Capillary rise is the rise in a liquid above the level of zero pressure due
to a net upward force produced by the attraction of the water molecules to a
solid surface (e.g., soil or glass). Tabor, in 1930, recognized that frost heaving
required substantially more water than was naturally available in the soil
pores (characterized as "moisture content"). He noted:
"The average soil seldom contains as much as 50 percent water, but if all the water in such a soil were to freeze in situ, the change in volume could cause an uplift of less than 5 percent of the depth of freezing. The depth of freezing in the colder parts of the United States seldom exceeds 2 or 3 feet; yet a surface heaving of 6 inches is not uncommon and an uplift of a couple of feet has been reported."
This “extra” water comes from capillary rise. The capillary rise of water can be substantial, up to 6 m (20 ft) or more. The potential capillary rise can be estimated by the following:
|
where: |
hc |
= |
capillary rise (cm) |
|
|
C |
= |
constant (can range from 0.1 to 0.5 cm2) |
|
|
e |
= |
void ratio |
|
|
D10 |
= |
soil particle size, 10 percent finer passing (cm) |
Thus, the smaller the soil grain size, the greater the
potential for vertical water movement.
A quote by Lobacz, et al. (1973) further illustrates the serious nature
of capillary rise:
"A potentially troublesome water supply for ice segregation is present if the highest ground water table at any time of the year is within 5 ft of the proposed subgrade surface or the top of any frost-susceptible base materials used. When the depth to the uppermost water table is in excess of 10 ft throughout the year, ice segregation and frost heave may be expected to be reduced."
Note that Lobacz et al. stated that a water table with a depth greater than 3 m (10 ft.) only reduces the potential for ice lenses.