A cubic yard of earth measured in its natural position swells to more than a cubic yard after it is excavated. This occurs because of an increase in voids.
Swell is expressed as a percentage of natural volume, for example, if 10 yd3 in the ground becomes 13 yd3 after excavation, the swell factor is 30%.
The following table lists swell factors for various engineering materials.
| Material | Swell (%) |
|---|---|
| Clay | |
| Dry | 40 |
| Wet | 40 |
| Clay and gravel | |
| Dry | 40 |
| Wet | 40 |
| Coal, anthracite | 35 |
| Coal, bituminous | 35 |
| Earth, loam | |
| Dry | 25 |
| Wet | 25 |
| Gravel | |
| Dry | 12 |
| Wet | 12 |
| Gypsum | 74 |
| Hardpan | 50 |
| Limestone | 67 |
| Rock, well blasted | 65 |
| Sand | |
| Dry | 12 |
| Wet | 12 |
| Sandstone | 54 |
| Shale and soft rock | 65 |
| Slate | 65 |
| Traprock | 65 |
