30 June 2001
SELECT MATERIALS AND SUBBASE COURSES
FOR FLEXIBLE PAVEMENTS
1. GENERAL. It is common practice in flexible pavement design to use locally available or other
readily available materials between the subgrade and base course for economy. The Navy and Marine
Corps designate these layers as subbases and require a minimum CBR of 30. The Army and Air Force
refer to these layers as subbases when the design CBR is above 20 and as select materials subbase
when the CBR is 20 or less. Minimum thicknesses of pavement and base have been established to
eliminate the need for subbases with design CBR values above 50. Guide specifications have been
prepared for select materials and subbases. Where the design CBR value of the subgrade without
processing is in the range of 20 to 50, select materials and subbases may not be needed. However, the
subgrade cannot be assigned design CBR values above 20 unless it meets the gradation and plasticity
requirements for subbases. In some cases, where subgrade materials meet plasticity requirements but
are deficient in grading requirements, it may be possible to treat an existing subgrade by blending in
stone, limerock, sand, etc., to produce an acceptable subbase. However, "blending in" cohesionless
materials to lower the plasticity index will not be allowed.
2. MATERIALS. The investigations described in Chapter 5 will be used to determine the location and
characteristics of suitable soils for select material and subbase construction. Limerock, coral, shell, blast
furnace slags (steel slag is not suitable), cinders, caliche, recycled concrete and asphalt, and other such
materials in addition to gravels and rock should be considered when they are economical and when they
meet the requirements of paragraph 4 entitled SELECTION OF DESIGN CBR. Do not use material
which has a swell of 3 percent or greater, as determined from the CBR mold, for subbase. These
materials will meet the LA Abrasion requirements of not more than 50 percent..
a. Select Materials. Select materials will normally be locally available coarse-grained soils.
Recommended gradation and plasticity requirements for select materials are listed in paragraph 4
entitled SELECTION OF DESIGN CBR.
b. Subbase Materials. Subbase materials may consist of naturally occurring coarse-grained soils
or blended and processed soils. Gradation and plasticity requirements for subbases are listed in
paragraph 4 entitled SELECTION OF DESIGN CBR. The existing subgrade may meet the requirements
for a subbase course or it may be possible to treat the existing subgrade to produce a subbase. Also,
admixing native or processed materials will be done only when the unmixed subgrade meets the liquid
limit and plasticity index requirements for subbases because it has been found that "cutting" plasticity in
this way is not satisfactory. However, it may be permissible to decrease the plasticity of some materials
by using lime or portland cement in sufficient amounts to meet the plasticity requirements of subbases.
In order to be considered stabilized for thickness design purposes, the soil must meet the minimum
strength requirements as shown in Table 7-1.
3. COMPACTION REQUIREMENTS. Subbases will be compacted to 100 percent of maximum
density as determined by ASTM D 1557. Select materials will be compacted to the densities shown in
Tables 6-2 to 6-7, except that cohesionless select materials will be placed at no less than 95 percent and
cohesive select materials at no less than 90 percent of ASTM D 1557 maximum density.
4. SELECTION OF DESIGN CBR. The select material or subbase will generally be uniform, and the
problem of selecting a limiting condition, as described for the subgrade, does not ordinarily exist. Tests