Page 98 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

tyre pressure. However, since those analyses in 1995, tyre pressures in South Africa have risen, and 520 kPa is no
longer representative of current loading. Theyse (2011) recommends a 650 kPa tyre pressure. A pressure of 750
kPa is widely used to account for current loading. These higher values affect the results, which should be noted
when comparing designs to the TRH4 catalogues.

7.1.7.2 Critical Layer and Structural Capacity Determination
The SAMDM works by determining the structural capacity of each layer in the pavement. The structural capacity of
the pavement is determined by the layer with the shortest life, termed the critical layer. The exception is when thin
asphalt layers are not considered in the determination of the critical layer, as discussed in Section 7.1.1.

For pavements that include cement stabilized layers, the analysis is done in two phases. The first phase is the
effective fatigue phase, and in this the cemented layer is analysed as a cemented layer, with a high resilient modulus
used as input. The structural capacity of this phase is determined by the effective fatigue of the cemented layer. At
the end of the first phase, the cemented layer is assumed to be in an equivalent granular state. The layer is not in a
terminal condition. In the second phase, the cemented layer is now analysed as an equivalent granular material.
The structural capacity of the pavement is a combination of the first and second phase. Should two cemented layers
be present, the pavement is likely to need analysis in three phases. Details on how to calculate the combined
structural capacity from the phases are included in Theyse, 1996.

7.1.7.3 Method Sensitivity
The 1996 SAMDM is very sensitive to the input values, and large differences in the structural capacity estimates are
found with small changes in the inputs. To prevent inaccurate and inappropriate results, it is important for the
following issues to be taken note of:
• The input values used should be realistic for the available materials. It is highly recommended that the input
values used are similar to those that are suggested for the applicable materials (see Table 27, Table 29, Table 34
and Table 36). In addition, it is important that, especially for rehabilitation design, the available materials
information is used to verify the design inputs. The material classification system for design discussed in
Chapter 9: 15 is useful for deciding appropriate material classes for existing pavement layers.
• The SAMDM can give inappropriate answers. It is important to check the results with some common sense
“does it pass the test of reasonableness?”, and against other methods including the catalogues in TRH4. A
CAPSA 2004 paper by Jooste, illustrates some weaknesses of the method.
• The method must be used in the way it was developed and calibrated. For example, the method specifies
that that granular shear transfer function calculates the safety factor in the middle of the layer. The middle of
the layer may not be the point at which the safety factor is the highest, but the transfer functions were validated
for the calculation in the middle of the layer, and the answers are inappropriate if another position is used.

Advantages of the SAMDM Disadvantages of the SAMDM
• Suited to new and rehabilitation design • Developed for new pavement design, and
• Evaluates the adequacy of individual layers and adapted to rehabilitation design with difficulty
the pavement system • Perceived to be biased towards certain
• Calibrated for South African conditions and pavement types
materials • Overly sensitive to small variation in input
• Accommodates different pavement types and • Input parameters not well related to routine
pavement compositions engineering parameters
• Accommodates changes in operating conditions, • Damage models outdated
such as axle loads • Relatively complex, only suited to computer
application
• Inconsistent results, stronger and thicker layers
(especially subbase) do not always lead to an
increase in structural capacity

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 87