Beer Bottles, Arches and Eggs
I have never built a dome or a barrel vault with adobe. The closest (and it is distant) is an arch I built 30 years ago trying to prove that I could do it without temporary supports. It almost worked.
I was reminded of this shortcoming in my technical repertoire when a colleague emailed to ask my opinion on stabilizing domes after the 2003 - 2004 earthquakes in Iran. Sadly, I had very little to offer in the way of advice. The question did prompt some observations, however.
One of the gurus of earthen architecture is an Iranian by the name of Nader Khalili who wrote an interesting book called Racing Alone. (The title comes from Khalili's epiphany provoked by his son who, at a young age, said that he didn't like racing with other children, but preferred to race alone.) Khalili observed that the most stable buildings with which he was familiar were the old adobe beehive kilns used to fire bricks. He correctly surmised that the reason they survive earthquakes and the onslaught of weather is that the materials have become fused together by the high temperatures generated within. They have become, in fact, ceramic.
Khalili went on to build domed structures of earthen bricks that he fired in much the same way the kilns were. I love the experiment, but on a practical basis find the approach untenable. Affordable housing, even in the 1970s when this work was going on, had to conform to certain parameters, among them speed and ease of construction as well as the clever use of resources. I don't believe Khalili's designs fulfill any of those requirements with the exception of cleverness. But if you consider the resources it takes to fire the building once it is built, the cleverness test fails as well; firing whole houses is an abysmal use of fuel.
Nevertheless, there you are; in Iran the domes and vaults that were most likely to survive the quakes were kilns and fired structures.
Firing is not the only technology that proved successful in seismic Iran. Khalili makes some very telling observations about earthen buildings following the Tabas earthquake of 1978. One is that older construction outperformed the newer for several reasons, including the stability of the local lime/mud mortar called shefteh. Stability, it should be noted, does not mean rigidity. It means that although the local mix bonded very tightly to the blocks, in an earthquake it allowed those same blocks to survive by providing them with a zone that they could slide over without shattering. Energy was thus dispersed without the masonry coming down.
Communicating with my Iranian colleagues about earthquakes and adobe has led to some other fascinating tidbits. Abolhassan Astaneh, who is now an engineering professor at UC Berkeley, notes that two minarets serve as weight cylinders to buttress the flanks of the arch leading to the great domed mosque in Isfahan. Astaneh notes with some humor that this pattern is seen on massive structures throughout the seismic middle east and does not result from the development of early stereophonic callings-to-prayer. The twin minarets provide for much more stable buildings than those with only one.
Adobe is a brittle material with low span strength. That means that unlike reinforced concrete and some stone it cannot be used in cantilevers or left to support itself across openings. But that does not mean it is unsuitable in seismic areas. Built with wooden lacing in the walls as the Macedonians do, or with straw slip-joints as Eqyptian technology dictates, adobe can be very successful in earthquakes.
Finally, domes and vaults built with brittle materials are successful in large part because of their mass and weight, but also because of the inherent strength of the form. Many years ago I won a bet with an employee that the rear end of a pump-pulling rig weighing several tons could be supported on two beer bottles. The trick was to get it balanced without the bottles moving and shattering, a feat made easier by the built-in hydraulic stabilizing legs. Later, when reading Khalili, I came across a passage that caused me to recall the experiment. He notes the stability of the domes and compares them to an egg stood on end under a camel's foot. The brittle shell, thin as it is, can theoretically support the weight because of the inherent characteristics of the design.
Don't be deceived by architects and code enforcers who will belittle the structural qualities of adobe in a dome or vault. Think instead of beer bottles and pump trucks, eggs and camels when you go forth to build.