To what depth should the bottom of the excavation be covered with suitably graded backfill material?

The installation and retrofitting of underground storage tanks (USTs) require careful adherence to specifications and standards for site preparation, which includes backfilling the excavation once the tank is placed. Backfill material serves several critical functions, including providing structural support for the tank and ensuring proper drainage away from it to mitigate the risk of corrosion and instability.

Covering the bottom of the excavation with about 1 foot of suitably graded backfill material aligns with recommended engineering practices. This depth is generally considered sufficient to provide a stable base that supports the tank while accommodating any settling that might occur over time. Furthermore, this depth helps in achieving adequate compaction and minimizing voids that could lead to structural failure or water accumulation around the tank.

Additionally, using backfill with proper grading ensures that the material can effectively support the tank while allowing water drainage, which is essential for maintaining the integrity of the UST system and preventing contamination of surrounding soil and groundwater. The specific requirement of 1 foot is often based on standard industry guidelines that balance stability, drainage, and material characteristics, making it a prudent choice for UST installation practices.

In summary, 1 foot of backfill material is the recognized depth that provides a balance of support and safety in UST installations