Foundation

There's a wonderful abundance of old adobe ruins scattered throughout the lonely places in New Mexico. Trying to reconstruct how they came to such an end can be informative. One observation that becomes apparent is a failure on the builder's part to have given the structure a proper foundation.

Think of it. If you're planning an earthbag structure - let's say 10' x 10' with eight-foot high walls - and if the bags average 25 lbs each, you'll be using approx. 1,000 bags with a combined weight of about 24,000 lbs (12 tons, or nearly 11,000 kilos). The compression of such a weighty structure may well cause differential compaction on the ground, depending on the ground's makeup (rocks here, sand there)... which may over time cause your structure to start sagging. You may not notice this until your plaster starts cracking and your doors & windows start jamming.

Your goal should be perfectly vertical walls, with the stress load being exactly perpendicular to the ground. The easiest solution is digging a trench where your walls will be, and filling it with something that won't shift or compact over time. Good, available choices include gravel, rocks, or small, broken-up chunks of concrete rubble.

How deep? It depends on the size of your structure & how solid (or mushy) your ground is, but 18"-24" (45-60cm) is a good size. If you want to dig it deeper (which might incidentally allow your first few courses to be below ground level, sitting atop your foundation material), then so much the better - doing so will increase your walls' stability. If your fill is irregularly shaped (such as rubble), it'd be a good idea to pound it with your tamper or a sledge, so the chunks will "lock" together & have less likelihood of shifting after bearing weight. Additionally, you can mix & pour concrete over your fill, adding enough water to the mix that it will seep down & fill the void spaces before it sets up.

---

Sifting

A good place to start once you've identified your source of dirt is to clean & sift it. At the least, you'll want to remove & pick out:

• large sticks & branches - any pokey items that will get in the way of filling your bags (and that might risk puncturing your bags);
• sizable rocks, clumps, and foreign objects;
• anything substantial that might rot out over time & leave a void in your bag (wood scraps, leaf clumps); 
• roots (that might start growing).

A simple way to do this is to construct or purchase a sifter, which is - at its simplest - a wood frame sized for a wheelbarrow (approximately 25" x 36", or 64 x 92 cm). A sheet of stiff wire screen is then stapled to it. For moderate sifting, we use one with a 1/2" (1.3cm) mesh.  

Placing the sifter over the wheelbarrow, we'll throw three or four shovelfuls on the screen at a time. Setting aside the shovel, we'll then brush the dirt/etc. across the screen (first with the shovel blade, then by hand. Gloves are a good idea). The sifted dirt falls into the wheelbarrow, while the stuff that remains can be sorted.

Rocks get tossed off to one side (useful for your trenching); wood scraps (if any) and branches & twigs get tossed off to another side, for burning or recycling; insects & worms can be gently removed, and hidden treasures are often revealed (old coins, lost items, etc.) . In my part of the country, the clay often forms into hard clods. This clay is good stuff, and I toss it into yet another pile to later tamp into dust & mix with my soil to enrich its binding qualities.

What's generally left on the screen is what we call "duff", organic material that we then pour into black plastic garbage bags for composting. What's left in your wheelbarrow should be pretty clean, well-sifted dirt.

To make plaster, you'll want to build or purchase another sifter, one with at least 1/4" (0.6cm) mesh or finer, and then run your already-sifted dirt through it for finer results. (Finer meshes will slow you down, but will be much nicer & cleaner.) You can then add water, sand, straw, and/or whatever else you like.

---

Planning

---

1 bag	3 in. high		1 bag length	16 inches long
4 bags	1 foot high		2 bags (end to end)	32 inches long
8 bags	2 feet high		3 bags (end to end)	48 in. (4 ft) long
12 bags	3 feet high		6 bags (end to end)	8 feet long
16 bags	4 feet high		9 bags (end to end)	12 feet long
20 bags	5 feet high		12 bags (end to end)	16 feet long
24 bags	6 feet high		15 bags (end to end)	20 feet long
40 bags	10 feet high		18 bags (end to end)	24 feet long
*** Note: see comments on "buttresses" under Wall Length (below)

---