No. 47, May 2000
Desert Architecture for a New Millenium
by Mary Hardin
"As instructors ... worked ... to develop a forming system that would allow their students to build the classroom facility, the wide applicability of such a system became apparent. Rammed earth wall systems are currently fairly expensive, as the necessary formwork constitutes a major investment and the labor is specialized.... An alternative method of forming walls incrementally, with formwork that could be managed by two or three people and then reused, was necessary for low-cost building." |
This case study illustrates a series of university/community collaborations leading from research idea, to full-scale improvisation in a Design/Build Studio at the University of Arizona (UA) School of Architecture (SoA), to a significant application in the Gila River Pima community of southern Arizona. The Design/Build faculty and staff of the SoA were interested in learning the parameters, limits, and potentials of building with rammed earth, a construction method very recently adopted into the Tucson, Arizona, municipal building code. As common with building codes, the text on rammed earth provides performance criteria but no recipes. Without a body of knowledge or experienced local tradesmen to provide instruction, novices must conduct full-scale experimentation. Questions about soil composition, forming methods, and strength and plastic tolerance began to shape a research agenda. True applied research projects, however, require actual opportunities to build. A series of collaborations(Back to top) In 1997, the instructors destined to lead students through construction of the chosen structure of rammed earth and insulated concrete block began to face the realities of functioning as building contractors with little budget for equipment and overhead. A major obstacle was the need to accomplish rammed earth work without investing in the expensive commercial formwork commonly used in contemporary projects. This led to a research goal that would eventually affect the community beyond the University itself. As instructors Richard Brittain and Associate Professor Mary Hardin worked with SoA shop master Mark Perry to develop a forming system that would allow their students to build the classroom facility, the wide applicability of such a system became apparent. Rammed earth wall systems are currently fairly expensive, as the necessary formwork constitutes a major investment and the labor is specialized. Contractors focusing on rammed earth construction form the entire building at once with steel-reinforced forms typically used for poured-in-place concrete, and tamp the earth/cement mixture in a brief, intensive period. An alternative method of forming walls incrementally, with formwork that could be managed by two or three people and then reused, was necessary for low-cost building. The efficiency of the large-scale forming could be traded for the manageable system if labor was plentiful and cheap. The problem of developing a low-cost forming system for the Design/Build studio was thus identical to the challenge of bringing rammed earth into the affordable housing arena. Rammed earth construction has positive thermal, environmental and aesthetic attributes. A historical building method in southwestern North America as well as in Central America, South America and elsewhere, it faded from use in the Southwest for hundreds of years but has recently been revived as a construction alternative for custom homes and other elite projects (Easton, 1996). The loadbearing system requires wall thicknesses of 12 to 24 inches (30.5 - 61 cm) which may taper in section from base to top. Having almost no insulation value, rammed earth walls instead provide thermal mass, slowing heat transfer from exterior to interior spaces during the day and from interior to exterior spaces at night. The rate of heat transfer through a rammed earth wall is about one inch (2.5 cm) per hour. In the desert, this thickness prevents the sun's heat from reaching the building's interior before nightfall. Substantial drops in air temperature at night cause the walls to cool off again before sunrise. The possibility of gleaning most of the construction material from the site adds to the economic and environmental benefits of rammed earth. Currently, however, the high overhead cost of forms and scaffolding as well as the high labor investment make it unaffordable for most people. Research into ancient forming methods, soil composition, and wall dimensions led to speculation about a contemporary construction system that could once again be employed in the vernacular architecture of the region. The specific challenge of designing formwork for the University classroom facility had implications for further, and ultimately more significant, research. Several rounds of formwork design and test walls prefaced the Design/Build Studio. The formwork research |
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(Back to top) The earth mix research(Back to top) The classroom facility(Back to top) |
