What About

Earthquakes?

We are often asked this question, especially by parties in California but also in many other locations. The question seems to imply that there is something about earth construction that is especially vulnerable to seismic activity. Perhaps this is the result of media photos of piles of adobe bricks in third world earthquake disaster zones.

Modern Earthbuilding

Like any other building material, earth walls must be designed to withstand the natural distrubances that are reasonably expected to take place in the location where they are to be built. Proper design is not intended to make a building "earthquake proof" (or flood proof or bomb proof, etc.); the objective is to make it resistant to some defined level of impact. Beyond that level, survivability is not assured or expected. There will always be earthquakes and other acts of nature that no building will survive. The obvious objective is to engineer structures so they are resistant in terms of the "normal" events of the location, while remaining affordable.

Just as with frame construction, or concrete based building materials, earth walls can be designed to meet or exceed expected seismic zone stresses. In modern times, two innovations have enhanced the integrity of earth walls:

• Reinforced concrete Footings and stem walls tie the bottom of the walls together, spread the weight over an area sufficient to support it and elevate the earth material above grade to avoid minor flooding.
• Reinforced concrete Bond Beams tie the walls together around the top (and also provide a strong anchor plate for roof trusses or beams (of which vigas are one example) that support the roof and also provide a lateral strengthening by joining opposite walls accross the open span.

Thinking back to the beginning paragraph of this discussion, it will usually be found that the mangled adobe remains illustrated in those images of unfortunate seismic disaster areas in underdeveloped countries came from crude buildings that probably had no substantial footing -- the adobes may have been placed directly on the ground -- and it is clear that no bond beam appears in those photos.

In areas where earthbuilding is commonplace, codes are available to specify how earth walls are to be built. Cast Earth is not yet included in the International Building Code, but builders should follow the code requirements for adobe and rammed earth and also obtain the certification of a licensed structural engineer.

Pre-Code Earthbuilding

Because earth construction is thousands of years old, a great deal of practical knowledge exists that has enabled the medium to survive seismic events over hundreds of years and more, even before proper footings and bond beams appeared. This in evidenced in the Western USA in the numerous adobe missions built as long as 500 years ago by the Spanish. These are plentiful in the Rio Grande Valley, Southern Arizona and along the central and southern coastal areas of California. On the West Coast they have obviously survived many earthquakes. Througout these regions there are also many old adobe homes that date back several hundred years.

To mention two prominent examples among many, one is not a mission: it is the Vallejo mansion complex at Petaluma, not far from San Francisco.

Another is the complex of clerical and lay adobe structures at San Juan Capistrano. An interesting bit of history and also some anecdotal evidence of the relative strength of earth compared to a high strength material (stone) is illustrated below:

	In 1800, the "Queen of the Missions", San Juan Capistrano, was completed under the direction of Father Juipero Serra. Because the mission was built of stone instead of the usual adobe, its builders probably expected that it would last through the ages. In 1812, this lovely structure was substantially demolished by an earthquake with the loss of about 40 lives. Immediately adjacent to the stone church is an older large adobe complex. Let's take a look at it.

Junipero Serra arrived in the area in the 1880s and built an adobe mission. Not only was there the chapel, but the complex included dormitories, kitchens, stables, shops and the accrutrements of an established frontier community. When the stone-built mission chapel was competed, it became the main religious focus -- until 1812, when the older adobe complex was left relatively intact as the lovely "solid as stone" new church collapsed. Fra Serra's old chapel remains in use today. In addition, many old adobe homes and commercial facilities dot the downtown area of today's San Juan Capistrano. Perhaps the Spanish friars and other settlers were better adobe builders than stonemasons. A contributing factor may well be the more flexible and less friable nature of adobe as compared to stone.

	None of this is evidence that an earth building will be more or less resistant to seismic impact than a "stronger" medium. However:
		It's hard to ignore the state of the beautiful old ruin relative to its even older two story adobe companion, which is still actively in use.

What about Rebar?

OK, so maybe it does look as if earth walls can take a beating. Why don't you just load them up with reinforcing steel like we do with concrete and make them much stronger?

Someday maybe we'll have an answer to that question but it will take some large scale controlled testing. Those of you who have followed the Cast Earth saga know there are some factors that make this seemingly obvious improvement questionable. Not necessarily wrong -- just sufficiently questionable as to suggest that it may even be dangerous to include much rebar in earth walls.

Why wouldn't putting steel in Cast Earth be an improvement? Maybe it is. But consider two physical factors first:

• Steel and earth have vastly different frequencies of vibration. It may well be that in an earthquake, the steel will flex and vibrate so differently than the Cast Earth (or rammed earth) that it actually will disbond from the substrate and help to shatter the structure. In concrete, the materials are more similar and the concrete is much more strongly bonded to the steel. Furthermore, concrete will crack from internal stresses if it is not well reinforced. Cast Earth is not a stressed material. Concrete will also crack because it shrinks upon setting, which adds to its stress. Cast Earth expands when it sets and does not sufffer shrinkage cracking.

• Earth and steel have very different coefficients of thermal expansion. As the materials change in internal temperature on daily and seasonal cycles, another source of instability may be introduced when the two materials move against each other during expansion and contraction.

Until the question is scientifically resolved, the preceding speculation is all we can use to discuss the topic. It is based on scientific principles, but it remains to be determined whether the theory is correct. In the meantime, recall that earth walls have stood the test of time without steel.