Among the following, the correct sequence of WBM construction will be: i. Preparation of subgrade ii. Spreading of Coarse aggregates iii. Application of binding material iv. Application of screenings v. Provision of lateral confinement
- Ai, ii, iv, iii, v
- Bi, iv, ii, iii, v
- Ci, ii, iv, v, iii
- Di, v, ii, iv, iiiCorrect
Solution
Concept
WBM (Water Bound Macadam) layers are built in a fixed order: subgrade, side confinement, coarse aggregate, screenings, then binding material.
Step-by-step solution
1. First prepare the subgrade (shape, camber, compaction). 2. Provide lateral confinement (shoulders or kerbs) so aggregates do not spread sideways. 3. Spread the coarse aggregates evenly and roll them. 4. Apply screenings to fill the voids, with dry rolling. 5. Apply binding material (with sprinkling and wet rolling), then set and dry. 6. So the sequence is i, v, ii, iv, iii, which is option D.
Explanation
In WBM, lateral confinement is given before spreading aggregates, so the stones stay in place during rolling. Screenings fill the gaps between coarse stones first. Binding material is added last to bind the layer with the help of water.
Why the other options are wrong
A: lateral confinement cannot be the last step. B: screenings cannot come before coarse aggregates. C: confinement must come before spreading aggregates, and binder must come after screenings.
Shortcut
Remember the order: Base, Border, Big stones, Small stones, Binder.
Common mistake
Students often think lateral confinement (shoulders) is done at the end, but it is done before spreading aggregates.
Correct answer: D) i, v, ii, iv, iii