2. MATERIALS. Subbase materials can consist of any of the following:
a. Naturally occurring course-grained soils such as gravel, well-graded
sands, and disintegrated granite.
b. Naturally occurring and processed materials such as limerock, coral,
caliche, quarry and mine wastes, slag, shell, and sand-shell mixtures.
c. Naturally occurring or processed materials which are stabilized with
cement, lime, or bitumen.
d. Mechanically stabilized mixtures in which borrow or processed materials
are blended with on-site soils to form a more dense and stable layer.
Mechanical stabilization of subgrade soils may be used only where the liquid
limit (LL) and PI of the existing soil meet the requirements for subbase.
3. SUBBASE CBR.
a. Laboratory Tests. Perform laboratory CBR tests on remolded soaked
samples as outlined in MIL-STD 621, Method 101. Laboratory CBR tests are to be
supplemented with grain size and plasticity requirements to arrive at a CBR
value for design. Where proposed subbase materials exist in place under
similar conditions, the in-place CBR value should be determined and used for
design.
b. Maximum design CBR. When laboratory CBR values have been determined, the
maximum value for use in design may be controlled by the following gradation
and plasticity limitations:
Max. Permissible
Gradation, Max. % Passing
Design CBR
Max. Size #10 Sieve
#200 Sieve
Max. LL
Max. PI
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50
3"
50
15
25
5
40
3"
80
15
25
5
30
3"
100
15
25
5
For additional general requirements and recommended gradations of subbase
materials, see "AASHTO Specification M-147" in Standard Specifications for
Transportation Materials and Methods for Sampling and Testing.
of maximum density.
Section 6. BASE
1. FUNCTIONS AND REQUIREMENTS.
do not meet the gradation, plasticity, and CBR requirements for base. Base
courses are required under the following conditions:
(1) Frost Action. Use base courses to prevent or reduce frost
penetration into subgrades.
(2) Subgrade Pumping. Provide a base course over subgrade soils
classified as CH, CL, ML, MH, and OL to prevent pumping.
(3) Uneven Support. Provide base courses to provide uniform support to
pavement slabs.
(4) High Volume Change Subgrade Soils. Provide base courses as
overburden on expansive soils to minimize potential heaving and faulting at
joints.
b. Flexible Pavements. A base course is a major load-carrying component in
a flexible pavement. The base must have high structural strength and be
densely compacted.
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