Structural collapse and strength of some Australian soils in relation to hard setting: II. Tensile strength of collapsed aggregates

The strength of soils is related to structural stability. Aggregate structure which collapses on wetting may set to a hard, consolidated layer on drying (hard setting). This process may be moderated by suction wetting and possibly by application of calcium, but the mechanism of moderation is not clear. We investigated the collapse-strength relationship and the mechanism by which wetting method and calcium act to reduce strength in hard setting and non-hard setting soils. Indirect tensile strength of aggregate beds that had been wetted with water or 10mM CaCI₂ by rapid flooding or at a suction of 200 mm, was measured after draining to various suctions and drying at 40°C. The greater the volume strain during wetting and draining, the greater the tensile strength. Beds that were suction wetted, either with water or calcium solution, showed minimal collapse and did not develop high strength on drying. Water-flooded beds had the greatest dry strength while beds flooded with calcium solution developed significantly lower strength. The critical factor determining tensile strength of the beds appeared to be the presence of large (>75 μm diameter) pores. The greater the volume strain on wetting, the smaller the proportion of larger pores and the greater the tensile strength on drying. Flood wetting caused more loss of large pores and closer packing of particles. Wetting with calcium solution did not affect the degree of collapse compared with that of water alone but did produce beds with larger pores than when wetted with water. Consequently the strength of the calcium-wetted beds was lower.