Page 241 - Geotechnical Engineering Soil and Foundation Principles and Practice

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

11.9.4 Insulating Against Freezing
The most obvious control measure against frost heave is to prevent freezing.
Foam plastic, usually high-density polystyrene called ‘‘Geofoam,’’ has a thermal
conductivity of about 0.026 W/K m, which is about one-hundredth that of soil.
However, after absorbing water the thermal effectiveness is estimated to be
reduced to about 30 to 40 times that of soil. In the 1980s a 51 mm (2 in.) thick
layer was successfully used to prevent frost heave on I-70 in Colorado where the
freezing depth was 1.8 m (6 ft).

The Stefan equation is not applicable to predict foam thickness because L for the
foam depends on a variable and unknown content of water. However, as a rough
guide the required thickness can be taken as 1/30 to 1/40 times the normal depth
of freezing. In the example cited above the thickness would have been 45 to 60 mm
(1.8 to 2.4 in.), which closely agrees with the case history.

Adding salt or calcium chloride to soil also can reduce or prevent frost heave by
lowering the freezing point, but the effect may last only one or two seasons until
the salts leach out and become pollutants.

11.9.5 Drainage
The most common methods used to control or reduce frost heave in roadbeds is to
raise the grade by scooping out ditches, and provide a drainable subgrade soil.
As a general guide, the distance from the bottom of a pavement to the highest
probable elevation of the water table should not be less than 2 m (6 ft), and a
larger distance is desirable if it can be obtained at a reasonable cost. Increasing the
distance to a water table decreases the potential gradient and the rate of upward
movement of capillary water. Another advantage of a high-crowned road grade
is that snow blows off.

11.9.6 Granular Subbase
Another approach is to replace frost-susceptible soil with a coarse-grained soil
that has a low capillary attraction for water. Ideally the subbase should extend to
the maximum freezing depth, but as a practical matter this usually is not feasible.
Therefore a granular subbase often is combined with drainage by ditches and/or
tile lines.

11.9.7 Cutoff Blanket
A third approach is to replace upper subgrade soil with a nonexpansive clay
having low permeability. This has the disadvantage that clay tends to be weak.
An alternative is the use of an impermeable geomembrane. In either case it is
important to also control and minimize infiltration of surface water by providing
surface drainage.

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