Excavation 85FIGURE 9.36.Sketch of Spread Footing.
16 hoursTotal hours14 equipment hours2 hours for mobilizationEquipment hours357 bcy>25 bcy per hour14 hoursEXAMPLE 9-16 SPREAD FOOTING EXCAVATION
There is one spread footing shown in Figure 9.32. The details of this
spread footing can be found on sheet S8.1 detail 16 in Appendix A.
From those details, given a slope of 1.5:1, Figure 9.36 can be sketched.
Since the spread footing is square, the general excavation can
be found by squaring the average cut width and by multiplying that
by the depth.
General excavation (bcy)347 bcf>27 cf per cy13 bcyGeneral excavation (bcf )8.33¿8.33¿5.0¿347 bcfGeneral excavation (bcf ) 8 ¿ 4 – 8 ¿ 4 – 5 ¿ 0 –Total cost$260$316$576 Labor cost16 hours$19.75 per hour$316Equipment cost16 hours$16.25 per hour$2609–10 Special Excavation
Usually the special excavations are the portions of the work
that require hand excavation, but any excavation that
requires special equipment to be used for a particular por-
tion other than general (mass) excavation may be included.
Portions of work most often included under this head-
ing are footing holes, small trenches, and the trench-out
areas below the general excavation for wall and column foot-
ings, if required. On a large project, a backhoe may be
brought in to perform this work, but a certain amount of
hand labor is required on almost every project.
The various types of excavation must be kept separately
on the estimate; and if there is more than one type of special
or general excavation involved, each should be considered
separately and then grouped together under the heading
“special excavation” or “general excavation.”
In calculating the special excavation, the estimator must
calculate the cubic yards, select the method of excavation,
and determine the cost.9–11 Backfilling
Once the foundation of the building has been constructed,
one of the next steps in construction is the backfilling
required around the building.Backfillingis the putting back
of the excess soil that was removed from around the building
during the general excavation. After the topsoil and general
and special excavations have been estimated, it is customary
to calculate the amount of backfill.
The material may be transported by wheelbarrows, scrap-
ers, front-end loaders with scoops or buckets, bulldozers, and
perhaps trucks if the soil must be transported a long distance.
The selection of equipment will depend on the type of soil,
weather conditions, and distance the material must be moved.
If tamping or compaction is required, special equipment will
be needed, and the rate of work per hour will be considerably
lower than if no tamping or compaction is required.
One method for calculating the amount of backfill to be
moved is to determine the total volume of the building
within the area of the excavation. This would be the total
volume of the basement area, figured from the underside of
the fill material, and would include the volume of all foot-
ings, piers, and foundation walls. This volume is deducted
from the volume of excavation that had been previously cal-
culated. The volume of backfill required is the result of this
subtraction. The figures should not include the data for top-
soil, which should be calculated separately.
A second method for calculating backfill is to compute
the actual volume of backfill required. The estimator usually
makes a sketch of the actual backfill dimensions and finds
the required amount of backfill.
The following examples illustrate how to calculate back-
fill quantities.EXAMPLE 9-17 BACKFILLING THE BASEMENT WALLS
Using the sketches in Figures 9.37, 9.38, and 9.39, the following vol-
ume calculations can be performed.
Footing volume (cf) 1 ¿ 1 ¿ 26 ¿ 1 – 2Building volume (cf )32.33¿26.083¿ 8 ¿6,746 cfBuilding volume (cf ) 32 ¿ 4 – 26 ¿ 1 – 8 ¿
Total Footing volume (cf ) 52 cf 65 cf 117 cfFooting volume (cf) 1 ¿ 1 ¿32.33¿ 2 65 cfFooting volume (cf) 1 ¿ 1 ¿ 32 ¿ 4 – 2Footing volume (cf) 1 ¿ 1 ¿26.083¿ 2 52 cf