Page 221 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
220 CHAPTER SIX

6.5.1 Water Movement in Soils
At some elevation below every building site, there is water in the
ground because of rain seeping into the soil and because of the natural
water content of the earth. This groundwater may be close to the sur-
face or far below grade. The top elevation of groundwater is called the
groundwater level or water table. Water table varies with climate,
amount of rainfall, season, and, to some extent, with type of soil. The
water table follows the general contours of the land but is closer to the
surface in valleys and farther from the surface on hills and ridges.
Water moves laterally through the soil by gravity flow to lower eleva-
tions. The direction of groundwater flow is always in the direction of
lower elevations until the water emerges in a spring, stream, or other
open body of water (Figure 6-30).
A soil boring test can identify the soil types which will be encoun-
tered below a building site, as well as the elevation of the water table.
Since the water table can vary with climate and amount of rainfall, it
is important to understand that the water table listed in a geotechnical
report should not be taken as an absolute. If soil tests are performed
during the rainy season, the elevation of the water table may be at its
highest expected level, but if the tests are done during a period of
drought, the water table may be unusually low and not representative
of the normal conditions which would be encountered. If data from a

NATURAL
SLOPE

DRY RAIN

SATURATED
SURFACE SPRINGS
DRAINAGE
STREAM
GROUNDWATER
FLOW
WATER
TABLE

FIGURE 6-30
Groundwater. (from Callendar, John H., Timesaver Standards for Architectural Design Data, McGraw-Hill, New York).

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