WBM Road Construction Procedure

Construction of water bound macadam road involves the following 6 basic steps as given below.

1. Preparation of sub-grade
2. Preparation of sub-base
3. Preparation of base
4. Preparation of wearing course
5. Preparation of shoulders
6. Opening to traffic

Step – 1 (Construction of Sub-grade)

Sub-grade act as a cushion for other layers i.e. In order to achieve durable road sub-grade should be strong. Sub-grade is provided by digging up the sub-soil and the level of the sub-grade is decided by subtracting the total thickness of the pavement from the finished level of the road pavement. The sub-grade is thoroughly compacted by rollers weighing 8 tonnes by sprinkling water one night before. Low spots which develop during rolling must be made up and brought to the grades as required. In rocky regions the sub-grades are not rolled whereas in region of clay soils, a layer to natural sand, moorum or gravel, is provided over sub-grade and is duly packed.

Step – 2 (Construction of Sub-base)

On a well compacted sub-grade, spread 10 to 20 cm size boulders or broken stones, or over burnt bricks in layers of 15 cm thickness and total width of the sub-base to be kept 60 cm wider than pavement width, projecting 30 cm on each side. The sub-base should be compacted by a roller to provide an even surface.

Step – 3 (Construction of Base)

On the prepared sub-base or directly on the sub-grade, as the case may be, the specified materials of the base course is spread and proper grade, thickness and cross sections maintained as per design shown on the supplied drawings.

Step – 4 (Preparation of Wearing Course)

This course may be laid in one or two layers according to the total designed thickness and the thickness of each layer should not exceed 10 cm. this component being very important, the following steps may be taken systematically.

1. Check the defective portions/patches of the newly laid base course i.e. soling and rectify them
2. Provide either bricks on end edging or earthen kerbs strong enough to prevent the new road material from spreading outward and also to retain water used in consolidation of the wearing course.
3. Spread the road metal evenly over the prepared base to the specified thickness and hand pack them so that the finished surface is brought to the required camber.
4. Spread the coarse aggregate over the surface and roll it dry with a suitable roller till interlocking of the aggregate is achieved with sufficient void space. The rolling is started from the edges and gradually shifted towards the centre.
5. After dry rolling, spread the screening materials (stones upto 12 mm size) with uniform rate so that voids of coarse aggregates get filled properly. This is achieved by dry rolling and brooming alternatively, till the voids of the coarse aggregates are filled.
6. After spreading the screening material, sprinkle sufficient quantity of water, sweep the surface and roll it with roller again.
7. Now apply the binding material in two to three thick layers at a slow and uniform rate. Each layer of binding material is rolled after adding sufficient water. The slurry is swept in with brooms to fill the void properly. The moving wheel of the roller should be cleaned with water. Continue the operations of spreading of binder, sprinkling of water, sweeping with brooms and rolling till the voids get filled and slurry forms a wave before the moving wheel of the roller.
8. After proper compacting allow it to dry over night. Spread a layer of sand or earth, about 6 mm thick and roll the surface again after sprinkling water lightly.
9. The surface may be allowed for 7 to 10 days of curing.

Step – 5 (Construction of Shoulders)

While curing the pavement surface, prepare the shoulders by filling earth to the specified cross slope and compact them properly by rolling or by tamping. Width and thickness of the shoulder should be as per specification.

Step – 6 (Opening to Traffic)

After properly drying, the road pavement may now be opened to traffic, ensuring that the traffic is distributed uniformly over the full width of the pavement.