I want to build an underground storage cellar 9' wide x 14' long x 8' deep. Can I dig (with a mini-excavator) and then basically build a shed in the hole? Then build a deck over the top? I understand the surrounding earth exerts pressure on the walls. Will simple 2x4 stud wall and plywood siding will be able to withstand the pressure? Are 2x6 studs needed, or even enough? Or is it necessary to build cement block walls?