Page 81 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

6. STRUCTURAL CAPACITY ESTIMATION
The purpose of structural pavement design methods are to provide a method for the unbiased estimate of the
structural capacity of alternative design options, with the aim of selecting the most economical option and that
ensures the traffic demand will be met.

Structural design methods vary greatly in the amount of detail required and the level of analysis involved. The
methods are generally:
• Empirical methods, based on observations of the performance of pavements.
• Mechanistic-empirical (ME) methods, which analyse the pavement as a mechanism, and link mechanistic
parameters to the structural capacity through empirical observations of performance.
• Catalogues with standard designs for general conditions. These are developed either empirically or using ME
methods, or a combination of both.

The next three sections, Sections 7, 8 and 9, include the common empirical
and mechanistic-empirical design methods for flexible, rigid and block Design Catalogues
pavements. Section 6.1 gives some general comments on Mechanistic- These catalogues available in
Empirical Design Methods. TRH4 (1996) are “tried and
tested”, and should be used to
TRH4 contains catalogues of pavements for different combinations of benchmark any pavement
pavement type, road category and design structural capacity for flexible design.
pavements. The flexible pavement designs are all based on the South
African Mechanistic-Empirical Design Method of 1995 (Theyse et al, 1995 and
Theyse et al 1996). These catalogues are “tried and tested”, and should be used to benchmark any pavement
design. The M10 manual (1995) contains some catalogues for rigid pavements. UTG (1982) contains catalogues for
block pavements.

It is important to keep in mind that there is no single structural capacity value associated with a pavement, but
rather a range of appropriate values. This is because of the variation in all the variables used to determine the
structural capacity, resulting in a distribution in the structural capacity assessment. In addition, all assessment
methods are estimates, and no one method provides an absolute prediction. The structural capacity estimate is
therefore a single point estimate taken from a possible distribution of values and most often reflects the average
value. The use of stochastic simulation techniques to estimate a structural capacity distribution, such as the Monte
Carlo simulation technique, is useful. An example of the use of such a method is given in Jooste (1997).

6.1 General Comments on Mechanistic-Empirical Design Methods

Classical mechanistic-empirical (ME) methods have been used for many years. Many aspects of this type of analysis
are common to all ME methods and to rigid, flexible and block pavements. Some general comments on these
methods are discussed here, with details on specific methods given in Sections 7.1 for flexible pavements, 8.2 for
rigid pavements, and 9.4 for block pavements.

Mechanistic-empirical methods analyse the pavement as a mechanism by assuming a material model for the
pavement type (see Chapter 2: 3 and 4.2) and calculates engineering parameters such as stresses and strains. The
engineering parameters are then linked to the structural capacity of the pavement through observations of
performance. These methods follow a logical structure and adhere to the laws of solid mechanics, but have
absolutely no “intelligence” incorporated in the design method. Therefore, although the design method enables the
design engineer to develop a pavement design with sufficient structural capacity, it may not necessarily be the
optimal design. The “intelligence” in the system is provided by experience and sound engineering practice.
Fortunately, much of the sound engineering practice in South Africa has been captured in national and industry
guideline documents.

Figure 30 shows the typical components of a classical mechanistic-
empirical design method.
ME Methods
Although ME design methods enable a
pavement design with sufficient structural
capacity, it may not necessarily be the
optimal design. The “intelligence” in
the system is provided by experience
and sound engineering practice.

Section 6: Structural Capacity Estimation
Page 70