The bags we use are the same kind of bags used most typically to package feed and grain (Fig. 2.12). The type and sizes we use most often are woven polypropylene 50-pound and 100-pound misprints with a minimum ten-by-ten denier weave per square inch.
The companies that manufacture these bags sometimes have mistakes in the printing process that render them unsuitable to their clients. Rather than throw the bags away, they sell them at a considerably reduced cost. The 50-lb. misprint bags come in bales of 1000 bags and weigh about 120 pounds (53-54 kg) per bale. The more you buy the lower the price per bale. Prices for the 50-lb. bags average about 15-25 cents each, or from however much you're willing to pay to single-digit cents per bag for large orders (tens ofthousands).
The average, empty "lay flat," 50-lb. bag (the term used by the manufacturers) measures approximately 17 inches (42.5 cm) wide by 30 inches (75 cm) long. When filled and tamped with moistened dirt we call it a working 50-lb. bag which tamps out to about 15 inches (37.5 cm) wide by 20 inches (50 cm) long and 5 inches (12.5 cm) thick, and weighs 90-100 pounds (40-45 kg). The typical lay flat 100-lb. bag measures 22 inches wide by 36 inches long (55 cm by 90 cm). A working 100-lb. bag tamps out to about 19
2.13: The 100-lb. and way-too-big bags can also be used to surround the window and doorways in conjunction with the narrower 50-lb. bags/tubes for the walls.
inches (47.5 cm) wide by 24 inches (60 cm) long and 6 inches (15 cm) thick, and weighs a hefty 180-200 pounds (80-90 kg). In general, whatever the lay-flat width of a bag is, it will become two- to three-inches (5-7.5 cm) narrower when filled and tamped with earth. These two sizes of bags are fairly standard in the US. Twenty-five pound bags are usually too small to be worthwhile for structural purposes. By the time they are filled and folded they lose almost half their length. In general, we have not bothered with bags smaller than the 50-lb. variety.
Larger bags, up to 24-inch lay-flat width (which we refer to as way-too-big bags), can also be purchased for special applications such as dormered windows in domes or a big fat stem wall over a rammed earth tire foundation.
This provides additional support for the openings, while giving the appearance of a wider wall. By using the wider bags or doubling up the 50-lb. bags, we can flesh out the depth of the windowsills for a nice deep seating area (Fig. 2.13).
It has recently come to our attention that bag manufacturers have been putting what they call a "non-skid" coating onto the polypropylene fabric. These treated bags and tubes should be avoided. The "non-skid" treatment reduces breathability of the fabric, keeping the earth from being able to dry out and effectively cure. When inquiring or purchasing bags, be sure that the bags you order do not have the "non-skid" treatment applied.
Gusseted woven polypropylene bags are slowly becoming available in misprints. Gusseted bags resemble the design of brown-paper grocery bags. When filled they have a four-sided rectangular bottom. They are like having manufactured pre-diddled bags (refer to Chapter 3). The innovative boxy shape aids in stacking large amounts of grain without shifting. Someday all feedbags will be replaced with this gusseted variety and diddling will become a lost art.
Burlap bags also come in misprints. Burlap bags will hold up exposed to the sun in desert climates for a year if kept up off the ground, and as long as their seams have been sewn with a UV resistant thread. Otherwise, they will tend to split at the seams over time. In a moist climate they are inclined to rot. Stabilizing the earth inside them with a percentage of cement or lime could be an advantage if you want the look of a masonry wall to evolve as the bags decompose. Burlap bags come in similar dimensional sizes as the poly bags (Fig. 2.14). In the United States, they are priced considerably higher. The cost continues to escalate in the shipping, as they are heavier and bulkier than the poly bags. Contrary to popular assumption, natural earthen plaster has no discriminating preference for burlap fiber. Most burlap bags available in the US are treated with hydrocarbons. Some people have adverse physical reactions to the use of hydrocarbons including skin reactions, headaches, and respiratory ailments. Unfortunately, hydrocarbon treated bags are the type of burlap bag most commonly available to us in North America. Untreated burlap bags are called hydrocarbon free. The fabric is instead processed with food grade vegetable oil and remains odorless. Hydrocarbon free burlap bags require more detective work to locate but are definitely the non-toxic alternative. Perhaps as we evolve beyond our political biases, plant fibers such as hemp will be available for the manufacturing of feed bags. Bag manufacturers can be found on-line or in the Thomas register at your local library (refer to the Resource Guide at the back of this book).
