Rammed earth is a structural wall system built of natural mineral soils compacted in thin layers within sturdy formwork. The strength and durability of the wall results from the densification of a clay, sand, and gravel matrix. The mass of the wall provides superior thermal and acoustic properties.
To build a rammed earth wall, moist soil is compacted layer at a time inside of a strong mold or form. The forms can be removed immediately and the fresh wall is capable of supporting loads. It continues to cure and gain strength slowly over time. In the early days, rammed earth was primitive. Forms were small, cumbersome, and imprecise; soil was taken directly from the base of the wall and carried to the forms in baskets; the soil often included sticks and small boulders; joints between wall sections were crude and jagged. Early rammed earth was used primarily as an efficient conversion of an available resource into a serviceable wall, one which would keep marauders out of the village or cold and damp out of the hut. The rammed earth method remained virtually unchanged for several thousand years.
Over time, refinements to formwork and soil preparation began to elevate the method from the primitive and merely serviceable to a position of respect and wider acceptance. Eighteenth and nineteenth century architects, builders, and agronomists in Europe and the United States introduced elements of standardization and science that resulted in major improvements to the technology. Reasons for use continued to be economy and convenience of resource. Rammed earth remained in the backwater of the construction vernacular for two hundred more years until it was rediscovered in the mid 1970’s.
This twentieth century re-birth of the technology was prompted by premonitions of an impending global environmental crisis. The first hints of an end to oil, an expanding ozone hole, and health concerns stemming from toxins in houses inspired a search for natural alternatives in energy production and building materials. Rammed earth, built of unprocessed mineral soil and capable of storing passive solar energy, presented an ideal building alternative.
Recent improvements in all aspects of rammed earth technology have combined to give it increased respect within the architectural community and to place it near the top of the list of green building alternatives. Although rammed earth can serve as simply a resource-conserving thermal flywheel, in other applications it is celebrated as functional art.
Several factors affect the final appearance of a rammed earth wall. The most significant variable is the mix design itself: the proportioning of fine and coarse particles, the color of the mineral soil, the angularity of the gravel. Other controls over final appearance are formwork, delivery method, lift depth, strata alignment, compaction technique, curing, sealing, surface treatments, and cosmetic remediation.