Rammed earth, also known as pisé, is a building method used to create strong foundations, floors, and walls by pressing together natural materials like soil, chalk, lime, or gravel. This technique is very old, but it has become popular again recently because it helps protect the environment.

Pisé can also describe a type of sculpture material. These sculptures are often small and made using molds. They have been commonly used in Central Asia, Tibet, and sometimes in China.

Buildings made with rammed earth can be found in many places around the world, including areas with mild climates, wet regions, dry deserts, mountainous areas, and tropical zones. The presence of suitable soil and a building design that works well with the local weather are two important reasons why this method is used.

Building process

Making rammed earth involves pressing a wet mix of subsoil that has the right amounts of sand, gravel, clay, silt, and sometimes a stabilizer into a formwork, which is a temporary frame or mold used to shape the wall.

In the past, materials like lime or animal blood were added to make the soil stronger.

The soil mix is poured into the formwork to a depth of about 4 to 10 inches and then pressed down until it takes up about half its original space. This process is repeated in layers or sections until the wall reaches the top of the formwork. In the past, workers used long poles to press the soil by hand, but today, machines powered by air are often used.

Once a wall is finished, it is strong enough to remove the formwork right away. This is important if workers want to add a texture to the wall’s surface, such as using tools like wire brushes, carving, or molds, because the wall becomes too hard to work with after about an hour. The strength of rammed earth increases as it hardens over time. Walls made with cement added to the soil need at least 28 days to fully harden.

In modern rammed-earth buildings, walls are built on top of traditional foundation parts or a strong concrete base.

The process begins with a temporary frame called "formwork," usually made of wood or plywood, to shape each section of the wall. The frame must be strong and well-supported, and the two sides must be held tightly together to prevent bending or warping from the pressure. Formwork is an important part of building rammed-earth walls. In the past, wooden planks tied with rope were used, but today, builders often use plywood or steel (or both) for formwork.

Characteristics

The strength of rammed earth when pressed depends on things like the type of soil, how big the soil particles are, how tightly it is packed, how much water is in the mix, and the kind and amount of stabilizer used. Well-made cement-stabilized rammed-earth walls can have a strength between 5 and 20 MPa (700 to 3,000 psi). Using more cement may increase strength, but it can also change how much water passes through the walls. Properly built rammed earth can last for thousands of years, as shown by many ancient buildings still standing today. In areas with frequent earthquakes, rammed-earth walls are often reinforced with steel bars.

Adding cement to soil mixtures with little clay can help rammed-earth buildings hold more weight. In 1925, the United States Department of Agriculture found that rammed-earth structures can last forever and cost less than two-thirds of what standard wooden-frame houses cost.

A major advantage of rammed earth is its ability to absorb and release heat, similar to brick or concrete. It takes in heat during the day and gives it off at night, which helps keep indoor temperatures steady and reduces the need for air conditioning or heating. In cold areas, rammed-earth walls can be insulated by adding materials like foam or fiberglass panels inside or outside the walls. Depending on the binder used, walls may also need protection from heavy rain and insulation to prevent moisture damage.

Rammed earth can help control humidity if walls with clay are left exposed inside a building. Humidity levels stay between 40% and 60%. The weight and clay in rammed earth allow buildings to "breathe" better than concrete buildings, which helps prevent condensation and heat loss.

Rammed-earth walls look and feel like natural earth. Some people avoid using waterproof coatings like cement paint because they stop walls from releasing moisture, which is important for keeping the walls strong.

Cracks or blemishes can be fixed by applying the same soil mixture as a plaster and smoothing it with sand.

The thickness of rammed-earth walls varies by location and building rules. Non-load-bearing walls can be as thin as 6 inches (150 mm), while load-bearing walls can be as thick as 24 inches (600 mm). The thick, dense walls help block noise. They are also naturally fireproof, resistant to termites, and do not contain harmful chemicals.

Environmental effects and sustainability

Buildings made with rammed earth may be more sustainable and better for the environment than other building methods, depending on several factors, such as how much local material is used. Rammed-earth buildings that use soil found nearby have low energy use during production and create little waste. The soil used is usually from below the top layer of soil, which helps protect the topsoil for farming. If the soil removed during foundation work can be used, the cost and energy needed to transport materials are very low. However, this requires testing the soil to make sure it is suitable. Rammed earth can have a small environmental impact if the amount of cement used is limited and if materials are locally available. Often, materials like crushed rock are used instead of pure earth.

Rammed earth can help buildings use energy more efficiently because its density, thickness, and ability to control heat make it good for passive solar heating. It takes about 12 hours for heat to move through a wall that is 35 cm (14 inches) thick.

