Effectiveness of resistivity cone penetration tests in salt-treated highly sensitive clay

Abstract

Salt wells filled with potassium chloride may be used as landslide mitigation in highly sensitive quick clays, as the changed pore-water composition permanently improves the remoulded shear strength. To verify improvements, resistivity cone penetration tests were conducted around salt wells installed at Dragvoll, Trondheim, Norway. The cone-test measurements for electric resistivity (or conductivity), tip resistance and pore pressure were combined with the results from laboratory tests on piston-core samples extracted around the wells. Salt content, soil and pore-water conductivity, pore-water composition and geotechnical properties were studied. Quick clay is defined as that having a remoulded shear strength of <0·5 kPa, and the soil conductivity is often <100 mS/m. Increased tip resistance was detected at soil conductivities exceeding 200 mS/m as a result of increased salt content, corresponding to a laboratory-determined remoulded shear strength of 3·5 kPa. At Dragvoll, improved non-quick clay is detected by a normalised tip resistance of 3·5 and a pore-pressure parameter of 0·9. Improved geotechnical properties around salt wells may be verified by cone testing, but a site-specific interpretation model based on geotechnical properties from laboratory tests correlated to cone-test results may be needed.