Earth Materials Guidelines

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.

• 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, cement, 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 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. However, cement produces the strongest block. 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 caliche 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 soil and caliche 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 several times. A bulldozer, front end loader or tractor equipped with a shovel or back hoe will be necessary for on-site extraction of soil materials. A large flat area with good drainage is necessary for hand molding of blocks. The building footprint should be accessible by truck for rammed earth construction.

4.0 Caliche and Soil Block Construction

4.1 Materials

• Caliche is used in our area as a road base material, and in the production of cement and lime. Although not commonly used as a building material, there are historical as well as current examples of caliche for construction. For an in-depth treatment of the subject, see The Caliche Report (see Resources.)

• Caliche occurs in abundance in the Austin area. It is possible to get caliche from the construction site. However, if this is not possible, caliche may be purchased from area suppliers. Be sure to test the source. The use of soil as the basic block material is also possible, but will have slightly different stabilization demands. The same methods described here can be used with soil block.

4.2 Block Production Methods

• A bulldozer or front end loader will be needed to extract caliche or soil on-site. Between 5% and 10% cement must be added to the caliche block, depending on the quality of the caliche. For mixing, caliche must be dried and screened. Some soil may not require this step. The soil components are mixed in a mortar mixer or concrete mixer.

• 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 caliche 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, 800 blocks can be produced per hour. Compressed soil blocks can be used immmediately. 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.

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 its quick drying speed.

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 and caliche block.

• Design of structural walls using caliche or soil material block must take into account wall height and thickness, size of block, insulation value, and the desired style and finish. Wall height-to-thickness ratio must be adequate for stability.

• The relatively low insulation value of soil or caliche block may make additional insulation necessary. The most cost effective wall thickness for insulation value is 12 inches.

• Soil or caliche block structures need not have the "pueblo" style if this is not desired. Many architectural styles are possible. 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.
• Vertical reinforcement is difficult with solid block walls, but can be accommodated with the use of reinforced concrete columns at corners, wall openings, and at intervals in the wall. In this case, the soil block becomes an infill panel. Alternatively, walls made more than one block thick may have internal reinforcing between blocks, and have additional insulation between panels. With this method, care must be taken to ensure that the lower block courses are completely dry before additional courses are added.

• Soil blocks are typically stuccoed to prevent them from getting wet. Clear finishes may be applied on the interior.

5.1 Preparation and Transport of Soil

• Rammed earth soil mixes must be carefully prepared by screening, pulverizing, and mixing. Pulverizing is important to ensure a uniform mix and to break up any clumps.

• Transporting the soil mix to the forms is a demanding tasks. Large quantities of soil must be moved and transported vertically for placement in the forms. This process is not the same as pouring concrete, because the material is not liquid. Traditionally, workers passed baskets or buckets of earth up to where it was needed. Hoists can also be used effectively for this task.

5.2 Form work

• Form work for rammed earth must be stable and well-built in order to resist pressure and vibration resulting from ramming. Small, simply designed forms that are easy to manage are most effective. Ease of assembly and dismantling should be considered when designing forms. A variety of materials can be used for forms, including wood, aluminum, steel, or glass fiber.

• Systems for keeping form work in position vary. Small clamps adapted from concrete form work techniques work well, although small holes are left when the clamps are removed. Other methods include locking hydraulic jacks, or form work built on posts. For more discussion of form work design, organization and moving, see the Earth Construction Primer, and Adobe and Rammed Earth Buildings listed in Resources.

5.3 The Ramming Process

• Once a soil "lift" of 6 to 8 inches in thickness is in place, the soil is rammed. Ramming can be accomplished manually or mechanically. Manual ramming is an ancient technique using a large, specially shaped tool with a long handle called a rammer. Rammers weigh around 18 pounds, and have heads of wood or metal. Differently shaped heads are designed to perform ramming for various form shapes, especially for corners.

• Mechanical impact ramming uses pneumatic ramming machines. Only rammers specifically designed for soil are effective (rammers which are too powerful or too heavy will not work). Such equipment is quite expensive, but impact ramming is highly effective, and if the soil mixture is good, creates high quality rammed earth. Rammed earth will begin to cure immediately, and can take from several months to several years, depending on weather and humidity to complete the process.

5.4 Design Methods

• Rammed earth walls have low tensile strength, and should be reinforced by providing a bond or collar beam. Beams can be constructed of concrete, wood or steel. Vertical reinforcing may also be done, and may be required by some building officials.

• All openings in rammed earth walls, such as windows and doors, must have lintels to span the opening width. Water flow and moisture control is critical to protect structural walls. Special detailing to accommodate manufactured windows may be necessary to accommodate wall thickness. All openings for doors and windows will require a frame. Wood, as opposed to metal, is recommended due to the corrosive action of moisture from the soil material. Lintels can be concrete, stone or wood. Careful attention to both roof and opening details is necessary to protect the structure from water damage.

• Foundations required by most codes are concrete reinforced with steel. Soil block material may be used as a filler material between piers of a reinforced concrete pier and beam foundation. Historically, many structures built with earth materials had no foundation, or used sand and gravel foundations. The latter are excavated trenches filled with two parts sand to three parts gravel. Trench bottoms should be graded to provide positive drainage. Soil material block should not be used in below grade walls unless supported on both sides. Natural moisture from the ground may infiltrate the block, resulting in reduced compressive strength.

7.1 Plaster

• Mud plaster is usually applied in two coats for both exterior and interior surfaces. The addition of straw is recommended in the mud plaster mix. This will help to reinforce the plaster, allowing for thicker coats and surface leveling. In addition, this will decrease the tendency for cracking of the plaster as it dries. High clay content soils in mud plaster may tend to result in a poor bond of the plaster to the wall.

• The finish coat is made of screened, fine materials. This layer is applied as thinly as possible while achieving full coverage. Plaster can be troweled or floated to achieve a variety of textures, and reapplied as many times as necessary to achieve the desired affect or to make repairs. When dry, the mud plaster surface will take a hard, firm set similar in hardness and texture to conventional plaster.

• The same stabilizers used in the preparation of the structural soil mix may be used to stabilize the plaster. Thorough mixing of the plaster mix is necessary to avoid an uneven finish.

7.2 Stucco

• Traditional cement stucco may be used on walls for a low-maintenance finish. While this may seem desirable, cement stucco also has disadvantages in that it has a different expansion coefficient than the wall material. This may eventually lead to separation from the wall, and may conceal structural erosion problems which may result from leaky pipes or roofs. Stucco netting is recommended to accommodate any settling and cracking of the stucco. Exterior stucco walls should not be painted with traditional exterior paints, which will increase moisture impermeability. A final colored coat of stucco or texture finishes may be used decoratively. For more information on both interior and exterior cement stucco preparation and application, see Adobe and Rammed Earth Buildings (Resources section).

7.3 Interior Walls

• Interior earth walls may be painted more successfully, and may also be treated with sealing compounds to reduce the tendency for dust to develop and rub off on furniture and clothing. Oil-based varnishes and resinous liquids can be diluted for such use. If paint is to be used, a sealing or sizing coat should be applied first. Whitewash can be prepared with equal parts of lime and white cement mixed with water. Natural earth pigments may be added to this.

• In addition to stucco or plaster, interior walls may also be treated with a variety of veneers including gypsum wall board or other interior veneers.

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