1. The Mathematics of Earthwork Volumes
Excavation calculations begin with the geometric bank volume (the actual empty pocket in the earth) and adjust for material expansion factors:
The Fundamental Earthwork Volume Equations:
Loose Spoil Volume (LCY) = Bank Volume × (1 + Swell Factor %)
Example: A foundation excavation of 40' long, 20' wide, and 5' deep in standard clay (15% swell):
Bank Volume = (40 × 20 × 5) ÷ 27 = 148.1 BCY · Loose Volume = 148.1 × 1.15 = 170.4 Loose Yards (LCY)
2. Standard Soil Swell & Compaction Factors
Undisturbed soil contains tightly packed minerals. Once disturbed, voids are filled with air pockets. Material volumes expand significantly:
| Soil / Excavation Material | Swell Factor (%) | Compaction Shrink (%) | Common Density Profile |
|---|---|---|---|
| Clean Sand / Gravel | 10% – 12% | 10% – 12% | Packs extremely well. Perfect for under-slab prep. |
| Standard Earth / Topsoil | 15% – 20% | 15% – 20% | Standard backyard dirt. Moderately expansive. |
| Heavy Clay | 20% – 25% | 20% – 25% | High water retention. Extreme expansion dangers. |
| Hard Rock / Slate | 30% – 40% | 0% | Blasted rock fragments create large hollow air gaps. |
3. Dump Truck Capacities Reference
Hauling bids require translating total Loose Cubic Yards (LCY) into truckload trips:
- Single Axle Dump Truck: Holds around 5 to 6 cubic yards. Excellent for small residential driveways.
- Standard Tandem Dump Truck: The contractor workhorse. Holds exactly 10 to 12 cubic yards.
- Semi / Tri-Axle End Dump: Large commercial hauling. Holds 15 to 18 cubic yards.
4. Frequently Asked Questions (FAQ)
Q: Why did my backfill dirt not fit back into the hole?
A: Once soil is dug up, it expands by up to 25% due to loose air voids. Unless you compact the backfill in 6-inch lifts using a heavy vibratory plate tamper, you will have a massive surplus dirt mound!
Q: What is a bank cubic yard?
A: A bank cubic yard (BCY) is one cubic yard of dirt in its natural, undisturbed state in the ground before mechanical digging occurs.