A dome pushes outward at its base, so earthbag domes larger than 3 m in diameter need external buttresses or additional thick courses at the base to stop the walls spreading. Earth-berming the lower third of the dome also buttresses it and adds thermal mass, but buttress dimensions for earthbag are not established by test, so judge the size from the load and watch for cracking.
Part of Earthbag Building
Buttressing is the mass added outside a dome’s base to resist the sideways push that the curved roof creates.
The Dome Wants To Spread, And The Base Must Refuse
A dome carries its weight down along the curve, and at the base that force arrives as a downward push plus an outward push. If the base cannot resist the outward component, the dome spreads at the base and cracks open at the crown. Masonry domes are handled with a ring at the base that resists the spreading; the principle is explained in Dome Building, which describes a tension ring or thick walls and buttresses.
Earthbag walls resist this in three ways, per the parent article: thicker base courses, external buttresses, and earth against the outside. Barbed wire in the courses adds tension capacity in the ring direction, and that is its job in a dome as much as in a straight wall; see Barbed Wire Between Courses.
What You Need
- Extra bags and gravel for thickened base courses or buttress stubs.
- Spoil for berming, if the site allows it.
- Barbed wire continued through the buttress courses so they are tied to the wall.
Method: Choose A Restraint
Step 1: Decide by size. Domes up to 3 m are treated as inherently stable by the parent article. Above 3 m, plan restraint.
Step 2: Option A, thick base. Build the lower courses wider than the upper ones, so the wall base has more mass against the outward push.
Step 3: Option B, external buttresses. Build short wall stubs at intervals around the circle, tied into the dome courses with wire in each course.
Step 4: Option C, berming. Pile earth against the lower third of the dome. This provides buttressing and thermal mass, and the earth must be drained; see Underground and Earth-Sheltered Buildings.
Step 5: Watch for movement. Cracks opening at the base joints or around the crown mean the base is spreading; stop loading, add restraint.
Common Mistakes
- Buttresses not tied into the wall. They peel away.
- Berm without drainage. The earth holds water against the plastered dome.
- Ignoring size. Domes of 4 m and larger require engineering knowledge, as the parent article states.
Safety
Spreading walls fail suddenly
A dome that starts to spread can collapse without much warning. Do not shelter under a cracked dome, and prop and restrain it before repairs.
What’s Next
- Learn next: Finishing and Protection — plaster once the structure is stable
- Combine with: Bond Beam and Top Plate — the ring at the top of straight walls