Estimating in Building Construction

9–9 General Excavation

Included under general (mass) excavation is the removal of
all types of soil that can be handled in fairly large quantities,
such as excavations required for a basement, mat footing, or
a cut for a highway or parking area. Power equipment such
as power shovels, front-end loaders, bulldozers, hydraulic
excavators, and graders are typically used in this type of
project.
When calculating the amount of excavation to be
done for a project, the estimator must be certain that the
dimensions used are the measurements of the outside face
of the footings and not those of the outside of the build-
ing. The footings usually project beyond the wall. Also, an
extra 6 inches to 1 foot is added to all sides of the footing
to allow the workers to install and remove forms. The esti-
mator must also allow for the sloping of the banks to pre-
vent a cave in. The amount of slope required must be
determined by the estimator who considers the depth of
excavation, type of soil, and possible water conditions.
Some commonly used slopes—referred to as the angle of
repose (Figure 9.28)—are given in Figure 9.29. The mini-
mum angle of repose or slope is set by OSHA standards,
which govern construction safety. If job conditions will
not allow the sloping of soil, the estimator will have to
consider using sheet piling (Figure 9.30) or some type of
bracing to shore up the bank. Any building or column
footing projection is a separate calculation, and the result
is added to the amount of excavation for the main portion
of the building. Allow about 1 foot all around for working
space.
The actual depth of cut is the distance from the top of
grade to the bottom of the fill material used under the con-
crete floor slab. If topsoil has been stripped, the average

Excavation 81

FIGURE 9.28.Angle of Repose.

FIGURE 9.29.Earthwork Slopes.

depth of topsoil is deducted from the depth of cut. If the

fill material such as gravel is not used under the concrete

floor, the depth is then measured to the bottom of the floor

slab. Because the footings usually extend below the fill

material, a certain amount of excavation will be required to

bring the excavation down to the proper elevation before

footings can be placed. This would also be included under

the heading of “general excavation,” but would be kept sep-

arate from topsoil.

Before estimators can select equipment, they will have

to determine what must be done with the excess excavation—

whether it can be placed elsewhere on the site or whether it

must be hauled away. If it must be hauled away, they should

decide how far. The answers to these questions will help

determine the types and amount of equipment required for

the most economical completion of this phase of the work.

To determine the amount of general excavation, it is

necessary to determine the following:

1. The size of building (building dimensions).


2. The distance the footing will project beyond the wall.


3. The amount of working space required between the
   edge of the footing and the beginning of excavation.


4. The elevation of the existing land, by checking the exist-
   ing contour lines on the plot (site) plan.
   5.The type of soil that will be encountered. This is
   determined by first checking the soil borings (on the
   drawings), but must also be checked during the site
   investigation (Section 4-6). Almost every specification
   clearly states that the soil borings are for the contrac-
   tor’s information, but they are not guaranteed.


5. Whether the excavation will be sloped or shored. Slope
   angles (angles of repose) are given in Figure 9.29.


6. The depth of the excavation. This is done by determin-
   ing the bottom elevation of the cut to be made. Then
   take the existing elevation, deduct any topsoil removed,
   and subtract the bottom elevation of the cut to deter-
   mine the depth of the general excavation.
   When sloping sides are used for mass excavations, the
   volume of the earth that is removed is found by developing
   the average cut length in both dimensions and by multiply-
   ing them by the depth of the cut. The average length of the
   cut can be found as shown in Figure 9.31, or the top of cut
   and bottom of cut dimensions can be averaged. Either
   method will result in the same answer.