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South African Pavement Engineering Manual
Chapter 10: Pavement Design

Figure 16. Southern Africa Macro-Climatic Regions Based on Thornthwaite’s Moisture Index

The moisture condition affects the weathering of rock, the durability of weathered material and in combination with
drainage conditions and the surfacing layer integrity, the moisture content and stability of unbound granular layers.
Cognisance must also be taken of local micro-climates within the macro-climatic regions, as these may deviate
substantially from the macro-climatic norm. An example is where orographic rainfall occurs in mountainous areas.

Temperature conditions largely affect the design of surfacing layers, particularly hot mix asphalt. Prediction of the
temperature in these layers using ambient temperatures and solar radiation, for South African conditions, has been
developed (Viljoen, 2001, Denneman, 2007a and b).

3.8 Practical Design Considerations

Poor drainage and insufficient compaction are probably responsible for more pavement failures than poor structural
and material design. Problems related to the subgrade below the material depth must be considered. Other
practical considerations that may impact on the design are variable cross-sections and paved shoulders. These
considerations are discussed in the following sections.

3.8.1 Drainage
Efficient drainage of a pavement structure plays a vital role in Drainage Design
the performance. It is, therefore, imperative that the drainage Efficient drainage is an essential part of good
design of the pavement structure is completed in great detail. pavement performance. The following
Adequate surface and subsurface drainage should be provided. guideline provides details for drainage design:
SANRAL’s Drainage Manual provides guidelines for drainage
th
design. • Drainage Manual, 6 edition, SANRAL,
2013. Download at www.nra.co.za.
Effective drainage should be provided to at least the material
depth. Special care should be exercised in cuttings. Cuts and
fills on side-slopes often lead to drainage problems. Proper drainage facilities on the embankment side are well
worth the investment. See Chapter 7: 4 and 5 for more on cuts and fills.

Section 3: Design Considerations
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