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Developing a working method with the rammed earth forms and earth mixing equipment required intensive experimentation. Initial setting of forms and squaring, plumbing, and clamping was tedious until a logical sequence became obvious. Incorporation of small chamfer strips to create reveals between the rammed earth and concrete was time-consuming and caused logistics problems. For example, the steel angles screwed into the inside of the formwork at the top of the walls had to be site-welded to ensure proper alignment with lintels; therefore, a portable welding rig had to be lifted onto scaffolding and moved around the walls. Earth mixing had to be done manually, as no earth-moving equipment was available; this slowed progress and caused over-tamping of some wall sections. But, as construction proceeded, the students developed a rhythm for synchronizing the mixing of earth batches, the moving of scaffolding and forms, and the tamping. The two-person system of incremental forming became increasingly reliable. As they gained experience, students were able to make suggestions for revised formwork, details, and earth mixing techniques. The new challenge(Back to top) Traditional Pima dwellings |
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(Back to top) The Pimas, also known as Akimel O'odham or River People, have lived along the Gila and Salt Rivers throughout their history. These rivers irrigated their fields and provided building materials. The Pimas believe themselves to be descended from the Hohokam (a Pima word meaning "those who are gone") (Webb, 1959), who were renowned for their canal systems and earth buildings. The Hohokam migrated from Mexico into southern Arizona around 300 BCE, joining archaic bands of nomadic hunter-gatherers already living in the region. The Hohokam introduced farming and irrigation knowledge as well as permanent community locations and longer-lasting dwellings woven of slender branches and plastered with mud. Around 1250 CE, the Hohokam began to build adobe-walled houses that evolved into large, three- or four-story apartment buildings. One major cultural icon for southwestern Native Americans is the ruin of an ancient structure called "Casa Grande" today. It was a four-story watchtower or observatory built by the Hohokam in the mid-1300s and became the first archeological preserve in the United States. Constructed of layers of caliche mud, the walls are 4.5 feet (~1.5 m)thick at the base and endure because of their mass and compaction. In 1350 CE, the Hohokam population began to decline for unknown reasons (presumably drought) and scattered into groupings of small houses once again. These communities became the Papago (or Tohono O'odham) and Pima tribes, who lived this way until encountered by the Spanish in the 1600s (Gregonis & Reinhard, 1979). The Pimas built of arrowweed (Pluchea sericia), willow (Salix spp.) and cottonwood (Populis fremontii), which required moderate rainfall. Until the 19th century, the two most common building types were the "ki" and the "vato." The ki was a slightly excavated, brush- and mud-covered structure with a domed adobe-plastered roof. This was used for shelter in cool weather. The vato was a four-posted arbor covered with cactus ribs and arrowweed. This was where families cooked, ate and slept during the warmer times of the year (Easton & Nabakov, 1989).
Later Pima and Papago houses were rectangular, flat-roofed structures with a post-and-beam frame covered with arrowweed and mud. Changes in housing practices since the 1880s were largely caused by constant pressure from church and government groups, but the sandwich houses are not part of any government-sponsored development plan and retain Pima characteristics (Van Villigen, 1970). They include locally available materials and employ locally known techniques while reflecting the arrival of milled lumber. The walls are built of mud and straw packed into a frame of heavy vertical posts and lighter horizontal cross-pieces spaced a few inches apart or staggered. The mud fills the frame cavity and squeezes out between the cross pieces, forming a composite wall. Most sandwich houses are plastered inside and out with a coat of mud, which needs frequent repair. The packed mud also needs frequent repacking, especially after summer rains wash out areas of the walls. The roofs are framed with mesquite (Prosopis spp.) posts, crosshatched with saguaro (Carnegiea gigantea) ribs, and thatched with arrowweed and mud. Sandwich houses are still the most common Pima dwelling; new ones are constructed as a matter of preference and also economy. Contemporary rammed earth techniques differ due to available technology and building code requirements, but the genealogy remains obvious. The reliance on earth from the site, intensity of labor required, and uncomplicated techniques involved make rammed earth a good fit for the Pimas with their high unemployment and housing shortage. Revisions(Back to top) Formwork revisited |
(Back to top) Earth mix redesigned(Back to top) The rammed earth dwelling(Back to top) The family has a strong affection for their present home, although it is very small and in poor repair. They do not wish to see it razed by the tribal Housing Authority when their new home is complete, and hope to keep it on as a storage building or guest quarters. It is over 70 years old and was built by the late grandfather of the family. The appearance of the mud and saguaro rib walls is valued by this family, who asked for a similar appearance in some part of their new home. This challenge led to several experiments with strips of milled lumber and cactus ribs and different methods of embedding them into the tamped earth or attaching them to the formwork. The goal was to leave one face of the cactus ribs revealed after the forms were removed. Initial attempts to tie strips and ribs into forms using hemp or wire failed, as did