Discussion

MR J. P. PELLISSIER, CSIR, South Africa In view of Mr Cheney's recorded movements over an impressive period of 27 years (Paper 22), the problems encountered in South Africa with some desiccated active residual clays will be of interest.

Due to a combination of geologically ancient soils (up to 200 million years) and a climate that has existed in its present condition for about 1 million years, very deep residual clays have developed in certain areas: about 30 m deep for Kimberley and 15 m deep at Vryburg. The whole profile is desiccated, since a water able does not exist within any zone of influence. These clays normally show low heave potential (approximately 4-5%) but are coupled to extremely high swelling pressures (>1500 kPa). In this type of clay there is virtually no stable depth and the whole clay profile can be activated to heave, especially under larger structures.