Data Source and Methodology
This calculator uses standard geotechnical engineering formulas for volume and material estimation. The default swell and shrinkage factors are common industry estimates. For mission-critical projects, values should be confirmed by a site-specific geotechnical report.
A primary reference for soil property data, including swell factors, is the U.S. Department of the Interior, Bureau of Reclamation's "Earth Manual, Part 1" (Publication EM-1110-2-1906). All calculations are based rigorously on the formulas derived from these established principles.
The Formulas Explained
Understanding the math ensures you can trust the results and adapt them to your project.
1. Bank Volume ($V_{bank}$)
This is the in-ground, undisturbed volume of the excavation area.
2. Spoil Volume ($V_{spoil}$)
This is the "loose" volume after excavation, accounting for soil expansion (swell). This is the volume you must haul away.
3. Compacted Fill Volume ($V_{compacted}$)
This calculates the "bank" volume of material needed to fill a hole, assuming it will be compacted. It uses the shrinkage factor.
4. Truck Loads
This is a simple division of the total spoil volume by your truck's capacity, rounded up to the nearest whole number.
Glossary of Variables
- Bank Volume
- The volume of soil as it sits undisturbed in the ground (in its "bank" state). All measurements are based on this initial volume.
- Swell Factor (or Bulking)
- The percentage increase in volume that occurs when soil is excavated. Digging breaks up the soil and introduces air voids, making it "fluffier" and larger.
- Spoil Volume
- The final, loose volume of the excavated material (Bank Volume + Swell). This is the metric used to plan and pay for material disposal.
- Shrinkage Factor
- The percentage decrease in volume that occurs when soil is re-compacted as fill, relative to its original bank state. Compaction removes air voids, often making the soil denser than it was originally.
How It Works: A Step-by-Step Example
Let's plan a basement excavation with the following details:
- Length: 40 ft
- Width: 30 ft
- Depth: 8 ft
- Soil Type: Common earth (25% swell factor)
- Truck Capacity: 12 cubic yards
- Calculate Bank Volume (in feet):
$V_{bank} = 40 \text{ ft} \times 30 \text{ ft} \times 8 \text{ ft} = 9,600 \text{ cubic feet}$ - Convert Bank Volume to Cubic Yards:
Since 1 cubic yard = 27 cubic feet: $V_{bank} = 9,600 \text{ ft}^3 / 27 = 355.56 \text{ cu yd}$ - Calculate Spoil Volume (with 25% swell):
$V_{spoil} = 355.56 \text{ cu yd} \times (1 + \frac{25}{100}) = 355.56 \times 1.25 = 444.45 \text{ cu yd}$ - Calculate Truck Loads:
$\text{Loads} = \lceil \frac{444.45 \text{ cu yd}}{12 \text{ cu yd}} \rceil = \lceil 37.04 \rceil = 38 \text{ loads}$
Conclusion: Although the hole is only 355.56 cu yd, you must plan and pay for the removal of 444.45 cu yd of loose soil, which will require 38 truck loads.
Frequently Asked Questions (FAQ)
What are typical swell factors for different soil types?
Swell factors vary significantly. Always use a geotechnical report for accuracy. However, common estimates are:
| Soil Type | Swell Factor (Approx.) |
|---|---|
| Sand or Gravel | 10% - 15% |
| Common Earth (Loam) | 20% - 30% |
| Clay | 30% - 45% |
| Solid Rock (Blasted) | 50% - 75% |
Why is swell factor so important for budgeting?
Trucking and disposal costs are typically billed by the load or by volume. If you budget based on the 355.56 cu yd "bank" volume from our example, but you actually have to haul 444.45 cu yd, your hauling costs will be 25% over budget. Factoring in swell is critical for accurate cost estimation.
When do I use the "Shrinkage Factor"?
Use shrinkage when you are planning to use excavated material (or new material) as structural backfill. For example, if you need to backfill a trench that has a 100 cu yd "bank" volume, and the soil has a 10% shrinkage factor, you will need $100 \times (1 - 0.10) = 90 \text{ cu yd}$ of *original bank material* to fill it. Conversely, if you are buying 100 cu yd of *loose* fill, it will compact to a smaller volume.
What is the "angle of repose"?
The angle of repose is the steepest angle at which loose material (like a spoil pile) will remain stable without slumping. This is a critical safety factor in excavation, as it determines how far back from the edge a spoil pile must be placed to prevent it from collapsing back into the hole. This calculator does not compute the angle of repose, as it is highly material-specific.
Can I use this for circular excavations (e.g., a caisson)?
This calculator is designed for rectangular or square areas. For a circular excavation, you would first calculate the bank volume using the formula for a cylinder ($V = \pi \times r^2 \times h$) and then apply the swell/shrinkage factors manually using the formulas provided above.
Tool developed by Ugo Candido. Geotechnical content verified by the CalcDomain Engineering Editorial Board.
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