Observations on Quick Clay

Certain glacial and postglacial clay strata, long static in almost flat terrain, with little or no warning may flow spontaneously with destructive consequences. Samples of such clays, known as “quick clays,” have been examined from Scandinavia, Canada, and the northeastern United States. These have been compared with similar clays in areas where quick-clay movement has not been reported.

The principal minerals in the clay-size fraction of the quick clays observed are sheet-layer silicates. Illite (hydromica) and chlorite occur most frequently, interlayered illite-montmorillonite is common, interlayered chlorite-montmorillonite occurs occasionally, and interlayered illite-vermiculite was found in one sample. Quartz, feldspar, hornblende, and calcite predominate in the coarser-size fractions and are found to some extent in the clay-size fractions.

Most of the clays examined are of marine origin, but one of established fresh-water origin was included. The quick clays contain an average by weight of 57.3 per cent less than 2-μ. material, and the nonquick clays average 31.6 per cent less than 2-μ material. Chlorinity tests indicate that pore-water salinity of marine quick clays, about 35 g per 1 when deposited, becomes 0.2–15 g per 1 at flowage. The pore-water content of quick clays noted exceeds 44 per cent and in some clays may be as high as 80 per cent.

Major critical conditions which contribute to quick-clay movement appear to be: substantial quantities of layer-lattice silicates of colloidal size (about 40 per cent or more by dry weight), substantial pore water (44 per cent), a reduction in electrolyte concentration below 5 g of salt per 1, and the addition of a dispersant. Under such conditions a clay becomes critically sensitive and may flow spontaneously or as the result of a shock.

In quick clay of marine origin it has been suggested that a random orientation of clay particles is probably induced by coagulation during deposition. This orientation appears to provide stability to such clay strata. Removal of an electrolyte by natural leaching with fresh water appears to render the clay unstable. When the clay structure undergoes collapse, flowage is believed to result.

Field conditions suggest that flowage of non-marine quick clay may be attributed to the action of a dispersant such as humic acid.