GLACIOMARINE CLAYS: CHEMISTRY, SENSITIVITY AND LANDSLIDES

Sensitivity development in fine-grained, glaciomarine sediments is dominated by the influences of chemical factors, including mineralogy. Flocculation of the low-activity, silt- and clay-sized particles under marine conditions produces a randomly oriented, high-water-content structure that survives as accumulation and consolidation continues. Iron oxide cementation may augment the strength. With glacial retreat and uplift above sea level, rainfall infiltration through the surface and from the base displaces the high-salinity, Na-dominated, pore water. The undisturbed strength is little affected, but the remolded strength gradually decreases, until below ~2g/l salinity the remolded material behaves as a liquid. Such sediment is called ‘quick clay’. From the initial exposure of the sediment surface, concentrated erosion creates valleys and soil profile development commences on all the exposed surfaces. Drainage of the slopes, oxidization of the near surface zones, and vegetative growth combine to cause chemical and physical changes. Chemical changes include: the presence of O₂ and CO₂; release of K, Ca and Mg into solution; and formation of iron oxides and soluble organic compounds. Wetting/drying and freeze/thaw dramatically change the macro structure to produce a ‘nodular’ (blocky) structure that extends to 1.5 to 5+ m depth on old sea-bottom surfaces, and to 10 to 30+ m on riverbank slopes. Where they occur, the chemical and physical weathering reduce the sensitivity and change the mode of failure from full-volume collapse of the flocculated quick clay, to failure along well-defined planes in medium-sensitivity clay, and to dilatant failure as blocky units in low-sensitivity, physically weathered material.