Page 32 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Guidelines for Upgrading Low Volume Roads (SARB, 1993a)
• Towards Appropriate Standards for Rural Roads: Discussion Document (SARB, 1993b)

Details on subsurface drainage are provided in:
• The SANRAL Drainage Manual

3.7.1 Geology and Climate
The geology and climate of a region largely determines the characteristics of the in situ subgrade for new roads.
Geological issues are discussed in much detail in Chapter 6, Road Prism and Pavement Investigations: 6 and 8, and
Chapter 7, Geotechnical Investigations and Design Considerations: 3.
3.7.1.1 Flexible Pavements In Situ Subgrade

(i) Material Depth
The material depth denotes the depth below the surface of the finished road that soil characteristics significantly
affect pavement behaviour. Above this depth, the pavement strength must be sufficient for the traffic imposed
stresses. Below this depth, the traffic imposed stress conditions are assumed to have dissipated and the material
quality exceeds strength requirements. The moisture condition above the material depth has a major influence on
the material strength, and needs to be controlled by providing adequate surface and subsurface drainage. The
material below the material depth must be competent to support the pavement structure. The material depths
shown in Table 3 are recommended for the different road categories for flexible pavements. Chapter 9: 2 provides
additional discussion on the material depth.

Table 3. Recommended Material Depth for Each Road Category

Road Category Material Depth (mm)
A 1 000 – 1 200
B 800 – 1 000
C 800
D 700

Material depths are normally not considered for concrete pavements, provided that the support is consistent.

(ii) Minimum Subgrade Strength and In Situ Subgrade Delineation
A key pavement design principle is that the subgrade provides an adequate foundation for the pavement layers. A
minimum Californian Bearing Ratio (CBR) of 15% is generally required for flexible pavements.

To determine the adequacy of the in situ subgrade, it is divided into sections based on the CBR, using the ranges in
th
Table 4. The 10 percentile CBR value is determined for each region, and must exceed the minimum value in the
range, i.e., 90% of the CBR results should exceed the specified lower limit. If the strength of a particular subgrade
section does not meet the minimum strength requirement, layers of increasing quality are imported to ensure that
the minimum is achieved. Following the recommendations from Table 4, the pavement foundation is built-up, as
illustrated in Figure 15. When the road is constructed on a fill, the material should be controlled to at least the
minimum material depth specified in Table 3.

Figure 15. Importing Layers to Obtain Minimum Subgrade Strength

Section 3: Design Considerations
Page 21