Page 236 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 231

11.8.4 Control of Frost Heave
Frost heave can be reduced or prevented by interfering with any one of the
required conditions. For example, freezing of foundation soils is prevented by
placing the foundation below the maximum depth of annual freezing or by
keeping a building heated during winter. Pavement can be laid on top of an
Õ
insulating layer such as a few inches of Styrofoam .
Because so many different factors contribute to frost heave it is not always
predictable, and a road that suffers damage one year may not be damaged the
next. The reason may be lowering of the groundwater table, or perhaps rapid
freezing that does not give sufficient time for moisture to move to a rapidly
progressing freezing front. Snow is an insulator that in theory can either
contribute to frost heave by slowing freezing, or prevent frost heave by preventing
ground freezing, depending on weather conditions.

11.8.5 Diary of a Frost Heave

As the frost line, or line separating frozen from unfrozen soil, penetrates
downward, water in the soil pores freezes, which from the standpoint of matric
potential dries the soil out. Liquid water therefore moves to the freezing front.
Some water may also move as vapor, which creates a loose array of ice crystals
inside voids called ‘‘frazzle ice,’’ but this does not make a major contribution to
heave.

As water reaching the frost line becomes supercooled, freezing is triggered by
ice crystals already in the soil so that the crystals grow. The tenacity with
which the molecules are held causes a rejection of foreign matter, so the ice is
relatively clear. Heave-producing ice layers therefore grow along the frost line as
long as they are fed from below by capillary water. The frost line and ice layers
normally are parallel to the ground surface but can be inclined or vertical behind
a wall.

As the weather becomes colder and freezing proceeds faster than capillary
conductivity can supply water to the freezing front, freezing penetrates deeper and
encapsulates the ice layer in frozen soil. If freezing slows or pauses, conditions are
reinstated for production of a new ice layer. This cycle can continue and build
layer upon layer of ice, as shown in Fig. 11.14. The total frost heave therefore is
the sum of all ice layer thicknesses (Fig. 11.15).

With the onset of warmer weather, thawing for the most part takes place
from the ground surface down, creating temporary boggy conditions until
thawing progresses all of the way through the frozen soil layer so that excess
water from thawing of the ice layers can drain downward. In areas of frost
heave, roads often are protected during seasonal thawing by embargoing heavy
truck traffic.

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