TM 5-809-1/AFM 88-3, Chap. 15
CHAPTER 4
SITE INVESTIGATION
4-1. General
c. Exploration and classification. If field recon-
naissance and analysis of existing subsurface infor-
Once the floor slab load capacity requirements have
mation are insufficient to provide the necessary data
been established, an investigation of the existing
for floor-slab design, an exploration program should
conditions at the site must be made. Conditions to
be initiated according to provisions of TM 5-81 8-1/
be considered include an investigation of the
AFM 88-3, Chap. 7. All soils should be classified in
subgrade, climatic conditions, the need for and
accordance with MIL-STD-619. Sufficient
availability of base course materials, and the
investigations should be performed at the proposed
concrete strength properties likely to be
site to facilitate the classification of all soils that will
encountered in the locale.
be used or removed during construction; other
pertinent descriptive information should also be
4-2. Subgrade conditions.
included.
a. Importance of subgrade conditions. The sub-
d. Performance data. For the design of rigid floor
grade provides a foundation for supporting the floor
slabs in areas where no previous experience regard-
slab and base courses. As a result, the required floor
ing floor slab performance is available, the modulus
slab thickness and the performance obtained form
of subgrade reaction k to be used for design
the floor slab during its design life will depend in a
purposes is determined by the field plate-bearing
large part, on the uniformity and bearing capacity of
test. A description of the procedure to be followed
the subgrade. It is desirable, if economically feasible,
for this test and the method for evaluating test
to thoroughly investigate the subgrade to assess the
results are given in MIL-STD-621. Where
maximum support potential for the particular sub-
performance data from existing floor slabs on grade
grade. In unheated structures, the possibility of frost
are available, adequate values for k usually can be
heave emphasizes the importance of uniformity of
estimated on the basis of soil type, drainage
soil conditions under the floor slab.
conditions, and frost conditions that prevail at the
b. Initial investigation. Preliminary investigations
proposed site. Table 4-1 lists typical values of
of subgrade conditions at the site of proposed
modulus subgrade reaction for various soil types
construction should be performed to determine the
and moisture contents. Values shown may be
engineering characteristics of the subgrade soils and
increased slightly if the density is greater than 95
the extent of any peculiarities of the proposed site.
percent maximum CE 55 density, except that a
The general suitability of the subgrade soils is to be
maximum of 500 pounds per cubic inch will be used
based on classification of the soil, moisture density
for design. These values should be considered as a
relationships, expansive characteristics, susceptibil-
guide only, and their use in lieu of the field plate-
ity to pumping, and susceptibility to detrimental
bearing test is left to the discretion of the engineer.
frost action. A careful study of the service history of
The fact that the materials are shown in the table
existing floor slabs on similar subgrade materials in
does not indicate suitability for use. Suitability must
the locality of the proposed site should be made.
be determined for the particular job conditions.
Factors such as ground water, surface infiltration,
soil capillarity, topography, rainfall, drainage condi-
tions, and the seasonal change of such factors also
may affect the support rendered by the subgrade.
4-1
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