Arid Lands Newsletter No. 47, May 2000
Desert Architecture for a New Millenium

Rammed earth constructions: Trans-cultural research in the Sonoran Desert

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

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In 1996, the UA's Athletics and Recreation Department requested the SoA's assistance in designing a new classroom facility. In return, SoA adjunct faculty member Richard Brittain suggested a design/build project, and a partnership was formed. A 4th year Design Studio led by Brittain worked to design an environmentally conscious, low-cost classroom facility that could be built by novices in the construction trades. One semester was spent consulting with instructors and students in the Recreation Department and preparing schematic designs. A second semester of design development and construction documents, completed by an independent study group of students involved in the first phase, readied the project for ground-breaking. This collaboration in itself produced benefits for all concerned. Students participated in many professional activities, from client meetings to programming exercises, code checks and budget reviews. The Recreation Department received, in the words of its Director, "much, much more than we would have settled for."

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

rammed earth plywood forms
Thumbnail link to image of rammed earth plywood forms, ~22K file

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Formwork design and testing focused on the goals of easy mobility and reassembly. Early prototypes developed by Brittain and Perry used plywood walls stiffened with steel sections (later replaced by aluminum to lighten the forms' weight). Aluminum angles allowed the plywood pieces to bolt together easily and doubled as handles for moving the forms. However, the pressure built up during tamping made disassembling the forms very difficult. The sides bowed in spite of the stiffeners, the assembled forms were hard to move around, and they could not be stacked one upon the other. This forced a working sequence of ramming walls in horizontal courses, with the drawback of a small amount of horizontal form creep in the direction of the wall-building. After consulting with noted rammed earth expert David Easton and reviewing precedents for ramming walls in vertical piers (ancient and contemporary Chinese, Moroccan, and Australian methods), plywood walls, pipe clamps, and stiffening boards were used in a simpler configuration. After a few test runs with the revised formwork, fine-tuning of pipe spacing and placement allowed actual construction to begin.

The earth mix research

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Composition of the earth mix also had to be tested and revised in several iterations. Generally, earth from the given site is tested by sieve and settlement to determine its composition in terms of particle sizes. Then, admixtures are designed to result in a well-graded mix. This mix is then combined with cement and water, and tamped into test cylinders for curing and compressive strength trials. Determining the percentage of cement to total mix requires much testing to achieve the compressive strength required by building code. Color pigments also affect the final strength of the mix due to their fine particulate nature. In the case of the classroom facility, the earth mix was to be imported from a sand and gravel company because the walls required more earth than the excavation of footings would provide. Brittain, Hardin and students began with an adobe mix and added engineered fill and cement. Numerous test cylinders were tamped and crushed before a reliable mix was discovered.

The classroom facility

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Twenty-eight students registered for the 1997 Design/Build Studio that was to construct the classroom facility. Teams were formed to produce shop drawings for each wall and roof plane. Students organized and placed materials orders, met deliveries, and practiced skills such as welding, mixing mortar, and laying block. Carefully dimensioned sketches filled notebooks as students planned and prepared for each day's exertions. Tool belts lost their sheen, thumbs wore bandages, vocabularies grew. Faculty and students from the Recreation Department joined in, shoveling dirt and steering pneumatic tampers. The entire crew was energized by this client participation. As the walls rose, the forming system was rethought, revised, and constantly improved until results became consistent.

beveled openings and chamfered reveals
Thumbnail link to image of architectural details in wall, ~19K file

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

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As students shaped the classroom facility, the faculty began to realize the potential benefits of the new forming system to low-income communities of the region. Professor Hardin wrote a grant proposal for an educational partnership between the SoA and a Native American community in dire need of additional housing. The Gila River Pima community of southern Arizona had rejected government-built housing that had no affinity with their traditional building methods. Representatives of the tribal District Six Housing Committee had attended student presentations of environmentally sensitive housing proposals, and had already requested assistance from the SoA. Committee members were enthusiastic about the notion of a partnership that would train members of their community to build rammed earth houses for themselves, using low-cost formwork and indigenous building materials. When the Kellogg Foundation funded the UA/Community Partnership grant in 1998, a new collaboration was formed.

