Page 109 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

log   ∆ PSI  
−
×
−
log SC Z × R S + o 9.36 log (SN + ) 1 − 0.20 +  4.2 1.5  1094 + 2.32 log (M ER ) 8.07 (30)
=
×
0.40 +
(SN + ) 1 5.19
where SC = Structural capacity of the pavement (Standard Axles)
Z R = Standard normal deviate
S 0 = Combined standard error of the traffic and performance predictions
SN = Structural number of the total pavement thickness
∆PSI = Difference between the initial (PSI 0 ) and terminal (PSI t ) serviceability indices
M ER = Effective roadbed resilient modulus adjusted for seasonal variation (psi)

SN aD= 1 1 + a D m + 2 2 2 a D m (31)
3
3
3
where SN = Structural number of the total pavement thickness
th
a i = i layer coefficient (per inch)
th
D i = i layer thickness (inches)
th
m i = i layer drainage coefficient

Table 43. Layer Coefficients
Material Ranges for South African
Materials
Asphalt concrete 0.20 – 0.44
Crushed stone 0.06 – 0.14
Cemented-treated material 0.10 – 0.28
Bituminous-treated material 0.10 – 0.30

The AASHTO design guide of 1993 provides recommendations and predictive equations relating the layer coefficients
of different material groups to other engineering parameters. The method also requires that drainage coefficients
are applied to account for the quality of the drainage and the time the materials are exposed to near saturation
conditions.

When using the AASHTO SN method to estimate the initial structural capacity in the initial assessment phase, the
subgrade modulus derived from FWD backcalculation, or from the DCP penetration rate, may be substituted for the
effective roadbed modulus. However, care should be exercised when using these values in a rehabilitation design
situation since South African subgrade stiffnesses are often much higher than those the method caters for, which
may result in unrealistically high structural capacity outputs. The change in riding quality is calculated from the
appropriate percentile value for the current riding quality (PSI) and the terminal riding quality (PSI).

Advantages of the AASHTO Method Disadvantages of the AASHTO Method
• Models riding quality deterioration • Empirical: derived from data collected at one
• Models available for flexible and rigid pavement site in the USA
design • Not sensitive to quality of base
• Relatively simple to apply • Not sensitive to pavement balance, switch layer
• Applicable to new and rehabilitation design positions and get same result
• Principles used in the HDM IV economic analysis • Outdated: derived from data collected almost 50
software years ago
• Relatively quick and easy to use, and provides a • Developed for foreign conditions and materials
good check of pavement designs done by other • Uses imperial units
methods

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 98