Page 33 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Table 4. In Situ Subgrade Delineation for Flexible Pavements

CBR (%) of Action
1
delineated
subgrade
sections
• In situ subgrade of a G7 standard and of sufficient strength to support structural layers.
> 15
• Rip and recompact to 93% of modified (mod.) AASHTO density.
• In situ subgrade of a G9 standard.
7 to 15 • Rip and recompact in situ material to 93% of mod. AASHTO density.
• Import a 150 mm thick layer of G7 standard material.
• In situ subgrade of a G10 standard.
• Rip and recompact in situ material to 93% of mod. AASHTO density.
3 to 7
• Import a 150 mm thick layer of G9 standard material.
• Import a second 150 mm thick layer of G7 standard material.
• Chemical/mechanical stabilization
< 3 • Or, remove and import new material.
• Or, add additional cover to place poor quality in situ material below material depth.
Note
1. CBR at 93% modified AASHTO density

3.7.1.2 Concrete (Rigid) Pavements
Slab support influences the performance of rigid pavements. It is more important to have a uniform slab support
than a strong, but variable, support. The prevention of voids under the slab due to erosion, plastic deformation and
subsidence and swelling clays is also important. A strong foundation support is not necessarily required, because of
the high stiffness, and therefore, good load spreading ability of concrete.

3.7.1.3 Block Pavements

The in situ subgrade requirements for block pavements are the same as flexible pavements.

3.7.2 Meteorological Environment
The meteorological environment is divided into macro-climatic regions with different moisture and temperature
conditions. Figure 16 shows the macro-climatic regions for moisture conditions based on Thornthwaite’s Moisture
Index (Paige-Green, 2012). Thornthwaite is a function of evapo-transpiration, i.e., dependent on vegetation and
rainfall. It determines the moisture deficit and is more accurate than Weinert’s N value, which is season dependent.
Thornthwaite also provides a more sensitive differentiation in the climatic regions than Weinert. The index is
interpreted as shown in Table 5.

Table 5. Interpretation of Thornthwaite’s Index
Index Range Climatic Region
< – 40 Arid
– 40 to – 20 Semi-arid Dry
– 20 to 0 Dry Sub-humid
0 to 20 Moist Sub-humid Moderate
> 20 Humid Wet

Drainage
Drainage is an extremely important consideration for
pavements! Water is the primary cause of premature
failure, accelerated distress and reduced structural
capacity.

All aspects of drainage are comprehensively covered
in SANRAL’s Drainage Manual and not repeated in
SAPEM. Download the Drainage Manual from
www.nra.co.za.

Section 3: Design Considerations
Page 22