EXCAVATION CALCULATION BASIC INFORMATION AND TUTORIALS


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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