Drainage rock (aggregate, gravel) when placed correctly will give stability to the utility/irrigation box. Drainage rock placed underneath, inside and around the box will allow excessive water to penetrate, drain and prevent settling of the synthetic turf. If the Irrigation valves, connectors or pipe becomes damaged and starts to leak, the drainage rock will keep the area stable for years to come.
Many hardware stores sell all types of rock in small bags. This makes it convenient since you will only need a couple of bags for its completion.
Exit drains, French drains and Catch Basins must have stable compaction surrounding its inlets. Excessive water run-off that flood near drain exits will cause failure in the subgrade and will prevent the drain from being serviceable.