Step 3
Determine the equivalent number of standard axles incident on the road over
its design life, based on the commercial vehicle usage. Based on a standard axle
230 Highway Engineering
Wearing course
Basecourse
Roadbase
Surface course
Binder course
Base
Step 4
Determine the subbase thickness. This is dependent on both the CBR of the
subgrade and the cumulative number of standard axles over the design life of
the highway. For a cumulative number of standard axles of 1 million (1msa), a
minimum subbase thickness of 150mm is required where the CBR is greater
than 6%, rising to 440mm where the CBR is 2%. Where the CBR is less than
2%, an additional 150mm of subbase should be added to that required for a
CBR of 2%. The CBR of the subbase should be at least 30%. If the CBR of
the subgrade is in excess of this value, no subbase need be used