Page 103 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• The ELTS for a layer is determined as the minimum of the support stiffness multiplied by the modular ratio limit
and the maximum allowed layer stiffness.
(v) The Base Confidence Factor

The type of material in the base layer is an important determinant of the performance of the pavement because the
base is the main load bearing element and failure of the base effectively constitutes pavement failure. Experience
has shown that there is a limit on the types of base materials considered for any given traffic situation. In particular,
suitable design options are significantly limited as the design traffic increases.

In the PN method, the appropriateness of the base material is controlled by the Base Confidence Factor (BCF), which
is used to adjust the layer contribution to the PN. Table 40 contains BCF values.
(vi) Thickness Adjustment for Cemented Layers
For cemented layers, the layer contribution to the PN is adjusted for thickness to account for crack propagation.

7.2.3 Pavement Number Calculation
Appendix C of TG2 contains the full details of the PN calculation along with a worked example. The main steps are
summarized below. In a pavement design situation, the steps described are applied for each uniform design section.
For rehabilitation design situations, it is thus presumed that the designer has detailed information on the existing
pavement layer properties for each uniform section.

Step 1: Check to ensure that the design method is applicable for the design situation.
Step 2: Determine the layer thicknesses, and available material properties for each layer. Determine the
design equivalent material class (DEMAC) using the guidelines in Chapter 9: 15 or Chapter 3 and Appendix A
of TG2. To prevent the use of unrealistic layer thicknesses, maximum and minimum limits are given.
Values outside these limits have not been validated.
Step 3: Combine layers with similar properties to obtain a five layer pavement system, including the subgrade
(i.e., four layers plus the subgrade). Check that the layer thicknesses do not exceed the maximum for
design purposes. Guidelines are given in TG2 on increasing or decreasing to 5 layers.
Step 4: Determine the ELTS of the subgrade by means of the given values. Adjust the stiffness for the climatic
region and depth of subgrade cover.
Step 5: For each layer above the subgrade, determine the modular ratio limit and maximum allowed
stiffness.
Step 6: Use the modular ratio limit and maximum allowed stiffness to
determine the ELTS for each layer by working up from the Inputs in the PN Method
subgrade.
Step 7: For the base layer, determine the Base Confidence Factor The values used for the ELTS,
modular ratio, layer thickness limits
(BCF). and BCF are specific to the PN
Step 8: For each layer, calculate the layer contribution to the PN method and should not be adjusted
using the ELTS, layer thickness and BCF (for base layers). by the designer.
Step 9: Add the layer contributions for each layer to get the PN.

The values used for the ELTS, modular ratio, layer thickness limits and BCF are specific to the PN method and should
not be adjusted by the designer.

7.2.4 Pavement Capacity Calculation
The calculation of the pavement capacity depends on the Pavement Number and the Road Category. The
relationship in Equation (29) is used, in conjunction with the constants in Table 41. The relationship between the PN
and the pavement capacity does not give a pavement life prediction, but rather provides a lower limit for which the
pavement should carry the desired traffic sufficiently. Figure 40 shows the criteria in a graphical format.

The criteria are only applicable to Category A and B roads and for design capacities between 1 and
30 MESA. For Category C and D roads, a catalogue of design is recommended, see TG2.

7.2.5 Comment on the PN Method
The PN method was developed from data from in service pavements and from the TRH4 catalogues. None of the
pavement structures available had asphalt surfacing layers thicker than 50 mm. Experience with the PN number is

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 92