Describe stepwise construction procedure for water bound macadam roads
1 Answer

The construction procedure of W.B.M. road is discussed below:

  1. Preparation of subgrade : The subgrade is prepared to the required grade and camber.

  2. Preparation of the base course : After preparing the subgrade or sub base, the required type of base course is constructed with specified materials in conformity with lines, grade and thickness.

  3. Preparation of intermediate and wearing course: The preparation of intermediate and wearing course of a WBM road is done in following steps:

    (i) Preparing the surface :- The surface of the newly laid base course on which some traffic has been allowed, is checked and the defective portions are rectified.

    (ii) Providing edging or earthen kerbs :- After preparing the surface brick-on-end edging is provided along the outer edges of the carriage way of the road.

    (iii) Spreading of coarse aggregate :- The road metal is spread evenly over the prepared base to the specified thickness.

    (iv) Dry rolling :- After spreading the course aggregate, dry rolling is done by means of a suitable roller. The rolling should be started from edges and gradually shifted towards the centre after properly rolling each strip.

    (v) Spreading of screenings :- After dry rolling, a blindage layer consisting of stone screenings (12 mm grits) is spread at a slow and uniform rate so as to ensure filling of all voids.

    (vi) Wet rolling :- After spreading the screenings, the surface is sprinkled over with sufficient quantity of water, swept and rolled.

    (vii) Application of binding material, watering and rolling :- After the application of screening and wet rolling, the binding material is applied successively in two or more thin layers at a slow and uniform rate. After each application, the surface is freely sprinkled with water and rolled with 6 to 10 tonne roller.

    (viii) Finishing the surface :- After the final compaction, road surface is allowed to dry overnight.

    (ix) Setting and drying :- The surface is then allowed to cure for 7 to 9 days.

  4. Preparation of shoulders : During curing, the shoulders are prepared by filling earth to the specified cross slope. These are then properly compacted by rolling or tamping.

  5. Open to traffic : After drying, the road is opened to traffic

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