Which material should NOT be present in the backfill for optimal storage tank safety?

For optimal storage tank safety, it is essential to ensure that the backfill material is free of rocks or debris. The presence of these materials can create uneven pressure against the tank, potentially leading to structural integrity issues. Rocks or debris might introduce sharp edges that can scratch or penetrate the tank's surface, leading to leaks or failures over time.

In contrast, materials like crushed rock, dirt, and sand are commonly used for backfilling because they provide a stable and supportive environment for the tank. These materials can help distribute weight evenly and allow for adequate drainage, further protecting the tank from potential damage. Selecting appropriate backfill materials is crucial in maintaining the safety and longevity of storage tanks.