Page 240 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 235

(b) How many degree-days above 08C will be required for complete thawing, assuming no
change in moisture content and no direct heat from the sun?

Answer: From Table 11.1, k u ¼ 2.6 W/m K,so
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
13:1 2:6 F
0:6 ¼ , or F ¼ 62 degree-days C:
77,000
Because of the difference between k t and k u , in this case it will require 62 degree-
days of thawing to compensate for 50 degree-days of freezing. As previously
mentioned, the calculations do not take into account additional water drawn into
the soil by frost heave, which mainly will affect the rate of thawing.

An application of the Stefan equation is illustrated in Fig. 11.17 and shows several
periods when thawed soil was on top of frozen soil that would prevent downward
drainage. Also, by simply counting the thawing and refreezing periods at different
depths it will be found that the number of freeze-thaw cycles is a maximum at the
ground surface and decreases with depth, which is an important consideration for
pavement design in freezing climates.

Question: In Fig. 11.17, during what months might it be expected that most of the
frost heave would occur?
Answer: November and December. Why?

Figure 11.17
Freezing depths
calculated using
the Stefan
equation indicate
8 cycles in the
road base course
during one winter
season. (After
K. P. George.)

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