The tubes, also called "long bags" or "continuous bags," are also made of woven polypropylene (Fig. 2.15). We use the flat weave variety rather than the style of tubes that are sewn on the bias. Tubes are what manufacturers make the feed bags from prior to the cut and sew process. Since they are not misprints the cost can be slightly higher per linear foot than the bags. The rolls can weigh as much as 400-600 lbs (181-272 kg) depending on the width of the material. They come on a standard 2,000-yard (1,829 m) roll, but sometimes the manufacturers are gracious enough to provide a 1,000-yard (914 m) roll. Tubes are available in all the same widths as bags. Tubes behave like the bags in that they lose two to three inches (2.5-3.75 cm) of their original lay-flat width when filled and
2.14: Burlap bags have a nice organic look that can be appreciated during construction.
Burlap bags are floppy compared to polypropylene bags. As a result, they tend to slip easily out of the bag stand while being filled. To avoid this annoying habit, pre-soak the burlap bags to stiffen them up prior to placing on the bag stand and filling.
THE ADVANTAGE TO KEEPING THE BAGS IN GOOD CONDITION ARE:
• In case of a flood or plumbing accident, the dirt will remain in the wall instead of a mud puddle on the floor.
• The bags are often easier to plaster over than the soil inside of them. An earthen wall likes to be covered with an earthen plaster that is similar in character. Sandy soil walls like a sandy soil plaster. A sandy soil plaster though, is not as resistant to erosion as a clay-rich plaster mix. Maintaining the health of the bag expands our plastering options.
• The bags provide tensile strength by giving the barbed wire something to grab onto. More bag, more grab.
tamped. Although 25-lb. bags are usually too small to use structurally, narrow 12-inch (30 cm) wide tubes (designed to become 25-lb. bags) make neat, narrow serpentine garden walls and slimmer walls for interior dividing walls inside earthbag structures.
Tubes excel for use in round, buried structures, free-form garden and retaining walls, and as a locking row over an arch (Fig. 2.16). Their extra length provides additional tensile strength for coiling the roof of a dome. They are speedier to lay than individual bags as long as you have a minimum crew of three people (refer to Chapter 3). Outside of the US, tubes also are available in burlap fabric and perhaps cotton. Our personal experience is limited to woven polypropylene tubes available in the US and Mexico.
Polypropylene bags are vulnerable to sun damage from UV exposure. They need to be thoroughly protected from sunlight until ready to use. Once you start building, it will take about three to four months of Utah summer sun to break them down to confetti. This can be a motivating factor to get the bag work done quickly with a good crew if maintaining the integrity of the bags is at all a priority. Most suitable rammed earth soils will set up and cure before the bags deteriorate. Even after the bags do break down a quality soil mix will remain intact. Still, there are advantages to keeping the bags in good condition.
While our little Honey House dome was still being finished a flash flood filled it, and all our neighbors' basements, with 10 inches (25 cm) of water. The base coat of the interior earthen plaster melted off the walls from 12 inches (30 cm) down. Since the floor had yet to be poured, the floodwater percolated into the ground.
The bags that were under water were soft enough to press a thumbprint into but not soggy. We supposed that under the extreme amount of compression from the weight of the walls above, the earth inside the bags were able to resist full saturation. As they dried out they returned to a super hard rammed earthbag again. The bag stabilized the raw earth even underwater. Had the bags been compromised by UV damage, it could have been a whole other story.
Nader Khalili had a similar experience in the sunken floor of one of his earthbag domes. Floodwater filled it about two feet (60 cm) deep for a period of two weeks. He documented the effects in conjunction with the local Hesperia building department and made the same observations we had. In essence, the bag is a mechanical stabilizer, as opposed to a chemical stabilizer such as cement, added to the earth. The bags provide us with a stabilizer as well as a form while still granting us the flexibility to build with raw earth in adverse conditions.
One way to protect the bag work during long periods of construction is to plaster as you go (refer to Chapter 13). Then, of course, there is always the method of simply covering the bag work with a cheap, black plastic tarp for temporary protection.
Another way to foil UV deterioration is by double bagging to prolong protection from the sun. Back filling exterior walls also limits their exposure to UV damage. It is possible to purchase woven poly bags with added UV stabilization or black woven poly bags designed for flood and erosion control. These will not be misprints, however, and will be priced accordingly. Polypropylene is one of the more stable plastics. It has no odor, and when fully protected from the sun has an indefinite life span. Indefinite, in this case, means we really don't know how long it lasts.
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