Earthbag/ Contained Earth
Earthbag is a geotextile earthen wall formed in woven materials. It creates walls that are modular (in fabric bags or tubes) or solid/ monolithic (in mesh tubes) using a layered, immediate construction technique.
Damp natural subsoil is contained in the fabric tubes. Barbed wire laid between courses adds critical friction to resist forces. Walls are plastered or stuccoed to protect from rain.
Barbed wire gives better tensile strength and flexibility than most other earthen walls. When stressed, earthen walls fail suddenly with a brittle type of collapse. We haven’t seen earthbag walls collapse yet. They fail very slowly and warp a lot. But they are surprisingly tough if built carefully with the right materials.
Earthbag can be built with any fill, in any way.
Contained sand, contained gravel and contained earth are all types of earthbag.
Sandbag or Contained Sand (CS)
Often used by civil engineers under railroads or roads, to fight beach erosion, or in other site structures, these bags or tubes (called soil bags) are filled with loose subsoil materials. The fill may contain sand and other particles, but not enough clay to be cohesive (solidify in the bags).
Early earthbag builders used bags made to hold sand and filled them with earth that contained sand and some clay. But although sand fill could make flood-resistant walls, it is not used for structural earthbag building walls because of its low strength. CS becomes unstable at about 4′ (1.2 m) height unless a lot of extra reinforcement is added. It is used as wall infill between studs for narrow-wall earthbag.
Gravel Bag or Contained Gravel (CG)
Gravel fill is not damaged by water but is more stable than sand. Where gravel is available, several courses are used as base wall. Barbed wire does not attach well to gravel bags, but they can be confined by horizontal wire wrapping for extra strength.
Most earthbag walls are built with slightly damp soil that is just tacky enough to form solid masses when tamped in the bags. These courses dry around barbed wire barbs that pierced the fabric to extend into the fill. Strong dried soil holds barbs securely and keep courses in place stacked on top of each other. Pressure on one end of a cohesive earthbag wall is shared along the wires throughout the wall. If barbed wire extends from walls through buttresses and around corners the building has less weak points than unreinforced earthen walls.
Adobe walls are traditionally built with buttresses extending out of the corners for better stability. Conventional earthbag buildings often use buttresses for the same reason.
Vertical and angled pieces of rebar (reinforcing steel) can be hammered into the damp walls. Short pins usually stick up from the foundation. Angled 2′ (60 cm) pins are usually driven in wall tops to attach a bond beam. Sometimes rebar is also inserted vertically near corners and openings.
Contained Earth (CE)
Contained earth is conventional earthbag with soil strength and reinforcement chosen to resist a specific level of hazard. All CE has soils of compressive strengths at least 190 psi (1.3 MPa). Simple field tests of soil strength are used to estimate building soil qualities.
Partially reinforced contained earth has conventional vertical inserted rebar. The first rod is driven in at mid-wall height and does not connect to a concrete footing. The second rod is driven in at the wall top and overlaps the first rod. The steel adds some strength.
Fully reinforced contained earth uses either a single entire rebar, or else leaves gaps to splice overlapping rebar together. Tests to date show that shear strength is at least 30 percent higher when rebar is entire or spliced.
Additional improvements to CE reinforcement range from small pins made of metal mesh placed at stress points, to cords tying barbed wire out walls at corners, to cutting bags to fit around footing-anchored vertical rebar so that barbed wire attaches to the steel. BSI is in process of evaluating the strengths and developing guidelines for these different techniques with different soil strengths.