Page 234 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 229

potential. The system then is depressurized and the moisture content of the soil
determined. By performing the test with different values of differential pressure,
one obtains a desorption curve that gives moisture-tension relationships in the
approximate range 1 to 15 bars.

A modification of this system by Hilf of the U.S. Bureau of Reclamation uses a
probe with a ceramic tip that is inserted into a soil specimen that is under external
air pressure. In order to minimize water transfer, a null-type measuring system is
used and connected to a vacuum gauge. As water is sucked out of the ceramic tip
the gauge becomes negative, so to prevent cavitation the external air pressure is
increased to a null point where water does not move into or out of the soil
specimen. That pressure is the matric potential.

11.8 FROST HEAVE

11.8.1 Where to Look
There probably is no more dramatic illustration of the effects of movement of
capillary water than frost heave, where the ground surface may spontaneously rise
many inches or tens of millimeters in a single winter season. Frost heave normally
is limited to northern climates where freezing occurs, but also can be a serious
problem under cold-storage lockers, where installations often become unusable in
a matter of one or two decades.
It once was assumed that the frost damage was caused by expansion of water
upon freezing, but this 9 percent volume change is not sufficient to explain heaves
measured in fractions of a meter. A close examination of the soil involved in frost
heave will reveal a series of layers of pure ice separating soil layers, as shown in
Fig. 11.14. The cumulative thickness of the ice layers represents the total amount
of frost heave, which can lift and crack building floors and foundations and
pavements, and prevent doors from opening.

11.8.2 Spring Breakup
Frost heave is only part of the problem because when the ice layers and lenses
melt, the soil becomes overly saturated with water and can turn into mud. In
unpaved roads such areas are called ‘‘frost boils.’’ They also occur under paved
roads where they become evident when a truck breaks through. It is for this
reason that a road may be embargoed for heavy loads, until the ground has
thawed and the excess water has drained out.

Thawing from the ground surface downward aggravates an already serious
problem by preventing excess water from draining downward. Thawing that is
hastened by rain can speed drying by allowing drainage downward.

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