Rammed earth
Rammed earth construction is an ancient building technique that utilizes natural soil to create sturdy walls. It dates back to at least 5000 B.C.E. and has been employed globally, with notable examples such as sections of the Great Wall of China and traditional homes in Morocco. Unlike adobe, which uses dried mud bricks, rammed earth involves compacting a mixture of soil—typically clay, gravel, sand, and silt—into forms to create solid walls. The method is recognized for its affordability and the thermal mass properties that help regulate indoor temperatures, making it energy efficient. Rammed earth structures are also resistant to fire and pests, though they can be labor-intensive to build, which may increase costs in regions with higher labor expenses. The technique saw a resurgence in the 1970s and remains popular, particularly in the western United States and parts of Australia and New Zealand. While there are challenges, such as ensuring adequate drainage and navigating building codes, rammed earth is increasingly being recognized for its sustainability and potential to reduce heating and cooling costs in construction.
Rammed earth
Rammed earth construction is similar to adobe making. Both involve using soil to build structures. Rammed earth construction dates to at least 5000 B.C.E. Cultures around the world have used it, including in building the Great Wall of China, which is more than two thousand years old.
Overview
The exact origins of rammed earth are unknown, but the Roman Empire helped to spread the building technique across Europe. It was used widely in France and Germany, and immigrants from those countries took the skill to North America. German builders used rammed earth construction in New York and Pennsylvania. Monticello, the Virginia home of Thomas Jefferson, is a rammed earth home. Researchers believe that many antebellum Southern mansions are made of rammed earth, but even the owners are usually unaware of this fact.
The U.S. government encouraged rammed earth construction during the 1920s. The Department of Agriculture published a book on how to build these structures, and many buildings were erected in the South. Universities researched the methods and materials and provided information to builders and the public. The Civilian Conservation Corps (CCC) and Works Progress Administration (WPA), both instrumental aspects of President Franklin Roosevelt's (1882–1945) New Deal, used the technique. The CCC and WPA provided employment for young men as the United States struggled to emerge from the Great Depression (1929–1939).
The Farm Security Administration hired an engineer, Thomas Hibben, to design rammed earth homes for rural areas. They offered an inexpensive option for poor farmers. A number of these residences were built in Alabama and are still in use a century later. The construction method enjoyed a resurgence in popularity during the 1970s, especially in Arizona, California, Colorado, and New Mexico. In the twenty-first century, about 20 percent of new homes built in the West are rammed earth projects.
The U.S. Agency for International Development builds rammed earth houses in many parts of the world. It is commonly used in Australia and New Zealand.
Construction Process
Adobe structures are made using bricks of mud. These bricks are formed and left to dry before construction begins. Rammed earth structures also use a mixture of soil components, but they do not require extensive drying time. The earth blend may include clay, gravel, sand, and silt. The soil mix should not include too much clay, however. Some modern construction adds a small amount of cement, creating stabilized rammed earth.
Experts say the soil mix should have a compressive strength of 2,068 kilopascal or kPa (300 pounds per square inch, or PSI). The moisture level should be about 10 percent, which is much lower than adobe. Walls will be dry by the time the forms are removed. Many builders use tractors or other large equipment to blend the soils, which must be used quickly before they dry out.
Builders erect a frame, usually a plywood structure, for the walls. The walls are typically 46 to 61 centimeters (18 to 24 inches) thick, though stabilized rammed earth walls may be thinner. The forms should be in place before the earth is prepared.
The soil mix is tightly packed, or rammed, into the wall frame. Layer upon layer is added and pressed, like sediment that slowly builds in a river bed and eventually becomes sedimentary rock. Builders put about four to six inches of soil into the form, compress it, and then add the next layer. Professional builders use machines connected to air compressors, but an individual might use a hand rammer. Too much pressing could damage the wooden form, so builders must be cautious.
After the soil is rammed in, the forms are removed. If properly pressed, the soil walls will be stable and remain standing. The builders continue setting up frames and ramming soil into them until construction is complete. If desired, walls may be covered with stucco or tile or painted.
A new technique does not use a two-sided wall form. Pneumatically impacted stabilized earth, or PISE, instead uses a single wall form. A high-pressure hose sprays the soil mixture against it, essentially ramming it horizontally instead of vertically.
Advantages and Disadvantages
Rammed earth structures have several advantages. In places where lumber is scarce or expensive, rammed earth is affordable. The materials usually are readily available, or soil may be purchased and brought to the building site.
These structures remain cool in the heat and warm at night because they have good thermal mass, meaning they absorb and hold heat energy and release it when the air cools. Builders may maximize the thermal flywheel effect by situating a structure so windows receive morning sun in winter but are shaded in summer. The walls also may be insulated. The thermal mass benefit can mean lower heating and cooling bills.
They withstand earthquakes well—and may be reinforced in earthquake-prone areas—and are fire-resistant because earth walls are not flammable. They also contain no food sources for rodents or insects.
One disadvantage of rammed earth construction is high labor cost. The work is labor-intensive, so it is often most economical in parts of the world where cheap labor is readily available. It is often very expensive in the United States; a rammed earth structure could cost from 5 percent to 15 percent more than a wood-frame house. In many areas, however, this additional cost is offset by reduced heating and cooling expenses.
Though rammed earth structures can withstand rain, in very wet areas a larger roof, with a deep overhang, may be necessary to protect the walls. The structure should be located in an area with good drainage, and gutters and other precautions should be installed to direct water away from the building.
While construction loans, insurance, and mortgages for rammed earth structures are commonly available in areas where these structures are common, people in other areas may have to do additional research before beginning a structure. Building codes and other information may be unavailable or difficult to find.
Bibliography
Downton, Paul. "Rammed Earth." Your Home. Government of Australia, 2013. Web. 16 Feb. 2016. http://www.yourhome.gov.au/materials/rammed-earth
Easton, David. "What Is Rammed-Earth Construction?" Mother Earth Living. Ogden Publications, Inc., Sept./Oct. 1999. Web. 16 Feb. 2016. http://www.motherearthliving.com/green-homes/the-dirt-on-rammed-earth-construction.aspx
Edmonds, Molly. "How Rammed Earth Homes Work." How Stuff Works. How Stuff Works, a division of InfoSpace LLC, 19 Aug. 2008. Web. 16 Feb. 2016. http://home.howstuffworks.com/rammed-earth-home.htm
"History of Rammed Earth." Soledad Canyon Earth Builders. Soledad Canyon Earth Builders. Web. 15 Feb. 2016. http://www.adobe-home.com/about-rammed-earth/history-of-rammed-earth/
McMeekin, John O. "How to Build a Rammed Earth House." Mother Earth News. Ogden Publications, Inc., Sept./Oct. 1973. Web. 16 Feb. 2016. http://www.motherearthnews.com/diy/rammed-earth-house-zmaz73sozraw.aspx