It should be noted from the design charts in LR1132 that not all highway
pavement materials are suitable for traffic levels over 20 million standard axles.
A standard long-life design is given for bituminous pavements with a lean-mix
concrete roadbase. In the case of pavements with bituminous roadbases, designs
for more than 80 msa are not included in the standard charts. At these traffic
levels, the end result of deep-seated failure is generally severe traffic disruption.
In order to avoid fatigue failure, a layer of rolled asphalt is placed at the posi-
tion in the pavement where resistance to fatigue is most essential – at the bottom
of the bound layer. This will help ensure that failure is likely to occur in the
form of surface deformation rather than deep-seated fatigue failure. Consequent
damage will be less extensive, confined to the upper regions of the highway
pavement.
Figure 8.6 shows the general composition of a bituminous pavement designed
to carry traffic in excess of 80 msa. The total thickness of the bound
layer should be approximately 400 mm for cumulative traffic in the range of
100 msa.236 Highway Engineering
Cumulative traffic (msa)Thickness of bound layers (mm)0.01 1.0 100100200400300Subgrade CBR = 5%
Subbase thickness = 225 mm
Probability of survival = 85%RoadbaseSurfacingSurfacingRoadbaseFigure 8.5Design curve for highways with lean concrete roadbase. (Crown copyright 1984)