Page 241 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.43

SOIL MOISTURE

Water Table. Subsurface water exists in three states or zones. The lowest of the series is hydro-
static or free groundwater. Its upper surface is known as the water table, or groundwater level. It
follows the contour of the land in a general way, but tends to be farther under the surface in hills
and pervious soils than in hollows and fine-grained soils. If it rises to or above the surface, it
makes swamps, ponds, or springs.
The actions of this water are controlled by gravity, causing it to seek lower levels by the resis-
tance of the soil to its movement, and by fresh supplies of water reaching it from the surface.
The water table may be static, or fluctuate only slightly, or it may shift up and down widely in
response to season or rainfall.
Soil which is saturated with groundwater is usually unstable under load, will turn to mud if dis-
turbed, and does not permit the growth of roots of most plants.
If a hole is dug below the water table, it will fill with water, unless clay seals it off.

Capillary Zone. The capillary zone lies above the water table. It may be a few inches deep in
coarse sand, and 8 feet or more in fine-grained soils. It contains a substantial quantity of water
that is held above the gravity surface by capillary attraction and other forces tending to attract and
hold it in the finer soil spaces.
The amount of contained water diminishes from the bottom to the top of this zone.
Capillary movement in coarse soils is rapid, in fine ones quite slow. Raising or lowering the
water table may raise or lower the capillary zone.
Medium and fine soils in the zone usually contain too much water for stability, and may be
subject to frost heaving. In climates where rainfall exceeds evaporation, this zone offers the best
conditions for root growth. In arid regions, the water may deposit alkali in the soil and render it
unfit for cultivation.

Upper Zone. The upper or hygroscopic zone contains water which is in very thin films on the
particles, or is in chemical or physical combination with them. Some of this water is hygroscopic—
absorbed from the atmosphere—and is greatest in amount when humidity is high.
These small quantities of water often give the soil maximum stability, by acting as a cement
or binder. Much of the water is too firmly attached to be removed by plant roots, or any method
but oven baking.
This zone may also contain varying quantities of rainwater, moving downward by gravity or
capillarity, or adhering to soil particles. This is available to plants and may be found in sufficient
quantity to make the ground unstable.

SUBSURFACE DRAINAGE

Purpose. Subsurface drainage lowers the water table. Deep drains, or those in porous soil, will
lower the capillary surface also.
Soil must be drained when its water content makes it incapable of supporting roads or other
structures on it, or causes frost heaving.
Playgrounds, golf courses, and other recreational areas may require draining to dry up spots
that remain wet and soft long after rains.
Farmland drainage may serve to eliminate wet spots that cannot be worked as early as the sur-
rounding land; to speed up the drying and the warming of soil in the spring; to encourage plants
to form deep root systems, with resulting increase in vigor and drought resistance; and to leach
out harmful substances which may accumulate in the soil.

Methods. Groundwater level may be artificially lowered by open channels or ditches, or by
buried pipe or porous material. Such pipe is generally referred to as tile, even though it might be
made of other materials.