Page 220 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 215

Solution of eq. (11.3) for h yields
2T
h ¼ cos ð11:4Þ
r g
With perfect wetting, ¼ 08 so cos ¼ 1.0. Dimensions in the above equation will
2
be seen to be consistent since by definition 1 N ¼ 1 kg-m/s .
Example 11.3
Predict the height of rise of water with surface tension 72 mN/m, in a clean capillary tube
0.1 mm in diameter.
Answer: From eq. (11.4),
2 x 72 mN=m
h ¼ cos 0
6
3
0:5ð10Þ 4 m ð10Þ g=m 9:81 m=s 2
¼ 0:294 m ¼ 294 mm
In solving an equation such as this it is essential that units be included to avoid decimal
error.
According to eq. (11.2) the height of capillary rise is inverse to capillary diameter,
so as an approximation
0:03
d e ¼ ð11:5Þ
h
where d e is defined as a capillary equivalent pore diameter in mm, and h is in
meters.

Example 11.4
What is the capillary equivalent pore diameter in a soil where the height of capillary
saturation above the groundwater table is 2 m?
Answer: Soil pores obviously are very irregular and do not have a uniform diameter.
According to eq. (11.5) the equivalent diameter is d e ¼ 0.03/2 ¼ 0.015 mm or 15 mm, which is
silt-size.

11.3.8 The Vadose Zone
Because the height of the zone of capillary saturation is somewhat erratic, being
dictated by the maximum pore size, this zone also is called the closed capillary
fringe. Additional capillary water is held above that zone in an open capillary
fringe that is held by smaller pores and can extend to the ground surface. Open
and closed capillary fringes are illustrated in Fig. 11.4. The unsaturated zone of
soil including the open capillary fringe is called the vadose zone (from the Latin
vadum for shallow water, as in ‘‘wade’’).

Negative pore pressure in the closed capillary fringe is readily calculated from the
distance above the groundwater table, but is less easily found in the open zone

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