Page 189 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

BASEMENTS 4.29

An overloaded storm drain may push water into an otherwise dry excavation, unless a check
or shutoff valve is provided.

Well Points. A satisfactory but expensive way of predraining the area is to use well points,
which are discussed in the next chapter. Points may be driven outside of the digging line on the
north and west, and probably, by special permission, in the sidewalk. Seepage from the east might
be blocked by the building. If not, arrangements should be made to put well points in its basement.
Open Pumping. Digging may be done without predraining and water pumped out of the hole as
it appears. If the water is very dirty, and quantities are small or moderate, a diaphragm pump
should be used. If the inflow exceeds the capacity of a diaphragm, about 1,500 to 3,000 gallons
per hour, several may be used. More often, in holes of this size, centrifugal pumps are employed.
Best results will be obtained by locating centrifugal pumps as close to the water level as possible,
as their discharge is more efficient than their suction. Holes should be dug so that the inlet will be
a foot or more below the water surface. Sucking air in shallow water may be reduced by floating
a piece of board over the inlet, where it will block the formation of whirlpools which would conduct
air down to the inlet center, or by arranging the hose so that it rises vertically out of the water.
Pumping may be done on a 24-hour per day basis, or only during or just before digging opera-
tions. If pumps are to be shut down overnight and on holidays in very wet holes, it may be wise to
take them up each time, or to put them on floats for protection against unexpected rises in water level.
Other equipment should be moved up to a safe level when work is shut down at the end of the day.

Caving. Caving of banks and undermining of adjacent structures must be guarded against, par-
ticularly in connection with pumping. Caving banks involve hazards to people and equipment,
and to adjoining structures, and increase the amount of excavation and backfill necessary.
Some materials, such as dry sand, will not stand in vertical walls, and digging must be figured
to include natural slopes from the foundation line outward to the surface, or provision made to drive
sheeting, or erect other barriers, to hold it from sliding. Sands or sandy soils containing the right
amount of moisture will stand vertically, but they cannot be trusted, as drying will result in sur-
face disintegration and sliding, and heavy rainfall may increase their weight and undermine them
by washing grains out at the bottom so that massive caving will follow.
Silts, clays, and loams usually stand well, if not too wet, but if resting on a saturated layer
draining into the excavation, may be undermined so as to fall. Vibration of machinery or street
traffic may cause clay to creep or flow.
Gravel may stand or may slide, depending on the shape and grading of the coarse particles,
presence of cementing material, and the amount of fines. Angular gravel of several sizes, with just
enough fines to stick it together, will stand wet or dry unless subject to excessive water flow, or
wave action. Very clean gravel, particularly if it includes a large proportion of cobbles and rounded
pebbles, may slide in much the manner of dry sand.
Causes of Caving. Danger of caving continues for days or sometimes weeks after the cut is
made. In its natural state the soil is in both static and dynamic balance—static, because of inertia
and the manner in which its particles are fitted and stuck together, and dynamic, because the
weight overlying soil or structures exerts a sideward as well as downward thrust, which is met by
equal counterthrusts from surrounding soil on the sides and below.
When a cut is made, the soil pressure toward it is balanced only by the soil inertia. This may
hold it permanently in place, or the pressure may deform the soil and cause breaking apart and
rearrangement of its particles, gradually weakening it until it falls. The effect may be likened to
the collapse of a building under the weight of snow on its roof, which may occur hours or days
after the storm and even after part of the snow is gone.
Groundwater is very effective in both holding and bringing down banks. While in very thin
films it serves as a glue or binder. In contact with clay minerals it forms a lubricant, making it easier
for particles to change position in response to pressure to such an extent that certain plastic clays
will flow slowly. In larger quantities, water will seep or flow through the soil, carrying fine particles