How many feet of suitably graded and compacted backfill material should cover the bottom of a fiberglass tank excavation?

The correct answer indicates that a fiberglass tank excavation should be covered with a minimum of 3 feet of suitably graded and compacted backfill material. This requirement is based on ensuring that the tank is supported adequately by the backfill to prevent settling and to properly distribute the weight of the tank and any overlying structures or soil.

Having 3 feet of well-compacted backfill provides a stable foundation that helps to maintain the integrity of the tank throughout its lifespan. It also aids in managing groundwater and reducing the possibility of floatation when the tank is empty. Proper compaction is essential because it minimizes voids that could lead to potential issues such as tank movement or distortion over time.

The thickness of the backfill is dictated by factors like the tank's size, soil conditions, and external loads that might be applied. A minimum of 3 feet helps ensure that the structural and environmental requirements for long-term operational safety and stability are met. Therefore, this quantity of backfill material is critical for effective installation and retrofitting practices.