Page 138 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 133

Calcium ions are attracted and held between layers of an expansive clay, with
some additional ions held at the edges. Because the ions are loosely held, they are
exchangeable, which means that other ions can be substituted and can markedly
change the clay properties.

The changes in expansive clay d-spacing are stepwise and show hysteresis, which
means that the humidity must be over 90 percent for a calcium clay to fully
˚
expand, and below about 50 percent humidity for it to dry to a spacing of 15.4 A.

6.3.12 Can Foundation Pressure Prevent Expansion?
How much pressure can be exerted by hydration of an expansive clay? Is the
pressure really enough to lift a building, and if so, how heavy?

To answer this a special compression device was mounted on a diffractometer,
and it was found that re-expansion of an air-dry calcium montmorillonite from
˚
15.4 to 19.6 A was prevented with an applied pressure of 4 MPa (40 tsf).
In contrast, a more ordinary foundation pressure of 0.4 MPa (4 tsf) allowed
90 percent expansion (Handy and Demirel, 1987). The relationship was linear, so
the heavier the foundation pressure, the less the expansion. Therefore, under the
same moisture conditions, floors are lifted more than adjacent load-bearing
foundations.

One way to prevent future expansion is to pre-wet clay so that it reaches
equilibrium expansion prior to construction. However, settlement then can occur
when the clay dries out. One of the more common causes of damage is from
drying shrinkage of soil around the edges of a floor or shallow foundation,
particularly if the situation is aggravated by suction from tree roots. Damages
may occur years or even decades after construction.

6.3.13 Sodium Clay and Lime Treatment

The expansive nature of clay is greatly increased if interlayer Ca 2þ ions are
þ
substituted by ions having a lower positive charge, in particular sodium (Na )or
þ
hydrogen (H ). Sodium clay occurs naturally in alkaline conditions such as in
playas, and hydrogen clay occurs in some forested A horizons with acid soil
conditions. The severe expansive properties of a sodium clay can be reversed by
adding a calcium salt, in particular hydrated lime, Ca(OH) 2 , so that Ca 2þ ions
þ
replace Na . Lime treatment also benefits calcium clays for reasons discussed
later in this chapter.

6.3.14 Expansive Clay Mineral Names

Montmorillonite is named for a village where it was first described in France.
Later it was discovered that the chemical composition of expansive clays

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