How should the manhole be designed regarding the fill pipe?

Designing the manhole to provide flexibility regarding the fill pipe is important for several reasons. Flexibility in the design allows for movement that could occur due to soil settling, temperature fluctuations, and changes in liquid level within the tank. This flexibility helps to prevent stress on the fill pipe, reducing the likelihood of damage or failure over time.

Additionally, a flexible design can accommodate slight misalignments that may occur during installation or as the surrounding environment changes. It ensures that any mechanical stresses are minimized, thus maintaining the integrity of the tank system. Properly designed flexibility can also enhance maintenance accessibility, allowing technicians to work on the fill pipe more easily when inspections or repairs are necessary.

In comparison to the other options, which may restrict movement or complicate access, a flexible design is critical to ensuring long-term reliability and safety of the UST system.