What should support underground piping be made of?

When considering the support for underground piping, compacted soil or sand is the most appropriate choice. This material provides a stable and even base that can effectively distribute the weight of the piping system. Compacting the soil ensures that there are fewer voids and reduces the likelihood of settling or shifting over time, which could lead to potential damage or leaks in the piping.

Dirt, while it may seem like a natural option, does not provide the same level of stability as compacted soil or sand. Loose dirt can shift and settle unpredictably, which could compromise the integrity of the pipe.

Gravel, though it might seem suitable due to its drainage capabilities, does not support the pipes uniformly and can allow for more movement compared to compacted materials, potentially leading to misalignment or breakage.

Concrete, while strong and stable, is typically not used as a direct support for underground piping due to reasons such as thermal expansion or contraction and the potential for cracking, which could cause additional stress on the pipes.

Therefore, compacted soil or sand offers the best balance of support, stability, and adaptability for underground piping systems.