Lesson 5/Learning Event 1
Learning Event 1
APPLY PRINCIPLES OF PAVEMENT DESIGN USING THE CBR METHOD
Soils display variation in dry densities when compaction occurs at different magnitudes and at different
soil moisture content. Intuitively, this means different strengths are obtainable at various combinations
of soil moisture and levels of compactive effort. For some soils, the moisture content which gives the
greatest dry density also gives the greatest strength. The generalization, more density gives more
strength, does not hold for all cases. The object of the CBR Test Program is to define in the laboratory
the soil placement conditions (moisture content and dry density) which gives the greatest measure of
strength.
Prior to defining a test program, the engineer should have a basic understanding of the principles of
pavement design using the CBR method. Figure 33 is a sketch of a typical flexible pavement upon
which the design wheel load has been imposed. As indicated by the dashed line, the stresses caused by
this load are dissipated with depth into the pavement structure. This means the further a soil is beneath
the load, the less soil strength (lower CBR) is required to support that load. The pavement design curves
allow for determination of thickness or protective cover required to keep each layer of material from
failing under the imposed load. In making a design, the engineer must determine the strength of the soil
and the associated field control measures, namely, soil placement moisture content ranges and
compaction ranges which insure a design strength. Also of note is the fact that design specifications
impose minimum levels of compaction (as shown in Table 8) which serve to minimize settlement under
traffic loads. As indicated in both Figure 33 and Table 8, the soils in a flexible pavement have been
named depending upon their strength and/or use within the structure.
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