What is a 'WBM' road? | Meaning of WBM road | WBM road illustration

A 'WBM' road stands for 'Water Bound Macadam' road. It derives its last part of the name from an Engineer named Macadam who proposed and designed the structure of this road section. The other part of the name stands relevant as this road type essentially uses the principle of binding the various in-situ layers of boulders, stones, fine aggregates and fillers as moorum, stone dust, sand, etc.

A typical WBM road section comprises of the following basic structure:
  • Subgrade preparation : This includes removal of the top soil, vegetation and loose sub-surface (if any) up to the desired reduced levels. The subgrade should be well consolidated and compacted to a camber of 1 in 60 or 1 in 48.
  • Sub-base and base preparation : This is the base course and is also called soling. It comprises of well compacted base made out of rolling boulders/ kankar/ overburnt brick ballast in three layers, viz., the larger sized boulders (sized 63mm to 40mm) evenly placed as the bottom-most layer; medium sized aggregates (sized 40mm to 25mm) evenly placed as the middle layer; and smaller sized aggregates (sized 25mm and down) evenly placed as filler material as the top layer. The layers should be compacted well longitudinally in pre-determined sections by a 10 tonne roller with the final compacted layer being about 125mm thick. A second course of such sub-base commonly known as base and comprising of three layer system should be provided over the well compacted 1st course so that the total compacted thickness of the sub-base and base is about 250mm. The compacted base should be dry-rolled followed by saturating it well with water and subsequent wet-rolling until the base becomes cohesive and fully saturated.
  • Surface dressing : The sub-base & base are allowed to be bone dry over a period of time. The loose dust and dirt are removed by a combination of wire brushes, hard brush brooms and soft brushes so as to expose clean metal surface to a depth of 6mm to 12mm, but without loosening the embedded stones.
  • Asphalting : The temporary and semi-permanent roads involve only the above mentioned steps. However, asphalting is done for making the roads all-weather and durable. The process includes binding the surface with alternate bituminous tack coats and applying a 40mm layer of bituminous concrete (commonly known as 'premix'). The final layer of 25mm thick bituminous mix of fine aggregates and a seal coat are applied as a wearing course and to make the road water tight for enhanced durability.


-  Image source (screenshot of the mentioned webpage) & recommended related information webpage:  MACADAM.htm

-  Query sent by Vivek Kumar

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