Page 90 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

granular materials do not behave as a fully elastic material, but rather as an elasto-plastic material. An elastic and
plastic deformation component exists for each load application. The cumulative plastic deformation caused by the
load repetitions appears as a rut at the top of the granular layer. Therefore, the sum of the principal stresses
calculated in the centre of a granular layer using non-linear elastic theory defines the stress state. The model allows
for the calculation of the permanent deformation in each granular layer of a specific strength for any number of load
repetitions. The structural capacity of a pavement structure can thus be determined by limiting the sum of the
permanent deformation calculated in each pavement layer to the maximum permanent deformation allowable on the
surface of the pavement (10 or 20 mm depending on the road category). This concept makes the approach very
attractive for use in rehabilitation design, where measured rut levels can be effectively used to realistically calibrate
the system.

An example of the function relating permanent deformation to stress state and stress repetitions is shown in
Figure 33 for a G6 material at 90 percent design reliability.

While this model was developed using sound engineering, it is not widely used in practice. This is primarily because
it has only been incorporated into the Cyrano software package, and not into other software packages.

G6 Material at 90% Lower Confidence Limit

Figure 33. Relationship between Stress, Permanent Deformation (Strain) and Repetitions

(iii) SAPDM

Theyse (2008b) has recommended shear strength properties and transfer functions for unbound granular materials
for use in the SAPDM. These properties are listed in Table 31. A notable change is that G5 and G6 subbase quality
material are subdivided according to coarse and fine material, and the product of the Bar Linear Shrinkage (BLS) and

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 79