| Abstract: | The geographic association between earthflow occurrence, sensitivity of the sediment and salinity of the interstitial fluid has been examined in an area of exposed marine (Champlain Sea) deposits in the Lachute region northwest of Montreal, Quebec. Mapping of earth resistivity, as a means of calculating porefluid salinity, revealed that areas of large earthflows (>20 ha) coincided with sediment of very low porefluid salinity. Earthflow scars were absent from intervening areas of higher salinity (>5 gl?1). Mapping of the thickness of the clay deposit by seismic refraction showed, furthermore, that low porefluid salinity occurred only in areas of relatively thin (<25 m) deposits resting upon more permeable drift or rock. Finally, laboratory tests confirmed that high sensitivity was restricted to areas of low porefluid salinity. The study thus provides a geomorphological verification of the importance of ‘salt depletion’ as a prerequisite for earthflow occurrence, and, by implication, presents mapping of porefluid salinity as a method of delineating zones which should be safe from landslide retrogression in this type of sediment. |
