If you’re building is the same as the picture? I’m no t sure it’s built strong enough with the right thickness of boards 2x10 or 2x12 I would trust it to be safer! But the next concern is moisture/condensation or water. Which could cause dangerous mold to grow in your underground building.
If you ever seen the bootleggers on TV, one of them decided to build an underground still site. Unfortunately it didn’t go as planned when it rained everything underground got saturated and wet and had started growing mold everywhere. They had to scrap the whole project!
I would highly recommend digging down at least 2 ft deeper then what you want and hopefully digging a trench to the outside to drain off any excess water that may build up in your underground house? Fill in the floor up to the height you want with gravel so the water can drain through the gravel and out your drain pipe. If you’re ground is clay, the clay will hold water and won’t drain off very quickly without some kind of drainage system!
If you accomplish these two issues, I consider covering everything under ground ( floor and walls in heavy plastic before covering the walls or floor with any other materials? Plastic should help keep moisture down to a minimum! With you the best of luck!
This is actually something I have wanted to do myself for a storm shelter and storing food/veggies! But there’s a lot to consider before actually doing the work! I hate doing things that’s doesn’t work out and ends up a waste of time! That sucks!
