Page 294 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.30 THE WORK

SPILLWAYS

Construction. Ponds which are made by excavation only, and do not raise the original water
level, usually overflow through stabilized streams or channels that do not require any artificial
protection against erosion. If an earth or masonry core dam is used, however, an artificial over-
flow channel, called a spillway, must be prepared.
A spillway may have a surface of any material that will resist the destructive action of the water which
might flow across it. A steady flow calls for a structure, usually of stone or concrete, but occasionally
wood, metal, or asphalt. A spillway that carries water rarely, as one which is intended to care for
floods in excess of the capacity of a masonry or pipe spillway, or to provide for occasional overflow of
a normally static pond, may be planted with grass or other well-rooted vegetation.
Spillway size may be calculated on the basis of the area drained, type of land and vegetation,
and rainfall records in the same manner as culverts. However, a greater margin for safety should
be allowed.
It is good practice to keep the spillway and dam separate if possible, as each is a source of
weakness to the other. A recently constructed dam ordinarily lacks the stability necessary to support
heavy masonry, and it is difficult to get a leakproof bond between dirt and stone. Any leaking
through or around a spillway will be much more destructive to an earth dam than to a long estab-
lished subgrade. On the other hand, practical and aesthetic considerations frequently require
placement of the spillway in the dam.
If the dam including a spillway has a masonry core, the two structures can be combined. However,
the core must be widened or buttressed, or the spillway provided with additional foundations as firm as
the core wall. If the spillway is supported by a thin core wall and dirt fill, and the fill settles, the spillway
will be left supported only at the core, and may break, or may twist and break the core. A preferred
method is to extend the core footings far enough to carry piers to support the spillway.
If the overflow is to be carried around the dam, standard practice for masonry structures may
be followed. Two more or less parallel walls carrying the water race, which may be a curve or a
series of steps, is a standard type of construction. The structure is strongest if built of reinforced
concrete well tied together, but stone and mortar make a more attractive appearance. The fill
under the water race should be clean sand or gravel with good bottom drainage if the ground
freezes in winter.

Settlement. If the spillway is to be part of a newly made dam, it may be based on the fill mate-
rial, or may have footings in the native soil underneath the dam. In the first case, any settlement
is liable to tilt or break the spillway and to settle away from it, leaving channels for leakage. In
the second case, the masonry will stand firm while the dam settles under it and away from it. If
the structure includes a core wall long enough to tie into the earth on each side, such settlement
may not be serious.

Grouting. Leakage under a masonry spillway surface, resulting from dirt settling away from it, may
be stopped by drilling holes in the masonry and pouring or pumping a cement and water grout into them.
A grout injector may be an air pressure tank or a pump. The tank is provided with an agitator to
prevent separation. It is partly filled with grout and tightly closed. Compressed air is piped into the
top of the tank, forcing the grout out through a pipe or hose in the bottom. The tank is opened and a
fresh batch of grout poured in as often as necessary. Air should not be allowed to enter the outlet hose.
Special pumps may be purchased, or a fluid grease dispenser or a tractor grease gun used.
Pumping can be continuous, with extra grout added as necessary.
The holes are drilled or punched to a depth where the leaks are suspected. The grouting tube
may be fitted with a rubber collar to fit the holes and held in place by hand, if low pressures are
used. For high pressures, a threaded iron pipe is cemented into the hole some days before and the
grout pipe coupled to it.
The grout forced underground may penetrate and seal the leaks, may be washed away by water,
or may escape to the surface of the ground. If possible, the pond level should be lowered to stop
the water flow during grouting. The whole area should be watched for the appearance of grout,
particularly at the leakage points.