What percentage of the tank should be filled before the backfill is compacted?

The correct percentage of the tank that should be filled before the backfill is compacted is 60%. This practice is essential for ensuring the integrity and stability of the underground storage tank (UST) installation. When a tank is filled to this level, it allows for proper support and prevents any possible deformation or movement of the tank during the backfilling process.

Filling the tank to 60% also aids in achieving an even distribution of pressure around the tank walls, which is crucial in preventing structural damage. This approach provides a balance between supporting the tank and allowing for adequate compaction of the backfill material, which enhances the tank's stability and minimizes the risk of sinking or shifting over time.

Compacting the backfill while the tank is underfilled or overfilled could lead to varying pressure distributions, potentially resulting in misalignment or other structural issues in the UST system. Therefore, maintaining this specified filling level is a standard practice in UST installation to ensure safety and compliance with regulatory standards.