An underground bunker can keep your family safe in the event of tornadoes and other natural disasters. If you are worried about surviving a nuclear accident or attack, the bunker design below is specifically built to protect against radiation poisoning. Before you build, you'll need to know how much space and what necessities you'll need for the amount of time you expect to stay in your bunker.
Excavate the pit for your underground bunker with a backhoe. You want the pit to be a minimum of 2 feet deeper than the height of your bunker, and 2 feet wider than the bunker's width. Excavate an area of your pit on one side that is deeper than where you plan on having your shelter. This will serve as the leach field for your sewer system.
Set your survey stakes into the bottom of the pit to mark the bunker's outside corners. Tie a string between each stake so you can see where the bunker's walls will be.
Excavate trenches along the outside edge of the bunker walls you have marked with the survey stakes and string. Make the trenches about 1 foot wide and 1 foot deep. Fill the trenches with 6 inches of gravel. This will serve as drainage for your bunker, keeping moisture from the surrounding earth from penetrating your shelter.
Hand-dig a shallow trench from your slab to the leach field and run a PVC pipe so that it extends horizontally from where you want to place your toilet on the slab, then follows a downward grade to the leach field. Backfill the leach field when you are done.
Set your 2-by-6-foot boards along your string line and stake them into place upright. That will make a box that is 6 inches deep. Place your wire mesh in the box and pour concrete over this until the box is full. Use a piece of wood to level the concrete and smooth it. That will be the floor of your bunker.
Snap lines on the dry concrete floor with your chalk line that show where the walls of your underground bunker will be. Mark the outside of the wall and not the center. Mark where the door will go and stand the door frame up, using the wood from the concrete slab to hold it in place.
Lay out the cinder blocks and see whether you need to cut any block to make it fit. Make sure you leave about 1/2 inch between the blocks to allow for the mortar. When you have your block layout, mix the mortar according to the instructions on the bag and begin to lay your courses (a course is one layer of block). Every four feet of height, stop laying block, cut a series of pieces of the No. 4 rebar, 6 feet in length. Drop the bar down into your wall every 8 inches. Fill the wall with concrete, keeping the rebar centered in the block. When the concrete has set, begin laying more courses of block using the rebar that is stocking out to "lock" the sections together. When you are on the last course, the one that will meet the roof, stop and do not fill it with concrete yet.
Cut joist pockets into the last course of block that will allow you to lay a 4-by-4-foot joist from wall to wall to support the roof. Place the joist pockets no more than 36 inches apart. You will only have to do that in two opposite walls, not all four; pick the two walls that have the least amount of distance between them. Build small wooden boxes out of your 2-by-6-foot wood and place them in the joist pockets you have made to keep the concrete from filling them.
Two courses down from the course you cut your joist pockets in, lay out where you want your vent pipes to pass through the wall, core drill the block and put a small section of PVC pipe through to prevent the space from being filled with concrete. Now, cut more rebar that will end about 2 inches below the top of your block, drop it in place and fill the last section with concrete. Let the concrete dry.
Pull the wood boxes and PVC pipe sections from the wall. Lay your joists in, from wall to wall, until all your joist pockets are full. Run support joists from the floor of your bunker up to the roof joists. Bolt the joists together using mending plates. Run a series of vertical joists directly up along your block wall; bolt the joists directly into the wall.
Run a section of PVC pipe through the vent holes and up beyond the surface level of the earth at least 8 inches. You will need to bend the pipe with a PVC bender. Install gooseneck hoods on the pipe end that will be above ground.
Lay your plywood sheets across the roof joists and nail into place. Then lay two layers of block. They don't need to be mortared; just butt them tightly together. Cover the roof with a thick layer of earth, making sure the earth fills the blocks.
Hang your steel door in the door frame
Place your ladder from the surface of the ground down to where the entrance of your bunker is. Backfill the hole with earth, leaving an area clear around the entrance.
Before you pour the concrete slab of your underground bunker, use a small core driller to drill a pipe into the earth in a corner of your slab. Drill until you hit a water table and then attach a hand pump to this. You will have a constant supply of fresh water. Extend your walls and roof to cover your ladder and install a trapdoor to cover the ladder entrance.
Do not place your vertical roof bracing farther than 36 inches apart or the roof might collapse under the weight of the block and earth.