Page 73 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

The final step of the detailed assessment phase is to update the uniform section reports from the initial assessment
phase with more reliable information. Then, the cause and mechanism of distress can be pinpointed. The input for
the structural pavement design should also be presented in a concise manner.

5.2 Design Considerations
Before embarking on the design, the designer should be aware of material availability and cost, as well as practical
and functional considerations that influence the final rehabilitation decision.

5.2.1 Material Availability and Cost

The existing pavement is a major source of available material in a rehabilitation design. The potential utilisation of
the existing pavement material through recycling must be investigated.

5.2.2 Constructability
Factors that need to be considered include: Reusing Materials in
• Traffic accommodation Existing Road
• Widening of the paved surface The existing pavement is a
• Bridge and overpass clearance major source of available
• Layer thickness constraints material in a rehabilitation
design. The potential
• Availability of resources utilisation of the existing
• Potential for deep in situ recycling pavement material through
• Environmental impacts of the selected rehabilitation option recycling must be investigated.

5.2.3 Performance Adequacy
Drainage conditions and the selection of an appropriate surfacing or wearing course for specific characteristics should
be kept in mind. For example, porous asphalt is used to reduce spray and noise.

5.2.4 Maintainability
Pavements have a long lifespan, including several maintenance and rehabilitation cycles. Maintainability
considerations are often determined by the road authority’s resources and policies, such as budget, maintenance
capabilities, maintenance policies and political policies. It is unwise to opt for a rehabilitation option that impairs the
future maintainability of the facility.

5.3 Structural Pavement Design

The structural design of a pavement, and subsequent economic analysis, should only proceed once sufficient
information has been gathered during the design investigation.

5.3.1 New Design
The purpose of the structural design of a new pavement is to develop a number of potential designs with sufficient
structural capacity to match the design traffic demand, while adhering to the principles of pavement design. Refer to
Section 2 for the principles of design and Sections 7, 8 and 9 for structural design methods. Appropriate structural
design methods for new design include:
• TRH4 catalogue and other industry catalogues
• DCP method, Section 7.3
• SAMDM for flexible pavements, Section 7.1
• Mechanistic-empirical design method for concrete pavements, Section 8.2
• Pavement Number method, Section 7.2
• Mechanistic-empirical design method for block pavements, Section 9.4
Maintainability
5.3.2 Rehabilitation Design It is unwise to opt for a
rehabilitation option that
The philosophy of rehabilitation design is to fully utilise the remaining strength of impairs the future
existing layers. The structural design is done for each of the subsections maintainability of the facility.
requiring structural strengthening. From a practical point of view, it is not
advisable to have short lengths of slightly different designs to be constructed

Section 5: Pavement Investigation and Design Process
Page 62