Page 259 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.61

position. It can usually be prevented by arranging the end of the pipe to hang vertically or attaching
a shield over the inlet, or by throwing a square or round piece of flat wood in the water, which will
tend to center in the whirlpool and block the air passage.
A pump will work best if a foot or two of water is kept over the inlet. In most excavations, the
bottoms should be kept as dry as possible. It therefore is advisable to dig a sump pit for the pump
hose, and to cut through any ridges that prevent water from flowing into it from the pit.
Dirty Water. If the water flowing into the excavation is dirty, it indicates that soil is being brought
in from outside the excavation. Continued pumping may cause caving of banks due to undermining;
or even may cause sinking of adjoining buildings or roads. It is wise to keep such pumping to the min-
imum required for the work and to finish the job as rapidly as possible, even at extra expense. It may
be necessary to dry the area by well points, or to block the water off by grout, chemicals, or freezing.
Contractors’ liability and property damage insurance ordinarily does not cover damage to
structures by undermining, even in the “comprehensive” policies. A special endorsement is nec-
essary, and inspection of the job is usually required.
Well Points. A well-point pump is a centrifugal pump with rather close-fitting parts, and often
with an auxiliary air-vacuum pump, and which can work efficiently in spite of a fairly high pro-
portion of air in the intake lines.
A well point is a section of finely perforated pipe that is sunk into the ground by jetting, driving,
or drilling. It is attached to ordinary iron pipe, which rises to the surface and is connected by other
lines to a pump that usually takes care of a number of points.
When the pump is running, the groundwater in contact with the well point is drawn through the
holes of slits into the pipe and pumped away. The holes are so small that only very fine particles
of earth will pass through them. The continued suction gradually removes all such particles from
the area immediately around the pipe, leaving the coarser ones. This makes a porous screen with
an outside area several times larger than that of the point, and improves its gathering efficiency.
Each well point will remove groundwater from a cone of depression around it, the slope of
which depends largely on the porosity of the soil.
If well points are placed in a line so that their cones overlap, a continuous band of soil can be
dewatered, as in Fig. 5.52.
A ditch could be dug in this band without encountering groundwater, regardless of otherwise
saturated conditions throughout the area.
The well points may also be set in a square pattern to dry up the ground for a basement or similar
excavation. It sometimes is possible to dewater such an area by using points as a curtain drain
where the source and depth of the water are known.

FIGURE 5.52 Well points.