Page 225 - Geotechnical Engineering Soil and Foundation Principles and Practice

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

compaction is halted when most but not all of the air has been squeezed out.
A compacted soil therefore is not a saturated soil, as some air remains trapped
even if the soil is submerged.

Unsaturated conditions dominate in soils such as dune sand and coarse loessial
silt that are so permeable that they do not support a groundwater table. Surficial
zones in most soils are not saturated, and an unsaturated zone can extend very
deep in deserts and semiarid areas where there is little rainfall to replenish the
groundwater supply.

As previously mentioned, a correction is required to the Terzaghi effective stress
(eq. (11.2)) in unsaturated soils because pore water does not occupy the complete
cross-sectional area of the soil pores. A simplified version of a correction
originally proposed by Bishop is as follows (Fredlund and Rahardjo, 1993):
0
¼ u ð11:14Þ
where is a number between zero and one, and the other variables are as defined
in eq. (11.2).

When a soil is saturated, ¼ 1, and eq. (11.14) reduces to eq. (11.2). When a soil is
completely dry, ¼ 0 and total stress equals effective stress, as obviously there is
no influence from pore water pressure if there is no pore water. However, the
relationship to percent saturation is not linear, and has been found to depend on
soil type and even on the direction of the applied stress according to the void
orientation and particulate packing arrangement.

The increase in cohesive strength on drying was shown by a classic experiment by
a co-worker of Atterberg, whose contributions are presented in the next chapter.
In 1914 in Germany, Johansson dried soil briquettes and measured the force
necessary to split them with a wedge. His results are shown in Fig. 11.8, where
strength steadily increases down to a moisture content of 14 percent at point A,
where there is a break in the curve. At that point the soil suddenly became lighter
in color, indicating the entrance of air into the pores and supporting the
introduction of a factor as in eq. (11.14).

The complete Bishop equation includes pore air pressure, which under most field
conditions is negligible. However, this inclusion is necessary for laboratory testing,
where, in order to apply an external pressure to soil specimens, they are sealed in a
rubber membrane such that included air cannot escape and also is compressed.

11.5.2 Pressure, Head, and Potential

In fluid mechanics pressure is one component of total head, which is the sum of
pressure head, elevation head, and velocity head. Because fluid flow in soils is
relatively slow, velocity head is negligible.

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