Page 114 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

E80s for bituminous cover 175 mm or thicker.
• The surface deflection does not give a good indication of the remaining life when structural weakness is
present in and upper cemented layer, supported by a sound lower cemented layer.
• Surface deflection early during the life of pavements with cemented layers does not give a good indication of the
remaining life, if the grading of the aggregate for the cemented layers is such that the material has little
mechanical stability. Cracking of the layer results in disintegration, with an associated rapid reduction in
remaining life.

Figure 45. Using the TRRL Chart to Determine Remaining Life for a Self-Cementing Granular
Base Pavement

(ii) Overlay Design
Overlay design charts are provided for the same pavement types as for remaining life. The thickness of the overlay
depends on the pavement type, the standard deflection before rehabilitation and the expected future traffic. The
design charts are based on a 90% probability of achieving the design life. An asphalt overlay thickness of 40 mm is
considered the minimum. Figure 46 shows an example for pavements with granular bases.

The following comments apply to the use of the TRRL surface deflection method:
• The method is an empirical method developed from data collected only in England.
• The method is based on Benkelman beam deflection. Although a conversion from Deflectograph to
Benkelman beam deflection is provided, it is based on a single correlation between these two deflection types,
which probably does not hold for all pavement types.
• The method provides for different deflection-performance relationships depending on pavement type.
• The method was developed for a 62 kN axle load while the design standard in South Africa is an 80 kN axle
load. Although Jordaan (1989b) adjusted the charts to accommodate the deflection at an 80 kN axle load, the
adjustment is linear. It is known from field observations that the relationship between axle load and pavement
deflection is not linear.
• Although deflections are adjusted for temperature, no adjustment is recommended for seasonal (moisture)
variation.
• The method was developed using data from sections that all had inadequate subgrade cover, designed to fail
within a reasonable time. The method is therefore suited for pavements that have inadequate cover, which
needs to be provided.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 103