Page 97 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Table 37. Transfer Functions for Subgrade and Selected Layer Permanent Deformation

Permanent Deformation

) (28)
(a−10 log ε v
N PD = 10

where N PD = Standard axles to set level of permanent deformation
ε v = Vertical compressive strain at top of layer
a = Constant, given below
Constants
A
Reliability Level
10 mm Terminal Rut Depth 20 mm Terminal Rut Depth
95% (Category A) 33.30 36.30
90% (Category B) 33.38 36.38
80% (Category C) 33.47 36.47
50% (Category D) 33.70 36.70

7.1.6 Analysis Positions for Critical Stress and Strain Parameters
The location in the pavement structure at which the critical parameters are calculated depends on the material type
and the assumed mode of failure. Figure 37 summarises the critical parameters and locations used for a typical
South African pavement structure, as discussed in the previous sections. Note that generally a half-axle load is
assumed, because the other side of the axle is out of the zone of influence. The critical parameters are calculated
both underneath and in-between the loads. This is because the deeper into the pavement, the more the two zones
of loading influence, represented by the “cones” Figure 37, interact. Standard practice is to calculate the critical
parameter under and between the wheels, and then use the value which produces the shortest structural capacity.

Figure 37. Analysis Positions for Critical Parameters

7.1.7 General Comments on SAMDM 1996
The SAMDM is a powerful design tool. However, it does have some weaknesses, which must be considered when
using the method. There are also some important tips and checks useful for using the method. This section
attempts to discuss some of these aspects.

7.1.7.1 Standard Axle and Tyre Pressure
The SAMDM is generally used with standard axle loading. See Sections 3.5, 3.5.2 and 4.1.3 for definitions and
discussion on standard axles. The catalogues in TRH4 were developed using standard axle loading, with a 520 kPa

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 86