method of laying Water Bound Macadam (WBM) road

Describe the method of laying Water Bound Macadam (WBM) road.

Solution:-


The following method of laying WBM road are as followed:-

(i) Preparation of sub grade: The subgrade must be strong enough so that it does not settle under the loads and thus soil subgrade is compacted usually by rollers (of weight 80kN) after sprinkling water on to the soil. Any resulting depressions must be filled first before laying the second layer.

(ii) Construction of sub – base: Here in this step, crushed stones or over burnt bricks or boulders in thickness of 10 cm – 20 cm is laid. The sub – base is also compacted by rollers.

(iii) Construction of base: On the prepared surface of sub – base, a layer of base course is laid of specified material. The thickness, grade and cross – section are maintained as per the design and specifications.

(iv) Preparation of wearing course: This layer is the top must surface which undergoes large abrasion due to moving vehicles. This course is laid in one or more layers and thickness of this layer is kept as per the design. Generally this wearing course is of asphalt of tar.
After all these steps, shoulders are constructed by spreading earth on either side of the pavement at the specified slope and width. This is achieved by rolling and tamping.