STRUCTURAL COLLAPSE AND STRENGTH OF SOME AUSTRALIAN SOILS IN RELATIONTO HARDSETTING .1. STRUCTURAL COLLAPSE ON WETTING AND DRAINING

The surface structure of many Australian red and red-brown earths freq uently collapses (slakes) when dry, disturbed aggregates are wetted by rain or irrigation. The resulting fine matrix sets, on drying, to a s trong, cohesive layer of up to 200 mm thick (hardsetting). We investig ated the mechanism of collapse and the extent to which the structure o f aggregate beds from hardsetting and non-hardsetting soils collapsed when wetted by quick flooding or slowly with water at a suction of 200 mm, then drained in sequential steps of increasing suction and finall y dried at 40-degrees-C. After flood wetting, but before draining, no collapse was observed due to the small effective stress prevalent in t he flooded beds. After suction wetting, some collapse was measured owi ng to the effective stress (approximately 1.4 kPa) from the applied su ction. On draining, flood-wetted beds collapsed extensively (volume st rain >0.20), largely due to the disappearance of large pores (>75 mum diameter). Suction-wetted beds collapsed less (volume strain <0.16) an d retained more large pores. Hardsetting soils collapsed more followin g both flood and suction wetting (volume strain >0.20 and 0.10, respec tively), while non-hardsetting soils did not collapse as extensively ( volume strain <0.16 and 0.09, respectively). Results indicate that the mechanism causing collapse was independent of wetting method and invo lved two steps: (i) slaking of aggregates on wetting, and (ii) collaps e of the aggregate bed on draining as a result of development of effec tive stress within the beds.