What is a 'WBM' Road? | Meaning of WBM Road | WBM Road Illustration

What is a 'WBM' Road? | Meaning of WBM Road | WBM Road Illustration

A 'WBM' road stands for 'Water Bound Macadam' road. This type of road derives its name from the Scottish engineer John Loudon McAdam, who proposed and designed this particular road structure. The term 'Water Bound' is relevant as this road type uses the principle of binding various in-situ layers of materials such as boulders, stones, fine aggregates, and fillers like moorum, stone dust, and sand, with water acting as a binder.

Structure of a Typical WBM Road

A typical WBM road section comprises the following layers:

1. Subgrade Preparation:
• Process: Remove topsoil, vegetation, and loose sub-surface material to the desired level.
• Objective: Ensure the subgrade is well-consolidated and compacted to a camber of 1 in 60 or 1 in 48.
2. Sub-base and Base Preparation:
• Base Course (Soling): This involves creating a compacted base with three layers of different sized materials:
• Bottom Layer: Large-sized boulders (63mm to 40mm).
• Middle Layer: Medium-sized aggregates (40mm to 25mm).
• Top Layer: Smaller-sized aggregates (25mm and down) used as filler material.
• Compaction: The layers should be compacted well using a 10-tonne roller, resulting in a final compacted layer about 125mm thick.
• Second Course: Another three-layer system is added over the first course, making the total compacted thickness about 250mm.
• Water Saturation: The base is dry-rolled, then saturated with water, and wet-rolled until cohesive and fully saturated.
3. Surface Dressing:
• Drying: The sub-base and base are allowed to dry thoroughly.
• Cleaning: Loose dust and dirt are removed using wire brushes, hard brooms, and soft brushes to expose a clean metal surface to a depth of 6mm to 12mm without loosening the embedded stones.
4. Asphalting (for all-weather and durable roads):
• Bituminous Tack Coats: The surface is bound with alternate layers of bituminous tack coats.
• Premix Layer: A 40mm layer of bituminous concrete (premix) is applied.
• Final Layer: A 25mm thick bituminous mix of fine aggregates and a seal coat is applied as a wearing course, making the road water-tight and durable.

Illustration of a WBM Road Section

Layer	Material	Process
Surface Dressing	Cleaned Metal Surface	Remove dust and dirt, exposing a clean surface without loosening stones.
Asphalting	Bituminous Concrete, Premix	Apply tack coats, a 40mm bituminous concrete layer, and a 25mm seal coat for durability and water resistance.
Base Preparation	Smaller Aggregates (25mm and down)	Top layer as filler material.
Base Preparation	Medium Aggregates (40mm to 25mm)	Middle layer of the base course.
Base Preparation	Large Boulders (63mm to 40mm)	Bottom layer of the base course.
Subgrade Preparation	Compacted Soil	Removal of topsoil and vegetation, ensuring a compacted subgrade with a camber.

Conclusion

Water Bound Macadam roads are an essential part of civil engineering infrastructure, providing a cost-effective and durable solution for road construction. The meticulous preparation of subgrade, base, and surface layers, along with optional asphalting, ensures that these roads meet the necessary standards for both temporary and semi-permanent applications. Understanding the structure and process involved in WBM road construction is crucial for civil engineers and construction professionals aiming to build reliable and long-lasting roads.