Installing Arch Forms

Placing the arch form on top of the box form is a relatively simple process (as explained in Chapter 6). Install the arch forms deep enough towards the inside of the dome to accommodate the angle of the corbelling process. Use the compass arm to level the arch form along its long axis, the same way it was used to level the box form below it. Use a level along the width of the base of the arch form to level it from side to side. The wedges used between the box and arch forms can be tapped in or out in all directions to achieve this level. Once you are happy with the level of this form, begin the next row, which will also involve the fan bags (Fig. 12.20).

12.19: Later, as the rows were stepping in three or more inches, we used the compass to draw a line on the outside edge of the previous row, and used the line and the compass as our guide for stepping in the following row.

12.20: Incorporate the barbed wire from the wall in between every fan bag around the arch forms, as shown in the step-by-step illustrated guide, and integrate tubes with bags against the forms.

The fan bags around the arch forms for a dome are built pretty much the same as they are in our step-by-step wall-building chapter. The main difference is that, because the slope of the dome roof is stepping in every row, so too will the fan bags (Fig. 12.21).

Continue laying coils up against the fan bags. The step-in increases as the profile of the dome curves increasingly inward.

12.21: Placement of the inside edge of the fan bags should be aligned with the compass.

If there is any wobble at all, check to make sure the bags and coils are snug up against each other and the forms. If the dirt is either too wet and soggy, or too dry, or if the moisture throughout the mix is inconsistent, the soil will not compact properly and will feel unstable. For safety's sake, take the time to customize the soil accordingly (Fig. 12.22).

As the dome gets taller, straw bales placed around the perimeter with boards on top make a simple scaffold that contours to the shape of the dome and protects the bags from UV exposure.

Be sure to check periodically to see that the compass pole is still rigid, as the higher the horizontal arm is raised, the more torque is placed on the vertical pole, pulling it out of alignment. Stacked sand bags around the base help steady it. So does adding rope or wire tie-offs at a level over head height (Fig. 12.23).

If the center pole is not long enough to reach the entire height of the dome, it can be extended by adding a coupling (a double-ended sleeve) that fits over the top of the center pole, and another pole can be set into it. Duct tape wrapped around the pipes helps create a snug fit.

secure compass with cross-ties attached to wire

12.23: You will definitely need to shore up the compass pole with cross-ties if a coupling is used. Raise these cross-ties as the wall gets higher, to add more stability.

12.22: After tamping, each row should feel as secure as a sidewalk.

12.23: You will definitely need to shore up the compass pole with cross-ties if a coupling is used. Raise these cross-ties as the wall gets higher, to add more stability.

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