Environmental Engineering Reference
In-Depth Information
Regionally abundant
Locally abundant
Minor constituent of bedrock but may be
locally abundant in surficial materials
FIGURE 10.39
Distribution of expansive soils in the United States. They are most widespread in areas labeled “regionally
abundant,” but many locations in these areas will have no expansive soils, while in some unshaded portions of
the map, some expansive soils may be found. (From Godfrey, K.A.,
Civil Engineering, ASCE,
1978, pp. 87-91.
With permission.)
North and South Dakota, and Texas. The distribution of materials susceptible to swelling
in Texas is given in
Figure 7.54.
10.6.2
Swelling in Soils
Determining Swell Potential
Basic Relationships
The phenomenon of adsorption and swell is complex and not well understood, but it
appears to be basically physicochemical in origin. Swell potential is related to the per-
centage of the material in the clay fraction (defined as less than 2
m, 0.002 mm), the fine-
ness of the clay fraction, the clay structure, and the type of clay mineral (see
Section 5.3.3).
Montmorillonite has the highest potential for swell, followed by illite, with kaolinite being
the least active. Thus, mineral identification (see Section 5.3.3) is one means of investigat-
ing swell potential.
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Clay Activity
Swell potential has been given by Skempton (1953) in terms of activity defined as the ratio
of the plasticity index to the percent finer by weight than 2
m. On the basis of activity,
soils have been classified as inactive, normal, and active (see
Table 5.29).
The activity of
various types of clay minerals as a function of the plasticity index and the clay fraction is
given in
Figure 10.40.
It is seen that the activity of sodium montmorillonite is many times
higher than that of illite or kaolinite.
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