Geology, mineralogy, and geochemistry of Canadian soft soils: a geotechnical perspective

Abstract

A review of the recent research literature concerning the geology, mineralogy, and physicochemistry of soft soils in Canada is presented. Soft clays are considered from three viewpoints: (1) their three-dimensional distribution and chronological evolution, (2) the sedimentological processes critical in their formation, and (3) geochemistry and mineralogy as they directly influence geotechnical behaviour, especially soft soil sensitivity.Canada's soft soils are products of sedimentation in proglacial and postglacial lake basins that existed between 18 000 and 6000 years BP. They consist of both freshwater and marine clays, which usually behave quite differently, due to peptization or dispersion of the marine clays following postglacial crustal rebound that elevated all clay deposits. The evolution of the major lakes and seas is briefly reviewed in a Canada-wide context.The sedimentology review shows clearly that varved clays are normally features of bottom, heavy-density turbidity current origin, probably in very cold, ice-contact glacial lakes. Warmer postglacial lakes were probably characterized by low-density overflows, and thinly laminated clay deposits developed rather than varved clay sequences. Marine clay deposits also developed from low-density freshwater overflows followed by flocculation, organic agglomeration, and sedimentation. Modern examples are given to illustrate the three major sedimentological processes.Mineralogy and physicochemistry are reviewed in terms of their practical relevance to soft soil sensitivity. Factors influencing both the undisturbed strength and the remoulded strength are reviewed with special reference to recent Canadian research. Included are discussions of cementing agents, specific surface, amorphous and smectite contents, and zeta potential controls as they influence depositional flocculation and postdepositional peptization.