The field distribution piping should be surrounded with ______ and at least 2 inches deep under the pipe.

The correct answer is to use washed gravel that ranges from 3/4 inches to 2 1/2 inches in size for the field distribution piping. This type of gravel is optimal for drainage and support of the pipe. The properties of washed gravel allow for adequate water flow while preventing soil migration into the piping system, which is important for maintaining the functionality of the drainage system.

Washed gravel specifically is effective because it is free of dust and finer particles, which ensures that the voids between the rocks remain open for water to flow through easily, minimizing the risk of clogging. Furthermore, surrounding the pipe with a minimum depth of 2 inches of washed gravel provides proper support and stability, protecting it from potential damage due to surrounding soil pressure.

Other materials listed, such as broken limestone or fine marble chips, do not possess the same drainage properties as washed gravel, and they may not provide the structural stability needed when placed around piping systems. Additionally, a mixture of sand and gravel can lead to compaction and limit water flow, which could result in drainage issues.