This vessel is for containment only. It is not pressurized and it is sealed. The vessel is two piece ABS (most likely) plastic. The container serves as a vault. Imagine putting a sealed tuperware container underground. That is essentially it, but stronger of course. Just an anology.
If somebody knows of one, could you please point me in the direction of a standard that states the procedure for testing of plastic vessels underground in a trafficed area. I have ideas on how I might do it, but if there is something already established, I would much rather use that.