Page 12 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

1. INTRODUCTION
Pavement design is an engineering discipline, somewhere in between science and art. In contrast to scientists,
engineers do not require perfect models, but models that are “good enough” to produce fit for purpose designs.
Where the scientific knowledge is insufficient, it is supplemented with the experience and sometimes intuition (art) of
the designer. However, it remains the professional responsibility, and liability, of the design engineer to ensure that
the design is based on the accepted design practice of the time.

The loading on pavements consists of millions of relatively small magnitude loads causing the gradual or incremental
deterioration of the pavement, until the level of service becomes unacceptable. The stress imposed by the external
load is normally well below the strength of the material, resulting in gradual deterioration, rather than a catastrophic
failure caused by one load. In some cases, climatic effects are more detrimental than the effects of loading,
especially in low volume roads.

The basic objective of pavement design is to combine materials of sufficient strength in a layered system to provide
the desired functional and structural service levels over the design period, subject to the applicable traffic demand
and environment in which the pavement operates. The economic viability of a design is determined by:
• Functional and structural service levels
• Deterioration rates of these service levels
• Costs associated with the provision and maintenance of these service levels
• Savings by the road users resulting from improved service levels

Although the final design decision is dictated by the most economically viable design, the process of ensuring a
pavement with adequate strength is provided is critical.

The ability of a pavement to provide acceptable functional and structural levels of service defines the pavement
“supply”. The “demand” on a pavement depends on the environments in which the pavement operates, which are:
• Traffic environment, which is the primary “demand” imposed on the pavement by the vehicle traffic operating
on the road. The following factors are important:
− Axle load magnitude
− Contact stress
− Traffic volume
• Natural environment, may be subdivided into the geological environment and climatic environment, which
together determine:
− Available natural material sources
− In situ subgrade conditions
− Moisture regime in the pavement
− Temperature and parameters contributing to aging, specifically of asphalt layers
The natural environment plays an increasingly important role in low volume roads, as illustrated in Figure 1.
• Population environment, which is of particular importance in urban areas

Gravel Roads
Flexible & Rigid Pavements The design of gravel roads is not discussed
Flexible pavements or bituminous in this manual. However, most of the
pavements typically have asphalt or seal design principles discussed apply to gravel
surfacings. These structures are roads. A good reference for the design of
traditionally characterised by higher gravel roads is: TRH20: Unsealed
deflections or bending. Roads: Design, Construction and
Maintenance. 2009.
Rigid pavements or concrete
pavements act in a rigid manner relative Specifications for gravel roads are included
to flexible pavements, producing much in the Standard Specifications.
lower deflections and bending.

Section 1: Introduction
Page 1