What material should be used as the final layer in contact with the tank bottom?

• A. Clean and washed sand that is 3 to 4 inches thick
• B. Crushed stone mixed with sand
• C. Concrete slab at least 4 inches thick
• D. Asphalt-impregnated fiber board

Answer: (A)

The use of clean and washed sand as the final layer in contact with the tank bottom is favored due to its ability to provide a uniform and stable support for the tank. Sand is effective in distributing loads, helping to prevent settlement and uneven pressure distribution that could lead to structural issues over time. Additionally, the cleanliness of the sand minimizes the risk of introducing contaminants that could react with the tank material or the stored product. The thickness of 3 to 4 inches is typically sufficient to provide proper cushioning and support without adding unnecessary weight.

In comparison, while crushed stone mixed with sand can provide drainage, it may not offer the same level of stability and uniformity. A concrete slab, though sturdy, can be more rigid and may not accommodate any movement or settling that occurs naturally in soil, which could lead to cracking. Asphalt-impregnated fiber board could be useful in some applications due to its moisture-resistant properties, but it does not provide the same structural integrity and load distribution that sand does.