Earthbag Construction

Earthbag construction is earth building at its most expedient. Bags — typically woven polypropylene, occasionally burlap — are filled with soil, laid in courses, and tamped into dense, stable walls. The technique borrows from military sandbag fortification and agricultural flood control, adapted into a permanent building method where the bag serves as temporary formwork and the compacted earth within it is the wall.

The soil requirements are less exacting than for rammed earth or adobe. A mix of approximately 70 percent sand to 30 percent clay is typical, though earthbag construction tolerates a wider range of soil compositions because the bag itself provides initial containment. Gravelly soils, volcanic ash, and even cement-stabilized soil can be used where the local earth is unsuitable on its own. The bags are filled to approximately 85 to 90 percent of their capacity, leaving enough slack at the top to fold and close. Each filled bag weighs between 20 and 35 kilograms depending on size and soil density, and is laid lengthwise along the course, then tamped firmly with a flat-bottomed tamper until the bag flattens into a roughly rectangular section approximately 300 millimeters wide and 100 to 120 millimeters high.

Barbed Wire and Tensile Continuity

Between each course of bags, two strands of four-point barbed wire are laid along the full length of the wall. The barbed wire serves the same structural role as the straw in cob or the rebar in reinforced concrete — it provides tensile reinforcement across a material that is strong in compression and weak in tension. The barbs grip the woven fabric of the bags above and below, locking each course to the next and resisting lateral forces, particularly the outward thrust of wind or seismic movement.

Without the barbed wire, the wall is a stack of individual units with friction as the only resistance to sliding. With it, the assembly behaves as a unified mass. This is what makes earthbag construction viable in seismic zones — the barbed wire creates a continuous tensile network through the wall, redistributing lateral loads rather than allowing individual courses to separate and collapse. Four-point wire is preferred over two-point for its grip; the barbs must penetrate the bag fabric on both faces of each course to be effective.

Geometry and Curves

Earthbag walls follow curves naturally. Because the bags are flexible when placed and rigid only after tamping, they conform easily to circular and elliptical plans. Domed structures — where each successive course is laid slightly inward from the one below — are the natural culmination of this geometry. The dome is self-supporting during construction because each bag is locked to the course below by barbed wire and gravity, and the inward lean of each course is slight enough that the assembly remains stable without centering or formwork.

A full earthbag dome requires no separate roof structure. The dome itself is the roof, and once rendered with plaster, it sheds water by geometry alone. Corbelled domes — where each course steps inward incrementally rather than following a true arch — are more common than catenary curves because they are simpler to lay and do not require calculation of thrust lines. The maximum practical span for an earthbag dome without internal buttressing is approximately 5 to 6 meters. Beyond this, the horizontal thrust at the base of the dome exceeds what barbed wire and bag friction can resist, and either a bond beam or a buttress wall is required.

Foundations and Moisture

The foundation for an earthbag wall follows the same principle as all earth construction: the wall must be separated from ground moisture by a material that does not wick. A rubble trench filled with gravel, topped with one or two courses of bags filled with gravel rather than soil, provides both drainage and a capillary break. The gravel bags are laid below grade, and the transition to soil-filled bags occurs at or above the finished ground level.

The polypropylene bags themselves are not waterproof but are largely unaffected by moisture in the short term. Their vulnerability is ultraviolet radiation — unprotected polypropylene degrades in direct sunlight within one to two years, becoming brittle and losing tensile strength. This is not a structural concern for the finished wall, because the bags are temporary formwork. Once the earth inside is tamped and the courses are locked together with barbed wire, the wall's structural integrity depends on the compacted earth and the wire, not on the bag fabric. But the bags must be protected from UV exposure during and immediately after construction, either by rendering the walls or by covering exposed surfaces until plaster can be applied.

The Plaster Surface

Every earthbag wall requires a plaster or render coat. The reasons are both protective and structural. The plaster shields the bags from UV degradation, provides a weather-resistant surface, and — on earth-filled walls — creates the vapor-permeable skin that allows moisture within the wall to migrate outward and evaporate. Cement stucco, lime render, and earth plaster are all used, with the choice depending on exposure and climate.

Lime render is preferred for exterior faces in wet climates because of its durability and vapor permeability. Earth plaster is appropriate for interior surfaces and sheltered exteriors. Cement stucco provides the greatest weather resistance but is less vapor-permeable, and on earthbag walls its rigidity can cause problems — if the wall shifts or settles slightly, cement stucco cracks, while lime and earth plasters accommodate minor movement without failure. The first coat is applied directly to the bag surface, keyed into the woven fabric. Subsequent coats build thickness and provide the finished surface.

What the Bag Contains

The finished earthbag wall, once tamped, wired, and rendered, is functionally indistinguishable from any other massive earth wall. Its thermal behavior follows the same principles — high thermal mass, moderate conductivity — approximately 0.7 to 1.0 watts per meter-kelvin, comparable to cob and rammed earth — and a thermal lag proportional to wall thickness. A 300-millimeter earthbag wall provides a thermal lag of approximately six to eight hours, shifting exterior temperature cycles in the same manner as adobe or rammed earth of comparable thickness.

What distinguishes earthbag construction is not the material of the wall but the method of its assembly. The bag allows earth to be placed rapidly, with less skill than rammed earth requires and less drying time than cob or adobe demand. The trade-off is a wall that is somewhat less dense than rammed earth — tamping within a bag achieves lower compaction than ramming within rigid formwork — and that relies on barbed wire for its tensile network rather than on the fibrous continuity of straw in cob. These are differences of degree, not of kind. The earth is the same. The wall is the same. The bag, in the end, is the part that does not matter — it holds the earth in place long enough for the earth to hold itself.