Lesson 1/Learning Event 2
Subbase materials. Subbase materials may consist of naturally occurring coarse-grained soils or
blended and processed soils. You may use materials such as limerock, coral, shell, ashes, cinders,
caliche, and disintegrated granite as subbases when they meet the requirements described in selection of
Design CBR for select materials and subbases. As noted in the preceding paragraph, the existing
subgrade may meet the requirement for a subbase course or it may be possible to treat the existing
subgrade to produce a subbase. Also, as noted, admixing native or processed materials will be done
only when the subgrade unmixed meets the liquid limit and plasticity index requirements for subbases,
because it has been found by experience that "cutting" plasticity in this way does not work out
satisfactorily. Material stabilized with commercial admixes may be economical as subbases in certain
instances. Portland cement, cutback asphalt, emulsified asphalt, and tar are commonly employed for this
purpose. Also, it may be possible to decrease the plasticity in some materials by use of lime or portland
cement sufficiently to make them suitable as subbases.
Compaction of Select Materials and Subbases
These materials can be processed and compacted with normal procedures. Compaction should be
specified in accordance with the criteria described in Figure 4.
Selection of Design CBR for Select Material and Subbase
Tests are usually made on remolded samples; however, where existing similar construction is available,
CBR tests should be made in-place on material when it has attained its maximum expected water content
or on undisturbed soaked samples. The procedures for selecting test values described for subgrades
apply to select materials and subbases. The CBR tests are supplemented by the gradation and Atterberg
limits requirements for subbases as indicated in Table 3. Suggested limits for select materials are also
indicated. In addition to the requirements shown in the table, the material must also show in the
laboratory tests a CBR equal to or higher than the CBR assigned to the material for design purposes.
Cases may occur in which certain natural materials that do not meet the gradation requirements may
develop satisfactory CBR values in the prototype. Exceptions to the gradation requirements are
permissible when supported by adequate in-place CBR tests on construction that has been in service for
several years. The CBR test is not applicable for use in evaluating materials stabilized with chemical
admixtures, and they must be rated by judgment in terms of an equivalent CBR. Ratings as high as 50
can be assigned these materials when proper construction procedures are followed.
Base Course Factor to be Considered
The purpose of a base course or courses is to distribute the induced stresses from the wheel load so that
they will not exceed the strength of the subgrade. Figure 5 shows an idealized representation of the
distribution of stress
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