Page 229 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
224 Geotechnical Engineering

of soils. The most troubling soils for frost heave are those that combine per-
meability and capillary potential, which are silts containing small amounts
of clay.

11.5.9 Effect of Dissolved Salts and Chemicals
An increase in the amount of dissolved salts in the soil water slightly increases
its surface tension and thereby lowers the matric potential of the soil, but the
effect is relatively minor. On the other hand a chemical additive that changes
the wetting angle, such as a bituminous compound, can halt capillarity.
In road embankments this also can be achieved by incorporating a layer of
impermeable plastic, but that has the disadvantage of also preventing infiltration
of rainwater.

11.5.10 Effect of Clay Mineralogy

Because of their high affinity for water and the associated diffuse double layer of
ions and water molecules, active clay minerals such as smectite can profoundly
influence matric potential. As the water content is decreased by drying, adsorptive
forces increase, and the last water adsorbed is held by an osmotic potential of
thousands of bars.

11.6 SORPTION CURVES

11.6.1 A Simple Model that Illustrates Capillarity in Soil

A simple and direct way to determine the equilibrium water content in soil in the
capillary fringe or vadose zone is to fill a tube with soil and suspend the bottom in
water. After a sufficient amount of time has elapsed the soil can be extruded,
cut into sections, and the moisture content of each section measured and, if
appropriate, its strength determined.

A typical result is shown in Fig. 11.9. The height above the free water surface
represents the matric potential. The curves for till and loess are based on more
precise measurements of matric potential, discussed below. As can be seen, soil
layers tend to reach equilibrium moisture contents that are characteristic of each
layer, and this system can model soil layers under a pavement. These experiments
also demonstrate the difference in capillary conductivity of the two soils, since in
the experiment shown in the figure it took approximately three times as long for
the wetting front to rise in the glacial till as in the loess. Note also that the loess
has a substantially higher capacity for capillary water, even though it contains
less clay than the till.

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