The short answer
Tonnes = length x width x compacted thickness x compacted density. One kilometre of pavement 10 m wide, with 200 mm of GSB and 250 mm of WMM, needs about 4,500 tonnes of GSB and 5,900 tonnes of WMM, before allowing for the wider sub-base under the shoulders and for handling loss.
Every quantity on the material side of a road job starts from four numbers on the cross-section drawing. Get them into tonnes early and the plant, the fleet and the cash flow can all be sized from the same figure.
01The formula
Multiply length by width by compacted thickness, all in metres, to get the compacted volume. Multiply that by the compacted density to get tonnes. For planning, 2.25 tonnes per cubic metre for GSB and 2.35 for WMM are reasonable. Replace them with the maximum dry density from your own laboratory once the source is approved.
02Tonnes per kilometre, per metre of width
| Compacted thickness | GSB at 2.25 t/m3 | WMM at 2.35 t/m3 |
|---|---|---|
| 100 mm | 225 | 235 |
| 150 mm | 338 | 353 |
| 200 mm | 450 | 470 |
| 250 mm | 563 | 588 |
| 300 mm | 675 | 705 |
Multiply by the width of the layer in metres. A 200 mm GSB layer 12 m wide is 450 x 12 = 5,400 tonnes per kilometre.
03A two-lane road with paved shoulders
- Pavement width
- 10 m
- GSB, 200 mm: 1,000 x 10 x 0.20 x 2.25
- 4,500 t
- WMM, 250 mm: 1,000 x 10 x 0.25 x 2.35
- 5,875 t
- Tipper loads at 18 t
- 576
- Granular layers per km
- 10,375 t
Check the drawing before using 10 m for the sub-base. GSB is normally carried wider than the layers above it, often across the full formation, so that water draining through it can escape at the embankment slope. If the drawing shows GSB 12 m wide, its figure becomes 5,400 tonnes.
04A four-lane divided road
Treat each carriageway separately and add them. With each side 9 m wide including its paved shoulder, the pavement is 18 m in total.
- Total pavement width
- 18 m
- GSB, 200 mm
- 8,100 t
- WMM, 250 mm
- 10,575 t
- Tipper loads at 18 t
- 1,038
- Granular layers per km
- 18,675 t
Service roads, junctions, lay-bys and toll plaza aprons are on top of this, and on some packages they add a good deal.
05Allowances
- Extra width. Whatever the drawing shows for sub-base beyond the pavement edge.
- Handling loss. Two to three per cent for stockpile bottoms, spillage and trimming.
- Level correction. Where the layer below is low, the layer above is thicker. Tight control of subgrade levels saves real tonnes here.
- Camber and superelevation. Thickness is uniform, but check that widths at curves and the extra width at widening sections are included.
06From tonnes to a programme
A 50 km two-lane package at 10,375 tonnes a kilometre needs about 5.2 lakh tonnes of GSB and WMM, before the bituminous and concrete stone. One 250 TPH plant on two shifts holds 25,000 to 30,000 tonnes a month, so that is roughly nineteen producing months for one plant. If the granular layers have to be down in twelve, the answer is a second plant or a stockpile started early. The reasoning behind the monthly figure is in what a 250 TPH plant really produces.
For ordering by volume instead of weight, and for the difference between loose and compacted measure, see brass, cft, cubic metre and tonne.