Highway Engineering

LR1132 (Powell et al., 1984) revised RN29 by redefining pavement failure,

thereby delivering a thicker but longer lasting highway likely to be in a less

deteriorated state after 20 years.

The design criteria adopted by LR1132 were:

(1) The subgrade must be able to sustain traffic loading without excessively

deforming. This is achieved by limiting the vertical stress at formation level.

(2) Bituminous or cement bound materials used in the flexible pavement must

not be subject to fatigue cracking. This is achieved by limiting the

horizontal tensile stresses at the bottom of the bituminous/cement bound

roadbase.

(3) The load spreading capability of granular subbases should be enough to

provide an acceptable construction platform.

(4) When a pavement is composed of a considerable depth of bituminous

material, its creep must be restricted in order to stop the rutting which

arises from internal deformation.

Some of the stresses referred to above are illustrated in Fig. 8.2.

232 Highway Engineering

Moving wheel load

Bituminous layers

Unbound / cement-bound

granular layer

Horizontal tensile strain

Fatigue crack

Subgrade Vertical compressive stress / strain at formation

Figure 8.2Critical stresses/strains in a bituminous highway pavement slab.

(Crown copyright 1984)

In contrast with RN29, where the failure condition was presented as a 20 mm

rut with severe cracking/crazing, LR1132 defined the end of a pavement’s design

life as indicated by a 10 mm rut depth or the beginning of cracking in the wheel

paths. These less severe indications were chosen on the basis that they are the

precursors of significant structural deterioration. They mark the latest time

when the application of an overlay will have maximum effect and will be