Environmental Engineering Reference
In-Depth Information
Figure 7.16.
Estimation of dispersivity from exchangeable sodium percentage of the pore water, and
total ionic concentration of the seepage water (Ingles and Metcalf, 1972).
ICOLD (1990) indicate that soils with ESP of 10% or above, which are subject to hav-
ing free salts leached by seepage of relatively pure water are classified as dispersive. They
indicate that Australian research showed that soils are dispersive if SAR exceeded 2.
ESP and SAR are determined by chemical analysis of the soil pore water extracted by
vacuum from saturated samples.
Ingles and Metcalf (1972) suggest that to assess dispersivity potential, the exchangeable
sodium percentage and total ionic concentration of the water e.g. the water to be stored
in the dam, should be determined. Then for montmorillonite and illite clays, Figure 7.16
is used to determine whether the soil is in a flocculated, or dispersed (deflocculated) state,
and whether a change in pore water chemistry during filling of the reservoir can lead to
dispersion.
In Figure 7.16 filling of the storage for Dam A maintains a stable flocculated state, but for
Dam B, dispersion would occur. This can be readily related to the diffuse double layer con-
cept, where a change in concentration of salts in the soil water can lead to a larger diffuse
double layer, and a tendency to disperse (Dam B), but exchange of sodium cations by say
calcium would retain a stable flocculated structure even though the total concentration
was reduced (Dam A). McDonald, Stone and Ingles (1981) produced
Figure 7.17
to
include some work by Moriwaki and Mitchell (1977).
This indicated that the Ingles and Metcalf (1972) boundaries were reasonable except that
dispersion could occur in montmorillonites at higher ESP than indicated by the earlier