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Terry Collins
Sustainability consultant
Asked a question last year

What is the difference between Granular Sub Base (GSB) & Wet Mix Macadam (WMM) in Pavement structures?

What is the difference between Granular Sub Base (GSB) & Wet Mix Macadam (WMM) in Pavement structures? What is the maximum size of the aggregates to be used in GSB & WMM?

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Raul Jimenez
Senior Project Manager

GSB is Sub-base course and mostly not considered in the main crust of the designed section of road. The prime work of GSB is drainage system and secondary to support the load transfer to the sub-grade.

WMM is a part of designed road section, thickness arrived from design calculations to withstand design load and ultimate transfer to sub0grade. WMM is mixed with designed proportion of aggregates and water content, laid and compacted in layers till designed thickness achieved.

The maximum aggregate size is limited to 65mm in GSB and WMM is limited to 20mm.

Flenn Hale
Construction Manager

SUB BASE (GSB)–  it is being laid in road work because it plays two major roles 

1- it works as a drainage layer 

2- it provides a good sub-base to laid base layer on it.

The material to be used for the work shall be natural sand, crushed gravel, crushed stone, crushed slag, or a combination thereof depending upon the grading required use of material like brick metal, kankar and crushed concrete shall be permitted in the lower sub-base. The material shall be free from organic or other deleterious constituents.

Wet Mix Macadam consists of laying spreading and compacting of clean, crushed, well-graded granular materials on a prepared and approved Granular sub-Base. The material is well mixed with water and rolled to a dense mass. It shall be laid on one or more layers as per line and level, grade and cross-section shown in the drawing or as directed by the Engineer. The thickness of single compacted Wet Mixed Macadam (WMM) Base shall not be less than 75 mm. The maximum thickness of a single compacted layer base can be up to 250 mm upon approval of the Engineer.

Ricardo Kuhn
Construction Foreman

Hi, In simple words, the wet-based macadam is constructed by the same technique but the material used for binding is different. wait pacemaker dam has been replaced by water-bound macadam in the swift process and uniform. But wait for base macadam and bituminous macadam use the same technique of construction. A granular sub base is in their adopted for highway construction which can be constructed above the subgrade and below the crust material. 

The largest size of crane used for GSB is approx 65 mm.in17 recent type of construction engineers would like to use the crushed stone because it having a particular call centre job and specific range is this can ensure about the good quality. a granular sub base is used as a layer and compacted because of controlling moisture. granular sub base was adopted to lead initially and water-bound macadam is spread on top of the granular sub-base. Hope this is beneficial for you.

Theodore Ray
Planning and development surveyor

GSB :- Granular sub base or GSB is a typical layer adopted in highway construction, which is laid above the subgrade and below the crust materials. Naturally available material consisting of various grades may be used for GSB, the largest grain size up to 65mm. In modern highway construction, engineers like it to be crushed stone having a particular percentage for a specific range. And this assures uniform quality. Thus for GSB the material is laid to be the desired thickness and well compacted by controlling the moisture. 

WBM :- Water bound macadam or WBM is a type of road construction where coarse stone chips having a particular range of grading (generally 65mm range) are bought to the construction site and laid to a thickness of about 100mm. The spread layer is well compacted and later stone dust or other non sticky fine material is spread over the surface. Then water is sprinkled liberally so that the fines fill the voids in the compacted aggregate. The layer is again compacted.

Jacqueline Wade
Environmental engineer

Granular Sub Base (GSB)

Agg are used below base coarse & above subgrade layer is known subbase layer, due to granular crushed agg its called granular subbase.


  • GSB act as a load bearing  & strengthing component of pavement.
  • GSB reducing to stresses applied on subgrade layer.
  • GSB providing drainage for pavement.
  • GSB providing resistance against rutting.


  • GSB material is a mix of sand & gravels, crushed stone & broken rock.
  • GSB material typically contain crushed stone > 50%.
  • Material having CBR > 30% & PI < 6%.
  • Water absorption < 2% & Agg impact value (AIV) < 40%.
  • Material should be free from clay & organic matters.

Other specifications

  • Drainage layer.
  • Use condn’s
  1. For lower sub-base: Grading III, IV
  2. For sub-base cum drainage layer: Grading V, VI.
  • Thickness ≮ 150mm
  • Size – max: 75mm
  • Agg impact value (AIV): 40%
  • Liq limit (LL): Max 25%
  • Moisture: 1 – 2% below OMC
  • Water absorption: Max 2%
  • FDD: 1 test/ 100sqm
  • Compaction: Min 98%
  • Level diff: ±10mm

Wet Mix Macadam (WMM)

WMM is a new tech on road constn & it has greater durability than WBM.


  • Agg’s need to be crusher run- Grading 1 (53mm – 75μ) for layer thkness > 100mm & Grading 2 for layer thkness < 100mm (26.5mm – 75μ).
  • OMC varies betn 2 – 6%.

Constn sequence: All components are pre-mixed in mixing plants

  • WMM is prepared in mixing plants. Having MC slightly greater than OMC. Accounting for transit/evaporation losses.
  • Mix is spread by a self-propelled paver finisher & in case of multilayer constn bottom layer may be spread using motor graders.
  • Rolling is done using roller having min static wt 100KN.

Expensive Method

  • WMM requires crusher run agg’s.
  • It req’s mixing plant. A paver (to spread the WMM mix) & a heavier roller (to compact the WMM mix). More constn machines means more money.

+ pts

  • WMM is labour extensive.
  • WMM uses more closely graded material, hence more stable.
  • Components are mixed in mixing plant before spreading of mix, making the laying operation simpler & faster (A paver being used for the laying operation also helps).
  • Easy to handle.
Kent Welch
Planning and development surveyor

Difference between granular sub-base and wet mix macadam in payment structure:

The wet mix macadam mostly consists of laying spreading and compacting of clean, crushed, well-graded granular material that is prepared and approved granular sub-base.

Single compacted wet mix macadam player shall not be less than 75mm.

Whenever we are using vibrating compacting equipment then compacted depth of a single layer of the course of the subway may be up to 200 mm.

Wet mix macadam shall be prepared in an approved of water macadam mixing plant.

Here replacing the aggregate fraction should be retained on 22.4 mm sieve with materials of passing from 22.4 and written on 4.75 mm.

While the addition of water, the evaporation loss shall be considered the mixed material shall be uniformly wet and segregation will take place.