Fluff Factor (Swell Factor) of Soil: Formula & Worked Example

Dig out 500 m³ of soil, and it will not fit into 500 m³ of trucks. Put that same soil back as a compacted embankment, and it will occupy less than 500 m³ again. Soil changes volume every time it is handled — it swells when excavated and shrinks when compacted. The factors that describe this are the swell (fluff) factor and the shrinkage factor, and getting them wrong means ordering too few trucks, underestimating borrow quantities, or mispricing an earthwork item.

This article defines the three volume states, gives the formulas, lists typical values, and works through a full example.

The Three Volume States

Earthwork quantities are always stated in one of three states. Mixing them up is the most common earthwork error.

• Bank volume (BCY/BCM) — soil in its natural, undisturbed state in the ground. This is what cross-sections and survey quantities give you.
• Loose volume (LCY/LCM) — soil after excavation, sitting in a truck or stockpile. Excavation breaks up the soil structure and introduces air voids, so loose volume is always larger than bank volume.
• Compacted volume (CCY/CCM) — soil placed and compacted in an embankment or fill. Compaction squeezes out the voids, so compacted volume is smaller than bank volume (for soils; rock behaves differently — see below).

The same mass of soil passes through all three states. Only the volume changes.

Swell (Fluff) Factor

The swell factor — also called the fluff factor — describes how much the soil grows when excavated:

Swell factor = Loose volume ÷ Bank volume

It is usually expressed as a percentage or as a multiplier. A soil with 25% swell has a swell factor of 1.25: every 1 m³ in the ground becomes 1.25 m³ in the truck.

So:

Loose volume = Bank volume × swell factor

Shrinkage Factor

The shrinkage factor describes how much the soil contracts when compacted into a fill:

Shrinkage factor = Compacted volume ÷ Bank volume

A soil with a shrinkage factor of 0.90 compacts from 1 m³ in the bank to 0.90 m³ in the embankment. The corresponding shrinkage percentage is:

Shrinkage (%) = (1 − shrinkage factor) × 100

So:

Compacted volume = Bank volume × shrinkage factor

Typical Values

Values vary with soil type, moisture, and compaction effort — always confirm against project specifications or local test data. The widely used reference values (after standard estimating references) are:

Soil type	Swell (%)	Swell factor	Shrinkage factor
Sand and gravel	10 – 18	1.10 – 1.18	0.92
Loam	15 – 25	1.15 – 1.25	0.88
Dense clay	20 – 35	1.20 – 1.35	0.90
Solid rock	40 – 70	1.40 – 1.70	— *

* Rock is the exception: blasted rock placed in an embankment occupies more volume than it did in the bank — a swell of 40–80% is typical, with little subsequent settlement. The shrinkage concept does not apply to rock fill the way it does to soil.

Worked Example: From Cut to Trucks to Fill

 

Problem: A road project requires excavating 500 m³ (bank) of clayey soil. The material will be hauled to an embankment and compacted. Use a swell of 25% and a shrinkage factor of 0.90. Trucks have a struck capacity of 14 m³.

Step 1 — Loose volume (what the trucks carry):

Loose volume = 500 × 1.25 = 625 m³

Step 2 — Number of truck loads:

Truck loads = 625 ÷ 14 = 44.6 → 45 loads

Step 3 — Compacted fill volume (what the embankment gets):

Compacted volume = 500 × 0.90 = 450 m³

So the chain is: 500 m³ in the ground → 625 m³ in trucks → 450 m³ of finished embankment.

Where These Factors Matter

• Truck and equipment planning — loose volume decides the fleet size. Underestimate the swell and trucks queue up; the excavator waits; the day rate burns.
• Borrow pit quantities — compacted fill requirements must be converted back to bank volume to size the borrow area.
• Mass haul and balancing — cut-to-fill balancing only works if both sides are in the same volume state (usually bank).
• Payment — contracts typically pay excavation in bank measure and embankment in compacted measure. The factors connect the two.
• Compaction control — the shrinkage assumption is verified on site by field density tests, which confirm the fill is reaching the specified density.

Frequently Asked Questions

What is the fluff factor of soil?
Another name for the swell factor: the ratio of loose volume to bank volume (V_loose ÷ V_bank). A fluff factor of 1.25 means excavated soil occupies 25% more volume than it did in the ground.

Is the swell factor the same for all soils?
No. Sands and gravels swell 10–18%, clays 20–35%, and solid rock 40–70%. Finer, more cohesive soils generally swell more.

Why is compacted volume less than bank volume?
Because compaction removes air voids more thoroughly than the soil’s natural structure does. Well-compacted fill is denser than the undisturbed ground it came from.

Does rock shrink when compacted?
No — this is the one exception. Broken rock in an embankment takes up more space than solid rock in the cut, typically 40–80% more, and stays that way.

Final Thoughts

Bank, loose, compacted — three numbers for the same soil, and every earthwork calculation depends on converting between them correctly. Keep the two factors straight (swell multiplies up for trucks, shrinkage multiplies down for fill), use the typical values as a starting point, and always tie the final quantities back to the volume state your contract pays in.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.