Raised floor assemblies may be constructed in any soil type. In fact, they perform very well even in problematic soils, such as expansive soils which often crack conventional slabs.

To ensure durability and trouble-free performance, a raised wood floor foundation system must be capable of accommodating all design loads and transmitting those loads to the soil without excessive settlement. Footings should be supported on undisturbed natural soils or engineered fill. Foundation systems supported on fills should be designed, installed, and tested in accordance with accepted engineering practices. For example, gravel fill used in foundation systems such as wood foundations should comply with local building code requirements.

Soil Conditions

The type of soil and the general grading conditions at the building site are important factors in determining foundation construction details, such as footing design, backfill, and drainage. Soils are classified depending on several physical and engineering parameters including their grain size distribution, liquid and plastic limits, organic contents, drainage characteristics, frost heave potential, and swell potential.

There are several types of classification systems: for example, the Unified Soil Classification System, the AASHTO Soil Classification System, and the U.S. Department of Agriculture (USDA) Classification System. The USDA (www.usda.gov) publishes soil maps that cover most counties and parishes within the U.S. These maps provide a general guide on the type of soils that may be found in any given region.

Ground materials can vary from rocks to loose sand or saturated clays. It is also important to note that soil properties can vary significantly from one site to another, and even within a single site. It may be necessary to consult a geotechnical engineer when any unusual or unknown soil conditions are encountered.

Considerations for Problematic Soils

In poorly drained soils, an open pier-and-beam foundation system is the best way to provide adequate ventilation for raised wood floor systems. This recommendation is especially applicable for sites having a high water table, or where extreme amounts of rain often fall in short periods of time.

For building sites where expansive clay soils are predominant, a geotechnical engineer should determine the requirements for footings, piles, and drainage around the foundation. In such cases, special design considerations may be necessary to avoid excessive expansion and shrinkage, which might otherwise adversely affect foundation and structure performance. Furthermore, piles or grade beam footings may be required for soil types with minimal bearing capacities. Regardless of soil type, crawlspace foundation systems have the benefit of minimum excavation and backfill.

When a raised floor system is built on soils that are highly compressible, a settlement analysis should be performed as these soils have the potential to settle at more than admissible values. Also, highly compressible and swelling soils should not be used as fills unless they are stabilized within each active zone by chemical, preloading, dewatering, or pre-saturation processes.

In all areas where problematic soils may be found, a geotechnical engineer should determine whether soil tests are needed to better characterize the engineering behavior of the soils. Tests may range from classification and index tests to consolidation and triaxial tests. These tests should be performed by an approved laboratory or geotechnical engineer using standardized methods.

Slope Stability

Soil slope stability is an important design consideration that is often difficult to predict. A history of slope failures at or near the site is a strong indication of the presence of a problem, and further investigation and careful design considerations may be needed. A geotechnical engineer can predict whether slope failures are likely to occur at a particular site based on the slope angle, the characteristic drainage and seepage of the site, the shear strength properties of the soils (friction angle or undrained shear strength), and the external loads.