ABSTRACT
The limestone powder was obtained from a specific calcrete deposit in the Northern Province of South Africa where it has been mined for more than fifty years. The cream coloured limestone went through a well-controlled milling process and is credited with optimal oil absorption, a good surface finish and excellent drying characteristic. The chemical and physical properties of the limestone powder are given in Table 1(a) and (b), while the grain size distribution is shown in Figure 1 together with other granular soils (quartz powder and glass beads) used in previous experimentation. Specimens of limestone powder of dimensions 38 mm diameter and 76 mm long were moulded at 6% water content from mixes made with distilled water, 1 M NaCl and 2g/l detergent solutions after these had attained moisture equilibration in sealed plastic bags. The choice for 1M NaCl and 2g/l detergent was in keeping with previous research (Katte & Blight, 2012). When these ionic and non-ionic solutes were dissolved in water, NaCl caused a slight increase (10%) in surface tension while detergent caused a 50% reduction in
1 INTRODUCTION
The shear strength of unsaturated soils has been a debatable topic among researchers for many years. The behavior of these unsaturated soils is intrinsically linked with suction. Most geotechnical and soil science literature mentions that suction is a function of osmotic and matrix suction combined, which is called total suction though there appears to be no convincing basis for this combination. For example, Sreedeep and Singh (2006) have concluded that most studies on suction wrongly approximate total suction to matrix suction. Other studies carried out on marine soils indicate that the high salt content contributes to a high suction which in turn greatly influences the physical and volumetric changes of soil (Noorany, 1984; Barbour and Fredlund, 1989; Fredlund and Rahardjo, 1993; Feng et al., 2003). The resurgence of interests in the capillary model with the works of (Justo and Vasquez, 2010; Blight, 2010; Katte and Blight, 2012; 2013) lends credence to the fact that more thoughtful and direct research is called for in understanding which of the two suction components affect soil strength and volume change. Most studies carried out so far used clayey soils of which the clay structure and mineralogy will affect experimentation. However, for granular soils, Katte & Blight (2012) showed that the shear strength of unsaturated soils is little affected by the presence of solutes in the pore water. Therefore, the shear strength of unsaturated granular soils is
specimens which were not sheared were re-exposed to atmospheres over distilled water, 1M NaCl and 2g/l detergent solutions for another three months, making a total of six months. After this, the specimens were sheared as in the case above. During triaxial testing, the suctions were not measured. These were assumed to be the same as the equilibrium suction after drying.
