The mechanical stability of clay soils as influenced by the moisture conditions and some other factors

To test whether flocculation is a prerequisite for granulation and a waterstable structure or whether drying (close contact) is sufficient, homoionic suspensions were prepared from pure sodium, potassium, calcium or magnesium montmorillonite, illite or kaolinite and from the respective soil clays. Forms other than Na were prepared from Na suspensions without drying; the ultimate salt concentration was below flocculation value.

Suction-moisture curves were established and reswelling (pF 1 after positive pressure) was studied. Sodium clays except latosol swelled again completely after drying; potassium clays, except pure montmorillonite, swelled again completely up to high suctions; calcium and magnesium clays showed partial reswelling; latosol none, except with Na at high pH; pure illites swelled again but the soil clays behaved like soil montmorillonite. Inversion shaking was used to estimate the stability of granules formed by complete or partial water desorption. The stability of calcium and magnesium clays, and soil clays depended slightly on salt concentration; with Na or K, stability depended strongly on salt concentration, except in potassium illite soil clay; stability of all latosols decreased with salt concentration. The stability test was also employed to estimate the destruction caused by working the soil at different water contents. The phenomena observed could be explained by MADELUNG forces between parallel planes of clay minerals at short distance and by edge to plane attraction.