Buy a large home sized propane tank. It comes with "feet" upon which it normally sits. After the trench is dug I plan to dig 6 foot deep by 2 foot round holes where each "foot" will go. Insert ample rebar then pour a slab 6 to 8 inches deep the length of the tank, filling the 6 foot holes. While the cement is still wet place the propane tank on the still very wet slab so that the feet sink well into the cement and then put more rebar under the cement surface but over the top of the "feet". After the cement cures the propane tank will be very securely anchored. I'll then fill in around the tank with only about 4 inches above the surface.
I'll then take a cutting torch and cut a "hatch" in the top angling the cut angle "outward" at a 45 degree angle all around the hatch circle. This will cause the "hatch" to fall inside the tank, and because of the angle with which it was cut, it will be impossible to remove the hatch cover from inside the tank. You eventually want the hatch to open from the inside with no possibility to open toward the outside, i.e. during firestorm when air valves are closed and low outside air pressure could "suck" the door open. When fitted closed from the inside it cannot open outward because of the angle of the interface between hatch and tank. Secure the hatch cover by welding hinges and latch from inside the tank.
Air vents will be essential. Two holes will be cut towards both ends of the tank on the top with a diameter of 6 or 8 inches each. Through these holes will be welded 1 foot high pipe segments. The pipe will have a wall thickness of 5/8 inch and extend into the tank 2 inches. On top outside of the pipes will be welded 3 equally spaced around the circumference 2x1x1 inch steel "stand offs". The 2 inch side will protrude 1 inch above the top of the pipe. Two "caps" will be fabricated by using 8 or 10 inch diameter 5/8th inch pipe cut into two 3 or 4 inch lengths. To one end of each of these "caps" will be welded 5/8th inch flat steel cut to the outside diameter of the "cap" pipe. These "caps will then be welded to the standoffs providing an air opening that will not allow rain to enter the pipes, even when driven by high winds.
Inside the tank, the protruding ends of these air pipes will be connected to valves which can be completely closed during any firestorm so that no air will be "sucked" out of the tank. During normal (no fire storm) time the valves will be fully open. To the other end one of the valves will be installed a hand cranked air blower normally used to provide a high flow of air to a steel forge. This will be used intermittently to refresh the air supply inside the tank. Also provided inside the tank will be a carbon dioxide detector to warn when the air is becoming "stale".
The tank will be set with a slight slope toward one end. Before the tank has been set in cement, a 2 inch hole will be cut into the bottom of the tank at the "low" end and a two inch diameter by 1 foot long pipe will be welded. The other end of the pipe will lead to a "dry well" 6 foot deep and 2 foot diameter filled with gravel. This will be outside the area to be cemented. This will provide a "drain" to the bottom of the tank to drain away condensation, gray water, and urine. The bottom of the tank will be fitted with a wood plank floor which will be 6 inches above the bottom most curvature of the tank. The planks will rest snugly against the curvature of the inside wall of the tank and 1X1 inch angle iron will be welded to the tank sides so as to hold the planks down snugly. The tank will be provided with at least 1 or 2 weeks supply of water and food. In addition, it will be provided with heavy mattress material on which to lie along with straps with which to secure myself in place in a prone position.
This shelter will be used until all chance of firestorms are past. It will also serve as a refuge in the event of severe weather.
Offered by Ron.