Page 261 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.63

Pressures required range from a few pounds over atmospheric for penetrating loose fine
deposits to several hundred for tough clays.
A single pipe with a nozzle or reduction in size at the tip may be used in probing for rock or
other obstructions. The tip reduction increases the velocity of the water and makes plugging less
likely if it is forced into soil that the water will not cut.
Single pipe holes are irregular in shape, as the exhaust water and spoil rise around the pipe and
will erode channels along the path of least resistance.
A better system is to use several water jets around the rim of a pipe so that washings can rise
through the pipe to the surface. Water may be supplied through separate pipes, or by welding one
pipe inside another, leaving a space between them for passage of water from an upper inlet con-
nection to the bottom jets.
There should be at least three jets, preferably four or more. They must be evenly spaced around
the circumference to prevent the pipe from drifting sideward toward the most effective erosion.
The pipe should be handled by a crane or some other type of hoist.
The lower end is sometimes fitted with teeth, and is lifted and dropped to loosen hard materials.
The nozzles must be well protected against contact with hard dirt, if this method is used.

BASEMENT DRAINAGE

Excavating contractors are often consulted about the feasibility of having a basement under a
house. The problem may be one of the cost of dealing with rock on the site, or a fear of water con-
ditions which would make the basement wet and unusable.
If proper procedures are followed, a basement can be kept dry in any location where water does
not spill in the windows or over the top of its wall. The cost ranges from the sometimes nominal
expense of installing subdrains, up to more than $1 per square foot for complete waterproofing of
floor and walls.
Soils and Locations. The tight soils such as clay or the various varieties of hardpan, tend to
become saturated in wet seasons, even near hilltops. The quantity of water they may carry, which
is the basis for deciding on drain size, may be very difficult to determine in a dry season. In gen-
eral, if the soil contains long streaks or lenses of sand or very fine pebbles, it may be assumed that
there is considerable flow through it. If there are spots near the building site which ooze water in
the spring, or in which water-loving plants grow, a serious drainage problem is indicated.
Difficulties are sometimes avoided by shifting the building site to a spot with better drainage,
or doing only shallow excavation and obtaining depth by filling around the walls. Drains should
still be used, as groundwater may rise into the fill.

Subdrainage. Drainage around the footings is a precaution that should always be taken if there
is any lower point to which water can be led. A porous soil such as sand or gravel can seldom hold
enough water to wet a basement, but it may be part of a waterlogged lowland or a gradual slope
up from one, or have water held in it by layers or lenses of clay.
The standard basement subdrainage consists of a line of land tile laid completely around the
outside of the footing, and preferably 1 foot to 18 inches below basement floor level. It should be
laid in a fine-crushed stone, protected with tar paper or hay, and backfilled promptly. Such tile has
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a downward pitch of ⁄ 2 to 1 percent from a point opposite the outlet.
The outlet may be land tile, but because of the danger of entrance of plant roots, glazed sewer
tile with cemented joints is better. It should slope down away from the building at 1 to 5 percent
grade to a disposal point. This may be a stormwater drain under the street, a stream, or lower ground.
A stormwater drain complication is that water entering the system at higher levels may back
up through the tile and saturate the ground around its walls temporarily.
When there is no stormwater drain, or connection to it is considered unwise, a discharge point
on the same property should be sought. It is often easy to get permission to lay pipe through a
neighbor’s yard, but impossible to get a formal easement to keep it there.