G Dispersion

Dispersed soils are those in which the clay fraction is or may become deflocculated or disaggregated. The clay particles in these soils have minimal electrochemical attraction and are not tightly bonded when

Table 7-3 Excavation characteristics of geologic materials (from Kirsten 1987)

Geologic material Excavation Equipment size characteristics flywheel horsepower saturated resulting in reduction of effective particle size and effective pore diameters. Dispersed soils are also characterized by high soluble sodium content. Whereas calcium ions promote flocculation, sodium ions enhance dispersion in clays. Dispersion tends to decrease permeability while flocculation tends to increase it. Dispersed soils occur in all regions of the United States. If dispersion is suspected, send representative soil samples to a laboratory for testing.

Typical characteristics of dispersed soils are:

• Relatively high content of soluble sodium and varying amounts of exchangeable sodium.

• Highly erodible. Clay and colloidal fractions go readily into suspension and remain there. Surface exposures have appearance of melted sugar. Gullying or rilling is extensive.

• Shear strengths are lower than normal in CL, CH, and ML soils. Clay fraction goes into suspension within the pore fluid and reduces electrochemical attraction between particles.

• Generally have high shrink-swell potential and are thus subject to severe cracking when dried.

• Often occur in layers or lenses in a soil profile rather than as extensive masses of a mappable soil series.

Further information on dispersion is in Soil Mechanics Note 13, Dispersive Clays.

Geologic material Excavation Equipment size characteristics flywheel horsepower

Very soft to very

Hand pick and

stiff cohesive soil

spade or light

or very loose to


very dense


granular soil


Very soft rock to

Power tools or

moderately soft

easy ripping




Moderately hard

Hard to very

to hard rock

hard ripping

(rock excavation)

Very hard to

Extremely hard


ripping to blasting

hard rock

(rock excavation)

1/ Explosives may be an alternative to equipment.

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