Which of the following is NOT a common deficiency that affects the structural integrity of tanks?

Regular monitoring of the tank is essential for maintaining the structural integrity of underground storage tanks, making it a crucial practice rather than a deficiency. This practice allows for the early detection of issues such as corrosion, leaks, or structural weaknesses, ensuring that any problems can be addressed promptly to prevent failures.

In contrast, the other options describe deficiencies that can lead to serious structural issues if not properly managed. Using incorrect backfill material can create uneven support and increase the risk of tank movement or collapse. Similarly, inadequate or improper compaction of backfill materials can lead to settling over time, which can compromise the tank's stability. Voids under the lower quadrant of the tank can also result in uneven pressure on the tank, potentially causing structural failure. Therefore, while monitoring is a proactive measure that helps protect the tank, the other options represent conditions that negatively impact the tank's structural integrity.