A brief introduction to Stabilized Earth Construction
by Bruce King
exerpted in part from
Alternative Construction - Contemporary Natural Building Methods
edited by Lynne Elizabeth and Cassandra Adams
John Wiley & Sons, 2000
Reproduced with permission
Earth (exclusive of organic topsoils, which are never good construction material) covers the planet in a layer of varying thickness, and appears in a virtually infinite array of physical and chemical properties. Of prime concern to an earth builder is the particle gradation: how much of an earth supply is gravel (larger than 2 or 3 millimeters), sand (from one hundredth to 2 or 3 millimeters), and fines (less than 75 thousandths of a millimeter). The fines, in turn, contain silt (essentially very fine sand) and clay (leached minerals whose macroscopic properties change with wetting or drying). In historic adobe, cob, and rammed earth structures, the best building soils typically contained between 20 and 30% clay as the stabilizing binder, and relatively little silt. Such structural materials typically attain compressive strengths between 200 and 800 psi, and 300 psi is considered by some adobe codes to be a minimum (McHenry, "Adobe and Rammed Earth Buildings", 1984)
In modern times, lime, fly ash, bitumen, and especially portland cement are sometimes added as stabilizers, and in sufficient quantity can turn virtually any soil into buildable soil. With one of these added stabilizers, strengths of 1,000 to 2,000 psi are easily attained, and this author has experience with cement-stabilized rammed earth (8% by weight) reaching long term strengths in excess of 6,000 psi. In some cases, such as tire houses or the earth bag systems developed by Nader Khalili, little or no chemical additive is required, as the earth is stabilized by containment in durable surrounds. In other cases, such as compressed adobe block or rammed earth, strength is developed or greatly augmented by mechanical compaction.
The strength of most modern and historic earthen structures also relies on sheer massiveness; the relative weakness of the material is made up for by the low height-to-width ratios (eg two foot thick rammed earth walls only nine or ten feet high). Adobe and cob buildings may also incorporate buttressing, and sometimes curved walls (and, sometimes, vaulted or domed roofs) that are inherently stronger than straight walls connected at right angles. Lacking other design criteria, builders and engineers often use empirically derived height to width ratios such as those published in the Uniform Code for Building Conservation. Guidelines from unreinforced masonry practice are also helpful, such as limiting the size and proportions of piers and openings.
Seismic Zone |
2B |
3 |
4 |
One-story buildings |
12 |
10 |
8 |
Two-story buildings - first story |
14 |
11 |
9 |
Two-story buildings - second story |
12 |
10 |
8 |
| Source: Uniform Code for Building Conservation, 1994, Table A-1-G. |
In less seismically active areas (zone 2B or less in the parlance of the Uniform Building Code), the need for stabilization is driven more by durability than strength. Cultures of Asia, Arabia, North Africa and the American Southwest have gotten hundreds of useful years of service out of adobe buildings by simply replastering every year (the plaster is the same earthen material), usually as part of a cultural event in the Spring. In lieu of annual replastering, a modest amount of binder in the wall material, especially with good roof overhangs and a raised foundation, will provide decades of serviceable life. (The application of cement plaster to an earth wall emphatically will not; the relatively impermeable cement stucco will trap moisture, leading to an unseen and rapid erosion of the earth wall beneath.) Any plaster, in otherwords, must "breathe", ie be vapor permeable commensurate with its substrate. For earthen structures not highly stabilized with cement or lime, protection from moisture attack (especially in freeze/thaw climates) can be the primary design concern.
Design in high seismic risk areas is overwhelmingly dominated by earthquake loads, because seismic loads are in proportion to mass. In strong seismic events, poorly designed or stabilized structures often fail catastrophically, while the sturdier buildings (well-built and with low height to width wall dimensions) show surprising strength and ductility. Durable modern earthen structures in seismic risk areas tend to rely either on cement binders (typically five to ten percent of the earth mix) with reinforcing of steel or geosynthetic textile, or sometimes just on the very stable geometry of thick or curving walls. If reinforcing is used, the working stress design method is conservative and appropriate, as reinforcing steel will bond to cement-stabilized earth with strengths of 380 psi and higher (King, Buildings of Earth and Straw , 1996). Walls can be designed to cantilever off of wide foundations under out of plane load, and should in all cases be well locked together at the tops with some form of reinforced bond beam.
Centuries of earthbuilding experience as well as modern research and testing have shown that earthen construction can be safe, even in the most seismically active areas. Sometimes modest amounts of materials such as cement and reinforcing are needed, but always an understanding of the specific soil, the specific building method, and of engineering principles in general are required to insure safety and durability.
