Page 213 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING

Pore Water Pressure,
Capillary Water,

11 and Frost Action

11.1 PUTTING IT TOGETHER

11.1.1 Of Soil and Water
The preceding chapters dealt with the nature of soil minerals and their
relationships to water; this chapter emphasizes water pressures, both positive
and negative, that either tend to push soil grains apart, reducing their contact
friction, or pull them together, increasing friction. The first is called positive pore
water pressure; the second, negative pore water pressure or suction. The reference
pressure for both is atmospheric pressure, which tends to be equalized in soils
above a groundwater table that are more or less permeable to air.

11.1.2 Positive Pore Water Pressure
Because positive pore water pressure acts to push soil grains apart and reduce
their intergranular friction, it can be a key component of shearing strength.
Positive pore water pressure plays a key role in landslides, which therefore usually
occur after heavy rains when the groundwater table is high.

Positive pore water pressure also is subject to dynamic influences from
compression, which creates a gradient that moves water out of the soil and
allows it to compress. A gradient also exists across soil zones through which
seepage occurs because of the viscous resistance to flow. These factors are
discussed in detail in later chapters that deal with settlement of foundations and
seepage rates through and under dams and levees, and into open excavations.

11.1.3 Negative Pore Water Pressure

Soil suction is what holds a child’s sand castle together, until waves or tides
produce a feeling of sad inevitability as negative pore pressure is relieved and the

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