Hello there! I have an odd question for you. My partners and I will be digging (if all goes well) a large 55 acre pit that we want to use for storing barrels of oil in for a couple years before reselling them. I've done well trying to keep costs low but there's one part that I'm stumped on. Basically, we need some sort of roof on the facility but one that size can be very expensive. One thought I had is, instead of digging the hole and building a roof, we can dig it deeper as though it's a cave and use the dirt above us as the roof. However, I don't know how safe that is for such a large area. Is this safe or should we keep it an open pit that we then place a roof over? If the latter is the case, what kind of roof would be secure, waterproof, and cheap and how cheap, exactly, would it be?

Getting ready for the zombie apocalypse, eh? You're right - it's an odd project.

First, why not save a bunch of money and simply store the barrels above ground in an inexpensive warehouse?

As for a roof for your pit - you don't want one large, 55 acre roof. What you do instead is build supporting columns every 20 feet or so and use standard roof trusses between them. Rough guess is the roof would cost around $10/sq ft, or about $20 million or so to cover 55 acres. And no - your cavern idea is a non-starter.