Traditional Pima dwellings

Pima 'sandwich' house
Thumbnail link to image of traditonal Gila River Pima house, ~21K file

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Original Native American inhabitants of the Southwest used both rammed earth and wattle and daub techniques. Such techniques were replaced in the 20th century by a composite wall system of wood and packed mud. The Gila River Pima call these "sandwich" houses in English. Most residents of the reservation live or grew up in a sandwich house. Such houses require constant patching and replacement of mud, but are valued for maintaining a fairly stable interior temperature despite the wide diurnal temperature swings of the Sonoran Desert. They also hold considerable cultural value because they are a local tradition, built by their tenants with found materials from the landscape (cactus ribs, plant stalks, earth) that remain part of the landscape when the houses deteriorate.

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).

"A vato or shade was usually just a few yards from this cooking place. This shelter of a type still used by the Pimas was made with four or six upright forked posts that held cross-poles on which arrowweeds were placed to make the shade. This shelter was open on all sides and used in the summer time when the sun shines hot.

Beyond the vato was the olas-ki, or round-house, made of mesquite posts, willow and arrowweeds. This type of house is no longer used. It was enclosed all around, with a little dirt and straw on top to keep the rain out. The only opening was a small hole about two feet wide and four feet high which was used as a door." (Webb, 1959)

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.


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In 1998, as Professor Hardin and the next generation of students began designing a rammed earth dwelling for a Pima family, new considerations arose. The soil mixture had to incorporate soil found on the site, and the family wanted to integrate other traditional materials into the house. Also, Hardin wanted to revise the formwork to require fewer breakdown and set-up periods, as these took more time and labor than the tamping. A period of design and testing followed, until the 1999 Design/Build Studio felt prepared to begin new construction.

Formwork revisited

formwork details
Thumbnail link to image of formwork details, ~29K file

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The eight-foot-long (2.5 m) modular forms worked well with other common U.S. construction materials used after the wall construction. Typical modular dimensions of plywood and drywall fit neatly with the wall modules, and rough opening dimensions for windows and doors worked well in four- or eight-foot (1.3 - 2.5 m) increments. The original two-foot-high (61 cm) forms took too much time to move, realign and reset. While building the classroom facility, students began setting up two sets of forms and tamping four feet (1.2 m) of wall height at once. Initial concerns about the physical discomfort of standing within such a deep space to tamp proved to be unjustified, because tamping time was so brief. Raising the form height also allowed 4x10-foot (1.25x3 m) sheets of plywood to be used for formwork with no alterations except drilling holes for the pipe clamps. Fewer pipe clamps were necessary because the seam between forms was no longer there and did not need to be clamped on both edges. The technique of using lengths of polyvinyl chloride (PVC) as sleeves for pipe clamps that had to go through the wall was perfected during the classroom construction, after experiments with lubricated pipes and tapered pipes had proven difficult. The use of wedges or shims to take the building pressure between forms and pipe clamps as the earth was tamped was also arrived at during the classroom construction. These ideas were incorporated into the next incarnation of formwork assemblage, and took on new uses during construction of the family's residence. For example, the PVC sleeves left inside the walls functioned as conduit for the snap ties used to hold together the plywood forms for the concrete bond beam at the top of the walls. Such refinements and innovations could only have happened in this iterative cycle of inventing and testing. All of the well-documented paper ideas were transformed in the field.

Earth mix redesigned

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The earth from the Gila family's site was high in silt and clay due to the site's location in the floodplain between the mostly dry Santa Cruz and Gila Rivers. A sand and gravel company on the reservation had the necessary admixtures - natural clean sand material mined from the riverbeds and small pieces of granite, or crusher fines, left over from a crushing operation. Determining a suitable mixture of all these materials took several months of mixing, curing and crushing cylinders at a soils lab, and yet another potential research project resulted from this effort. It was observed that cylinders cured in the wet room broke at a slightly higher compressive load than dry-cured cylinders. The very small percentage of cement added to the mix apparently hydrated more fully with a longer period of higher humidity, and affected the compressive strength of the mix. This might affect future forming techniques if forms were wetted down, covered, and left on longer to allow more hydration.

The rammed earth dwelling

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The family's dwelling was configured as a simple rectangle (similar to the typical sandwich house), designed on an eight-foot (2.5 m) module to correspond with the form dimensions, and adapted to the family's preferences for orientation, view, and outdoor living practices. The overall process of configuration will not be outlined here as it is a study in itself. Considerations that directly affected the construction practices, however, have a place in this text.

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.

concrete bond form work
Thumbnail link to photograph of concrete bond form work, ~25K file

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.


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Easton, David. 1996. The rammed earth house. White River Junction, Vermont: Chelsea Green Publishing Company.

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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Author information

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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:

Additional web resources:

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Earth materials guidelines
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
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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