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
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 Classifi- cation Symbol
Soil Description
Drainage
Character-
istics1
Frost Heave
Suscepti-
bility2
Volume Change Potential Expansion3
Group I Excellent
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 Poor
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 Unsatisfactory
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 Soil Classifi- cation Symbol
Bearing Capacity (psf)
Undrained Shear
Strength1 (psf)
Angle of Internal
Friction (degrees)
Group I Excellent
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)
NA
Group III Poor
CH
600-1,200 (soft) —
3,000-4,500 (stiff)
250-500 (soft) —
2,000-4,000 (stiff)
NA
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.
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.