What should not be used for backfilling within 6 inches of direct buried cables?

When considering what should not be used for backfilling within 6 inches of direct buried cables, it's important to understand the impact of the chosen backfill material on the integrity and safety of the cables.

Machine compaction applies significant force to the soil, which can exert excessive pressure on the cables. This pressure may lead to damage or deformation of the cable insulation, jeopardizing its functionality and safety. In contrast, hand compaction allows for a gentler method of packing the soil around the cables, reducing the risk of harm.

As for loose fill material, while it may not offer as much stability as compacted soil, it is suitable for placement near buried cables, provided it's managed appropriately. Concrete, although not part of the correct answer, would also not be suitable for backfilling directly next to cables due to its heaviness and inflexibility, which could harm the cables as well. Thus, using machine compaction is inappropriate within close proximity to buried cables due to the potential for damage from excessive pressure.