Civil Engineering Reference
In-Depth Information
Ground surface
Water table
Fig. 2.1
Tendency of the water table to follow the earth's surface.
Fig. 2.2
Diagram illustrating types of subsurface water.
Terzaghi and Peck (
1948
) give an approximate relationship between the maximum height and the grain
size for a granular soil:
C
eD
h
c
=
mm
10
where C is a constant depending upon the shape of the grains and the surface impurities (varying from
10.0 to 50.0 mm
2
) and D
10
is the effective size expressed in millimetres.
Intermediate belt
As rainwater percolates downward to the water table, a certain amount is held in the soil by the action
of surface tension, capillarity, abdsorption and chemical action. The water retained in this manner is termed
held water and is deep enough not to be affected by plants.
Soil belt
This zone is constantly affected by precipitation, evaporation and plant transpiration. Moist soil in contact
with the atmosphere either evaporates water or condenses water into itself until its vapour pressure is
equal to atmospheric pressure. Soil water in atmospheric equilibrium is called hygroscopic water and its
water content (which depends upon relative humidity) is known as the hygroscopic water content.
The various zones are illustrated in Fig.
2.2.
