Earthbag construction is an inexpensive building method using mostly local soil to create structures which are both strong and can be quickly built. Earthbag development Features It is a natural building technique developed from historic military bunker construction techniques and temporary flood-control dike building methods. The technique requires very basic construction materials: sturdy sacks filled with organic material usually available on site. Standard earthbag fill material has internal stability. Either moist subsoil that contains enough clay to become cohesive when tamped, or a water-resistant angular gravel or crushed volcanic rock is used. Walls are gradually built up by laying the bags in courses—forming a staggered pattern similar to bricklaying. The walls can be curved or straight, domed with earth or topped with conventional roofs. Curved walls provide good lateral stability, forming round rooms and/or domed ceilings like an igloo. Buildings with straight walls longer than in length need intersecting walls or bracing buttresses. International standards exist for bracing wall size and spacing for earthen construction in different types of seismic risk areas, most notably the performance-based standards of New Zealand recommended by the ASTM International's earth building standards. Static shear testing shows that earthbag can reach similar strengths to New Zealand's reinforced adobe standards with specific soil strengths and reinforcement although unreinforced weak soil earthbag can have lower shear strength than unreinforced adobe. To improve friction between bags and wall tensile strength barbed wire is usually placed between courses. Twine is also sometimes wrapped around the bags to tie one course to the next, to hold in-progress structures together and keep courses well-seated on barbed wire prongs. Rebar can be hammered into walls to strengthen corners and opening edges and provide resistance against overturning. The structure is typically finished with plaster, either cement stucco on a strong mesh layer or an adobe or lime plaster, to shed water and prevent fabric UV damage. Finishes can differ from protected interior applications to exposed external applications. This construction technique is one of the most versatile natural building methods and can be used for benches, freestanding walls, emergency shelters, temporary or permanent housing, or barns and commercial buildings. Earthbag is frequently chosen for many small-to-medium-sized institutional structures in the developing world. Subgrade structures including underground and bermed dwellings (such as Earthships), cisterns, spring boxes, root cellars, and retaining walls can be built with stabilized soil fill or with additional reinforcement and water-resistant gravel