Adding cement to the soil can reduce some of the benefits of using rammed earth, such as low energy use during production. Making cement releases 1.25 tonnes of carbon dioxide for every tonne of cement made. Even though rammed earth has low greenhouse gas emissions in theory, the process of transporting materials and making cement can greatly increase the overall emissions of modern rammed-earth buildings. For example, a 300 mm thick rammed-earth wall with 5% cement produces slightly more emissions than a 100 mm thick concrete wall.

History

Evidence of ancient rammed earth use has been found in Neolithic sites, like those in the Fertile Crescent, dating back to the 9th–7th millennium BC. Similar evidence exists in China's Yangshao and Longshan cultures from around 5000 BCE. By 2000 BCE, rammed-earth techniques (夯土 Hāng tǔ) were widely used for walls and foundations in China.

In the 1800s, the book Rural Economy by S. W. Johnson helped popularize rammed earth in the United States. This technique was used to build the Borough House Plantation and the Church of the Holy Cross in Stateburg, South Carolina. Both are National Historic Landmarks.

A notable example of rammed-earth construction in Canada is St. Thomas Anglican Church in Shanty Bay, Ontario. It was built between 1838 and 1841.

From the 1920s to the 1940s, rammed-earth construction in the U.S. was studied. South Dakota State College built nearly 100 weathering walls of rammed earth and researched soil paints and plasters for over 30 years. In 1943, Clemson Agricultural College published a pamphlet titled Rammed Earth Building Construction. In 1936, the U.S. Department of Agriculture built experimental rammed-earth homes near Gardendale, Alabama, with architect Thomas Hibben. These homes were sold to the public with land for gardens and small livestock areas, helping low-income families.

Today, the U.S. Agency for International Development works with developing countries to improve rammed-earth house engineering. It also funded the Handbook of Rammed Earth by Texas A&M University and the Texas Transportation Institute.

After World War II, interest in rammed earth declined as modern materials became cheaper. Many contractors, engineers, and tradesmen consider rammed earth substandard.

A modern example is the façade of the Nk'Mip Desert Cultural Centre in southern British Columbia, Canada. As of 2014, it is the longest rammed-earth wall in North America.

Australia has developed a strong tradition of rammed-earth construction, especially in Western Australia. This method dates back to early colonial times, with use in all states and territories. It was most common in New South Wales, where the MacKnight family influenced building designs in the Riverina region.

In the 1970s, Western Australia began using rammed earth for homes, schools, offices, and community buildings. This method is now well-established and cost-effective in the state.

In the past 30 years, cement-stabilized rammed earth (CSRE) has become popular in Australia. It uses low-clay soil, water, and cement, making it much stronger than traditional rammed earth. It can withstand compression forces up to 40 megapascals, similar to concrete.

A challenge in Australia is the lack of a national building code for rammed-earth structures, which discourages some engineers and architects from using it.

Rammed earth was important during the rapid construction of the Daqing oil field in China. The "Daqing Spirit" reflected dedication to national goals, simple living, and integrated land use. The area was seen as an example of Marx's ideal communist society, where differences between town and country, workers and peasants, and manual and mental labor were eliminated.

In the mid-1960s, China promoted rammed-earth construction after Mao Zedong encouraged a "mass design revolution movement." This was part of the Sino-Soviet split, where China avoided Soviet-style materials and used local, low-cost rammed-earth techniques.

During the Third Front campaign, China built rammed-earth housing to save resources for industrial production. This led to the slogan, "First build the factory, then housing."

Julian Keable and his son, Professor Rowland Keable, have studied rammed earth in the UK.

Earth structures have been used for thousands of years across the world, but no building codes existed to support their use after the industrial era. In the late 1970s, British architect Julian Keable advised on building without cement for Tanzania's capital, Dodoma. He used traditional rammed-earth methods, leading to projects in Tanzania, Sierra Leone, Ghana, Kenya, Uganda, and Malawi. He later helped create a rammed-earth building code for Africa, which became a standard in Zimbabwe and other regions.

In Europe, especially in France, Britain, and Germany, traditional rammed earth is being used again in modern architecture. Examples include a three-story home in Austria built in 2008. Historically, rammed earth (called "pisé de terre" in French) was common in rural areas where other materials were scarce. Many old rammed-earth buildings still exist in France, Spain, and Germany. Today, rammed earth is linked to sustainable architecture and is used in showcase projects to highlight its environmental and aesthetic benefits, though it remains rare compared to traditional methods.