3.0 Soils for Rammed Earth, Earth Block, and Soil Material Construction

  • Soils that qualify for both Compressed Earth Block and Rammed Earth are common in most 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. The 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 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:
    • Strength
    • Low Moisture Absorption
    • Limited Shrink/Swell Reaction
    • High Resistance to Erosion and Chemical Attack
    • availability

         3.1 Soil Testing

    • 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.

         3.2 Chemical Soil Stabilization

    • Lime, portland , and pozzolan (high silica volcanic ash) can be used as chemical additives. Lime is most effective on clay soils, and can be used in combination with portland 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. 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 (source). Pozzolan exists in plentiful supply in Texas , but is not readily available commercially.
    • (The Center for Maximum Potential Building Systems (CMPBS) in Austin is experimenting with the use of pozzolan as an additive and offers considerable expertise in earth materials use. See the Resources section.)

         3.3 Strength of tested earth and caliche block

    • 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.

         3.4 Soil Handling

    • 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.

         4.0 Block Production Methods

    • A backhoe and or a front end loader will be needed to dig the soil on-site or handle soils imported. Soils are then tested to prove their compactability and to determine any needed additions such as sand or clay. The next step of hydrating and mixing has traditionally been the largest labor and time investment being done either by hand or with a front end loader. The use of concrete and stucco mixers have proven ineffectual for large projects such as a home, however there are earth mixing or blending machinery available that are especially cost effective for adding Portland or lime and for adding water in dry areas. Source
    • Sun Dried Adobe
    • Molding techniques may be in the form of monolithic walls (See Rammed Earth) or molded into blocks or bricks. For the latter, the mix is poured into molds, or pressure molded using special machinery. These methods provide for a variety of standard and custom size and shapes of block. With the hand mold technique, the prepared mix is poured into damp or oiled molds, spread evenly, and the molds are shaken slightly to ensure even filling of the forms. The blocks are then removed and allowed to cure before stacking.
    • Air curing must occur for 10-14 days before the block can be used in construction. Protection from direct sunlight for 5 days and protection from rain throughout the curing process are important. Drying bricks may be temporarily covered with tarps or plastic sheeting, but these must be removed for curing to continue. Once bricks are sufficiently cured, they can be set on end to continue drying.
    • With a wheelbarrow and gang forms, a crew of two can produce 300 to 400 bricks per day. With the addition of a plaster mixer and gang forms for 500 bricks, this production can be doubled. The addition of a front end loader with a driver will additionally increase production.
    • Compressed Earth Block
    • Compressed earth or soil block can be manufactured on site with a variety of block-making machines, including hydraulic presses, mechanical presses, and various combinations. Some mechanical presses are small enough to be operated by hand (Cinva-Ram, for instance). With a mobile industrial block machine, powered by a diesel engine, as many as 800 blocks can be produced per hour. Compressed soil blocks can be used immediately. They continue to cure and gain strength after they are installed. When green (before they are cured), they can be readily shaped or nailed into with hand tools.

Compressed Earth Block come in two basic types, The vertical press where the block are normally 10 x 14 (there are many variations) that are fixed with the height of the block nominally 3 which is variable due to the variability of the soil. These block are treated like Adobe in that they need to be mortared and cut to fit. The Horizontal Press are of a fixed dimension normally 4 x 14(again there are variations) with a length of the block variable from 2 to 12 depending on the machine. These blocks do not require mortar and can be dry stacked with ease by basic skilled workers, the block can also be custom sized to minimize cutting for electrical, plumbing and wall changes.

         4.3 Mortaring

    • Mortar for blocks must be applied to the entire surface of the block, as opposed to ribbon mortar beds often used with conventional brick. Full surface mortaring allows for maximum compressive strength. The same soil used in block making, mixed with water to form a slurry, is usually used as a mortar for binding blocks together into floors and walls. Cement can be added to the mortar mix, but this increases the cost. The main advantage of cement mortar is stabilization.

         4.4 Design Methods

    • Block size can be varied easily to accommodate a variety of designs. Walls can be sculptured, rounded, or formed into keystone arches to create custom effects. Relatively unskilled labor can be utilized in construction with compressed earth block.
    • Design of structural walls using any soil material block must take into account wall height and thickness, size of block, mass value (source), and the desired style and finish. Wall height-to-thickness ratio must be adequate for stability (source).
    • Because thermal mass equates to insulation in soil block a minimum of 12 inches is needed for a comfortable abode.
    • Earth block structures need not have the "pueblo" style if this is not desired. In fact a gable or hip roof can protect the home better while offering solar protection from western exposures. A bond or collar beam is necessary if the roof is supported by the walls. This will serve to spread the loads over the entire wall, and stabilize the tops of the walls from horizontal movement. (See code)

Plasters

    • Soil blocks are typically stuccoed or plastered  to prevent them from getting wet, however, any veneer or siding can be used on Pressed Earth Block as they can hold a nail or staple. Interior finishes are normally plaster (structolite) or earth plasters that are simple to apply and maintain. Petroleum based finished do not work well with earth unstabilized earth block and cement plasters do not stick to asphalt stabilized adobe. A common mix for a stabilized interior mud plaster is 5% Portland to 30% minimum clay fine screened with window screen. Exterior mud plaster will need 6 to 10% Portland with 30% minimum clay and 1/8 screen.

 

    • Fully stabilized structures do not require any exterior finish unless desired for aesthetics.