Page 530 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

10.36 THE WORK

If this rear drainage is not practical, the slope may be made to the side or to channels or drains in
the floor.
If the pit is sunken, drainage can sometimes be arranged into a deeper portion which will take
it off the working floor and allow it to soak away gradually. This will often be a pond dug under
the water table to supply the plant with water. See Fig. 10.25(C).
If pumping is necessary, it should be done from a sump that will hold a large volume of water,
and in which the pump can be protected against being covered if heavy rain falls while the pit is
shut down. Such a sump may serve as a storage reservoir for plant supply.
Runoff. A pit may be troubled by water running off surrounding areas, either during rains or in
the form of permanent streams, which should be diverted around the working areas or channeled
through them in such a way that it will cause minimum interference.
The best practice is to dig diversion ditches to lead the flow in other directions. However, if
the pit is expanding, these ditches will require relocating and may cost more than they are worth.
This is particularly likely if the ground is steep or rocky so that ditching is difficult.
Also, the water may be needed in the pit for plant or dredge supply.
If the water flows only occasionally, it can be led through the pit in wide shallow channels
which can be crossed by machinery and trucks at any point. If it flows often or continuously, it
should be in a ditch and taken through haul roads in pipes or on rock-paved fords.

GROUNDWATER

Layout, machinery, and methods in a pit may be affected by the water table, or level of groundwater.
This unseen water surface may be practically level, evenly sloped, or irregular. The water gener-
ally appears to be stagnant, but it is almost always in slow motion and will slope down from the source
to the outlet. The angle of this slope is the hydraulic gradient, which is determined by the resistance of
the material to the passage of water, the pressure and volume of the supply, and the relative heights
of inlet and outlet.
Porous materials, such as gravel and sand, have low gradients, and tight ones, such as silt or clay,
steep ones.
The water table tends to follow the slope of the land, but at reduced grades so that water will
be farther from the surface in hilltops than in valleys.
Above the true water table is the so-called capillary table, which is kept wet by water rising in
the spaces between soil particles. The finer spaces cause greater rise in fine-grained soils than in
porous ones. Capillary water gives comparatively little trouble in clean sand and gravel, but causes
serious softening of other soils.
Capillary water may come up higher when ground is compressed, as under a haul road.
Underground water ranges in quality from fine spring water to solutions of salt or chemicals
unfit for any use.
A shallow pit may be kept well above the water table, which is then of little interest except as
a source of water for processing. A pit carried sideward into a hill may cut into the table and have
drainage problems of varying severity. A downward cut will get into water eventually, except in
arid climates, or very tight ground.
Surface water falling into a depressed pit as rain, or flowing into it from adjoining areas, must
also be taken into account in coping with the groundwater.
Permanent plants and all-year haul roads should be above the highest water levels.
When it is necessary to use materials lying at or under the water table, they may be obtained by
wet digging, digging in dry seasons only, draining, pumping, and combinations of these methods.
Digging underwater. Any machine which can dig from the top of a bank can dig underwater to
some extent. However, there are a number of special difficulties, including inability to see the
work, weaker penetration because of decreased bucket weight and interference of water currents,
loss of material carried out of the bucket by water, and sloppy condition of the spoil. Wet banks
are also more liable to cave under a shovel.