Chapter 8
Structural Design of Pavement Thickness
8.1 Introduction,
One of the basic requirements for a pavement is that it should be of sufficient
thickness to spread the surface loading to a pressure intensity that the underly-
ing subgrade is able to withstand, with the pavement itself sufficiently robust to
deal with the stresses incident on it. Where required, the pavement should be
sufficiently thick to prevent damage to a frost-susceptible subgrade. Thickness
is thus a central factor in the pavement design process.
The thickness designs in this chapter are based on LR1132 (Powell et al., 1984)
for flexible pavements and RR87 (Mayhew & Harding, 1987) for rigid con-
struction. However, these basic designs are modified and updated based on later
research as detailed in HD 26/01 (DoT, 2001). The pavement thickness design
methodologies for the two different categories of pavement type are treated
separately below.
In order to reflect European harmonisation, the names of the various pave-
ment layers have been altered within the context of thickness design, as seen in
Fig. 8.1.8.2 Flexible pavements,
8.2.1 General
The pavement should be neither too thick nor too thin. If it is too thick, the
cost will become excessive. If it is too thin, it will fail to protect the underlying
unbound layers, causing rutting at formation level.
A flexible pavement is defined as one where the surface course, binder course
and base materials are bitumen bound. Permitted materials include hot rolled
asphalt (HRA), high density macadam (HDM), dense bitumen macadam
(DBM) and dense bitumen macadam with 50-penetration bitumen (DBM50).
Flexible composite pavements involve surface course and upper base materials
bound with macadam built on a lower base of cement bound material (CBM).
Wearing courses are either 45 mm or 50 mm of hot rolled asphalt or 50 mm of