Lesson 5/Learning Event 1
TABLE 8. SUMMARY OF COMPACTION REQUIREMENTS
Starting at the bottom of the pavement and moving toward the surface, a brief description of the
Subgrade. Subgrade is the in-place natural soil which supports the pavement structure. It may have a
CBR value ranging from 0 to 100 percent and specifications require that portions of it must be
Select Materials. Select material is occurring imported soil having CBR values less than 20 percent but
greater than the compacted subgrade. It is used to carry stress levels intermediate to those supported by
the subgrade and subbase material.
Subbase Material. Subbase material is imported cohesionless material having CBR values ranging
from 20 to 50 percent. It is normally obtained from natural gravel deposits and serves to carry
intermediate stresses in the pavement.
Base course is highest quality structural materials having strengths of nearly 100 percent CBR. For
pavement design, CBR values are normally assigned based upon gradation requirements rather than
CBR tests. Base course are always cohesionless and usually processed to obtain the desired gradation.