Lesson 3/Learning Event 3
FIGURE 22. CURVES FOR EVALUATING VALUES OF FROST PENETRATION
In order to further insulate the subgrade, to allow water to drain out of the flexible pavement structure,
and to prevent capillary movement of water up from the subgrade, a 4" layer of well draining material is
placed immediately above the subgrade. This "frost filter" is normally constructed of a graded sand.
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