Page 185 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

BASEMENTS 4.25

SHOVEL TEAMS

Two Power Shovels. This job is big enough to justify the use of two shovels of three-quarter size
or larger. The second shovel might ramp down from the sidewalk along the east side, and cut through
to meet the first one at the center. After this, one ramp might be used as an entrance and the other as
an exit; or one might be cut away. Or the second shovel might be brought after the first one reaches
the upper level, and using the same ramp assist it on that level or ramp down to the bottom.

Traffic. An external factor which may limit the number and size of shovels in such an excava-
tion is traffic congestion on the street. This may create a bottleneck that would leave a line of
empty trucks parked waiting in the street, with the shovels half-idle for lack of trucks to load. In
congested areas, traffic may be one of the principal problems of the digging.

Ramp Removal. When the digging is complete except for removal of the ramp, a backhoe or a
clamshell must be employed. This ramp may contain 300 to 500 yards, a sufficient amount to
make the use of the faster-digging backhoe better than the clam. If the hoe has an effective down-
ward reach of 16 feet, it will leave a bit of the foundation of the ramp for hand labor; but the clam,
in taking the whole ramp out, is apt to require a larger amount of hand labor assistance while
working. Another factor may be that the shovel at this job with a crane (clamshell) boom might
pick up extra work lowering materials into the excavation. The hoe rig is awkward to handle and
to transport when detached, so that it might be more economical to move the shovel to the yard to
change over and bring it back than to pick up the hoe attachment, bring it to the pit, and take away
the front shovel attachment. The clam rig could be loaded on a truck by a chain hoist or a tractor
loader, and moved with little or no blocking.
It might also be good business for the contractor to hire a backhoe or a clamshell rig and move
the shovel onto another job.
Whatever machine is used, it will probably stand on the ramp as it tears it up, as the driveway
is too narrow for swing space, and for safety at such height.

Teaming Front Shovel and Backhoe. If two shovels are to be used for the whole excavation, it
may be that the larger would be a front shovel and the other a backhoe, although under the ideal
conditions considered so far, this is not likely. In such a case, the front shovel ramps down on the
building side, unless danger of vibration damage is unusually severe. The strength of the founda-
tion wall might be checked, and permission obtained to brace it from inside if the weight of trucks
on the ramp seems to threaten it.
The backhoe is assigned to cutting the north and west walls of the pit because of its ability to
make a smooth straight cut without hand trimming. It would preferably start on the north side, cut-
ting from east to west, keeping the line in the manner described earlier, and digging out as much
of the center as can be conveniently reached, as in Fig. 4.22. The edge ditch should be made as
deep as it can reach, but the rest of the digging only 9 feet. The center digging is discontinued in
the last few feet of the north line as the shovel is then backed up against the building beyond the
driveway, and turns to get in digging position on the west line. Whether this corner can be cut
square will depend on the length and tail swing of the shovel, but in general, it cannot. In any case,
it cannot be squared to full depth.
The west line is ditched back to the sidewalk, with some additional material moved from the
center as in (C). This leaves room to install bank supports. The backhoe can then work a wide cut
back from any convenient starting place, taking care that its efforts, combined with those of the
front shovel below, do not cut off its exit.
The trucks carrying the spoil from the backhoe may be loaded sideward to the shovel for safety,
or from the back for convenience. If the body sides are very high, loading will be inconvenient and
spillage excessive. This difficulty may be reduced by loading directly behind the shovel, so that it
will have to walk over the spilled material, which will raise it so that loading will be easier. This
spillage needs some manipulation to make a smooth ramp, particularly if the soil contains boulders.