Which type of material is specifically prohibited as backfill for water mains?

Cinders are specifically prohibited as backfill for water mains due to their potentially harmful characteristics. Cinders, which are a byproduct of combustion processes, can create voids and air pockets around the water main, leading to insufficient support and possible structural issues over time. They can also retain moisture, which may promote corrosion of the pipes, especially if the cinders contain soluble materials or chemicals that can leach into the surrounding soil.

In contrast, soil, sand, and rock have properties that make them more suitable for backfilling. Soil and sand can provide adequate support and compaction around the pipes, helping to stabilize them and reduce movement. Rock, when used properly, can also create a solid foundation, provided that it is appropriately sized and placed to avoid causing damage to the water main. Each of these other materials adheres to engineering principles aimed at maintaining the integrity and longevity of water infrastructure.