What is an essential characteristic of materials placed under the tank shell?

• A. They must be lightweight
• B. They need to be impervious to moisture
• C. They should allow for rapid drainage
• D. They must possess a high thermal conductivity

Answer: (B)

The essential characteristic of materials placed under the tank shell is that they need to be impervious to moisture. This is crucial in order to prevent moisture from penetrating the underlying structure of the tank, which could lead to corrosion and degradation of the tank's integrity over time. Moisture can exacerbate issues such as corrosion in both the tank material and the subsoil, potentially leading to leaks or failures. Using impervious materials also helps in managing environmental concerns, as any leakage from the tank can be contained and mitigated more effectively.

Choosing materials that are moisture-resistant provides a stable foundation for the tank, ensuring that it will maintain its structural integrity throughout its operational life. This is particularly vital for tanks storing oil, where leaks could have significant environmental impacts.

Other characteristics listed, like being lightweight or having high thermal conductivity, although useful in different contexts, do not directly address the primary concern of moisture infiltration and its effects on tank stability. Similarly, while allowing for rapid drainage can be beneficial in some cases, it does not directly equate to the need for moisture resistance, which is the key focus for materials under the tank shell.