After the shell has been gunited and the pvc pipes been laid should the hole behind the pool shell where most of the plumbing lines lay be filled with sand or the original 'gumbo' dirt that was dug out of it?

After the shell has been gunited and the PVC pipes have been laid, it's generally recommended to fill the hole behind the pool shell with the original 'gumbo' dirt rather than sand. The compacted gumbo provides better stability and support for the pool structure, minimizing the risk of shifting or settling over time. Sand can shift and may not provide the same level of structural integrity. However, it's essential to follow any specific guidelines from the pool manufacturer or local building codes.