Figure 1-5. Decision-Making Process for Foundation Design
The four basic types of foundations— full basement, submerged crawl space, flush crawl space, and slab-on-grade—are shown in Figure 1-4. Of course, actual houses may include combinations of these types. Information on a fifth type of foundation—the shallow or half-bermed basement—can be found in the Building Foundation Design Handbook (Labs et al. 1988).
There are several construction systems from which to choose for each foundation type. The most common systems, cast-in-place concrete and concrete block foundation walls, can be used for all four basic foundation types. Other systems include precast concrete foundation walls, masonry or concrete piers, cast-in-place concrete sandwich panels, and various masonry systems. A slab-on-grade construction with an integral concrete grade beam at the slab edge is common in climates with a shallow frost depth. In colder climates, deeper cast-in-place concrete walls and concrete block walls are more common, although a shallower footing can sometimes be used depending on soil type, groundwater conditions, and insulation placement.
Most of the foundation types and construction systems described above can be designed to meet necessary structural, thermal, radon, termite and moisture or water control requirements in any climate. Factors affecting the choice of foundation type and construction system include site conditions, overall building design, the climate, and local market preferences as well as construction costs. These factors are discussed below.
The topography, water table location, presence of radon, soil type, and depth of bedrock can all affect the choice of a foundation type. Any foundation type can be used on a flat site; however, a sloping site often necessitates the use of a basement or crawl space. On steeper slopes, a walkout basement combines a basement foundation wall on the uphill side, a slab-on-grade foundation on the downhill side, and partially bermed foundation walls on the remaining two sides.
A water table depth within 8 feet of the surface will likely make a basement foundation undesirable. Lowering the water table with drainage and pumping usually cannot be justified, and waterproofing may not be feasible or may be too costly. A water table near the surface generally restricts the design to a slab-on-grade or crawl space foundation.
The presence of expansive clay soils on a site requires special techniques to avoid foundation movement and significant structural damage. Often, buildings placed on sites with expansive clay require pile foundations extending down to stable soil strata or bedrock. Similarly, sites with bedrock near the surface require special foundation techniques. Expensive bedrock excavation is not required to reach frost depth nor is it economically justifiable to create basement space. In these unusual conditions of expansive clay soils or bedrock near the surface, special variations of the typical foundation types may be appropriate.
Overall Building Design
The foundation type and construction system are chosen in part because of appearance factors. Although it is not usually a major aesthetic element, the foundation at the base of a building can be raised above the ground plane, so the foundation wall materials can affect the overall appearance. A building with a slab-on-grade foundation has little visible foundation; however, the foundation wall of a crawl space or basement can vary considerably from a minimal exposure to full exposure above grade.
The preference of foundation type varies with climatic region, although examples of most types can generally be found in any given region. One of the principal factors behind foundation preference is the impact of frost depth on foundation design. The impact of frost depth basically arises from the need to place foundations at greater depths in colder climates. This is due to the fact that water expands when it freezes, and if this occurs beneath the footing the building (or portions of the building) can be forced up toward the surface. For example, a footing in southern Minnesota must be at least 42 inches below the surface, while in states along the Gulf Coast footings need not extend below the surface at all in order to avoid structural damage from frost heave. Because a foundation wall extending to a substantial depth is required in northern climates, the incremental cost of creating basement space is much less, since it is necessary to build approximately half the basement wall anyway. In a southern climate the incremental first cost of creating a basement is much greater when compared with a slab-on-grade with no significant required footing depth.
This historic perception that foundations must extend below the natural frost depth is not entirely accurate. Buildings with very shallow foundations can be used in cold climates if they are insulated properly (NAHB 2004).
The foundation type and construction system are also chosen based on cost and market factors that vary regionally or even locally. Virtually any foundation type and construction system can be built in any location in the United States. The relative costs, however, are likely to differ. These costs reflect local material and labor costs as well as the availability of certain materials and the preferences of local contractors. For example, in certain regions there are many contractors specializing in cast-in-place concrete foundation walls. Because they have the concrete forms and the required experience with this system and because bidding is very competitive, this system may be more cost-effective compared with other alternatives. In other regions, the availability of concrete masonry blocks is greater and there are many contractors specializing in masonry foundation walls. In these areas, a cast-in-place concrete system may be less competitive economically because fewer contractors are available. More subjective factors that influence a designer's choice of foundation type and construction system are the expectations and preferences of individual clients and the home-buying public. These market influences are based not only on cost but also on the area's tradition. If people in a certain region expect basements, then builders generally provide them. Of course, analyzing the cost-effectiveness of providing a basement requires a somewhat subjective judgment concerning the value of basement space. These more subjective market factors and regional preferences tend to increase the availability of materials and contractors for the preferred systems, which in turn makes these systems more cost-effective choices.