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 floor foundation system must be capable of accommodating all design loads and transmitting those loads to the foundation soil without excessive settlements. 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. The selected engineering properties of soils are determined from several sources, including:

  • published soil maps by the USDA Natural Resources Conservation Service and other government offices
  • a review of borings from nearby sites
  • geophysical exploration (e.g., seismic reflection and refraction, cross-hole testing, electrical resistivity soundings, etc.)
  • in-situ testing (e.g., Cone Penetration Test, Standard Penetration Tests, Vane Shear Tests, etc.)
  • soil borings at the construction site
  • a test pit dug at the construction site

The USDA Natural Resource Conservation Service categorizes and describes soil types in four large groups depending on Unified Soil Classification System, their estimated engineering behavior, drainage characteristics, frost heave potential, and swelling potential (see Table 6). Suggested values for soil bearing capacities, undrained shear strength, and friction angles are presented in Table 7. These values are only estimated values to be used for light construction applications when other data are not available. It is also important to note that soil properties can vary significantly from one site to another and even within a single site. It is necessary to consult a geotechnical engineer when any unusual or unknown soil conditions are encountered.

Considerations for Problematic Soils

In poorly drained soils (Group III), an open pier-and-beam foundation system is the best way to provide adequate ventilation for raised 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 in Groups III or IV 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. For example, spread footings may need to be constructed below the layer of expansive soil, or piers may need to be supported on pressure-treated piles (or other pile systems) driven below the troublesome soil. Furthermore, piles or grade beam footings may be required for soil types with minimal bearing capacities (for example, soils in Group III and IV). 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 (e.g. plastic soils in Groups II, III and IV), a settlement analysis should be performed as these soils have the potential to settle 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 ofthe 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.

Table 6 Types of Soils and Engineering Characteristics
Soil Group Unified
Soil Description Drainage
Volume Change Potential Expansion3
Group I
GW Well-graded gravel, gravel-sand mixtures, little or no fines Good Low (F1) Low
GP Poorly graded gravels or gravel-sand mixtures, little or no fines Good Low (F1) to Medium (F2) Low
SW Well-graded sands, gravely sands, little or no fines Good Medium (F2) Low
SP Poorly graded sands, gravely sands, little or no fines Good Medium (F2) Low
GM Silty gravels, gravel-sand-clay mixtures Medium Low (F1) to High (F3) Low
SM Silty sand, sand-silt mixtures Medium Mekium (F2) to High (F3) Low
Group II
Fair to Good
GC Clayey gravels, gravel-sand-clay mixtures Medium High (F3) Low
SC Clayey sand, sand-clay mixtures Medium High (F3) Low
ML Inorganic silts and very fine sands, rock flour, silty fine sands or clayey silts with slight plasticity Medium Very High (F4) Low
CL Inorganic clays of low to medium plasticity, gravely clays, sandy clays, silty clays, lean clays Medium High (F3) to Very High (F4) Medium
Group III
CH Inorganic clays of high plasticity, fat clays Poor High (F3) High to Very High
MH Inorganic silts, micaceous or diatomaceous fine sandy or silty soils Poor Very High (F4) High
Group IV
OL Organic silts and organic silty clays of low plasticity Poor High (F3) Medium
OH Organic sands of medium to high plasticity, organic silts Unsatisfactory High (F3) High
PT Peat and other high organic soils Unsatisfactory High (F3) High

Source: Table modified from the U.S. Department of Agriculture (www.usda.gov).
1 Percolation rate for good drainage is over 4 inches per hour, medium drainage is 2 to 4 inches per hour, and poor drainage is less than 2 inches per hour.
2 After Coduto, D.P.(2001). Foundation Design. Prentice-Hall. F1 indicates soils that are least susceptible to frost heave, and F4 indicates soils that are most susceptible to frost heave.
3 For expansive soils, contact a geotechnical engineer for verification of design assumptions. Dangerous expansion might occur if soils classified as having medium to very high potential expansion types are dry but then are subjected to future wetting.


Table 7 Engineering Properties of Soils
Soil Group Unified
Bearing Capacity (psf) Undrained Shear
1 (psf)
Angle of Internal
Friction (degrees)
Group I
GW 2,700-3,000 NA 38-46
GP 2,700-3,000 NA 38-46
SW 800-1,200 (loose) NA 30-46 (loose to dense)
SP 800-1,200 (loose) NA 30-36 (loose to dense)
GM 2,700-3,000 NA 38-46
SM 1,600-3,500 (firm) NA 28-40 (firm)
Group II
Fair to Good
GC 2,700-3,000 NA 38-46
SC 1,600-3,500 (firm) NA 30-34 (dense)
ML 2,000 NA 30-34 (dense)
CL 600-1,200 (soft) —
3,000-4,500 (stiff)
0-250 (soft) —
1,000-1,200 (stiff)
Group III
CH 600-1,200 (soft) —
3,000-4,500 (stiff)
250-500 (soft) —
2,000-4,000 (stiff)
MH 2,000 1,600 NA

Source: Table modified from the U.S. Department of Agriculture (www.usda.gov), FEMA Coastal Construction Manual (www.fema.gov), and Bardet, J. (1997). Experimental Soil Mechanics. Prentice-Hall.
1 The undrained shear strength is also commonly referred to as cohesion in saturated clays.
psf = pounds per square foot     NA = not applicable

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. The International Building Code (IBC) provides some guidance on the placement of footings near slopes. For example, the 2003 IBC indicates the bottom of a footing should be located at a distance from the face of the slope equal to or greater than one-third the height of the slope.