Rammed Earth Construction

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Rammed Earth Construction

Background

Rammed earth is essentially manmade sedimentary rock. Rather than being compressed for thousands of years under deep layers of soil, it is formed in minutes by mechanically compacting properly prepared dirt. The compaction may be done manually with a hammer-like device, mechanically with a lever-operated brick-making press, or pneumatically with an air-driven tamping tool. Dynamic compaction using manual or power tampers not only compresses the soil, but it also vibrates the individual dirt particles, shifting them into the most tightly packed arrangement possible. When finished, rammed earth is about as strong as concrete.

Houses built of rammed earth have several advantages over wood-frame construction. The walls are fireproof, rot resistant, and impervious to termites. The solid, 18-24 in (45.72-60.96 cm) thick walls are nearly soundproof. The massive walls help maintain a comfortable temperature within the house, damping temperature swings that normally occur on hot summer days or cold winter nights. When designed and oriented to take the best advantage of solar energy, a rammed earth house can be comfortable with 80% less energy consumption than a wood-frame house. On the other hand, initial construction is about 5% more expensive than wood-frame construction because it is very labor intensive.

History

Humankind has used the earth itself to build homes and other structures for thousands of years. Jericho, the earliest city recorded in history, was built of earth. Temples, mosques, and churches were built of mud bricks and rammed earth throughout the ancient Middle East. Egyptian pharaohs ruled cities constructed of rammed earth. In the Far East, the technique was used not just for houses, but even for building ancient forerunners of the Great Wall of China. Romans and Phoenicians brought the technology to Europe, where it was used for more than 2,000 years.

In the United States, rammed earth construction enjoyed a period of popularity from 1780 until about 1850, when mass-produced fired bricks and sawed lumber became readily available. Houses could be built more quickly and easily with bricks and lumber, which were considered more modern and elegant materials than dirt. The supply shortages experienced after World War I and during the Great Depression brought rammed earth construction back into favor for two decades. Frank Lloyd Wright designed houses to be made of rammed earth.

When World War II ended, the country faced a large demand for housing, and wartime factories turned to manufacturing building materials that could be used for quicker types of construction. Rammed earth was brushed aside until it was repopularized during the environmentally conscious 1970s. A modified version of the technique, invented by Michael Reynolds, uses building blocks of discarded automobile tires rammed full of earth. These houses not only keep used tires out of landfills, but they can be built by inexperienced, first-time builders. When a homeowner uses the unpaid labor of himself, relatives, and friends, and when he can obtain many of the building materials for free, the construction cost can be held to less than half that of a wood-frame house.

For thousands of years, rammed earth construction was taught personally by one generation of builders to the next. In early twentieth-century America, such a network of experienced builders did not exist. The U.S. Department of Agriculture developed and published a manual titled Rammed Earth Walls for Buildings that showed people how to build their own homes. Research projects designed to improve the methods and quality of rammed earth construction were reported in academic journals, and more than 100 articles on rammed earth appeared in trade and popular magazines from 1926 to 1950.

Several more recent innovations increase the speed and ease of construction and enhance the structural integrity of the final product. For example, pneumatic tampers can be used to compact soil much more quickly than the traditional manual method. Easy-to-assemble forms allow walls to be built as solid panels rather than building them up as successive layers a foot or two at a time. Using time-honored manual methods, a four-person crew can erect 40-50 sq ft (12.19-15.24 sq m) of rammed earth wall per day; with power tools, the same crew can construct 300 sq ft (91.44 sq m) per day. David Easton, founder of Rammed Earth Works, has developed several earthquake-resistant designs to reinforce and structurally integrate the walls; the choice of design alternative depends on several considerations, including the distance to the nearest seismic fault.

Raw Materials

As the name implies, the primary material used in rammed earth construction is the earth itself. There are five basic types of soil (gravel, sand, silt, clay, and organic), and the dirt in a given location is generally some combination of all or most of these types. Historically, the longest lasting rammed earth walls were made of soil that was 70% sand and 30% clay. The soil from a new building site is tested to determine its suitability. Organic material must be removed from the soil and, if necessary, a different type of soil can be trucked in and mixed with the existing dirt to create a blend that will work. Cement may be added to the soil to increase both its strength and its resistance to moisture—usually at about one-fourth the ratio that would be used to make concrete.

Steel reinforcing bars are placed in the foundations and sometimes in the walls. Plywood is used to make the removable forms for standard rammed earth construction. Sheets of three-quarter-inch (1.9 cm) plywood are thick enough. High-density-overlay (HDO) panels, which have a thin, plastic coating on one side, work especially well because they release more easily from the wall after construction. This not only leaves a clean finish on the just-completed wall, but it leaves the form boards in good condition to be used on future projects.

Rammed-earth tire construction uses discarded automobile tires, aluminum cans, and cardboard in addition to compacted soil. About 1,000 tires are used to build the walls of a 2,000 sq ft (609.6 sq m) house.

Design

Rammed earth houses are custom designed to make the most energy-efficient use of the site. They can be successfully designed for many climate regions, including humid areas with cold winters. The size and placement of windows is an important factor in taking advantage of solar heating in the winter and cooling breezes in the summer. The house can be positioned to take advantage of hills that offer protection from storms. Shade trees or trellised vines offer relief from summer heat but admit warm sunlight in winter.

