What type of material should be avoided as backfill for water mains?

The choice of cinders as a backfill material for water mains should be avoided primarily due to their porous nature and potential for environmental issues. Cinders, which are by-products of coal combustion, can retain moisture and provide a pathway for water infiltration, potentially leading to corrosion of the water main. Additionally, the chemical composition of cinders can introduce contaminants into the water supply, posing a risk to public health.

When selecting backfill materials, it's essential to choose those that provide adequate support and stability while minimizing the risk of contamination. Materials such as sand, gravel, or clay can serve well for this purpose, as they allow for proper compaction and drainage while being less likely to introduce harmful substances into the water system.