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Water bound macadam road is a type of flexible pavement in which the base and surface layer contains crushed stone or broken rock pieces and materials are well interlocked with the help of a mechanical roller.

The construction procedure of WBM road are as follows

A. Collection of material: There are mainly three types of construction materials are required for the construction of WBM roads.

• i) Coarse Aggregate: Coarse Aggregate consists of a mixture of hard and durable crushed aggregate and broken stones. The aggregate used for every layer of the Water Bound Macadam Road construction should be well graded.

• ii) Screening: A screening is a construction material used in WBM road construction work to fill the void or space left between aggregate particles after compaction is done. Generally, construction materials used for screening purposes have smaller sizes compared to coarse aggregate.

• iii) Binding Material: A Binding Material that is going to be utilized for the WBM road should be checked and authorized by the working engineer and it should have minimum plasticity. 

B. Equipment: For the construction of WBM road excavator, grader, compactor, loader, dumper, water browser, distributor, etc are used.

C. Construction of pavement

• i) Subgrade or Foundation Preparation work: The subgrade layer contributes as a foundation layer of the road on which the base, subbase, and wear course are rested. The required filling and cutting are done to bring the desired R.L. of ground. Preparation of this subgrade layer for the grade & camber and, dust & other loose materials are cleaned up

• ii) Providing Lateral Confinement:

The lateral reinforcement or confinement is constructed before the laying of different layers of Water Bound Macadam materials. Before laying of aggregates the shoulders have a thickness similar to the WBM road properly compacted layers. But if the soil is unstable and the road is at a higher elevation or required to be filled, then the retaining wall can be constructed as lateral confinement. The main purpose of providing lateral confinement is that the road surface to be constructed is retained in between them and it becomes easy for the lay of coarse aggregates.

• iii) Spreading the Coarse Aggregates:  

To spreading the coarse aggregates over the surface of the subgrade, the lumped mass of coarse aggregates is stacked at the definite interval of the road through the required length of the road to be constructed. Then the coarse aggregates are evenly spread on a subgrade base with the help of different types of machinery like backhoe or dozer. The thickness of every layer should be such that after compaction it gives a 7.5 cm thick layer. The surface is checked from time to time to ensure the proper camber and grade of the road.

• iv) Rolling Process:

The process of rolling is carried out along the edges first and then moved to the center for the straight section of the road. For super elevated curves, rolling is finished from the lower end of superelevation and then moved to the high edge. Skilled operators must be used for driving the roller as the fault rolling operations lead to the formation of corrugations, the unequal finish of road surface, or wearing of the road in a few months of construction. The main objective of this rolling is to fix the coarse aggregate thoroughly over one another.

• v) Application of Screening (Fine) Material:

The next step is the spreading of the screening material. After the process of rolling is completed, screening material is applied to fill all the voids. The quantity of screening is generally used to fill about 50% of the total voids and the remaining 50% of voids are filled with filler materials. The process of spreading, booming, and rolling is carried out simultaneously for each layer till the surface of coarse aggregate seems firm and well compacted without any voids.

• vi) Sprinkling of Water and Grouting:

After completion of wet rolling with screening materials, the layer is water sprinkled and then again rolled so that the water erodes the surface finer and seeps and deposits it to the inner voids. The water sprinkling and process of rolling continued until all the voids are filled and a wave of grout flushed ahead of the roller.

• vii) Application of Binding Materials:

The same procedure is used for the application of binding materials as that of screenings. Here after each layer of water is sprinkled and a rolling operation is carried out. At the time of rolling operation, the wheel of the roller should be continuously watered to wash out binding material that gets stuck to the rollers’ wheels. The binding material is generally stone dust or lime dust depending upon the suitability and requirements of WBM road.

• viii) Leveling and Drying of Surface:

After the final rolling operation, the WBM is left to cure for a single day, and a day after if any depressions or undulations are visible then again the adequate amount of screenings and binding materials can be spread and compacted.

• ix) Preparation of Shoulders:

While curing the road, shoulders are constructed alongside by keeping proper slope with earth filling work. They are properly compacted.

• x) Open for Traffic:

After proper drying and without any depressions, the road is then made up for the traffic. For a few days, the traffic should be well disturbed over the full width of the road by placing obstacles longitudinally in the form of drums, barricades, etc.