TM 5-822-5/AFM 88-7, Chap. 1
CHAPTER 5
FLEXIBLE PAVEMENT SELECT MATERIALS AND SUBBASE
COURSES
5-1. General.
5-2. Materials.
It is common practice in pavement design to use
The investigations described in chapter 2 will be
locally available or other readily available materials
used to determine the location and characteristics
between the subgrade and base course for econ-
of suitable soils for select material and subbase
omy. These layers are designated in this manual as
construction.
select materials or subbases. Those with design
a. Select materials. Select materials will normal-
CBR values equal to or less than 20 are designated
ly be locally available coarse-grained soils (prefix G
select materials, and those with CBR values above
or S), although fine-grained soils in the ML and CL
20 are designated subbases. Minimum thicknesses
of pavement and base have been established to
groups may be used in certain cases. Limerock,
eliminate the need for subbases with design CBR
coral, shell, ashes, cinders, caliche, disintegrated
values above 50. Where the design CBR value of
granite, and other such materials should be consid-
the subgrade without processing is in the range of
ered when they are economical. Recommended
20 to 50, select materials and subbases may not be
plasticity requirements are listed in table 5-1. A
needed. However, the subgrade cannot be assigned
maximum aggregate size of 3 inches is suggested to
design CBR values of 20 or higher unless it meets
aid in meeting grading requirements.
the gradation and plasticity requirements for sub-
bases.
b. Subbase materials. Subbase materials may
in this way is not satisfactory. Material stabilized
consist of naturally occurring coarse-grained soils
with commercial additives may be economical as a
or blended and processed soils. Materials such as
subbase. Portland cement, lime, flyash, or bitumen
limerock, coral, shell, ashes, cinders, caliche, and
and combinations thereof are commonly employed
disintegrated granite may be used as subbases when
for this purpose. Also, it may be possible to
they meet the requirements described in table 54.
decrease the plasticity of some materials by use of
The existing subgrade may meet the requirements
lime or portland cement in sufficient amounts to
for a subbase course or it may be possible to treat
make them suitable as subbases.
the existing subgrade to produce a subbase.
However, admixing native or processed materials
5-3. Compaction.
will be done only when the unmixed subgrade meets
These materials can be processed and compacted
the liquid limit and plasticity index requirements for
with normal procedures. Compaction of subbases
subbases. It has been found that "cutting" plasticity
will be 100 percent of ASTM D 1557 density
5-1
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