Rammed earth house by martin rauch and roger boltshauser
Photo by Neille Hepworth
Soils for CEB and Rammed Earth
Soils that qualify for both Compressed Earth Block and Rammed Earth are common in many areas. Consider that most of the continents are granitic and decomposed granite is normally perfect having the ratios of feldspars to quartz that are appropriate for compaction. Basaltic soils are a little more difficult and many times require additional clay added. The basic formula is 30% clay and the balance loam and small aggregate. Caliche (which is usually a misnomer for decomposed limestone soils) is the common subsoil of the alluvial plain which dominates the south Texas landscape, much of the Midwest and most of the deep south as well as most of the Caribbean . In The Dominican Republic it is named for the coral reefs that underly the island and is somewhat compactable depending on the area. The use of decomposed limestone can be problematic unless modified with either the addition of clay, portland cement or lime if necessary.
Soils that are bentonitic or highly expansive are normally unsuitable for earth construction without modification. The shrink and swell capacity of these soils, related to their clay content can cause the block to be highly susceptible to moisture, even high humidity, however the acid test is how the clays actually perform under compaction and even poor performance can be offset by stabilization. Soil cracking after rainfall may indicate expansive soil. Soil must be tested to determine its suitability. The ideal is a block or wall that looks pretty and has a lot of strength but even ugly block and marginal soils can be used to build a structure that will last for centuries.
Desirable qualities for soil construction materials include:
Low Moisture Absorption
Limited Shrink/Swell Reaction
High Resistance to Erosion and Chemical Attack
Soil testing techniques vary, and include laboratory as well as field testing. Testing is done in three phases: laboratory testing, construction mix testing, and quality control testing. Laboratory testing should always be done early in the design process, using representative samples of soil intended for use. (See Resources section for laboratories.) Engineering properties for which soils are tested include permeability, stability, plasticity and cohesion, compactibility, durability, and abrasiveness. Shrinkage, swelling and compressive strength are important aspects of soil suitability.
Again, it is possible to alter soils to make them suitable for construction by stabilizing them. Stabilizing soil helps to inhibit the shrink and swell potential, and aids in the binding of soil components. Soil can be stabilized through chemical or mechanical means or both. For information on mechanical methods, see Section 5.0 on rammed earth.
Chemical Soil Stabilization
Lime, portland cement, and other pozzolans (high silica volcanic ash, rice hull ash, etc) can be used as chemical additives. Lime is most effective on clay soils, and can be used in combination with portland cement and pozzolan. Hydrated lime, as opposed to quick lime, should be used. Lime is inexpensive, but care must be taken to protect workers from breathing in lime dust. Cement is relatively inexpensive, but requires large energy inputs in its production process and puts approximately an equal weight of carbon dioxide into the atmosphere. However, cement produces the strongest block and will substitute for clay poor soils where lime will not and the normal usage of between 5 and 10% minimizes the embodied energy especially when compared to concrete and lumber products * . Pozzolan exists in plentiful supply in many areas, and is sometimes readily available commercially in the form of coal fly ash .
Strength of tested earth
Unfired Compressed Earth Block with addition of 5-10% cement can easily pass the Uniform Building Code standards for compression with an average of 960 psi.
Rammed earth walls have been tested with a compressive strength of 30 to 90 psi immediately after forming. Ultimate compressive strength should reach 450-800 psi. If cement is added, compressive strength will increase.
The Uniform Building Code for single and two story buildings requires block bearing capacity of 300 psi bearing strength. Blocks manufactured with a hydraulic press have been tested with a bearing capacity immediately after production of 700 psi. Such soil block continues to cure, until blocks reach a typical bearing capacity of 1000 psi., far exceeding requirements of the Uniform Building Code and HUD standards. Cement can be added to the soil block mixture to reach a bearing capacity of 2500-3900 psi.
The use of earth as building materials is inexpensive for materials costs, but emphasizes labor in construction methods. The right equipment and coordinated labor are important in the soil material construction process. Even a small structure may require at least 15 tons of earth. This material must be moved and handled at least twice. A front end loader, skidsteer or tractor equipped with a shovel or back hoe will be necessary for on-site extraction of soil materials as well as processing the soil and loading the machinery. A large flat area with good drainage is necessary for handling and processing the materials as well as making the blocks. The building footprint should also be accessible by truck for rammed earth construction.