Excavation is measured by the cubic
yard for the quantity takeoff (27 cf # 1 cy). Before excavation, when
the soil is in an undisturbed condition, it weighs about 100 pounds
per cf; rock weighs about 150 pounds per cf.
The site plan is the key drawing for
determining earthwork requirements and is typically scaled in feet
and decimals of a foot. There is usually no reason to change to units
of feet and inches; however, at times they must be changed to
decimals. Remember that when estimating quantities, the computations
need not be worked out to an exact answer.
Swell and Compaction.
Material in its natural state is
referred to as bank materials and is measured in bank cubic yards
(bcy). When bank materials are excavated, the earth and rocks are
disturbed and begin to swell.
This expansion causes the soil to
assume a larger volume; this expansion represents the amount of swell
and is generally expressed as a percentage gained above the original
volume.
Uncompacted excavated materials are
referred to as loose materials and are measured in loose cubic yards
(lcy).When loose materials are placed and compacted (as fill) on a
project, it will be compressed into a smaller volume than when it was
loose, and with the exception of solid rock it will occupy less
volume than in its bank condition.
This reduction in volume is referred to
as shrinkage. Shrinkage is expressed as a percentage of the
undisturbed original or bank volume.
Materials that have been placed and
compacted are referred to as compacted materials and are measured in
compacted cubic yards (ccy). Bank, loose, and compacted cubic yards
are used to designate which volume we are talking about.
Figure 9.1 is a table of common swell
and shrinkage factors for various types of soils. When possible tests
should be performed to determine the actual swell and shrinkage for
the material.
FIGURE 9.1. Swell and Shrinkage
Factors. (Solid rock when compacted is less dense than its bank
condition.)
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