This article is the second in a series discussing the ongoing spiral straw bale project. Benjamin Pinover is co-writer of the series, as well as co-builder. In this article we discuss a number of possible foundations for natural buildings, focusing on the rubble trench technique that we used for this one.

The foundation is the most important part of any building, representing the highest embodied energy and structural load of any part of the building. The foundation roots the building to the ground, granting a stability often lacking in ordinary soil. In cold climates, freezing temperatures will heave the soil, while expansive soils rich in clay or sand will expand or contract depending on moisture content - hence the need to stabilize a building by digging down below the frost line or creating a solid footing in loose soils.

Conventional buildings use concrete, a mixture of cement and aggregates such as gravel and sand, to attach the building to stable soil. Concrete is the most common material used in foundations, but it is also one of the most ecologically costly. 

• The worldwide manufacture of concrete consumes 1.6 billion tons of Portland cement, 10 billion tons of sand and rock, and 1 billion tons of water each year, making the concrete industry the largest user of natural resources in the world. 

• The open pit quarrying of the raw materials needed for concrete can result in soil erosion, pollutant runoff, habitat loss, and ugly scars on the landscape.

Portland cement is the third most energy intensive material used in construction, after aluminum and steel.  Its embodied energy is about 2400 BTU per pound, or 5.6 MJ/kg. The total energy embodied in a cubic yard of concrete is about 2,600,000 BTU.

The cement industry produces massive amounts of air pollution: For every ton of cement clinker produced, a ton of carbon dioxide, a greenhouse gas, is released into the atmosphere.  This is about 7 percent of the total release of greenhouse gases in the world, an amount equivalent to the output of 330 million vehicles" (427, Allen).

Cement is a wondrous material, a liquid stone which can be formed to nearly any shape and which will harden quickly. However, there are several ways to limit the amount of cement that is used, and even a few ways of avoiding cement altogether.

A good foundation has two functions: it should keep water from entering into the building itself, and it should stay put. Cement does both of these things, by blocking water from draining past it and simply by being a massive anchor. However, a combination of techniques can be used to achieve stability and protection from water. For proper drainage a common technique is to replace the soil below the structure with gravel, or small rocks. Gravel allows water to drain away quickly, while providing a uniform footing for the solid mass of the foundation. On top of the rubble trench goes something extremely heavy and firm, something massive, that connects the foundation to the wall. This part, called the stem wall, can be made of stone, earth-bags, urbanite (recycled concrete), or a small cement cap piece.

 

For my building, I began digging in August of 2006 and continued until late September.  Each pick strike and shovelful brought me closer to my goal, but in the monotony and rhythm of the task, I found a kind of satisfaction.  A satisfaction that was of course mitigated by sore muscles and an aching back.  Ultimately, digging a foundation is the heaviest work involved in natural construction, and, on larger projects, the use of heavy machinery is the only economical way to go.  A backhoe can dig a complete foundation for a small house and fill the rubble trench with gravel in a day or two, by way of contrast, I spent nearly two months digging and gathering small stones.  In those two months, however, I worked each day in the open air on the site and gained a much deeper connection through the process of digging, smelling the dirt, listening to the quiet of the woods, working slowly, gradually, adapting the form of the building as I went; all of which the use of heavy machinery would have completely destroyed.

For the straw bale spiral we could not use a backhoe even if we had wanted to, as tightly curved shapes are not what heavy machinery do best. We only needed a trench underneath the load-bearing portions of the building, which are the straw bales and heavy posts. So we needed a two foot wide trench that spiraled around itself for 60 feet. Fortunately the Texas soil is akin to sandy flour, so digging 20 cubic yards of soil in a spiral was not unpleasant at all. Each shovelful was substantial, and at the bottom of the three-foot trench we hit some compacted soil of clay, sand and small rocks. The trench was lined with landscaper's cloth: a fibrous mesh that prevents soil from washing into the drainage and keeps roots from growing among the gravel. At the bottom of the trench we laid a drainage pipe that spirals down the slightly inclined trench. A day later, 18 tons of compressor rock (¾ inch to 2 inch rocks that wedge together under pressure) made the site look like the four days of digging had never occured.

On top of a rubble trench sits something massive. We ruled out one good technique, rammed earth tires, as being incredibly labor intensive. Also, I had health concerns about the brake pad asbesos powder that the tires may have consumed.. Another interesting alternative that we could have used is urbanite.  Urbanite is an extremely available and ecologically friendly material, using a ubiquitous waste product of every new construction.  Chunks of concrete, especially from waste pours at large building sites or road/sidewalk construction, are often of uniform thickness and have at least one and usually two flat edges, making them stable when stacked and easy to work with.  Urbanite foundations can be built like free standing stone walls or stacked inside a form to decrease the volume of poured concrete necessary. In this case, since the straw bale project is in a rural area, transporting urbanite to the build site would all but negate the positive impact of reusing a waste product.

We decided that given the weight of the bales and roof planned, a cap piece of concrete was advisable, as opposed to earth bags. We made form boards by staggering thin, flexible boards and then staking them into the ground. It is interesting to note that as the form boards were leveled around the spiral, they untwisted and stood rigid in their curve. As the last board was leveled with stakes, the curves of the forms were graceful and secure. Inside the forms we put steel remesh, which bonds to the concrete reinforcing it against cracking. Along the tight curves and corners, the steel taken from the side of the dearly departed bus added extra reinforcement.

With the form boards ready, we contracted the local concrete mixer to come with a truck and pour the cement cap piece. The mixer uses a 15% fly ash mix with limestone available locally for aggregate. However, disaster struck when the cement truck, only slightly larger than the dump truck that delivered the stone, could not navigate the turn into the build site. We decided that the best option at this point was to rent a mixer and make the cement ourselves.  As it turned out, mixing our own concrete allowed us to use excess stone from the rubble trench, as well as some of the sandy soil we removed while digging the trench.

We waited until after the sun's heat to start mixing; pouring buckets of rock, sand, Portland cement, and water into the loud grinding mixer. As we scooped the cement around the spiral, we saw the thin form boards beginning to bulge. Since thicker plywood would not bend, we had used thinner stock. Fortunately, the wood bulged between the stakes but did not breach. As we continued around, however, we soon realized that a few inches of bulging would increase our estimated volume substantially, and another harried cement run needed to be made. The frenzied pour lasted into the night, with car lights illuminating the frantic scene. As we poured we smoothed and leveled the concrete, placing the brackets and anchor bolts that would later secure posts and bales to the cap piece. The end result was slightly bulging, slightly bumpy, but solid, secure, and strong.

sources

http://www.concretenetwork.com/concrete/whatis/  

http://www.dancingrabbit.org/newsletter/Newsletter0898_Foundation.php

Natural Capitalism: Creating the Next Industrial Revolution, by Paul Hawken, Amory Lovins, and L. Hunter Lovins,   2000