To get a feel for the corbelling process, let's corbel a few rows of ten-foot long (3 m) tubes on the ground. Practice getting a "feel" for how tightly you can bend the tube into a curve. Tamp the first tube and measure the finished width and thickness (refer to Chapter 3 for techniques on laying tubes). You can omit the barbed wire for this test. Measure three inches (7.5 cm) in from the outside edge of the tube and draw a line on the tube for its full length. This line indicates how far the next row will be stepped in (corbelled). Fill and lay the next tube up to this marked three-inch (7.5 cm) line. Tamp down this second row and measure the thickness of this row. Did it get much flatter than the one below? Now try a third row, stepping it in four inches (10 cm). Tamp it and measure its thickness. If the rows tamp down significantly flatter the more they are stepped in, it's likely you have the right ingredients for following the compass recipe we have provided. Four inches (10 cm) is about the maximum that we feel comfortable overhanging (corbelling) a tube or bag (Fig. 12.1).
The stronger the corbel (that is, the further the rows are stepped in), the more likely the earthbag will be flatter than the preceding rows. The reason is that tamping forces more of the material into the part of the bag that has the least resistance, the part overhanging the previous row. Keep in mind there is a definite limit to the amount a row can be stepped in. This is determined by the width of the bags/tubes, the characteristics of the fill material, and the quality of the work being performed. Because of the soil, materials, skills, and other unforeseeable conditions that may present themselves, you as the builder/architect will have to adjust your design based on real life circumstances, rather than on what you have read in this book.
We used a variety of bag sizes to construct the Honey House, using the larger bags (way-too-big and 100-lb. bags) down low where they were easier to work with, and finished the dome off with narrower 50-lb. bags and equivalent-sized tubes. When building earthbag structures, and especially domes, use your largest bags near the base of the structure and progress to smaller bags as the walls increase in height. We want to distribute the weight of the whole building so that wider bags support the base that carries all the compressive force, while progressing to narrower and lighter bags towards the top.
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