Base course of road is the layer of material placed on a subbase layer and directly beneath the surface pavement (Rigid or Flexible). The primary function of the base course is to provide structural support and load distribution for the pavement layers above it.
It is a foundation course, consisting of crushed hard durable gravel, rock or stone fragments. It shall be clean and free from organic matters, lumps of clay and other deleterious substances. The material shall be of such a nature that it can be compacted readily under watering and rolling to form a firm, stable base for both Flexible and Rigid pavements.
• Construction Process of Aggregate Base Course
The base material is placed on top of the subbase and compacted to the specified density. The Aggregate Base Course shall comply with Grading and Quality requirements given in below table:
b) Water Bound Macadam (WBM) Road Base
Water Bound Macadam (WBM) road is a type of road construction method where material of base course are compacted and bound together with water. It consists of broken stones of irregular shape, keyed together by consolidation under a roller, the stones themselves being wedged together in position and prevented from moving by smaller stones. Water is used to wash smaller particles of stones and grit into voids from above. Stability of WBM depends upon binder material which holds the stones in position and prevents them from being moved or displaced under traffic.
• Construction process of Water Bound Macadam (WBM) Base
Following are the Construction Requirements of laying Water Bound Macadam (WBM) Base.
1. For existing road surface 50 mm x 50 mm furrows shall be cut at One meter (1M) interval at 45° to the centerline of the roadway before proceeding with the laying of Coarse aggregate. 2. Use hard, tough and durable stone. 3. Road metal to be graded, produced from broken or crushed quarried stone or large boulders, least dimension not less than 150 mm. 4. The completed Base course shall be maintained in an acceptable condition until necessary subsequent treatment is applied. 5. Spreading and Compaction 6. Crushed stone shall be deposited and spread over prepared surface to proper depth so that the compacted layer shall not exceeds 2 ½ times the thickness of Max Size of Aggregate. Each layer shall be inspected thoroughly before rolling to detect high or low spots. Crushed stones shall be added or shifted to provide a true surface. The coarse aggregate layer, after being laid to proper thickness, shall be lightly rolled, sufficient only to establish the required grade and level of the stones. 7. Lay to gradient, camber and super-elevation as required and dry roll, filling the depressions which may appear during rolling. Spreading of aggregate shall be followed by rolling with a smooth wheel roller weighing 10 Tons. Rolling shall begin at the lower edge of the shoulders to lock the stones firmly at the edge, then progress gradually towards centre line. Rolling shall continue until aggregate is well keyed and does not creep ahead of the roller. 8. After the initial rolling, dry screening shall be applied uniformly over the surface. Dry rolling shall be continued while screenings are being applied. The surface shall be swept with mechanical or hand brooms to aid spreading of the screenings. In no case, shall coarse aggregates be stored in heaps directly on the area where these are to be laid nor shall their hauling over a partly completed base be permitted, however dumpers shall be allowed at the construction area, where material will be spread quickly after dumping. 9. When the interstices in the coarse aggregate are filled with screening, the surface shall be sprinkled with water until it is saturated. The rolling, sprinkling and application of additional screening shall continue until a grout is formed that fills all the voids and forms a wave of grout in front of the roller. 10. When more than one layer is required to complete the Macadam Base course to the thickness as per the drawings, each layer shall be constructed as described.