efforts to create a reveal in the surface of the rammed earth with ribs exposed behind it. The desired result was finally accomplished by laying the ribs against the formwork one by one as tamping progressed, anchoring them into the rammed earth with 3-inch (1.2 cm) drywall screws, and brushing them with a wire brush to subtract the covering surface once the forms were removed. The saguaro ribs could not extend the full length of the forms because their ends would then be exposed and eventually pull free from the wall. Ultimately, the ribs were set in 12 inches (30.1 cm)from the end of the form. This also allowed the visual understanding that their purpose was ornamental, not structural. Another challenge was to incorporate the familiar materials of the vato into the roof of the deep porch, which would serve as outdoor living room. Reservation building officials had already outlawed use of traditional thatch materials inside the house due to concerns about flammability. However, laying cactus ribs and arrowweed on the porch rafters was acceptable to the officials. After much student experimentation, the final detail required lighter rafters at a closer interval at the entry section of the porch, in order to permit adequate thickness of split saguaro ribs and arrowweed but still end up at the same level as the rest of the porch for sheathing. | |
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The walls of the new family home were built in nine days with the participation of the Gila River Community construction crew. Crew members formed and poured the footings for the walls. Four to six of the crew worked with the students daily and continued the work after the University semester ended. During the first two days of wall building, the Gila crew mixed earth and cement, and observed the forming process. By the third day they were engaged in the forming and eventually adapted it as required, for example when the building inspector requested a recess to contain the electrical panel box. The last two days of wall building were done entirely by the Gila crew, as the Design/Build Studio turned to the challenge of forming for the concrete bond beam. The rammed earth formwork proved to be manageable by two people, although a third person was useful in tightening the clamps and checking for level and plumb. The cost of the sand and gravel admixture imported to the site was approximately US$400, and the formwork cost US$300, not including the pipe clamps (which were already on hand). The formwork is re-usable, although it does suffer from contact with the tampers over time and the edges get rough. Some of the rammed earth formwork was used in forming the bond beam; most was saved for the next house. As designed, the system works well for the single, low-cost house. To build houses in greater numbers might involve staggering the phases of construction to allow one component of a construction crew to pour footings while another follows and tamps walls, for example. New plywood forms would be required for about every third house. The reactions of the Gila family, their neighbors and the construction crew have been strong and positive because of the structure's resemblance to their traditional sandwich houses, in appearance, smell, and surface temperature. A second rammed earth house is already scheduled for construction by the trained crew, using the same soil mix and forms. In summary, these iterative cycles of research and building have led from technical requirements to a powerful design and construction experience on the University campus to an opportunity serve the larger community. Research ideas, when pursued in the context of design/build opportunities, can escape from the paper upon which they are conceived. It is the research ideas that in the end make the design/build experience meaningful beyond the skills, design understandings, and human interactions that are the immediate benefit for students. The research contributions carry their impact past the design/build project that is isolated in time and space, into the living community. References(Back to top) Easton, Robert and Peter Nabakov, 1989. Native American architecture. Oxford University Press, 1989. Gregonis, Linda M. and Karl J. Reinhard. 1979. Hohokam Indians of the Tucson basin. Tucson, Ariz.: University of Arizona Press. Van Willigen, John. 1970. Contemporary Pima house construction practices. The Kiva: Journal of the Arizona Archaeological and Historical Society, Inc. 36(1). Webb, George Buzzing Feather. 1959. A Pima remembers. Tucson, Ariz.: University of Arizona Press. |
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Mary Hardin is an Associate Professor in the School of Architecture at the University of Arizona. You can reach her for comment by email at:
mchardin@u.arizona.edu.
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Earth materials guidelines
http://www.greenbuilder.com/sourcebook/EarthGuidelines.html
These guidelines are from the Sustainable Building Sourcebook of the Austin, Texas Green Builders Program. While primarily aimed at those in the Austin region, the general information in the guidelines is of general interest. Covers other materials besides rammed earth.
Important facts about stabilized earth
http://www.rammedearthworks.com/facts.html
This page from the Rammed Earth Works, Inc., web site provides an overview of advantages, considerations, and disadvantages to stabilized earth construction.
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