How much relative density is required for cohesionless materials backfilled in trenches constructed through rock?

The requirement of 85% relative density for cohesionless materials backfilled in trenches through rock is rooted in ensuring adequate stability and support during and after construction. Cohesionless materials, such as sand, rely heavily on particle interlock and friction for stability, which can be affected by the degree of compaction.

A relative density of 85% strikes an optimal balance, maximizing the frictional resistance while minimizing the potential for settlement and instability. Achieving this level of compaction helps to prevent water infiltration and erosion that could undermine the trench or the surrounding structures. Higher relative densities, while potentially offering more stability, may not be necessary for every application and could lead to increased costs and effort without significant benefit.

In the context of the performance requirements for trench backfill, maintaining this relative density helps ensure that the cohesionless material can adequately support the loads applied to it, particularly when placed in a rocky environment where additional challenges may exist, such as uneven rock surfaces and variations in support.