Page 182 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

4.22 THE WORK

and clamshell attachments. This machine is somewhat undersize for the job but may be used unless
a premium is placed on speed.
The front shovel is used because of digging and loading efficiency. The ramp is located at
the street as there is no access to the other sides except through the driveway, which is too nar-
row for heavy trucks and will be undermined by the digging. The ramp is next to the driveway,
because if the other corner were used, the store building might be damaged by collision or
vibration.
Moving In. Arrangements are made to prohibit parking in front of the work area before machin-
ery is brought in.
The shovel is unloaded from the trailer onto planks laid on the street to protect the pavement,
or directly onto the sidewalk which is not protected because the trucking will destroy it anyhow.
Digging is commenced in the sidewalk, or at its rear edge, and sloped down at about a 1-on-5 or
20 percent grade, in a cut 30 to 40 feet wide. Trucks are first loaded when standing in the street,
parallel to the curb. As the digging progresses, they are backed across the sidewalk and down the
ramp, as in Fig. 4.20.
It is necessary to have one person, preferably two, assigned to prevention of tangles between
traffic and trucks. These should be police officers or contractor’s employees authorized by the
police to do this work.
Trucks should be backed into both sides of the ramp, as in Fig. 4.21(B), and faced directly
away from the shovel while being loaded. At least one truck should always be in loading position.

Trimming. As the cut progresses, the foreman checks the left edge for accuracy. Because of the
angle at which the bucket works, it cannot make flat cuts on the wall, and one or more laborers
should trim the face, working either from the top or from the ramp surface beside the boom.
Checking may be done by stretching a string along the digging line at the top, and lowering another
string to which a plumb bob is attached.
Bank Height. The front shovel or backhoe would be able to take the full 18-foot depth in one
cut, but this would not be good procedure. The shovel cannot cut a straight face higher than the
level at which the bucket teeth start to turn away from the bank, and the face above might over-
hang or break back beyond the digging line. Hand trimming is more difficult on a high face, and
caving is more likely and more serious than from a lower one.
The bottom is more likely to be muddy or to contain rock outcrops than higher levels, and it is
economical to remove as much soil as possible under good conditions, before tackling the diffi-
culties. In addition, trucks hauling from an upper level have an easier climb to the street.
Under average conditions, this job would be dug in two levels or benches of about 9 feet each.

Support for Bank. Most deep excavations for buildings in a built-up area need to have support
to guard against cave-ins. Certainly this is true when the excavation is alongside a building, as on
the east side, even though the foundation for that building goes below the new excavation. There
could be a cave-in above the excavation depth, which could affect the basement of the existing
building. The building to the west, with foundations above the depth of the excavation, needs to
be protected against any settlement that could lead to a lawsuit.
The method of support generally used is either to drive continuous-sheet piling or soldier piles
with timber lagging between them. This support system would need to be along the length of each
adjacent building and the street. It would not be needed along the north side of this excavation
where a cave-in would not cause a serious problem.
There are other methods for supporting an excavated wall of earth. The use of tiebacks, such
as with the sheet piling, is a popular method. A variation on tiebacks has been called soil nailing,
which starts by using about 2 inches of shotcrete, i.e., a mortarlike mix sprayed at high velocity
on the earth surface, and wire mesh on the first 3 to 6 feet of excavated depth. Then 4-inch holes
are drilled on an incline down into the remaining earth in a set pattern and depth. A lightweight
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reinforcing bar, perhaps of ⁄ 2 -inch diameter, is placed in the center of each hole, which is grouted