Rammed earth is an ancient building technique in which a damp mixture of soil, sand, and gravel is compacted into formwork in successive layers to create monolithic walls. Archaeological evidence dates the technique to at least 5000 BCE in China, where sections of the Great Wall were built using rammed earth. In modern practice, the technique has been refined for contemporary architecture through engineered soil mix designs, pneumatic tampers, and stabilising additives. Rammed earth is experiencing renewed interest as a low-embodied-carbon alternative to concrete and fired brick.
Soil Mix: 30-40% Clay/Silt, 60-70% Sand/Gravel
The ideal soil mix for rammed earth follows a well-graded aggregate curve similar to that used for road base material. Clay and silt content should be 30 to 40% by weight, with sand and gravel comprising the remaining 60 to 70%. The clay fraction acts as the natural binder: when moistened and compacted, clay particles align and form cohesive bonds through capillary forces and electrostatic attraction. The sand and gravel provide compressive strength and reduce shrinkage during drying.
Grading is critical. The maximum particle size should not exceed approximately 40 millimetres, and the particle size distribution should be continuous with no gaps that would create voids after compaction. On-site soils are rarely ideal without amendment. Builders typically source a base soil with adequate clay content and blend in imported sand and gravel to adjust the gradation. Soil testing, including sieve analysis and Atterberg limits, is essential before construction begins.
Stabilized vs Unstabilized
Unstabilized rammed earth relies solely on the natural clay binder and compaction to achieve strength. These walls can last centuries in dry climates, as demonstrated by surviving structures in Yemen, Morocco, and the American Southwest. However, unstabilized earth is vulnerable to erosion from driving rain and cannot be used in seismically active regions without reinforcement.
Stabilized rammed earth incorporates a hydraulic binder, typically Portland cement at 5 to 10% by weight of the dry soil mix. Cement stabilisation dramatically improves compressive strength, typically reaching 5 to 10 MPa compared to 2 to 5 MPa for unstabilized, and provides resistance to water erosion. The trade-off is embodied carbon: cement production accounts for the majority of the material's carbon footprint, and cement contents above 8% can negate the carbon advantage of using earth as the primary material. Lime stabilisation, using hydraulic lime at similar dosage rates, offers lower embodied carbon than cement while providing adequate stabilisation for many climates.
Thermal Performance
Rammed earth walls are typically 300 to 600 millimetres thick, giving them enormous thermal mass. The material stores heat during the day and releases it at night, creating a thermal lag of approximately 10 to 12 hours for a 450-millimetre wall. In climates with diurnal temperature swings exceeding 15 degrees Celsius, such as the high desert of New Mexico and Arizona, this thermal lag aligns daytime heat with nighttime release, moderating indoor temperatures without mechanical heating or cooling.
The R-value of rammed earth is low, approximately 0.4 per inch of thickness, meaning a 450-millimetre wall achieves roughly R-7 total. In cold climates, rammed earth walls usually require an exterior insulation layer to meet modern energy codes. The thermal mass benefit is substantial only when the wall is exposed to direct solar gain on the interior, as in passive solar design, or when diurnal temperature swings are large.
Modern Practitioners and Building Codes
Meror Krayenhoff, founder of SIREWALL Inc. based on Salt Spring Island, British Columbia, is the leading contemporary practitioner of rammed earth. SIREWALL uses engineered soil mixes with 7 to 10% cement stabilisation and proprietary formwork to produce walls with compressive strength exceeding 10 MPa, finished surfaces that require no plaster or sealant, and water resistance suitable for wet climates.
Building codes have evolved to accommodate rammed earth. New Mexico's building code has included rammed earth provisions since 1991, making it the most mature regulatory framework in the United States. Australia's AS 3700 Masonry Code and the Earth Building Association of Australia's guidelines provide design and construction standards. France has a national standard for rammed earth, and Zimbabwe published a national earth building code in 2001. The International Building Code does not include prescriptive provisions for rammed earth, meaning most US projects outside New Mexico require engineered design and approval as an alternative material.