Bulking, swell and shrinkage: three factors people confuse
The same soil has three different volumes. Order against the wrong one and the job stops.
Three volumes, one material
Bank measure is the volume in the ground, undisturbed. This is what your drawings and cross-sections give you, and it is the only one of the three you can calculate from geometry alone.
Swell is the expansion the moment you dig it. Excavation breaks up the consolidated matrix and introduces air voids, so the material takes up more space in a truck than it did in the ground. You always need more haulage capacity than the hole volume suggests.
Shrinkage is the reduction when loose material is placed in layers and rolled. Compaction drives the air back out, so the finished compacted volume is well below the loose volume delivered.
Two rules cover most of what you need:
- Hauling spoil away: bank volume × swell factor = loose volume to haul.
- Importing material: compacted volume × shrinkage multiplier = loose volume to buy.
They point in opposite directions, which is exactly why they get mixed up.
Typical swell factors
From bank to loose, for materials common in Gulf road work:
- Clean dune sand / fine sand: 10% to 15% swell (factor 1.10 to 1.15). Low cohesion means little change in packing.
- Sabkha / saline coastal soil: 20% to 30% (factor 1.20 to 1.30), depending on moisture, salt crust bonding and silt content.
- Crushed limestone base (GAB / CAB): 20% to 25% (factor 1.20 to 1.25).
- Hard rock, blasted limestone or dolomite: 40% to 65% (factor 1.40 to 1.65). Irregular fragments cannot nest, so the voids between them are large.
Rock is where estimates go badly wrong. Assuming 20% for blasted rock when it is nearer 50% leaves you short of trucks by a third of the fleet.
Estimating truck loads
- Loose volume = bank volume × (1 + swell).
- Usable truck capacity = the lesser of the box volume and the legal axle weight limit divided by the loose density.
- Trips = loose volume ÷ usable capacity, rounded up.
Step 2 is the one people skip. A tipper box might hold 18 m³, but filled with dense wet aggregate at 2,000 kg/m³ that is 36 tonnes — far past a legal payload of 24 to 30 tonnes. The truck runs part-empty by volume, and your trip count rises accordingly.
Two other things move the number: rain-soaked material or saturated sabkha gains mass without gaining volume, so trucks hit the weight limit with the box two-thirds full; and uneven loading, heaped against struck-off, puts real variance into every trip.
Compaction yield
From 1.0 m³ of loose crushed aggregate base delivered to site, expect roughly 0.78 to 0.82 m³ compacted in the road — a shrinkage multiplier of about 1.22 to 1.28.
The working rule: to finish 1.0 m³ of compacted base, order about 1.25 m³ loose.
The expensive version of this mistake
A contractor won a highway package needing 80,000 m³ of finished compacted limestone base. Procurement ordered 80,000 m³ loose from the quarry — the drawing quantity, one for one, with no shrinkage factor applied.
That 80,000 loose became about 64,000 compacted. The job ran out of aggregate at 80% complete. Paving stopped for five days waiting for the quarry to lift production and find haulage. Pavers, rollers and crews stood idle on standby rates, and the remaining 20,000 m³ was bought at emergency spot prices with expedited freight.
One multiplier, applied at the purchase order stage, would have cost nothing. Left out, it cost material, standby and freight premiums together.
This guide describes general practice and typical figures. Your project specification, job mix formula and laboratory results govern the actual values on your job. Nothing here is a design or a substitute for the engineer of record.