The construction of Nubian style adobe brick domes is ingenious, but extremely difficult to replicate with earthbags. We are limited to the corbelling technique, based on the nature of working with the bags themselves. By corbelling the rows of bags or tubes, we maintain a flat surface to stand on while filling and tamping the bags in place — meeting FQSS principles. As the bags or tubes are stepped in every row a little at a time (gradually decreasing the diameter), the walls eventually meet overhead to form the roof.
Earthbag domes need to be steep-sided as the rows can only be stepped in a specific amount that is determined by the size of the bags (or tubes) used. Corbelling an earthbag dome results in its characteristic shape (as tall as it is wide), resulting in an essentially lancet or parabolic style arch in the round (Fig. 11.9).
11.9: The Honey House, a corbelled earthbag dome in Moab, Utah.
All of the design strategies used for typical brick dome building are simplified when designing an earth-bag dome. The simplest way to provide the most solid form of buttressing is to begin the springline at or below grade level. The springline begins at the point where the first row of bags is stepped in. By beginning the springline close to the ground, we omit the need for a concrete bond beam as the surrounding earth provides us with a natural tension ring sufficient to buttress the perimeter of the dome.
We actually do two things to aid stability: lower the springline, and steepen the profile. By raising the profile to a steep angle, the horizontal forces are minimized. The steepness of the profile helps to direct the gravitational forces down towards the ground rather than out to the sides.
As a rule of thumb, earthbag domes are designed with a compass formula that produces a shape as tall as it is wide. For example, a 20-foot (6 m) interior diameter dome will also be 20 feet (6 m) high at its peak, from springline to ceiling. We have chosen a simple formula using an architectural compass that creates a subtle curve with a steeply pitched profile. For the sake of security coupled with the limitations of the earthbags themselves, we like to use this formula for designing a dome.
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