of 80 kN, the required value is obtained from the product of the cumulative
number of commercial vehicles and a term called the damage factor which varies
for different road types. The maximum value of this conversion factor is 1.08,
used for motorways and trunk roads designed to cater for over 1000 commer-
cial vehicles per day in each direction.
For a motorway:
Equivalent No. of standard axles =No. of commercial vehicles ¥1.08 (8.1)
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 (1 msa), a
minimum subbase thickness of 150 mm is required where the CBR is greater
than 6%, rising to 440 mm where the CBR is 2%. Where the CBR is less than
2%, an additional 150 mm 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.
Step 5
Determine the roadbase and surfacing thickness. This parameter depends purely
on the cumulative number of standard axles over the pavement’s design life. For
cumulative standard axles in excess of 10 million, the surfacing should be
100 mm thick (60 mm basecourse plus 40 mm wearing course). If dense bitumen
macadam is used, a roadbase thickness of just under 150 mm is required to cater
for 10 msa, giving a total bound thickness of 250 mm.
8.2.3 LR1132
Road Note 29 was the sole officially recognised pavement design methodology
throughout the 1970s and early 1980s. While it was considered to be generally
effective, it had certain inherent deficiencies:
It was seen as unresponsive both to improvements in the quality of available
raw materials and to changes in construction processes.
The RN29 method is valid for designs up to 40 msa. Many highways were,
by the mid 1980s, well in excess of 50 msa, with some approaching 150 msa
over their 20-year design life.
The 20-year design life implied that, after this period, a surface rut of 20 mm
or more, or severe cracking or crazing had developed. The pavement was
then considered to be in a failed state and in need of major strengthening
or partial reconstruction. It has been shown that attempting to strengthen
a pavement damaged to such an extent did not automatically result in
satisfactory performance afterwards.
Structural Design of Pavement Thickness 231
