Before any foundation can be poured, the site must be graded and excavated. Earthwork estimating is the process of calculating the volume of soil that needs to be cut (removed) or filled (added) to prepare a site for construction.
Because soil changes volume when it is disturbed or compacted, earthwork volume calculations require specialized estimating formulas.
Three States of Soil Volume
Soil is measured in three distinct states. Understanding these states is critical for accurate hauling and compaction estimates:
1. Bank Cubic Yards (BCY): The volume of soil in its natural, undisturbed state in the ground. Excavation quantities are measured in BCY from civil grading plans.
2. Loose Cubic Yards (LCY): The volume of soil after it has been excavated. Soil swells when dug up because air enters the voids. LCY is used to estimate the number of dump trucks needed to haul soil off-site.
3. Compacted Cubic Yards (CCY): The volume of soil after it has been placed and compacted on-site. Soil shrinks under compaction equipment, reducing its volume below its natural state. CCY is used to estimate structural fill requirements.
Soil Swell and Shrinkage Formulas
Estimators apply swell and shrink factors to convert between soil states:
- Swell Factor: Typically ranges from 15% to 30% for standard soils. Clay swells more than sand.
`Loose Volume = Bank Volume x (1 + Swell %)`
- Shrinkage Factor: Typically ranges from 10% to 20%.
`Compacted Volume = Bank Volume x (1 - Shrinkage %)`
For example, if you excavate 1,000 BCY of clay with a 20% swell factor, you will need to haul:
`1,000 BCY x 1.20 = 1,200 LCY` of loose soil.
Calculating Cut and Fill Volumes
Estimators use digital takeoff software to compare the existing site elevations against the proposed finish grade. The software calculates:
- Cut Volume: Areas where the existing grade is higher than the design grade, requiring soil removal.
- Fill Volume: Areas where the existing grade is lower, requiring soil placement.
- Net Volume: The difference between total cut and total fill. A site is "balanced" if the cut volume equals the fill volume, eliminating the need to haul soil in or out.
Final Thoughts
Earthwork takeoff accuracy depends on understanding soil reports and grading sheets. Failing to account for swell and shrinkage factors will result in incorrect hauling quotes, leading to budget overruns for site logistics.