Glossary: Swell Factor
The percentage increase in volume that soil, gravel, or stone undergoes when excavated from its natural compact state, caused by the introduction of air voids.
Earthwork Material Expansion and Density Shifts
When soil is in its natural ground state (referred to as Bank Cubic Yards or BCY), it is compacted by historical pressure. Excavating it breaks the structure, introducing voids and expanding the volume (referred to as Loose Cubic Yards or LCY). Swell factor calculations prevent trucking transport overruns:
- Common Swell Factors: Standard sand swells by 10% to 15%, common loam/clay swells by 20% to 30%, and hard blasted rock can swell by 50% or more.
- Shrinkage Factor: Conversely, when soil is placed and mechanically compacted, its volume decreases relative to bank volume, which is critical for structural backfilling.
- Equation: Loose Volume = Bank Volume × (1 + Swell Factor %).