Page 221 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
216 Geotechnical Engineering

Figure 11.4
Zones of capillary
water and the
vadose zone.

because part of the capillary cross-section is occupied by air. A correction
therefore must be made to the effective stress equation to account for this effect,
as discussed later in this chapter.

Example 11.5
Soil at a depth of 3.05 ft (1 m) is in a zone of capillary saturation that extends 2.02 ft
3
3
(0.67 m) above the groundwater table. The unit weight of the soil is 125 lb/ft (19.6 kN/m ).
(a) What are the total and effective stresses at this depth? (b) Is the soil stronger or weaker
than that at the level of the groundwater table?
2
Answer: Total stress is 3.05 125 ¼ 381 lb/ft (1 19.6 ¼ 19.6 kPa). Pore water pressure is
2
2.02 62.4 ¼ 126 lb/ft ( 0.67 9.81 ¼ 6.6 kPa).
2
0
The effective stress is ¼ u ¼ 381 ( 126) ¼ 507 lb/ft (19.6 { 6.6} ¼ 10.0 kPa).
Because of the higher normal stress and intergranular friction the soil is stronger in the
capillary zone.

11.4 POSITIVE AND NEGATIVE PRESSURES FROM SURFACE TENSION

11.4.1 Pressure Inside a Soap Bubble

Calculating the pressure inside a soap bubble may seem a bit whimsical, but even
whimsy can have purpose. Furthermore the concept and calculation are so
simple, and the result is sure to enliven a flaccid conversation. Does the pressure
required to make a bubble increase, decrease, or stay the same as the bubble grows
larger?

From Fig. 11.5 the internal pressure over a diametric area equals 2 times the
surface tension around the circumference because both an inner and an outer
surface are involved:
2
r p¼ 2T 2r ð11:6Þ

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