Pumice is a light, porous volcanic rock often used for scouring, smoothing, and polishing. It is rock filled with tiny air pockets. As a result, it works well as an insulative layer. By premixing pumice rock with suitable rammed earth quality dirt at a 50:50 proportion, we have made earthbag blocks that weigh one-third their original weight. We haven't done any "official" tests on the insulative quality of these mixes. We only assume that with the additional trapped air spaces we are getting some kind of insulating effect (Fig. 4.16).
4.16: The pumice/earthbag cures hard and flat and holds together like a typical rammed-earth mix.
By combining the pumice with 50 percent earth, we are still able to tamp the mix into a compacted block that holds together like rammed earth. Filling the bags with pumice alone produces a lumpy bag full of loose material that refuses to compact while lacking the weight that we rely on for gravity to hold it in place. We prefer to maintain the structural integrity of the wall system first, and then figure out ways to address insulating options.
The pumice should be of a size range between three-quarter-inch (1.9 cm) up to one-inch (2.5 cm) in diameter. According to Tom Watson, the designer of the "Watson Wick" (a natural gray and black water living filtration system), a small-sized pumice will wick moisture up from the ground like a sponge, whereas a larger diameter pumice will drain moisture away. Scoria, another type of rock produced from vol-canism, may be substituted for pumice. Experiment with the ratio of earth to scoria to find what works best for your project.
Large quantities of pumice and good clean tam-pable earth can be premixed with a backhoe or tractor loader and heaped into a pile to be moistened, tarped, and ready for wall building as with a regular earthen soil. Extra water will need to be added to account for the increased amount absorbed by the pumice. For stem walls, or any place where moisture may be a problem, the pumice/earth combo can be stabilized with the same percentage of cement or lime suitable for the soil being used.
Design Considerations for Bermed and Buried Structures
For clarification, when we refer to a "buried structure," we tend to think more in terms of a sunken floor, at most about four feet (120 cm) deep. A "bermed structure" is usually buried into a slope (preferably with a southern exposure). A bermed structure can also be built at grade level on a flat plain with its north side buried by piling up earth around it, making a sort of man-made slope. A structure that is completely underground we refer to as "subterranean."
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