360R-06 Design of Slabs-on-Ground

DESIGN OF SLABS-ON-GROUND
360R-9
prior loading determine the load-deformation relationship.
The relationship also depends on the width of the loaded
area, shape of the loaded area, depth of the subgrade, and
position under the slab. In addition, time may be a significant
factor because any deeper compressible soils may settle due
to consolidation, and near-surface soils may settle due to
shrinkage from alternate wetting and drying. Nevertheless,
the procedures for static nonrepetitive plate load tests
outlined in ASTM D 1196 have been used to estimate the
subgrade modulus.
3.4.2 Plate load field tests—Determination of the modulus
of subgrade reaction on representative subgrade in place
with a 30 in. (760 mm) diameter bearing plate, which is
recommended by ASTM D 1196, is time-consuming and
Table 3.2—Laboratory classification criteria for soils (Winterkorn and Fang 1975)

360R-10 ACI COMMITTEE REPORT
expensive. Several days are generally needed to plan and
execute a load-testing program. Large loads may be needed
to obtain significant settlement of the plates. Adjustments
should be made for nonrecoverable deformation and any
plate deflections. Because the load-deformation results are
nonlinear, either an arbitrary load or deformation should be
assumed to calculate k. This is illustrated in Fig. 3.2.
Several tests over the project area are required to obtain
representative k values, which generally result in a range of
k values. A correction is generally necessary to account for
future saturation of cohesive soil subgrades, and this requires
sampling and laboratory tests. It is usually impractical to
conduct field tests on subgrade soils at their expected range
of densities and moisture contents. It is also impractical to
test the various possible types and thicknesses of base
courses and subbases on a representative subgrade. It is
difficult to test during adverse climatic conditions. Smaller
plates, such as 12 in. (300 mm) diameter, have been used, but
the diameter of the plate influences the results, and this is
difficult to take into account in reporting a k value. Typically,
these tests are made directly on an unconfined natural or
compacted subgrade or on a thickness of compacted subbase
or base course over a subgrade. The physical characteristics
of the base course and subgrade material are necessary to
properly interpret the plate bearing test results. At a
minimum, these data should include gradations, moisture
contents, densities, and Atterberg limit of the materials in the
supporting system. Before initiating a plate load field test, it
is advisable to consult a geotechnical engineer familiar with
site conditions to estimate cost and time required and the
probable results.
3.4.3 American Association of State Highway Officials
(AASHTO) approach—For rigid pavements, AASHTO has
developed a design procedure using the following theoretical
relationship between k values from plate bearing tests and
M
R
, the resilient modulus of the subgrade
k (lb/in.
3
) = M
R 
(psi)/19.4 (in.-lb units)
k (kN/m
3
) = M
R
(kPa) × 2.03 (SI units)
The resilient modulus is a measure of the assumed elastic
property of soil taking into account its nonlinear characteristics.
It is defined as the ratio of the repeated axial deviator stress
to the recoverable axial strain. It is widely recognized as a
method for characterizing pavement materials. Methods for
the determination of M
R
are described in AASHTO Test
Method T307. The value of M
R
can be evaluated using a
correlation with the older and more common California bearing
ratio (CBR) test value (ASTM D 1883) by the following
empirical relationship (Heukelom and Klomp 1962)
M
R
(psi) = 1500 × CBR (in.-lb units)
M
R
(kPa) = 10,342 × CBR (SI units)
This approximate relationship has been used extensively for
fine-grained soils having a soaked, saturated 96-hour CBR
value of 10 or less (Heukelom and Klomp 1962). Correlations
of M
R
with soil properties such as clay content, Atterberg
limits, and moisture content have also been developed.
The effective k value used for design as recommended by
AASHTO for rigid pavements is dependent on several
different factors besides the soil resilient modulus, including
subbase types and thicknesses, loss of support due to voids,
and depth to a rigid foundation. Tables and graphs in the
AASHTO “Guide for the Design of Pavement Structures”
may be used to obtain an effective k for design of slabs-on-
ground. The k values obtained from measured CBR and M
R
data using the AASHTO relationships can yield unrealistically
high values. It is recommended that the nomograph relation-
ships contained in 
Fig. 3.3 
be used to validate the results of
correlated k values derived from AASHTO correlations.
3.4.4 Other approaches—Empirical relations between soil
classification type, CBR, and k values have been developed
by the Corps of Engineers, and this is illustrated by 
Fig. 3.3
.
These relationships are usually quite conservative. All of
these test methods and procedures have been developed for
pavements and not for slab-on-ground floors for buildings.
Nevertheless, correlations such as these are widely used to
approximate the subgrade support values for slab-on-ground
design and construction.
3.4.5 Influence of moisture content—The moisture content
of a fine-grained soil affects the modulus of subgrade reaction
k, both at the time of testing and throughout the service life
of the slab. Nearly all soils exhibit a decrease in k with an
increase in saturation, but the amount of reduction depends
chiefly on the texture of the soil, its density, and the activity
of the clay minerals present. In general, the higher the moisture
content, the lower the supporting capacity, but the relationship
is unique for each type of soil. The more uniform the moisture
content and dry density, the more uniform the support. Thus,
providing good site surface drainage and drainage of the
subgrade is very important. Experience has shown that high
water tables and broken water or drain lines have caused
slab-on-ground failures.
Laboratory tests can be performed to evaluate the influence
of moisture by molding test specimens to various uniform
Fig. 3.2—Plate load-deformation diagram.

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