Page 214 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 209

castle collapses. The sand must be moist in order to be able to build a sand castle
in the first place. The transitory nature of the part of soil strength that depends on
moisture content gives it the name ‘‘apparent cohesion.’’

Negative pore water pressure also sets up a pressure gradient that acts to suck in
more water, which if available is responsible for frost heave. As this is closely
related to capillarity it is discussed in this chapter, along with the depth of freezing
and permafrost.

11.2 EFFECTIVE STRESS

11.2.1 Equilibrium Pore Water Pressure

A scuba diver knows from personal experience that water pressure increases with
depth under water. The same increase exists in saturated soils, since voids are
interconnected and transmit pressure. In a static situation, the pore water pressure
is simply calculated from
u ¼
w h ð11:1Þ

where u is pore water pressure,
w is the unit weight of water, and h is the depth
below a groundwater table.

11.2.2 Equation for Effective Stress

One of the most important insights by Terzaghi was to recognize that, because
water pressure on soil grains tends to push the grains apart, it can simply be
subtracted from total stress to obtain what he defined as effective stress.
This actually is a simplification because it assumes that water pressure acts
across the entire cross-section of soil instead of only on the area represented by the
voids. However, true grain contact areas are very small and make up only a small
fraction of the entire cross-section. This assumption greatly simplifies calculations
and, as it tends to overpredict the influence from positive pore water pressure, is
on the safe side. By considering soil stress on a total area basis, one can write:
0
¼ u ð11:2Þ
0
where is the effective stress, is total stress, and u is the pore water
pressure. This is the Terzaghi equation for effective stress, and even though
it is deceptively simple, it is one of the most important relationships in soil
mechanics.

Example 11.1
3
3
3
The unit weight of a saturated soil is 125 lb/ft (19.6 kN/m or 2.0 Mg/m ). What are the
total and effective stresses at a depth of 3.05 ft (1 m)?
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