Page 194 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

4.34 THE WORK

FIGURE 4.28 Sinking a caisson.

personnel and material. Much of the digging is done by hand, and in deep work at high pressures
workers may be limited to less than an hour of work at a stretch, with long periods spent in enter-
ing and leaving the high-pressure work chamber. Depths up to 100 or 110 feet can be reached.

ROCK

Bedrock. If bedrock is encountered that is too hard for the shovel to tear apart, it must be blasted.
Generally it is best to complete the earth excavation first, to reveal the full extent and as much of
the grain and quality of the rock as possible, before going to work on it.
Sometimes, however, drilling and blasting are started as soon as the rock is found, and the shov-
el doing the earth excavation can be utilized for handling logs or blasting mats. This may save
shovel time, as under city conditions it is not often practical to blast rock fast enough to keep a
shovel busy, and a shovel whose only duties are handling mats and removing blasted rock is likely
to be idle most of the time. On the other hand, earth-hauling trucks will be stopped while the shovel
places mats and during blasting.
Backhoes and small dozers are good machines for cleaning the bulk of earth off ledges, but
there is almost always need for hand work also.
Procedures for the rock blasting and removal are outlined in Chap. 9. However, it should be empha-
sized that blasting near streets and buildings is a much more dangerous and specialized job than the
same work in a quarry or a country highway cut. Elaborate precautions must be taken to prevent
material from flying, and large blasts, or small blasts following each other quickly at regular intervals,
must be avoided because of danger of concussion and vibration damage to nearby buildings. Jobs must
be inspected in advance by the insurance company in order to set a rate in line with the risks.
Boulders. Boulders in the soil slow excavation by creating digging resistance and complications,
and often by difficulties of disposal after they are dug out.
The hydraulic backhoe, such as a Gradall, usually does the best job in proportion to its size.
The comparatively narrow, toothed bucket with wrist action can work around and under large
objects, and has great prying force in its closing action.
It is competent at picking them up, but only if they are small enough to be held in or on the
bucket, or clamped between it and the stick. Larger ones may be held against the wall of the exca-
vation, or a specially dug slope, and pulled to the top.
The front loader is clumsier at digging out boulders, but does much better at removing the big
ones. The wide, deep bucket can pick many up directly, or by crowding against a bank while lifting
and curling the bucket. Extra large ones that would fall out can sometimes be chained, or just
pushed out of the way to wait for equipment to break them.