14 Feb 03
The first step in designing an effective reinforced pavement system is to determine the
properties of the subgrade including the grain-size distribution, Atterberg limits, and in situ shear
strength or bearing capacity. The in situ shear strength can be measured directly using vane
shear devices or indirectly using bearing capacity correlations from California Bearing Ratio
(CBR) or Dynamic Cone Penetrometer (DCP) tests. The design subgrade strength is defined as
the 75th percentile strength of the top 18 in. of the subgrade. The 75th percentile strength is the
value at which 75 percent of the recorded soil strength readings are higher than this value.
Figure 4 can be used to convert design strengths from cone index values and CBR to shear
strength (C) in psi.
C = 4.8 psi
Figure 4. Relationship between cone index, CBR, and shear strength (C) (TM 5-518-8).
2.1 Geosynthetic Applicability Assessment For Aggregate-Surfaced Pavements
2.1.1 Subgrade CBR < 0.5. For design subgrade CBR strengths of 0.5 or less, the primary
application is mechanical subgrade stabilization. At these soil strengths, the use of a nonwoven
geotextile is recommended for separation, and a biaxial geogrid is recommended for aggregate
reinforcement. At these low material strengths, the full depth of the aggregate fill should be used
and no reduction in aggregate thickness is recommended. Thus, the unreinforced aggregate