**6.3 Backfilling**

Backfilling along the length and breadth of the trench is done with suitable materials. Such materials may be sourced from alluvial deposits and treated, if necessary, to satisfy the design requirements. Suitable materials must have been stabilized after long transportation in fluvial regime amidst dissolution and abrasion of original rock. Granite is not recommended. From research finding [10, 11], gravelly sand is most recommended, followed by fine sand and then mixed sand in that order. The results of such evaluations are graphically illustrated in **Figures 5** and **6**. The diameter at 30% passing (D30) and Uniformity coefficient (Cu) of a suitable material must be relatively high, **Table 1**.
