ratio of the sum of potassium, magnesium and calcium over the major cations exceeded 20%. The cur was improved to about 4-8 kPa 0.5 m from the wells within 3 years, and the liquidity index decreased from more than 3.4 to less than 1.2. A diameter of minimum 1.5 m around the wells was stabilized within 3 years. Thus, it is recommended to install the salt wells with a center-to-center distance of 1.5-2.0 m. The improved post-failure properties (cur and Atterberg limits) are considered permanent in an engineering time scale due to a lasting pore-water composition inhibiting development of high sensitivity. Salt wells can be installed without substantial disturbance of the soil, and can be used as landslide mitigation in quick-clay areas.

Re-establishing high salt concentration in leached low-saline, highly sensitive clays significantly improves their mechanical properties. Long-term effects on quick clay exposed to diffusion of potassium chloride (KCl) from salt wells installed in 1972 at Ulvensplitten, Oslo, Norway, are investigated. The increased undrained and remolded shear strengths, as well as increased Atterberg limits, remain 30 to 40 years later. The undisturbed shear strength increased from less than 10 to 25-30 kPa, and the remolded shear strength increased from less than 0.5 to more than 6 kPa. The liquid limit increased beyond the natural water content. Adding KCl to quick clay improved the properties to such an extent that it no longer appears as quick. Recent ground investigations in the area suggest a permanent effect on an engineering time scale. Consequently, the method may be suitable to prevent large flow slides in quick clay areas.

Retrogressive or progressive landslides in slopes with highly sensitive quick clays may extend several hundred metres from where the slide was initiated. Reintroducing high salt contents to the pore water of leached low-saline quick clays may inhibit the development of such large landslides. Wells filled with potassium chloride (KCl) were installed in a quick-clay deposit at Dragvoll, Trondheim, Norway, to investigate the impact of changed pore-water chemistry on the geotechnical properties. The clay ceased to be quick due to favourable changed ion composition in the pore water, even at low salt contents, thus inhibiting retrogressive development. The apparent pre-consolidation stress (p′c) increased with increasing salt contents and the immediate collapse in structure typically seen in quick clays was avoided at loads just beyond p′c by changing the pore-water chemistry. Quick clay treated with KCl may therefore work as a barrier against retrogressive development and reduce the risk of initiation of progressive landslides.

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