Rammed earth

Lore & Background

Rammed earth, or pisé, is an ancient construction method that involves compacting a damp mixture of subsoil—containing sand, gravel, clay, silt, and sometimes a stabilizer—into a formwork. Historically, additives such as lime or animal blood were used to stabilize the mix. The soil is poured into the formwork in layers of 10 to 25 cm and compacted to about half its original volume, a process repeated until the wall reaches the top of the formwork. Tamping was originally done manually with a long pole, but modern systems may use pneumatic tampers. After a wall is complete, the formwork can be removed immediately, and surface textures such as wire brushing or carving must be applied within about an hour before the wall hardens. Cement-stabilized rammed earth requires a minimum curing period of 28 days.

Reader's Guide

Rammed earth is significant for its sustainability and low environmental impact when locally sourced materials are used. It has high thermal mass, absorbing heat during the day and releasing it at night, which moderates temperature fluctuations and reduces heating and cooling needs. In colder climates, insulation can be added between layers. Rammed earth also regulates humidity between 40% and 60% if unclad clay walls are exposed to interior spaces, avoiding condensation and heat loss. The walls are fireproof, termite-resistant, non-toxic, and suitable for soundproofing. However, adding cement to the soil can increase compressive strength but also raises carbon emissions, as cement production generates 1.25 tonnes of CO₂ per tonne of cement. A 300 mm rammed-earth wall with 5% cement produces slightly more emissions than a 100 mm concrete wall. The technique has been used for millennia, with evidence from Neolithic sites in the Fertile Crescent and ancient China. In the United States, it was popularized in the 1800s through S. W. Johnson's book Rural Economy, and notable examples include the Borough House Plantation and St. Thomas Anglican Church in Canada. Research in the 20th century, including studies by South Dakota State College and Clemson Agricultural College, furthered its understanding. The US Agency for International Development has supported improving rammed-earth engineering in developing countries.

Did You Know?

• Rammed earth walls can be as thin as 6 inches for non-load-bearing walls and up to 24 inches for load-bearing walls.
• The Borough House Plantation in South Carolina contains the oldest and largest collection of 'high style' pisé de terre buildings in the United States.
• Adding cement to rammed earth can increase compressive strength but also raises carbon emissions, as cement production creates 1.25 tonnes of CO₂ per tonne of cement.
• Rammed earth can regulate indoor humidity between 40% and 60% if unclad clay walls are exposed to the interior.