You can build a cheap underground shelter almost anywhere. While codes vary, typically tiny uninhabited buildings, like storage sheds, are not subject to building codes. To qualify, the structure must be small, usually under 120, 150 or even 200 square feet, depending on the codes in your area.
Essentially, if you build simply and small, tons of possibilities open up.
One of the cheapest ways to build underground is with earthbags. $10 per square foot is possible. If you utilize recycled and found materials, construction costs can drop significantly from there.
As the name suggests, earthbags are large polypropylene sacks (picture the large rice bags sold at big box grocers) filled with dirt.
You stack earth bags like bricks, tamping them solid after each course. For added strength, strands of four-point barbed wire are placed between courses.
This building technique sounds chintzy, but has been around for thousands of years. Used primarily in desert climates, building with earth, or adobe, is still common in the American southwest, Mexico, the Middle East and arid parts of Africa.
There are a number of ways to build underground:
Digging a fully underground shelter requires the most work and is only suitable for dry, sandy, well-drained soil.
Partially underground building involves digging down a few feet, the covering the rest of the structure with dirt. This method is suitable for most locations and is a good way for the DIY builder to save the cost of a full excavation. Additionally, the excavated soil can be used to cover the rest of the structure, potentially eliminating the cost of having additional fill brought in.
Building a fully earth bermed underground shelter is good for rocky or clay rich soil, where the water table is high, or where flooding is a concern. This involves building the structure on level ground and piling truck loads of dirt on top.
While the topsoil in many areas is suitable fill for earth bag construction, you may want to weigh the pros and cons of digging it out of the ground yourself, versus having it delivered.
The fill material should be free of twigs, sharp rocks or other foreign matter, meaning you’ll have to sift topsoil before it can be used.
The other option is to have a product called road base delivered. It is already at an optimal sand to clay ratio and can save hours of backbreaking work digging and sifting.
To ensure the shelter’s walls are as stable as possible, the soil used to fill the bags should be about thirty percent clay and seventy percent sand, though recipes vary greatly among builders. Lime or Portland cement work well as stabilizers.
In any case, the fill should be slightly moistened prior to being shoveled into the bags to ensure it will set properly.
Though building with right angles is the norm, consider building in the round. A dome is extremely strong, allowing even load distribution, an important consideration when building an underground shelter. And since a dome is self-supporting, it doesn’t require expensive shoring or bracing like a straight-walled structure would.
Fortunately, earth bag construction lends itself perfectly to underground dome building. Earth bags are malleable, so can be shaped into curved walls with relative ease.
The reality of earth bag building is that it doesn’t require specialized training or building experience. A simple earth bag structure can literally be built with a few simple hand tools like a shovel, level and hammer.
It is backbreaking work though. And what you save in tools and material cost, you make up for in length of construction. It’s tiring, but not complicated, to build an underground shelter from earth bags.
For more detailed instruction, check out earthbagbuilding.com, run by Kelly Hart and naturalbuildingblog.com, run by Dr. Owen Geiger.
They’ve put together a treasure trove of earth bag building information, photos, diagrams, books, DVDs, how-to articles and videos and even earth bag building plans, many of which they offer completely free of charge. For instance:
How to Build an Earthbag Dome
Underground Shelter - Earthbag Plans
If you have the time, checkout the video collection below. (Just play the first one and you'll cycle through the playlist automatically.) The first is a basic, slide-show introduction to building an earthbag dome, followed by nearly 50 videos, with audio instruction, that cover all aspects of earthbag construction.
In addition to this wealth of info, they are just very nice guys who
are always willing to answer the constant stream of questions from
their eager readers.
If you are seriously considering building an underground shelter with earth bags, check out their sites.
And just so you know, neither of the above websites compensate us in any way. There’s good stuff there. Spread the word.