To what depth should the excavation be covered with suitably graded back fill material?

The excavation should be covered with suitably graded backfill material to a depth of one foot to ensure stability and proper support for the underground storage tank (UST). Covering the tank with this depth of backfill helps distribute the weight evenly and protects the tank from external pressures, such as soil settlement and frost heave. It also provides sufficient cushioning and protective measures against potential damage from load-bearing forces and environmental factors.

This one-foot depth is consistent with industry standards and practices aimed at maintaining the structural integrity of the tank and its installation over time, thereby minimizing the risk of leaks or failures. Depending on soil conditions, moisture content, and other environmental factors, ensuring that the backfill material is graded appropriately can also enhance drainage and reduce the likelihood of settling, which could compromise the UST's positioning.

Using a depth that is either less than or more than one foot may not provide the same level of protection or support, possibly leading to issues down the line with the tank's stability and functionality.