Page 201 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
196 Geotechnical Engineering

to enter the boring and reach a stable level. Normally measurements are made
after 1 hour, but they also may be repeated after 24 hours. The time required
depends on the ease with which water flows through the soil, being less for highly
permeable soils than for less permeable soils such as clays.

A groundwater table is not an underground lake unless it happens to be in a cavern,
and it is not flat like a table. Instead, the water table is higher under hilltops and
declines to the level of nearby lakes or stream valleys, where groundwater
sometimes will be seen emerging as springs. Swimmers in lakes often will
encounter zones of colder water caused by groundwater feeding into the lakes as
springs.

The most apt description of a groundwater table is as a ‘‘subdued replica of the
surface of the ground,’’ as shown in Fig. 10.2. The reason for this is the
cumulative nature of infiltration on a hillside, as infiltrating water that reaches
the groundwater table must seep laterally to find an escape path.

10.2.3 Perched Groundwater Tables
If downward-infiltrating water encounters a relatively impermeable soil layer, the
rate of infiltration may be slowed sufficiently to cause the water level to back up
into the overlying soil, causing what is called a ‘‘perched’’ groundwater table.
Perched water tables sometimes contribute to landslides, and can be a problem in
excavations.

A perched water table is illustrated in Fig. 10.2. Such conditions often can be
predicted from a knowledge of the soil strata and buried clayey paleosols, or
ancient weathered soils that have a high content of clay.

A perched water table also may exist in loose fill soil that is placed on top of less
permeable soil. Without the fill, the water would be free to run off. A sewage

Figure 10.2
Nomenclature
descriptive of
various
groundwater
conditions.

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