Page 163 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

BASEMENTS 4.3

cellar headroom, and thickness of the basement floor and gravel or crusted rock under it. If the
full area is to be dug to the bottom of the footings, their depth must be considered also.
On sloping ground, excavation depth will vary at different points.
Factors in determining first-floor level, including a way to probe for rock, are discussed in
Chap. 7.

Rock. Digging in ledge rock or large boulders may increase excavation cost three to five times,
or more. Its presence may result in raising the house, or substituting a slab or crawlway for a base-
ment.
If it is to be removed, dirt removal should be completed and the rock surface cleaned before
drilling.
Most basement jobs are so located that mats must be used to cover blasts, and particular care
taken to safeguard passersby and traffic, and to avoid damage to property.

DIGGING

The three standard machines for digging small basements are the front-end loader, the bulldozer,
and the backhoe excavator.

Bulldozer. A bulldozer can dig a basement very deeply if there is no heavy rock or mud, but it
is at its best if the excavation is shallow. This is because it must dig away a considerable amount
of the bank to ramp itself in and out, and the whole weight of the machine must come up out of
the hole with every pass. It can push much larger 1oads on a level or a moderate upgrade than on
the steep rises from a deep basement.
Digging techniques vary with the operator and the locality. Several methods will be described,
but they are presented only as samples and should be followed only where they give satisfactory
results.
It is good practice to leave a ramp that will allow trucks to back into the floor of the excava-
tion, for convenience in delivering foundation materials to the point of use.
Example. The first case to be considered is that of a basement 30 40 feet, 4 feet deep, dug in
a large, level, treeless lot, from which topsoil has been stripped. The dimensions given are of the
outside of the foundation walls, and an additional 18 inches on each side should be allowed for
wide footings and working space for the masons, so that the dimensions of the hole should be 33
43 feet. Stakes are set 6 inches outside the digging line, as in Fig. 4.3(A) to avoid accidents to
them. Any temporary guide pegs the operator needs are set along the edge, or just outside it.

OPEN-FRONT METHOD

Top Layers. The bulldozer is first worked along the short dimension, inside the stake lines. The
blade may be dropped at the front or south line for a fairly deep bite, and when filled, it is lifted
to ride the load over the undug ground until the north line is crossed. The dirt may be dropped at
the line or pushed a few feet back. The dozer then backs to the south line and takes another bite
in a strip adjoining or overlapping the first. It may work the whole width of the front line, as in
(A), or only a section of it, before digging the area over which the soil has been pushed. The back
edge of the cut is worked north by successive bites until the rear line of the excavation is reached,
approximately as shown in the cross section (B).
After completing the removal of the top layer, the dozer may cut out and pile a second layer in
the same manner, as in (C). This deeper cut will not extend to either digging line, as the slope
down and the slope up are kept inside these lines.
If the soil is so hard that the blade cannot be filled in a short distance, the dozer may be worked
over a short digging area only, for several passes, after which the pile of loosened dirt may be