Rammed Earth

Rammed earth (French pisé de terre, Chinese hangtu) is a building technique in which damp subsoil, a mix of sand, gravel, clay and silt, sometimes with a stabiliser, is compacted in layers inside temporary formwork to form solid walls. It is a walling rather than a roofing material, but rammed earth buildings depend heavily on their roofs for protection, and the roof structure must be designed to sit on and be fixed to earth walls.

Construction

• Layers: loose mix is placed 10-25 cm (4-10 in) deep and compacted to about half its volume; UK guidance cites finished lifts of 100-150 mm (4-6 in).
• Formwork: historically timber planks tied with rope; today plywood, steel or both. Walls are strong enough to strip quickly after compaction.
• Thickness: non-load-bearing walls can be as thin as 150 mm (6 in) and load-bearing walls up to about 600 mm (24 in). UK guidance gives 300 mm (12 in) as a minimum for lateral stability.
• Stabilised rammed earth: adding cement gives strengths of about 5-18 MPa (700-3,000 psi), and higher in some Australian practice, at the cost of lower permeability and the carbon emissions of cement.

History

Rammed earth was in use in the Fertile Crescent by the 9th-7th millennium BCE, and by the Yangshao and Longshan cultures of China by about 5000 BCE. By 2000 BCE it was common in China for walls and foundations. It was widespread in parts of Europe, especially France, Spain and Germany, and was popularised in the United States in the early 19th century.

Roofs on rammed earth buildings

Rammed earth decays when exposed to water, so the roof is its main defence:

• Overhangs: large roof overhangs at eaves and verges reduce wetting of the wall faces.
• Bond beams: a continuous beam of timber or reinforced concrete on top of the wall spreads roof loads and gives a fixing for rafters or joists. New Mexico's Earthen Building Materials Code, for example, specifies that a concrete bond beam be tied into the uncured wall with No. 4 reinforcing bars at no more than 24 in (610 mm) centres, and that a timber bond beam be fixed with 1/2 in (13 mm) hooked anchor bolts rammed at least 18 in (457 mm) into the wall.
• Roof types: earth walls carry both flat earth roofs in dry regions and pitched tiled roofs, as on the rammed-earth tulou of Fujian, China.

Performance

Like brick and concrete, rammed earth has high thermal mass: heat takes about 12 hours to pass through a 35 cm (14 in) wall, moderating day-night temperature swings. In cold climates insulation can be built into the wall. Fire resistance of around 30 minutes is readily achieved, provided the wall stays dry and stable. Flood-prone sites are generally unsuitable.