ENGINEERING GEOLOGY/Problematic Soils 559 Figure Breakup along the CapeTown Johannesburg road, South Africa, due to expansive clay SAR ¼ p Na 0:5Ca ỵ Mgị ẵ1 with units expressed in meq/litre of the saturated extract It has been suggested that an SAR value greater than 10 is indicative of dispersive soils, between and 10 as intermediate, and less than as non-dispersive However, dispersion has occurred in soils with values lower than The presence of exchangeable sodium is the main chemical factor contributing towards dispersive behaviour in soil This is expressed in terms of the exchangeable sodium percentage (ESP): ESP ¼ Figure Scanning electron photomicrograph illustrating the fabric of dispersed soil from Natal, South Africa value of the sodium adsorption ratio, the salt concentration, the pH value, and the mineralogy The sodium adsorption ratio (SAR) is used to quantify the role of sodium where free salts are present in the pore water and is defined as: exchangeable sodium  100 cation exchange capacity ½2 where the units are given in meq/100 g of dry clay A threshold value of ESP of 10% has been recommended, above which soils that have their free salts leached by seepage of relatively pure water are prone to dispersion Soils with ESP values above 15% are highly dispersive Those with low cation exchange values (15 meq/100 g of clay) have been found to be completely non-dispersive at ESP values of 6% or below Similarly, soils with high cation exchange capacity values and a plasticity index greater than 35% swell to such an extent that dispersion is not significant Severe erosion or worse, serious piping damage to embankments and piping failures of earth dams have occurred when dispersive soils have been used in their construction Indications of piping take the form of small leakages of muddy coloured water from an earth dam after initial filling of the reservoir The pipes become enlarged rapidly and this can lead to failure of the dam (Figure 6) Experience, however, indicates