Page 115 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• The deflection–capacity relationship is only valid for the types of pavements tested. The trend that the
deflection increases with accumulated traffic and deterioration is because the pavements did not have adequate
subgrade cover.
• The method does not allow for the evaluation of the adequacy of the individual pavement layers.
• The mathematical formulation for the design charts is not available and the application of the method relies
on plotting the deflection and past traffic on the charts by hand and visually reading off the remaining life and
required overlay thickness. The method is therefore not well suited for modern software systems for pavement
rehabilitation.
• The Benkelman beam and Deflectograph are becoming obsolete and the current preferred instrument for
deflection surveys is the Falling Weight Deflectometer.

Figure 46. Example of the TRRL Overlay Design Chart for Granular Base Pavements

Advantages of the TRRL Method Disadvantages of the TRRL Method
• Provides for different pavement types • Empirical: derived from data collected in
• Accommodates Benkelman beam and England
Deflectograph deflection • Outdated: based on Benkelman beam deflection
• Corrects deflection for temperature variation • Developed for 62 kN axle loads
• Relatively simple and easy to apply • Relies on correlations for converting
Deflectograph and FWD deflections to equivalent
Benkelman beam deflections
• Not convenient for software implementation
• Applies to rehabilitation design only

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 104