Highway Engineering

8.2.4 HD 26/ 01

The standard HD 26/01 (DoT, 2001) is based on LR1132 for flexible and flexi-

ble composite pavements, but with modifications and amendments to take

account of more recent research, new materials and the observed functioning

of in-service pavements.

The standard design life for all types of pavement is assumed to be 40 years,

provided appropriate maintenance programmes are in place. For roads surfaced

with asphalt, it is anticipated that surface treatment would be required every 10

years. (Note: in HD 26/01 all bituminous materials are covered by the generic

term ‘asphalt’.)

A 20-year design life may be seen as appropriate for less heavily trafficked

schemes.

LR1132 was based on observations of highways over a 20-year period. Later

research (Munn et al., 1997) indicated that cracking and deformation is more

likely to occur in the surfacing than deeper in the structure. Therefore, it was

surmised that a well-constructed flexible pavement would have a very long struc-

tural life provided such signs of distress are treated before they affect the struc-

tural integrity of the highway. HD 26/01 notes that, for long-life flexible

pavements designed to carry traffic for at least 40 years, it is not necessary to

increase the thickness of the pavement beyond that required for 80 million

standard axles.

For flexible composite pavements, the thickness of the cemented lower base

can be reduced as the strength of the CBM is increased.

For fully flexible pavements, two design charts apply, one for recipe mixes, one

for design mixes. Figure 8.7 is an illustration of the design thicknesses for four

different types of base (roadbase) material:

238 Highway Engineering

40 mm wearing course

Dense macadam upper roadbase

(depth determined by overall

depth requirement)

125 mm rolled asphalt lower roadbase

225 mm granular subbase

Subgrade (CBR = 5%)

Figure 8.6Pavement composition for traffic levels greater than 80 msa.