If you work with Fextex on your project, you can bury the fiberglass tank at least seven feet. We commonly see tanks being buried much deeper, 15'+ with proper engineering and installation. Not only should the structural integrity of the vessel be taken into consideration, but just as importantly, please consider your accesses. Are you going to be using PVC risers or fiberglass extensions? How many do you have, and how can we assure they stay watertight? These are often times the weakest link and most overlooked item of the installation.
How much does an underground fiberglass tank cost?