How much GSB and WMM does a kilometre of road need?

The bill of quantities measures granular sub-base in cubic metres, finished in position. The crusher sells in tonnes over a weighbridge. Somebody has to get from one to the other before a purchase order is written, usually by borrowing a density from the last project.

Multiply length by width by compacted layer thickness to get compacted volume, then multiply by compacted density to get tonnes. For 1 km of a 7.0 m wide carriageway, 250 mm of WMM is 1,750 cubic metres, about 3,850 MT at 2.2 t/m3. The same kilometre with 200 mm of GSB is 1,400 cubic metres, and its tonnage depends on a density only your lab can give you.

The method in three steps

1. Compacted volume. Length x width x compacted thickness, all in metres. Take each layer's width from the cross-section, not the carriageway width, because the lower layers are wider.
2. Tonnes. Compacted volume x compacted density in tonnes per cubic metre.
3. Allowances. Add for shoulders, tapers, widening and losses as your contract and your own records justify. Do not bury them in the density.

Work in compacted volume from the start. Loose material in a tipper body or a stockpile takes up more space than the same tonnes rolled into the road. For quantities handed over in brass or cubic feet, see converting tonnes, cubic metres and brass.

Which density to use

No Indian standard fixes the density of GSB or WMM. It changes with the rock, the grading and the moisture. The number that belongs in the calculation is the maximum dry density (MDD) your project lab found for that material in the IS 2720 Part 8 heavy compaction test, adjusted for the compaction achieved in the field. MoRTH Clauses 401 and 406 both require at least 98% of MDD.

Before the lab has run a test, there is one published figure for WMM that can be defended. The MoRTH Standard Data Book rate analysis for wet mix macadam, as reproduced in secondary sources, takes 225 cubic metres of compacted WMM as 495 tonnes. That is 2.2 t/m3, and the example below uses it.

For compacted GSB we found no figure with a source we would stand behind, so this page prints none.

Worked example for one kilometre

The assumptions, all chosen for illustration: 1 km of two-lane carriageway 7.0 m wide; 250 mm of WMM and 200 mm of GSB, thicknesses that appear in IRC:37-2018's worked examples and are design outputs, not requirements; WMM at 2.2 t/m3; a tipper carrying 15 MT net; the kilometre's WMM laid over 5 working days.

Under Clause 406 a single compacted layer of WMM may not exceed 200 mm, so 250 mm goes down in two layers, each at least 75 mm. And 15 MT is a round figure for the arithmetic. What a tipper may legally carry depends on its axles and its own weight, which tipper payloads and the overloading rules sets out.

How sensitive the answer is

On 1,750 cubic metres, every 0.1 t/m3 of density is 175 MT, and basalt and sandstone do not give the same MDD. Thickness matters as much. MoRTH Table 900-1 allows ±10 mm on the surface level of both layers. A WMM layer running 10 mm thick across the whole kilometre is 1,000 x 7.0 x 0.010, or 70 cubic metres. At 2.2 t/m3 that is 154 MT laid, inside tolerance, that the bill of quantities does not measure.

What the simple sum leaves out

• Wider lower layers. GSB normally extends beyond the carriageway, under the shoulders, and often to the edge of the formation so that it can drain.
• Paved shoulders and median-side strips. These carry their own crust. Shoulder widths differ between editions of the four-laning manual, so take them from your contract, not from memory.
• Tapers, junctions, service roads and curve widening. A package total is never the per-kilometre figure times the length.
• Losses. Stockpile floors, spillage, material rejected at the paver. No specification gives a percentage. Use the one your first two months of weighbridge totals against laid volume produce.

From tonnes per kilometre to plant months

A per-kilometre figure becomes useful when it is set against what a plant can make. One of our 250 TPH three-stage plants on a two-shift pattern is rated for up to 1,00,000 MT a month. Take a 40 km package with the WMM section above.

So the base course alone is about a month and a half of one plant running at its rating and making nothing else. On a real package the paver sets the pace and the WMM is drawn over many months, alongside GSB and aggregate. Setting the two side by side tells you whether one plant covers the programme or stock has to be built ahead of the paving front. The daily figure, 52 loads in the example, sizes the fleet: how many tippers a haul needs works that out from cycle time.

What to send with the enquiry

Send the cross-section with layer widths and thicknesses, the GSB grading number, the length, the month each layer starts and the kilometres a month the paving programme assumes. Add the site location, because lead distance moves a delivered rate more than anything at the plant does.

We quote aggregate supply per tonne delivered, and every load leaves with a slip from a calibrated weighbridge. The tippers and weighbridges are on the equipment page, and the limits used above are in wet mix macadam under MoRTH Clause 406. Specifications are revised, and the thicknesses and tolerances in your own contract are the ones to calculate with.