Quality Assurance Handbook for Rural Roads Volume-I - pmgsy

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Quality Assurance Handbook for Rural Roads
405. WATER BOUND MACADAM SUB-BASE/BASE/SURFACING
A Methodology
1. The surface to receive the WBM course should be prepared to the lines, grade and cross fall. It
should be made free of dust and extraneous material. Large irregularities, where predominant,
should be made good by providing profile corrective course.
2. Where the WBM is laid over a fine grained soil subgrade, it is advisable to lay a 100 mm thick
intervening layer of screenings or coarse sand.
3. Any existing bituminous surface over which WBM is to be laid shall be completely removed
before laying WBM layer.
4. The coarse aggregate should meet the physical and grading requirements laid down in Table
405.1 and Table 405.2. Coarse aggregate can be crushed or broken stone, crushed slag, over burnt
brick aggregate, kankar, laterite meeting the prescribed requirements.
5. The spreading of coarse aggregate shall be done from stockpiles along the side of the roadway or
directly from vehicles. In no case the aggregate shall be dumped in heaps directly on the surface
prepared to receive the aggregates nor shall hauling over uncompacted or partially compacted
base be permitted.
6. The coarse aggregate shall be spread uniformly on the prepared subgrade, sub-base or base, as
the case may be, to proper profile (by using templates placed across at 6.0 m intervals) in such
quantities that would give the required compacted thickness. The thickness of compacted layer
should be 100 mm for Grading 1 and 75 mm for Gradings 2 and 3. The appropriate quantity of
aggregates is given in Table 405.4. The surface should be checked with templates and all high or
low spots remedied.
7. Roll the surface with suitable road rollers till aggregates are partially compacted with sufficient
void space left for application of screenings. However, where screenings are not to be applied as
in the case of soft aggregates, compaction shall be continued until the aggregates are thoroughly
keyed. Rolling shall proceed from inner edge to outer edge at the super-elevated portions and
from the edges towards the centre in other portions. The edge should be first compacted with
roller running forward and backward.
8. Check the profile transversely and longitudinally with templates/ straight edge. Correct the
irregularities by loosening the surface, adding or removing the needed amount of aggregates and
re-rolling until the entire surface conforms to the specified camber/ cross fall and grade.
9. Apply screenings to completely fill the interstices maintaining a slow and uniform rate, in three or
more applications. The screenings should not be damp at the time of application.
10. Do not apply screenings so fast and thick as to form cakes or ridges on the surface.
11. Continue dry rolling and brooming till no more screenings can be forced into the voids of coarse
aggregates.
12. Sprinkle water on the surface taking care that the underlying layer is not damaged.
13. Sprinkling, sweeping and rolling should continue till aggregates are thoroughly keyed, well bonded
and firmly set in its full depth and a grout has been formed of screenings.