Lesson 1/Learning Event 2
Subbase Factors to be Considered
It is common practice in flexible pavement design to use locally available or other relatively cheap
materials between the subgrade and base course for economy. Those layers are designated, in this
manual, as select materials or subbases. Those with design CBR values of 20 are arbitrarily called select
materials, while those with CBR values of 20 and above are called subbases. Minimum thicknesses of
pavement and base have been established to eliminate the need for subbases with design CBR values
above 50. 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 of 20 or higher 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, it is emphasized that "blending in" cohesionless materials to
lower the plasticity index will not be allowed in any case.
Materials for Subbases
The investigations referred to previously will be used to determine the location and characteristics,
respectively, of suitable soils for select material and subbase construction.
Select material. Select materials will normally be locally available coarsegrained soils (prefix G or
S), although fine-grained soils in the ML and CL groups may be used in certain cases. Limerock, coral,
shell, ashes, cinders, caliche, disintegrated granite, and other such materials should be considered when
they are economical. Recommended plasticity requirements are listed in selection of design CBR for
select materials and subbases. These are suggested to insure a material that can be processed readily.
Materials not meeting these requirements may be considered where it can be shown that they can be
processed readily. A minimum size of 3 inches is suggested to aid in meeting grades.