Rammed Earth Homebuilding

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In very strong soils, the footing may be no wider than the wall itself, while in very weak soils, the footing may be twice or even three times as wide as the wall. Standard engineering guidelines assume soil strengths in the weak range, from 1,000 to 1,500 pounds per square foot (psf) bearing capacity On the other hand, site investigations and testing by certified geologists may discover soil strengths on the order of 2,500 psf or greater. A significantly stronger soil, one that could justify narrower footing widths, allows you to pour narrower footings, thus saving money on concrete costs. Soils reports can be expensive, however, so before contracting with a geologist, try to determine the potential savings in concrete you could expect.

Obviously, the larger the project, the greater the savings. The footing can either be poured separately or in conjunction with the stem wall. Where frost depths require a deep footing, it is simpler to excavate the trench to the precise width designated by the engineering design, install the required reinforcing steel, and then pour concrete to the specified thickness directly into the trench. Form boards or form panels can then be set on top of the spread footing to the dimensions specified in the engineering. Where footings are more shallow, it may be possible to complete the whole foundation system in only one concrete pour. In this case, forms for the stem wall are suspended above the footing trench on stakes which are pulled before the concrete in the footing has fully set.

Both the spread footing and the stem wall require horizontal reinforcing steel tied continuously around the perimeter of the building. Typically, three 1/2-inch rebar are spaced evenly across the bottom of the footing trench, and two 1/2-inch rebar are suspended three inches from the top of the stem wall. Intermediate horizontal courses of reinforcing steel are required if the distance between the top and bottom steel exceeds 18 inches. Unless engineering design or special seismic considerations warrant it, vertical reinforcing does not extend above the top of the stem wall.

The strength of the connection between the earth walls and the concrete foundation is a result of both mechanical bonding and the weight of the wall. To improve the mechanical bond between the base of the walls and the top of the stem, the concrete should be left very rough after it is poured. The deformations in the concrete serve as a field of mini-keyways into which the earth is locked as it is compacted. Where earthquakes are a hazard, the tops of the walls are tied together with a bond beam, poured directly on top of the earth using the formwork. Below we see concrete flowing from the grout pump and a finisher using a vibrator to ensure consolidation.

The Art of Formbuilding

Formwork is a major part of building with rammed earth. In fad, the time spent setting, aligning, and stripping the forms is usually greater than the time spent transporting and compacting the earth within the forms. For this reason, the efficiency of the forming system is key to the quality and affordability of building with rammed earth

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