DESIGN OF SLABS-ON-GROUND

360R-12 ACI COMMITTEE REPORT
moisture contents and dry densities. This is more practical
than attempting to find the influence of moisture by field
tests. Various test procedures, such as CBR, unconfined
compression, and triaxial shear, can be followed. Moisture
and density ranges chosen for testing should match those
anticipated in the field.
3.4.6 Influence of soil material on modulus of subgrade
reaction—Soils found at a building site are capable of
providing a range of subgrade support. This is again illustrated
by 
Fig. 3.3
. Clay soils, such as CL and CH materials, provide
the lowest subgrade support. Well-graded, noncohesive
soils, such as SW and GW material, provide the greatest
support. An increase in density by compaction can improve
a soil’s strength, but to a limited extent. Stabilization
methods can be used, but they will also have a limited range
of effectiveness. In addition, drainage conditions can change
the support capacity of most soils, but this can be most
significant for clays and silts. Frost action can also reduce the
support capacity of soils containing silt. Thus, the correlation
between soil classification and supporting capacity is useful
for estimating the range of capability but should be adjusted
for expected site conditions.
3.4.7 Uniformity of support—The design charts of PCA,
WRI, and Corps of Engineers (COE) indicate the influence
that the modulus of subgrade reaction has on the required
slab thickness. These design aids assume continuous slab
contact with the base and a uniform subgrade modulus.
Continuous intimate contact, however, is not achieved in
practice because of differences in composition, thickness,
moisture content, slab curling, and subgrade density. If the
joint recommendations given in 
Fig. 5.6
 are followed,
however, the curling stresses will be sufficiently low that the
PCA, WRI, and COE methods will provide reasonable
solutions. Cycles of load and climatic fluctuations of moisture
may increase or decrease k, but such change is usually not
uniform. Differences in subgrade support due to cuts and
fills or irregular depths to shallow bedrock are common.
Poor compaction control or variations in borrow material can
cause fills to provide nonuniform support. Attempts to
produce high subgrade moduli by compaction or stabilization
may yield nonuniform support unless strict quality-control
standards are implemented. Uniform high k values are difficult
to achieve. On some projects, a well-constructed subgrade
has been compromised by utility trenches that were poorly
backfilled. After the slab has been installed, densification of
noncohesive soils, sand, and silts by vibration may yield
nonuniform support. The shrinking and swelling action of
cohesive soils (GC, SC, CL, and CH) has caused cracks in
concrete slabs, even when design and construction precautions
were taken. Inspection and testing of controlled fills should
be mandatory. The lack of uniformity of support is a cause of
slab cracks. The importance of providing uniform support
cannot be overemphasized.
3.4.8 Influence of size of loaded area—The k value, if
derived from the plate load test, only provides information
relative to the upper 30 to 60 in. (760 to 1520 mm) of the
subsurface profile. Although this may be sufficient for the
analyses of floor slabs subjected to relatively small
concentrated loads, it is not sufficient for floor slabs
subjected to large, heavy loads. For example, a fully loaded
warehouse bay measuring 25 x 25 ft (7.6 x 7.6 m) could load
and consolidate soils to depths of 30 ft (9.1 m) or more if fills
have been used to develop the site. Settlement of slabs is not
uncommon on sites where fills have been used to produce
dock height floors or promote area drainage. The degree of
settlement experienced under such a loading condition
typically indicates an equivalent k value of only 20 to 30%
of that measured by a plate load test.
To properly consider the effect of heavy distributed loads on
slab performance, a more comprehensive evaluation of
subsurface conditions should be conducted. Such evaluations
may include the performance of soil test borings and laboratory
tests of subgrade materials or one of a variety of in-place
testing techniques. Such information can be used to develop
soil-support values, which account for long-term consolidation
settlements under sustained heavy distributed loads.
3.4.9 Influence of time—Time of load application and
elapsed time are important. Short, transient loads such as lift
trucks, produce smaller deformations than sustained loads;
therefore, a higher k value can be used for rolling loads. With
the passage of time, the subgrade and subbase will be subject
to load cycling. Applications of stresses from surface loads
may increase the stiffness of the subgrade and subbase, and
a higher k value will result. Unfortunately, this may also
produce nonuniform support because the areas of load
application will not usually be uniform.
Subgrade moisture change over time may also affect the
soil-support system. Stability through changes of climate,
such as protracted dry or wet weather conditions or cycles of
freezing and thawing, should be considered.

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