Page 31 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

3.5.4 Materials

Structural layers are designed to resist the high stress conditions imposed by external loads and to provide sufficient
protection of the pavement foundation during the structural design period. Material of appropriate quality therefore
needs to be sourced for the imported subgrade and structural pavement layers. Chapters 6, 8 and 9, and TRH3,
TRH14 and TG2 provide detailed information on the behaviour of specific material types. The use of a materials
checklist indicating the availability and cost of materials in the vicinity of the project is recommended.

3.6 Variability in Traffic Loading, Material Properties and Construction
A specific pavement design is based on certain estimates of the traffic loading that will be applied during its life,
properties of the material it will be constructed from and subgrade strength it will be constructed on. It follows that
the pavement design input parameters are not precisely known, and that a range of values exists for each of these
parameters. In addition, variability is built into the pavement during construction because material in quarries and
borrow pits vary spatially and with depth; testing is not sufficiently detailed to identify areas with poorer materials;
and, material selection is by visual inspection. Furthermore, compaction moisture content and compaction effort vary
from one construction section to the next, resulting in a variance in layer density and, therefore, strength. The
constructed layer thickness can vary up to a maximum of 27 mm from the design thickness and still be within
specification. In such cases where the input parameters are variable and construction techniques result in further
variability in the material properties, it is often better to determine the probability of a certain outcome rather than
try to predict the exact outcome. Confidence limits are, therefore, frequently used in pavement engineering to take
variability in materials strength parameters into account. The confidence limits are further coupled to a specific level
of service. The following are used in TRH4:
• 95% confidence limit for Category A roads
• 90% confidence limit for Category B roads
• 80% confidence limit for Category C roads

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The confidence limits imply that, for example, for the design of Category A roads, the 5 percentile values of the
material strength parameters are used in the design. It also implies that for a Category A road, there is a 5%
probability that the pavement will not be able to carry the traffic it was designed for, or that 5% of the pavement
area will fail before the design traffic has been carried.

Monte Carlo simulation is a method for taking variability in material strength parameters into account in pavement
design. The loading capacity of a pavement is calculated for material strength parameters that are randomly
selected from a statistical distribution of each strength parameter. This process is repeated for a large number of
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iterations and results in a distribution of loading capacities. The 5 , 10 or 20 percentile loading capacity for the
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pavement corresponding to Category A, B and C respectively can be determined from this distribution. The 5
percentile loading capacity indicates, for example, that there is a 95% probability that the pavement will be able to
carry this load.

The variability in traffic and construction is typically not currently taken into account in the design of pavements. It
appears that current construction quality control methods are insufficient to reduce variability caused by construction
to an acceptable level. This is a shortcoming, as premature pavement failures are often related to variability in
material strength and layer thicknesses introduced during construction.

3.7 Environmental Considerations
The environment in which the pavement is situated may be divided into:
• Traffic environment
• Natural (geological and meteorological) environment
• Population (rural or urban) environment

The traffic environment is the primary demand for which the pavement is designed. However, the natural and
population environments also affect pavement behaviour and performance, and must therefore be considered during
pavement design. The effect of environmental conditions on pavement performance is particularly important for light
pavement structures.

The differences between rural and urban environments usually do not have a significant impact on the structural
design of the pavement, but impact on the selection and design of the surfacing layer and drainage. Surfacing layer
selection and design is a specialised activity, for which the following documents provide guidance:
• TRH3: Design and Construction of Surfacing Seals

Section 3: Design Considerations
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