Concrete lasts longer than metal, especially under ground. However, in high moisture or heavy clay area’s it may need to be sealed and/or have gravel around it to act as a French drain. It should not be a problem in sand.
I’ve envisioned doing a concrete bunker myself, starting with a small culvert opening for crawl in access and opening up to a much larger one as an interior room. The round shape is structurally very strong and there are all types of “fittings” like 90-degree turns that add creativity.
The problem is they are expensive and very heavy and require heavy equipment to handle. More people are burying the shipping containers, which they can buy used for a few grand, and they work well and should last for 10 to 20 years.
A lower cost concrete alternative might be to pour a concrete bunker in place. There are molds you can buy, OR, a wild idea would be to dig a deep ditch to use dirt as the forms. Make the ditch a square, and pour the outer walls with provisions for a door. Then dig the center out about 18" deep and pour a 6″ thick roof with a thicker beam at the center (depending on bunker size). Then cover the roof with dirt and plant grass on it. Now tunnel down to the door and dig the dirt on the inside out. This is the hard part. A small crawl in culvert with a 90-degree turn here might be a good entrance. Then pour a floor sloped to the center with a drain made with post hole diggers and field line pipe. This will drain any condensation in to the ground and serve as a place to dispose of a hand grenade if necessary. Below are some pictures.
For step by step plans, e-mail us at PrepperHandbook2014@gmail.com
For additional information see the following links:
The Rule of 3 (set priorities by this)
Why we are ALL Preppers (for skeptics)
Food and water:
Getting Started (Water)
Edible Wild Plants:
Natural Disaster Preparations
Firearms and security:
Investing for Preppers (Financial Security)
Survival Pack (Security Patrol or Bug Out pack)
Wilderness Survival Pack (Video)