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Abstract:In southwest Louisiana, some surface silts (up to 3 feet deep) collapse under load after moisture is added. These soils were indistinguishable from nearby normal silts through routine tests. The deposits occur as low, gently-sloping ridges. Very similar collapsible soils were found in the well-known pimple mounds of the area. In the stable silts, vermiculite is the dominant clay mineral; in the collapsible silts, montmorillonite, Kaoline and illite are also present. Moisture is held in the pores of collapsible silts by electro-chemical forces. When collapsible silts were remolded with water or ethylene glycol (both polar liquids), no water passed through under 2 meters of head. In settled mixtures of collapsible silts and a solution of Calgon, (commercial brand of Sodium Hexameta phosphate) the supernatant liquid was black. The color change is attributed to lignins. Four criteria were established for identifying collapsible silts: an in-place unit weight less than 80 lbs/cu ft; a maximum dry unit weight less than 104 lbs/cu ft; a black liquid after the soil solids have settled out in a 3% solution of Calgon; and a total strain of at least 15% in the collapse test (a modified consolidation test of an oven-dried, undisturbed sample that is then saturated under pressure to 16 tons/sq ft).
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