The Manufacturing
Process

Rammed earth houses can be built in one of three basic ways. Individual, rammed earth bricks can be formed and used with standard building techniques; in fact, such bricks may be used to form the floors in a rammed earth house built with other techniques. Standard rammed earth construction involves erecting wood forms and compacting the prepared soil into these molds, which are removed after the walls are completed. The rammed-earth tire method is a commonly used alternative. The descriptions that follow are overviews of the standard and tire methods.

Preparing the site

1 An inch or two (2.5-5 cm) of topsoil is removed from the building site and stored so it can be replaced around the completed structure. Organic matter such as weeds and roots are removed and may be composted for use in post-construction landscaping. After the site is cleared, the outline of the house is staked out. The soil is excavated to a depth that guarantees a level surface; the excavation includes the floor area of the building as well as a 3 ft (1 m) surrounding buffer zone. A trench may be dug so that the walls will be anchored into the ground to a depth below the winter freezing line.

Laying the foundation

2 The foundation, which is made of reinforced concrete, consists of a footing that may be as narrow as the thickness of the wall or up to three times that thickness, depending on the strength of the underlying soil. The footing is extended above ground level to form a short "stem wall" that will connect the rammed earth walls to the footing. Depending on the architectural design, a slab floor may also be poured.

Analyzing the soil

3 A variety of tests are conducted to determine the suitability of the local soil for construction material. For example, a particle determination test reveals the relative proportions of sand and silt in the sample. A compaction test is performed by forming a ball of mud and dropping it from a height of 3 ft (1 m); the degree to which the ball disintegrates on impact reveals its usefulness for building. Other, more precise, tests can be performed at a geotechnical laboratory. If the native soil is unsuitable or inadequate for building, it can be blended with or replaced by soil from another source. Soil may be purchased from a quarry, or it might be available as refuse from a nearby construction site, in which case it could be delivered free or at a minimal cost.

Framing the walls

4 Traditionally, wood forms were used to build up walls 2 ft (0.6 m) at a time. After the mold was filled with fully compacted soil, it would be removed and reset to form the next section of wall. More efficient methods now allow forms to be constructed for the entire height of the wall (even more than one story). Horizontally, the framework may form the complete length of wall, or it may form shorter panels [e.g., 8 ft (2.44 m) long] separated by 6 in (15 cm) gaps that can be filled with reinforced concrete for enhanced structural strength. Framing is a major component of the construction process, in terms of both importance and time; it usually takes less time to fill and compact the soil within the forms than it does to set, align, and remove the framework.
In the case of rammed-earth tire construction, wood forms are not used. A row of used automobile tires is simply laid atop the concrete footing, perhaps centered around steel reinforcing bars that extend out of the footing. After each layer of tires has been filled and compacted, another layer will be added, offset by half the tire diameter from the layer below.

Tamping the soil

5 Traditional tampers are made of a heavy wooden block with a handle extending upward through its center. A more compact version can be made from a 4 in (10 cm) square steel plate welded to a section of 1 in (2.5 cm) pipe. A 4-6 in (10-15 cm) layer of moistened soil is placed inside the form, and a worker drops the tamper from a height of 12-18 in (30-46 cm). In fact, most of the work is now done quickly with pneumatic tampers, and manual devices are used only in tight spaces around electrical boxes or plumbing pipes. After many repetitions with the tamper over the entire surface of the layer, the noise made by the impacting tamper changes from a dull thud to a ringing sound. This happens when the soil has been compacted to about half of its original volume. At this point, another layer of prepared soil is added, and the tamping process is repeated. When the tamping is finished, the wood forms are removed.
The tamping process is different when tires are used as the framework. In this case, a sheet of cardboard is placed across the bottom of the hole in the tire, and moistened soil is shoveled into the tire. The dirt is packed by hand into the interior of the tire, and then it is compacted by repeated blows with a sledge hammer. Using this technique, about three wheelbarrow loads (350 lb or 158.9 kg) of soil can be packed into each tire. Pounding the dirt causes the tire's walls to bulge, interlocking the tire to the row below. As additional layers are added and the wall becomes taller, scaffolding must be constructed so workers have a place to stand while filling and pounding the tires.

Finishing the walls

6 Interior faces of walls are often finished with plaster. If such a coating is not applied, the wall should be treated with a clear, penetrating sealant to prevent dust from sloughing off. Because stone (even when manmade from rammed earth) is somewhat porous, it may be necessary to apply sealant to weatherproof the exterior faces of the walls in certain climate areas.
Rammed earth tire walls are finished by inserting aluminum cans into gaps between the tires and filling remaining voids with adobe (straw-reinforced mud). Earth is packed against the exterior face of the wall, creating a flat surface that completely conceals the tires. Wall interiors are finished with 2-4 in (5-10 cm) of plaster or stucco.

Byproducts/Waste

Rammed earth structures use natural resources efficiently. Those made of packed tires even make productive use of some of society's trash. Because the tires are sealed within 3 ft (0.9 m) thick walls, neither oxygen nor the sun's ultraviolet rays can react with them. This means they cannot catch fire and they do not release toxic chemicals. The structures qualify for better fire ratings than wood-frame buildings, and they do not smell of rubber.

The Future

During the late 1700s, a French builder named Francois Cointeraux founded a school in Paris to study and publicize rammed earth construction, which he called pise' de terre (puddled clay of earth). Today, David Easton has developed a new version of rammed earth construction he calls PISE (Pneumatically Impacted Stabilized Earth). It involves spraying the prepared soil under high pressure against a one-sided form. This technique can produce 1,200 sq ft (365.76 sq m) of 18 in (45.72 cm) thick wall per day, which is four times faster than a typical, four-person crew can fill box-like forms and compact earth with power tampers.