Using the drawing as a guide, adjust the compass inward to match the calculation for the first row to be corbelled. In our case, we shortened the compass arm one-half inch (1.25 cm). This corresponds to a total step-in of one inch (2.5 cm) for the full diameter of the dome. As is shown in the drawing, the profile of the dome steps in very gradually at first — almost imperceptibly (Fig. 12.17).
That's OK; it's a cumulative kind of process. At this point we began laying coils (tubes) for the walls, in conjunction with individual bags up against the forms.
With the compass adjusted to accommodate the first step-in, we loaded a 20-foot (6 m) tube chute, tied off at the bottom from the inside (see Chapter 3). Before laying this coil, it's best to go ahead and do the bag work around the door and window forms first and any buttressing. Get all the bags gently secured with mild tamping. Then begin laying the coil as snugly as possible up against the bags. After laying the coil, tamp it, and all the bags around the forms, until hard (Fig. 12.18).
12.18: Corbelling the roof is done by stepping in each row the specific amount determined by the compass profile.
The trick to laying a coil is to have plenty of dirt available to fill the tube quickly so as not to tire out the person holding the tube chute — your human bag stand. We also found it helpful to place sliders under the feet of the human bag stand, to keep them from sticking to the barbed wire, and to make it easier to position the tube accurately according to the compass.
Our goal was to step-in the rows, but to leave enough of a gap so that when they were tamped we would still be able to swing the compass arm around without it getting stuck on a slightly protruding bag. It's a good idea to avoid having to smack the bags outward to make the compass fit, as this is more apt to disturb the compaction of the enclosed dirt and loosen the fabric, deform the shape of the coil, and decrease its woven tension under compression (Fig. 12.19).
From this point on the procedure remains the same. Adjust the compass for the next row. Install any needed strip anchors next to the forms. Install barbed wire. Install bags around box forms, remembering to hard-ass these bags if not installing strip anchors. Lay coils in manageable lengths that will permit you to overlap the seams of the previous row. Fit the ends of coils tightly against each other by adding a couple of extra cans of dirt and shaking the dirt down well. Twist the end of the fabric tight and neatly tuck it under itself. Tamp the whole row. Repeat this process until you get up to the height where the arch forms need to be installed.
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