Page 96 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

7.1.5 Subgrade Permanent Deformation

(i) 1996 SAMDM
The subgrade, and lower selected layers are analysed for permanent deformation in the layer, which manifests as
permanent deformation, or wide rutting, on the pavement surface. The rutting is calculated using the vertical
compressive strain at the top of the layer, as shown in Figure 36. Transfer functions are available for both 10 and 20
mm of rutting in the layer. The 20 mm transfer function is conservative, in that if the subgrade or selected layer has
20 mm of rutting, then the rutting at the surface of the pavement is likely to be considerable higher than the typically
20 mm terminal rut depth. Therefore, the 10 mm transfer function is more commonly used, particularly for Category
A and B roads. The recommended resilient moduli are shown in Table 36, and the damage model in Equation (28) in
Table 37.

ε v

Figure 36. Critical Parameter and Location for Selected and Subgrade Layers

Table 36. Elastic Moduli (MPa) of Subgrade
Materials for the 1996 SAMDM

Selected Layers and Elastic Moduli
Subgrade Material Classes (MPa)
G7 120
G8 90
G9 70
G10 45

(ii) SAPDM
SAPDM uses new models for subgrade permanent deformation. These are described in a 2011 CAPSA paper by
Theyse et al, “Interim Revision of the South African Mechanistic-Empirical Pavement Design Method for Flexible
Pavements” and Theyse, 2008a. These models use the deflection at the top of the subgrade layer to predict the
permanent deformation, as this was found to be a better predictor than vertical compressive strain. New transfer
functions using deflection have been developed. These new models have not yet been widely implemented.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 85