Page 99 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

7.2 Pavement Number (PN) Design Method

The Pavement Number (PN) design method was developed as part of TG2 (2009) for the design of bituminous
stabilized layers in pavements. The method is, however, also applicable to granular and cemented materials. The
method is described in detail in Appendix C of TG2 and the background development by Jooste (2009). Although the
method is relative new, it is already being widely used in South Africa.

The PN method is applicable to Category A and B roads where the design traffic is between 1 and 30 MESA. The
method can be used in the initial and detailed assessment phase in the design of new and rehabilitated pavements.

The PN design method is a knowledge-based approach. It is based on the Structural Number concept, as used in the
AASHTO method (Section 7.4). However, some of the shortcomings of the Structural Number have been overcome
in the PN method. The PN method has the following advantages:
• Data from in-service pavements were used to develop the method. The type and detail of the data suggests
the use of a relatively simple method and precludes the use of a Mechanistic-Empirical design method.
• The method gives a good fit to the available field data.
• The method is robust, and cannot easily be manipulated to produce inappropriate designs.

This method relies on basic rules-of-thumb, which reflect well-established principles of pavement behaviour and
performance, and ensure an appropriate pavement design solution in most situations. The concepts in the rules-of-
thumb are quantified into specific rules with constants or functions associated with each rule. The rules-of-thumb
are briefly described in the following sections.

The constants shown for the PN method included here and in Appendix C of TG2 were the values used at the time of
publication of TG2. Although these values are well validated it may be necessary from time to time to make changes
to improve the system. If changes are made, the modified values will be reflected on
www.asphaltacademy.co.za/bitstab. It is therefore recommended that before commencing a Pavement Number
calculation, the website is checked for any changes in values or tests.

The PN method is designed to be used in conjunction with the material classification system described in Chapter
9: 15 and in TG2, Chapter 3 and Appendix A. Software is also available on www.asphaltacademy.co.za/bitstab.
However, it is simple to do the calculations in a spreadsheet.

7.2.1 Applicability of Pavement Number Method
Before the Pavement Number method is used, the designer must check that the following situations do not apply:
• Design traffic greater than 30 MESA: The method was calibrated using a knowledge base which was limited
to pavements that had accommodated less than 30 MESA. Thus, if the design traffic exceeds 30 MESA, the PN
method is inappropriate as it has not been validated at these high traffic levels.
• Presence of thin, weak lenses: If thin, weak lenses of material exist below the surfacing, or between
stabilized layers, especially within the upper 400 mm, then zones of high slip and shear develop, and the PN
calculations do not apply. In such situations, the structural capacity assessment of the PN method is not
appropriate, and special treatment of the affected weak lens must be undertaken.
• Subgrade CBR less than 3%: The knowledge base on which the PN method was calibrated did not include
any pavements that had a subgrade CBR less than 3%. The PN method should therefore not be used in cases
where the subgrade CBR is less than 3% at a depth 600 mm below the surface.

7.2.2 Rules of Thumb / Departure Points
The rules-of-thumb underlying the method reflect well
established principles of pavement behaviour and performance.
The following rules-of-thumb are used:
• Pavement system in general PN Method
− The structural capacity of a pavement is a function of the The PN Method is described in detail in TG2
combined long term load spreading potential of the (2009) in Appendix C. The method is
pavement layers and the relative quality of the subgrade. relatively new and although it has been well
− The relative quality and stiffness of the subgrade is the validated, it may be necessary from time to
departure point for design, as the subgrade is a key time to make changes to improve the
determinant in the overall pavement behaviour and method. Any such changes will be published
performance. on www.asphaltacademy.co.za/bitstab, and
registered users will be notified.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 88