What is WBM Road? Construction Procedure, Advantages, Disadvantages

What is WBM Road?

Full form of WBM is Water Bound Macadam. It is the layer of broken stone aggregates bound together by stone dust or screening material and water applied during construction and compacted by heavy smoothed wheel roller. 

It is the oldest type of highway pavement used in modern times is known as Macadam after the name of Johan Macadam, A Scottish Engineer.

Construction Procedure of WBM Road

Preparation of Sub-grade 

The site is cleared and weak spots corrected and rolled. To check the spreading of aggregate beyond the carriageway or to provide confinement to the loose aggregate while compacting by rolling either a trench cut in the sub-grade and proper camber is provided or earth or brick bunds are made.

On the clayey subgrade, a blanket of granular material of a thickness of about 10 cm is provided. The subgrade should be well-drained and checked for stability etc.

Spreading of Aggregates

The coarse aggregate is spread on the prepared surface of sub-grade evenly in layers. The thickness of each layer should be such that it gives 7.5 cm thick layer on compaction.

The profile of the road is checked by placing templates across the road every 6 meters. To ensure proper camber and grade the surface is checked from time to time.

Also, Read – Material Required for Construction of WBM Road

Dry-Rolling

The objective of this rolling is to key the coarse aggregate thoroughly. After spreading the coarse aggregate and checking all irregularities, the rolling is done by 6 to 10 tonnes, 3 wheeled power roller.

Usually rolling is started from the edges with roller running forwards and backwards till the aggregate is fully compacted.

Application of Screening

The next step is the application and keying of screening. after the coarse aggregates are set and keyed thoroughly by the rolling, the screenings are spread uniformly and rolled.

Generally spreading, brooming and rolling operations are carried out simultaneously. The quantity of screening is generally used to fill about 50% of the total voids and the rest 50% voids are filled with filler materials.

Wet Rolling

After spreading the screening, the surface of the layer is sprinkled with water and rolled again. The sprinkling of water and rolling is continued till all the voids are filled and a wave of grout flushed ahead of the roller.

This indicates that all voids are filled with chocking material. The quantity of water and screening are generally dependent upon many factors such as size and nature of aggregate, type of surface desired, etc.

Also, Read – Concrete Road Vs Asphalt Road – Which is Better?

Application of Fillers

The filler material is applied in two successive thin layers. the plasticity index of the filler material should not be more than 9.

After the application of the filler, water is sprinkled on the surface and the slurry is allowed to fill the voids.

Now rolling is done with 6 to 10-tonne roller. to wash down the binding material off the wheels of the roller water is poured on them.

Surface Finishing

The section prepared as discussed above is allowed to dry overnight and then a  thin layer about 0.6 cm thick of sand or earth spread over the surface. The surface lightly sprinkled with water and rolled again.

Shoulders

These are made of the same cross slope as that of pavement and are compacted by rolling. After proper drying and set of the WBM, It is opened to the traffic.

Also, Read – What Is Super Elevation?

Maintenance of WBM Road

WBM is the basic stage of the planned improvement of road surfacing. Wbm roads are damaged rapidly due to the heavy traffic and adverse climatic conditions.

The steel tyred bullock carts cause serve wear and tear to the WBM surface. In dry weather, dust is formed and in rainy season mud is formed on WBM road.

Fine particles are sucked up by fast-moving vehicle from the road surface resulting in loss of binding particles in the surface.

In the rainy season, the rain is soaked by the surface which makes the surface soft. In such conditions, the movement of vehicles makes the layer of surface loose.

In this situation, some aggregate gets displaced from their position causing ruts and potholes on the surface.

Remedies of Damage WBM Road

To prevent aggregate from getting loose from the surface course, a thin layer of moist soil should be spread over the surface periodically, particularly after the rainy season.

The dust nuisance can be effectively eliminated by providing a surface dressing of bituminous materials.

Also, Read – What is Road Gradient? – Definition and Types

Advantages of WBM Road

1. Construction Cost of WNM road is low.
2. No skilled labours are required for the construction of WBM road.
3. Locally available material can be used in the construction of WBM road.

Disadvantages of WBM Road

1. WBM roads required high maintenance.
2. WBM roads are less durable.
3. Inconvenience and danger to traffic if not properly maintained.

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