The short answer
GSB, granular sub-base, is the lower layer: a coarser material with a top size of 53 or 75 mm, placed on the subgrade to spread load and let water drain out. WMM, wet mix macadam, is the base course above it: a dense, closely graded crushed stone of 53 mm and below, mixed with water in a plant and laid by paver. Both are compacted to 98 per cent of maximum dry density. WMM carries tighter limits on strength and particle shape.
The two are laid one on top of the other, come off the same plant and look alike in a stockpile to anyone outside the trade. They are specified in different clauses, made differently and tested to different limits, and one cannot stand in for the other.
01Side by side
| GSB, Clause 401 | WMM, Clause 406 | |
|---|---|---|
| Position | On the subgrade | On the GSB, under the bituminous layers |
| Main job | Spread load, drain water sideways | Carry traffic stress |
| Material allowed | Natural sand, crushed gravel, crushed stone or a blend | Crushed stone |
| Top size | 75 mm or 53 mm, by grading | 53 mm, with 95 to 100% passing 45 mm |
| Gradings | Six, numbered I to VI | One |
| Passing 75 micron | Under 5% in I to IV | 0 to 5% |
| Strength | Impact value 40 max | Los Angeles 40 max, or impact value 30 max |
| Particle shape | No limit stated | Flakiness and elongation combined, 35% max |
| Plasticity of fines | Liquid limit 25 max, PI 6 max | PI 6 max |
| CBR | 30 min at 98% density | Not specified |
| Mixing | Blended and watered before rolling | In a mixing plant, at optimum moisture |
| Compaction | 98% of maximum dry density | 98% of maximum dry density |
| Layer thickness | Up to 200 mm in one layer with a vibratory roller | 75 mm minimum, 200 mm maximum |
MoRTH Specifications for Road and Bridge Works, Fifth Revision. Your contract's own schedule governs where it differs.
02Why a road needs both
A wheel load is concentrated at the surface and spreads as it goes down. Near the top the stress is high, so the material has to be strong, dense and well interlocked. That is WMM. Lower down the stress is much reduced, and a cheaper, coarser material will do. That is GSB.
The sub-base has a second task that the base does not. Water gets into every pavement, through cracks and from the sides, and it has to be let out before it softens the subgrade. An open-graded sub-base carries it to the edge. A dense base course above keeps most of it from getting in.
03How each is made
GSB is a blend of coarse and fine fractions taken after the secondary crusher, proportioned to fall inside one of the six grading bands. Where a suitable natural gravel exists it may be used with little or no crushing. The six bands and which to choose are in GSB grading under Clause 401.
WMM is made from single-size fractions, typically 40 mm, 20 mm, 10 mm and dust, fed in set proportions into a pugmill where water is added. It leaves the plant as a damp, uniform mix and goes straight to the paver. The grading and the rules for laying it are in WMM under Clause 406.
04Where each goes wrong
- GSB with too many fines. Soil from the top of the bench gets into the feed, the share passing 75 micron climbs past 5 per cent and the plasticity index rises. The layer now holds water instead of shedding it.
- GSB that segregates. A wide-graded material tipped from height separates into coarse and fine. Coarse pockets will not compact, and fine pockets hold water.
- WMM mixed dry. Without moisture the fines will not bind and the roller cannot reach density.
- WMM made from flaky stone. Flat particles break under the roller and the grading changes after it has been approved.
- Either one laid on a wet formation. The layer below pumps up through it.
05Can GSB be used in place of WMM?
No. GSB may contain natural rounded material, has no limit on particle shape and is allowed a much weaker stone. In the base course it would rut. Using WMM as sub-base is permitted in the sense that it exceeds the requirement, but it costs more and drains less well than an open sub-base grading, so nobody does it by choice.
06What it means for the order
GSB is cheaper per tonne because it needs less crushing and no mixing plant. WMM costs more for the opposite reasons. On a typical highway section the two together are the largest tonnage on the job, which is why the quantity is worth working out early: see how much GSB and WMM a kilometre needs.