Lesson 1/Learning Event 2
through two base courses. When the subgrade strength is low, the stress must be reduced to a low value
and a substantial thickness of base is needed. Where the subgrade strength is higher, a lesser thickness
will provide adequate distribution. Since the stresses in the base course are always higher than in the
subgrade (Figure 5), it stands to reason that the base course must have higher strength. Similarly, where
two or more different types of base courses are used, the better quality material is placed on top.
TABLE 3. RECOMMENDED MAXIMUM PERMISSIBLE VALUES OF GRADATION AND
ATTERBERG LIMIT REQUIREMENTS IN SUBBASES AND SELECT
MATERIALS
FIGURE 5. DISTRIBUTION OF STRESS IN BASE COURSES AND EFFECTS OF
SUBGRADE STRENGTH ON BASE-COURSE THICKNESS
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