Page 67 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

management systems. It may be necessary to revisit the design and life
cycle strategies at times during the pavement design process, when
more accurate information becomes available. Drainage

5.1.1 New Design Drainage is an extremely important
consideration for pavements! Water is
For new design, the formulation of the design strategy is followed by the primary cause of premature
these components of the design investigation: failure, accelerated distress and
• Detailed traffic investigation and design traffic estimate reduced structural capacity.
• Materials investigation All aspects of drainage are
• Environmental and practical considerations comprehensively covered in SANRAL’s
Drainage Manual and not repeated in
5.1.1.1 Traffic Investigation and Design Traffic Estimate SAPEM. Download the Drainage
Manual from www.nra.co.za.
Detail on the traffic investigation options and design traffic estimation
process are given in Section 4. Traffic survey options are limited in the
case of new pavement design because:
• The road is a completely new facility and a transportation study is required to estimate the future traffic.
• If the road is being upgraded from unpaved to paved standard, some indication of the future traffic may be
obtained from the traffic using the existing gravel road. Provision must also be made for traffic attracted to the
upgraded road.

5.1.1.2 Materials Investigation

The materials investigation potentially consists of three components for new pavement design:
• Centre-line soils investigation to delineate the in situ subgrade and to determine the subgrade class
according to Section 3.7.1.1. See also Chapter 4, Appendix A and Chapter 6: 8.
• Confirmation of material availability and cost from excavations, borrow-pits and quarries. A materials checklist
is useful. See Chapter 8, Materials Sources for more information.
• Assessment of the potential improvement of available materials through mechanical or chemical
stabilization. If the available material shows potential for improvement based on limited testing, and the
structural design indicates that the improved quality is required, the initial tests are supplemented with a proper
mix design.

The TRH4, Standard Specifications or TRH14 material classification systems are used to classify the subgrade. The
TRH14 system is summarized in the Appendix to Chapter 4 and the materials are discussed in Chapter 4: 2, 3 and 5
and Chapter 9. The material classification system for design, discussed in Chapter 9: 15, and in TG2 can also be
used.

5.1.1.3 Environmental and Practical Considerations
The potential impact on the design of the environmental and practical considerations must be considered before
proceeding with the structural design of the pavement. See Sections 3.7 and Section 5.2. This particularly applies to
the climate and traffic environments, and their impact on the selection of an appropriate pavement type and
surfacing, as well as drainage and compaction requirements.

5.1.2 Rehabilitation Design
The design investigation process for rehabilitation design is Rehabilitation Design
more involved than that of new design and is described in
detail in TRH12 for flexible pavements. The principles are the TRH12 is a good reference for most aspects of
same for rigid pavements. The aim of pavement rehabilitation rehabilitation design for flexible pavements.
is to restore deficient or failed pavements so that they can The current version is from 1997, but is due
carry the future traffic expected over the design period with for an update.
only periodic maintenance. The scope of pavement
rehabilitation, therefore, includes any of the following: • TRH12: Flexible Pavement
• Complete pavement reconstruction Rehabilitation Investigation and
Design. 1997.
• Partial reconstruction by strengthening existing layers
before resurfacing Although TRH12 is for flexible pavements,
• Asphalt, granular or concrete overlays most of the principles discussed apply to rigid
• Surfacing rehabilitation pavements.

Section 5: Pavement Investigation and Design Process
Page 56