Dispersive Soils
The dispersion in the soil occurs when the repulsive forces between the clay particles exceed the attractive forces, thus bringing about deflocculation, so in the presence of relatively pure water, the particles repel each other.
In untreated soils, there is a certain threshold velocity that does not lead to erosion of the flowing water. Separate particles adhere to one another and are removed by flowing water with a certain erosion force.
In contrast, dispersed soils have no threshold, colloidal clay particles are suspended even in calm water and, therefore, these soils are more susceptible to erosion and piping.
Dispersive soils contain a moderate to a high content of clay material but there are no significant differences in the clay fractions of dispersive and non-dispersive soils, except that soils with less than 10% clay particles may not have enough colloids to support dispersive piping.
Dispersed soils have a higher content (up to 12%) of dissolved sodium than ordinary soils. Soil particles in soils with high salt content tend to coalesce and coat around the silt and sand particles and the soil floats.
Propagation soils are usually found in semi-arid regions where annual rainfall is less than 860 mm (Bell and Walker, 2000). For a given corrosion fluid, the boundary between the flocculated and deflocculated states depends on the value of the sodium absorption ratio.
The sodium adsorption ratio, SAR, is used to quantify the role of sodium where free salts are present in the pore water and are defined as:
SAR = Na/ 0.5 (Ca+Mg)
Saturated extract with units expressed in milliseconds per liter. There is a relationship between the pore water-electrolyte density and the exchangeable ions in the soil layer absorption layers.
This relationship depends on the pH value and may be influenced by the type of soil minerals present. Therefore, it is not stable. Gerber and Harmse (1987) considered a SAR value as an indicator of more than 10 dispersal soils, intermediate between 6 and 10 and less than 6.
However, Aitchison and Wood (1965) considered soil dispersion beyond SAR2. Scattering erosion depends on the mineralogy and chemistry of the soil on the one hand, and the salts dissolved in the hole and the water on the other.
The presence of interchangeable sodium is the main chemical contributing factor to clay behavior. The percentage of exchangeable sodium, expressed in ESP:
ESP = exchangeable sodium/cation exchange capacity x 100
The units are fed into mech / 100 g dry soil. Soils above the ESP limit of 10 discharge their free salts from relatively pure water spills and are subject to dispersal. Soils with ESP values greater than 15% are more prevalent, according to Gerber and Harms (1987).
On the other hand, those soils with low cation exchange values (15 meq/100 g of clay) are non-dispersive at ESP values of 6% or below. Soils with high cation exchange capacity values and a plasticity index greater than 35% swell to such an extent that dispersion is not significant.
High ESP values and piping potential may exist in soils in which the clay fraction is composed largely of smectitic and other 2:1 clays. Some illicit soils are highly dispersive. High values of ESP and high dispersibility are generally not common in clays composed largely of kaolinite.
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Another property that controls the possibility of clays being dispensed is the total content of dissolved salts, TDS, in pore water.
In other words, the lower the content of dissolved salts in pore water, the greater the susceptibility to the diffusion of sodium saturated soils. Sherard et al. (1976) For this particular purpose, the total dissolved salts are milli-equivalents per liter of the total content of calcium, magnesium, sodium, and potassium.
They have designed a chart in which the sodium content is expressed as a percentage of TDS and plotted against TDS to determine the dispersion of soils (Figure 5.7a). However, Kraft and Axiardi (1984) showed that this chart has a poor overall agreement with the results of physical examinations.
In addition, Bell and Maud (1994) have shown that the use of dispersal strips to isolate dispersal soils has not proven reliable in Natal, South Africa. There, the determination of dispersion efficiency involves the use of a chart repeatedly designed by Gerber and Harms (1987).
Internal erosion of the dispersed soil leads to the formation of pipes and internal cavities on the slopes. Piping is initiated by the dispersion of clay particles along desiccation cracks, fissures, and root holes.
Piping has led to the failure of earth dams built with dispersive soil Fig . Indications of piping take the form of small leakages of muddy-colored water after the initial filling of the reservoir.
In addition, there is severe erosion damage that forms deep trenches on the shafts after rainfall. Fortunately, when dispersed soils are treated with lime, they are converted to non-dispersing if the lime is completely mixed with the soil.
Also Read: Ground Improvement