Step 4. Construct Earth Walls.
Construct two 4 x 8 earth walls; one at the front of the shelter and one at the rear. (The design of the earth wall is similar to the forms used to pour concrete walls.)
Measure, mark off, and stake the position of the two earth walls. (Detail 4.1) Verify the distance between the front and rear earth walls is precisely 80 feet apart (inside edge). Dig a small trench for the base of the earth wall. (Detail 4.2) This trench should be 6 inches wide and 6 inches deep. (Each sheet of plywood will be buried 6 inches in the ground to anchor it in place.)
Create two sidewalls for earth wall. (Detail 4.3) Sandwich the long side of a ¾ inch thick sheet of plywood between two 2 x 4s and nail. Turn the plywood around and repeat the process.
The next step is to create front and back walls. (Detail 4.4) Lay 2 sheets of plywood next to each other. Nail an 8 foot 2 x 4 to the plywood, 6 inches below the top edge. Nail an 8 foot 2 x 4 to the plywood, 6 inches above the bottom edge. Nail a 5 foot 2 x 4 to the center of the plywood. These 2 x 4’s will be nailed only to one side of the plywood and this side will face the outside of the earth wall.
Drill ten ½ inch holes through the 2 x 4’s and the plywood of the front and back walls. (Detail 4.4) On the top and bottom 2 x 4’s the holes should be drilled 6 inches from the edge of the plywood. On the middle 2 x 4, the holes should be drilled in the exact center of the sheets of plywood.
Raise the 4 sides of the earth wall. Make sure the tops are level and nail through the plywood into the sidewall 2 x 4’s. (Detail 4.5) You have now constructed the shell of the earth wall. Move the shell over to the 4-foot by 8-foot trench and place the shell in the trench. Verify the base of the shell will be buried 6 inches. If not, dig or fill the trench accordingly.
Cut 5 pieces of wire rope. Use 5/16 inch galvanized cable, technically called aircraft cable, but commonly called wire rope. This rope is rated for 1960 lbs. Cut the wire rope into 5 pieces each approximately 16 foot in length. (Wire rope can be difficult to cut. Generally I cut it with a hacksaw with a Carborundum blade. Another method is to lay the wire rope on a large rock and split it with a cold chisel and a hand sledgehammer.)
The next step is install wire ropes into the shell of the earth wall to provide the wall strength. (Detail 4.6) For this step you will need the 5 pieces of wire rope, 5 eye-to-eye turnbuckles and 10 wire rope clips. (I recommend using ½ x 9 inch eye-to-eye turnbuckles rated for 1800 lbs. There are several types of turnbuckles. Some eyes are formed by bending the metal to form a hook. These turnbuckles tend to be unrated or have a low rating and I do not recommend their use. The kinds you are looking for have cast eyes and have ratings.) Take each of the five eye-to-eye turnbuckle and open to obtain maximum spread. (Turnbuckle1) On the backside, loop each wire rope through it’s designated predrilled holes. Take the ends of the wire ropes and push them into the corresponding predrilled holes on the front wall. Place one side of the wire rope through one eye of an eye-to-eye turnbuckle and place the other side of the wire rope through the other eye. Use two wire rope clips to clamp each side of the wire rope securely in place. (Turnbuckle2) Each of these wire ropes will now form a loop. The rope should be snug. If there is too much play, loosen the wire rope clips on one side and pull it tight and retighten the wire rope clips. Turn the turnbuckle 3 or 4 turns to make the wire rope snug.
After the shell of the front and rear earth walls are constructed and in place, remeasure the distance between the two walls. They should be exactly 80 feet apart. If not, adjust. Fill in the small trench at the base of the earth walls and compact soil.
Use loose lumber to brace the sides of the earth wall. Push one side of the shoring support into the wall of the trench. Push the other side tight against the earth wall shell and nail. These braces will prevent the sidewalls from giving way during the earth wall backfilling operation.(Detail 4.7) (In step 7, the sides of the trench will be filled with granular soil, which will provide permanent support.)
Fill the earth walls with granular soil up to 6 inches from the top of the plywood, and compact this earth. Tighten the turnbuckles, to keep the walls from bowing.
On the inside wall install two vertical braces.(Detail 4.8) A 5 foot 2 x 4’s will be used for each brace. The braces must be installed one foot in from the edge of the plywood. The two braces will run parallel to each other. From outside edge to outside edge, the two braces must be exactly 6 feet apart.
Congratulations! You have completed the construction of the two earth walls that will become the ends of the personnel shelter.