Highway Engineering

expected to make best use of the original structural quality of the pavement. In
other words, the design life as thus defined is the latest time at which the appli-
cation of an overlay will deliver another few years of high quality motoring.
This is termed pre-emptive overlaying, a process carried out at the onset of criti-
cal structural conditions within the pavement. If application is postponed to a
point later in the pavement’s life, it may well have deteriorated to a stage where
extensive pavement reconstruction will be required.
Since the LR1132 approach maximises the use of the existing pavement’s
strength, a pavement of more uniform strength will result. In addition, as dete-
rioration can be predicted without too much difficulty, ultimate reconstruction
can be more easily planned. This definition of design life results in LR1132
designing a pavement having an additional period of serviceable life before
major reconstruction, a period that would not be available if Road Note 29 were
used.
A design life of 20 years is normally employed.
Given the adoption of the design life concept as detailed within LR1132, the
cumulative number of equivalent 80 kN standard axles to be carried during the
design life of the highway must now be estimated. Observed or estimated 24-
hour commercial vehicle flows must be converted to annual flows. If there is
more than one lane in each direction, an allowance must be made for the pro-
portion of this traffic travelling in the nearside lane, assumed to be the lane car-
rying the majority of commercial vehicles (RN29 makes this same assumption).
The annual traffic is then multiplied by the vehicle damage factor – an estima-
tor of the damage effect of an average commercial vehicle.
The design procedure can be summarised as follows.

Step 1
Calculate Tn, the total number of commercial vehicles using the slow lane over
the nyears design life, as follows:

(8.2)

where
F 0 =initial daily flow (base year)
r =commercial vehicle growth rate
n =design life
P=proportion of commercial vehicles using the slow (nearside) lane
P=1 if it is assumed that all vehicles use the nearside lane.

Step 2
Calculate the damage factor,D.
In order to convert Tninto equivalent standard axles, it must be multiplied by
the vehicle damage factor,D, calculated for the mid year of the design life,Fm.
The damage factor is calculated as follows:

TF

r

r

n P

n

=

()()+ -

365

11

0

Structural Design of Pavement Thickness 233