Discussion on ‘Mackintosh-vs-vane estimation of undrained shear strength correlation for a sabkha clay of Saudi Arabia’ by D. Hossain & K. M. Ali

Abstract

A. N. James and A. L. Little write: Hossain and Ali have presented an interesting set of results obtained for an Obhor sabkha clay near Jedda, suggesting a tentative correlation between undrained shear strength (measured by a field vane test) and the M -value (number of blows for 0.3 m penetration) obtained using a Mackintosh probe. :Figures 2–4 of the paper show clearly that one of the difficulties in dealing with ‘sabkha’ is the considerable scatter of results and the variation in the nature of the basic material comprising the sabkha. In a previous paper, Hossain & Ali (1988) divided the upper 5 metres of the Obhor Sabkha into two main zones: Zone I: a landward zone, predominantly clayey beneath a thin ( c . 0.5 m) layer of loose sand; Zone II: a seaward zone, predominantly sandy with a thin ( c . 0.5 m) cemented sandy crust. A small subzone, IIA, is described as ‘clay over sandy sabkha’. For the upper soft clay (at Zone I) they give liquid limits between 20% and 50 0.000000or the top crust and 36% to 60 0.000000or the main claybody with plasticity indices ranging from 5 to 20 for the crust and 20 to 35 for the main body; ‘clay’ is an acceptable description for this type of soil. The medium sensitivity referred to by the authors may reflect conditioning by ion exchange processes, brought about by gypsiferous pore water or by changes in salinity (Mitchell 1976) . James & Little (1986) gave some results from an extensive