Page 455 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.55

Soil Excavation. Digging is started with a clamshell, which can dig soft soil unaided, and remove
hard soil and rock after they have been loosened. One or two signalmen direct the operator’s
movements, as he or she cannot see the bottom, and any wrong move with the heavy bucket might
be disastrous to the workers. The clamshell is ordinarily not used below a 25-foot depth.
The next stage may be to replace the digging bucket with a light bucket or container that is
lowered to the floor, and loaded by hand or by equipment suited to the cramped work space. The
container is raised out of the shaft by the hoist line, swung to the side, and dumped by a trip device
or by hand. This may be used to a depth of 100 feet, or a direct transition may be made from the
digging bucket to use of the headframe hoist.
A special small clamshell may operate from a platform close to the bottom, loading the containers
that are lifted past it to the top by the main hoist.
Blasting. In shaft rock blasting, all the holes are tight—that is, there is no open face to permit
sideward throw of the rock—so that close drilling and heavy loading are the rule. It is necessary
that the rock be cut back cleanly to the digging lines and important that overbreak be kept to a
minimum, because of the high expense of removing muck and the frequent requirement of filling
all spaces outside the lining.
Figure 9.44 shows typical drilling patterns for shaft and tunnel work. A set of two or more con-
verging angle holes (wedge holes) is drilled, and other sets of straight or slightly angled holes next
to them, until the rim has been reached. The wedge holes are heavily loaded, so that they crush
and kick out the rock between them, making an opening into which the rock around can move
sideward when the next ring of holes is fired. These in turn make space for the next set. Firing is
best done by short-period delays.
In (B) the floor is lowered on only one side in each shot. This permits drilling to be resumed
on one side while muck is being loaded from the other.
Figure 9.45 shows a burn hole shot, with the center holes parallel instead of angled.
The blast is fired from the top, after all workers and equipment are out of the shaft, except that in
very deep work some equipment might be merely raised far enough to be out of immediate danger.
After the explosion the bottom will be full of fumes, which would take a long time to dissipate
naturally. These may be blown out by lowering the tool air lines with the ends open, or extending

FIGURE 9.44 Wedge drilling in shaft.