Page 228 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 223

is laid. Measurements indicate that such accumulation occurs slowly and that 3 to
5 years may elapse before the maximum accumulation is reached. The pavement
or building also serves as an insulator that reduces the range and rate of change
of temperature in the underlying soil. Even subgrades under pavements in arid
regions have been known to become very wet, and the soil may lose a substan-
tial part of its bearing capacity by accumulation of capillary moisture from a
water table below. These factors point to the relevance of capillary characteristics
of soils under buildings and pavements.

11.5.6 Capillary Siphoning
A siphon normally is thought of as a tube filled with fluid and connecting a higher
with a lower fluid level so that the weight of the fluid in the lower part of the
siphon causes it to flow out and draw more fluid in at the top. If a host of tiny
capillaries is substituted for the tube, they also can act as a siphon, but the action
is relatively slow.

Capillary siphoning can cause an unexpected lowering of the water level behind an
earth dam if there is a continuous soil layer running over the top of the impervious
core. The core material therefore is carried to the ground surface even though it is
substantially higher than the highest water level behind the dam.

11.5.7 Effect of Temperature

The surface tension of water is inversely related to temperature, so cooling
increases the attraction of soil for capillary water. Thus, cooling during fall and
winter tends to draw more moisture into soil subgrades under pavements, whereas
summer heating reduces the soil affinity for water. Emergency water supplies in
deserts may be collected by excavating a shallow hole and covering it with a
plastic film so that water moving upward toward the cooler night temperatures
condenses on the underside of the film. The film is depressed by a weight in the
middle to make a drip point into a cup placed in the hole.

11.5.8 Ground Freezing
A sudden and drastic influence from temperature occurs when water freezes,
as the surface tension (surface energy) of ice is very high and the vapor pres-
sure very low. Freezing temperatures therefore attract both capillary water and
water vapor that migrate to a freezing zone to form ice crystals and lenses.
These generally are oriented horizontally and lift the overlying soil due to
frost heave. Water is replenished by upward movement of water from the
groundwater table.

The amount of frost heave also depends on the capillary conductivity and
matric potential of the soil, which in turn relate to its gradation. This relationship
is discussed in the next chapter, which is on the engineering classification

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