Page 85 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

7. STRUCTURAL CAPACITY ESTIMATION: FLEXIBLE PAVEMENTS
There are many structural capacity estimation methods available for flexible pavements. Each has their own
advantages and disadvantages, and may only be used for appropriate situations. The following methods are
discussed in this section:
• South African Mechanistic-empirical Design Method (SAMDM)
• Pavement Number (PN) Mechanics of Materials
• Dynamic Cone Penetrometer (DCP)
• AASHTO Structural Number (SN) The mechanics of materials,
including stresses and strains and
• FWD deflection bowl parameter method materials models, are discussed in
• FWD Structural Number (SN) Chapter 2: 3 and 4.2. How these
• TRRL Surface Deflection concepts are applied to pavement
• Asphalt Institute Surface Deflection materials is also discussed.

This section briefly discusses each of these methods, and provides a reference for each design method where full
details are available. The advantages and disadvantages of the methods are summarised, and the applicability to
various pavement situations is mentioned. It is the responsibility of the designer to select the most appropriate
method(s) for a particular design situation.

The input parameters required by the selected design method should be determined for the actual materials available
for the project. It serves no purpose to select values from published data, because that only results in a general
design similar to those already contained in a design catalogue. Mechanistic-empirical methods do, however, provide
the scope to study the impact of changes in certain input parameters, such as the loading conditions and pavement
composition, on the structural capacity estimate.

7.1 South African Mechanistic-Empirical Design Method (SAMDM)
The 1996 version of the South African Mechanistic-Empirical Design Method (SAMDM) has been widely used in South
Africa for many years. The historical development of the method is discussed in Theyse et al (1996). Ranges of
typical resilient modulus and material strength input values are published for South African road-building materials.
Damage models were calibrated for each of the main material groups used in South African road construction.

The damage models currently provided for by the 1996 version of the SAMDM are:
• Hot mix asphalt fatigue
− Fatigue of thin (< 50 mm thick) surfacing layers: continuously and gap-graded
− Fatigue of thick (> 75 mm thick) base layers
• Unbound granular base and subbase layer
− Permanent deformation
• Cemented base and subbase layers
− Crushing failure
− Effective fatigue Latest AASHTO Design Guide
− Permanent deformation In the USA, a mechanistic-empirical design
• Subgrade permanent deformation guide has been developed by AASHTO for all
typical pavement types. The method is also
The SAMDM is currently under review. Details on the known as NCHRP 1-37, and was published in
anticipated characteristics of the revised method can be found 2004. The method is complex, and needs to be
in Theyse et al, 2007, with some interim recommendations calibrated for climatic areas and specific
and models included in Theyse et al, 2011. Where possible, projects.
some brief details on the interim SAMDM (SAPDM) are
provided. Further updates are likely, with the revised SAPDM The method utilises a recursive procedure, to
likely to be published in 2013. calculate the deterioration over the life of the
pavement.
Substantial research has recently been done on bitumen
stabilized materials (BSMs) (Long and Theyse, 2003; Jooste et Aspects of this AASHTO method that are
al, 2009), unbound granular materials including crushed stone applicable and useful to South Africa are being
and natural gravel, and South African subgrades (Theyse, included in the SAMDM revision, which is
2001) to improve the mechanistic-empirical models. The known as the South African Pavement Design
improved models for these materials will be incorporated as Method, SAPDM.
part of the method revision.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 74