Page 200 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 195

10.2 GROUNDWATER

10.2.1 Overview

Soils and pervious rocks contain a vast subsurface reservoir of water that has
percolated downward to a zone of saturation. Not only is groundwater a major
source for water supplies, but it helps to maintain lakes and streams during
periods of low rainfall or drought. Water stored in soil pores is essential for
survival of terrestrial plants and animals. The importance of groundwater may
not be fully appreciated until a supply becomes depleted, which currently is a
matter of considerable concern in areas where more water is being extracted for
irrigation than is being replenished by rainfall and snow melts. Climatic change as
a result of deforestation also can have a devastating influence on water supplies,
as in the constantly expanding Sahara. It is humbling to realize that North Africa
once was the breadbasket for Rome.

10.2.2 Location of the Groundwater Table

Water infiltrating into soil is drawn downward by gravity until it reaches a
zone of saturation, where it starts to seep laterally to the nearest outlet that
normally is a stream valley. The zone of saturation extends above the
groundwater table because of capillary forces similar to those that draw water
up into a fine tube. The level of the groundwater table therefore is measured in an
open boring that does not support capillary water. This level, also called the
phreatic surface, represents the elevation at which the pressure in the soil water is
equal to atmospheric pressure. Figure 10.1 is a schematic diagram showing the
distribution of soil water.
The level of the groundwater table is measured and included in boring logs. Since
saturated soil is removed during the boring operation, time is required for water

Figure 10.1
Rain infiltrating into
soil recharges the
groundwater.
In fine-grained
soils a zone of
capillary saturation
occurs above the
groundwater table.

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