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The site's topography and microclimate determine how easily the building can be surrounded with earth. A modest slope requires more excavation than a steep one, and a flat site is the most demanding, needing extensive excavation. A south-facing slope in a region with moderate to long winters is ideal for an earth-sheltered building. South-facing windows can let in sunlight for direct heating, while the rest of the house is set back into the slope. In regions with mild winters and predominantly hot summers, a north-facing slope might be ideal. Every site differs, but generally southern exposures offer more sun and daylight throughout the year than north-facing slopes. Most designs can be built to take advantage of each site.
The type of soil at your site is another critical consideration. Some types of soil are more suitable than others for earth-sheltered construction. For example, the best soils are granular, such as sand and gravel. These soils compact well for bearing the weight of the construction materials and are very permeable, which means they allow water to drain quickly. The poorest soils are cohesive, like clay, which may expand when wet and has poor permeability. Soil tests, offered through professional testing services, can determine load-bearing capability of soils and possible settlements that may occur.
Radon is an invisible and odorless radioactive gas produced naturally when uranium in rock decomposes. It is found in the soil and in outdoor air in harmless amounts, but can reach dangerous levels when trapped. Radon levels are another factor to consider in locating your home. Areas with high concentrations of radon can be hazardous, although there are methods that can reduce radon buildup in both conventional and earth-sheltered dwellings.
The groundwater level at your building site is another important consideration. Building above the water table is almost essential. Choosing a site where the water will naturally drain away from the building is the best way to avoid water pressure against underground walls. The site should be surveyed for low spots and areas where water will collect. Seasonal or regular surface water flows should be channeled away from the structure. Drainage systems must be designed to draw water away from the structure to reduce the frequency and length of time the water remains in contact with the building's exterior. Underground footing drains similar to or greater than those required by a house with a basement are necessary in many cases.
Construction Materials
The construction materials for each type of structure will vary, depending on characteristics of the site, climate, soils, and design. However, general guidelines show that houses more deeply buried require stronger, more durable construction materials. Materials must provide a good surface for waterproofing and insulation to withstand the pressure and moisture of the surrounding ground. When soil is wet or frozen, the pressure on the walls and floors increases. Pressure also increases with depth, so materials such as concrete and reinforced masonry, wood, and steel are all suitable. To reduce your transportation costs, it helps if appropriate materials are locally available.
Concrete - Concrete is the most common choice for constructing earth-sheltered buildings. Not only is it strong, it is also durable and fire resistant. Several forms of concrete are used. Lightly reinforced concrete, which is poured and reinforced at the site, is used for noncritical structural elements such as concrete foundations, floor slabs, and exterior walls with less than 6 feet (1.83 meters) of earth cover. Precast reinforced concrete can resist loads at any reasonable depth and can be used for floors, walls, and roofs. Concrete absorbs and stores heat, helping to prevent temperature swings that can damage some building material.
Precast concrete components are cured at a plant or on-site location before they are used, thereby decreasing construction time and cost in comparison to cast-in-place forms. The uses and advantages of precast and cast-in-place concrete are similar, except that precast concrete works best in simple or repeatable shapes. Special care must be taken to make the joints between sections watertight.
Concrete can also provide supplemental strength in other types of earthen construction. For example, a concrete topping can be added to wooden roof planks, and cement "parging" (or coating) can be added to walls with masonry construction before waterproofing.
Masonry - Masonry (i.e., brick or stone) can be used for walls that will receive vertical or lateral pressure from earth cover. It is reinforced with steel bars that are put in the core of the masonry in places of high stress, such as weight-bearing walls or walls with earth against them. Masonry generally costs less than cast-in-place concrete.
Wood - Wood can be used extensively in earth-sheltered construction for both interior and structural work including floors, roofs, and exterior walls. Wood is attractive for its color and warmth, and complements tile and masonry, as well as concrete walls, floors, and ceilings. However, using wood as a structural material requires wooden frame walls, which must withstand lateral pressure, be restricted to a burial depth of one story. Beyond this depth, the rapidly increasing cost of wood construction restricts most builders from using it as a structural material. Although wood can cost less than other materials, it does not offer the strength that a material such as steel does, so it may not be the best choice for structural material in some houses. Wood must also be treated with preservatives to prevent damage from moisture. If your structure can make practical use of wood as a framing material, employing carpenters who can rapidly construct a timber frame for an earth-sheltered house can decrease labor costs.
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