HARDPANS IN PLEISTOCENE SEDIMENTS - A FORMATIVE ANALYSIS OF STRENGTH COMPONENTS

The occurrence of hardpans, a dense, relatively impervious layer found in unconsolidated sediments, has been described in a variety of geologic settings. The formation of hardpans and synonymous fragipan horizons in soils is commonly ascribed to the downward leaching of calcareous, siliceous or ferruginous substances that precipitate out of solution and rigidly cement soil particles. In the northeast, the occurrence of fragipans in glacial deposits has been attributed to compaction related to the weight of glaciers, to permafrost, and other processes active during the Pleistocene.  

To assess the mechanisms responsible for imparting strength to these layers, soils with fragipans representing a wide range of glacial sediments were tested. Compression tests performed on undisturbed cores and remolded soil demonstrated strength largely derived from internal friction forces arising from the close packing and interlocking of granular constituents. Remolded compression tests performed at various moisture contents further emphasize the importance of considering capillary tension forces in any analysis of shear behavior for densely packed soils. Dissolution studies conducted on cores confined to triaxial cells further demonstrated reduced sample cohesion upon preferential removal of cementing agents. Subsequent dispersion of matrix clay showed that the binding effect of the clay fraction is related to the adsorption of amorphous sesquioxides.  

The principal conclusion of this study is that pan rigidity and strength results from the mutual interaction of several factors. High soil densification, which serves to increase the number of solid contacts and reduce distance between granular constituents, is essential. The resulting close packing promotes grain interlocking and surface friction while maximizing intergranular cohesion by amorphous substances and moisture films concentrated at grain contacts.