Factors Influencing Design Traffic Loading

The term Design Traffic Loading is one that reflects the calculation of a cumulative repetitious load that pavement is subject to prior to structural failure.

In a flexible pavement configuration, the repetitious wheel load can induce bottom-up fatigue cracking in bound materials and rutting and shape loss in sub-grade materials.

These wheel induced distress mechanisms include:

• Asphalt – Fatigue Failure
• Subgrade – Rutting Failure
• Cemented Material – Fatigue Failure

Given the importance of accurately determining the design traffic loading, careful consideration is typically given to the following:

Historical Heavy Vehicle volumes

Investigating and generating an understanding of historical heavy vehicle volumes are required to determine a daily HV count for the year of opening.

This is important as it is the basis in which the design traffic loading is calculated and can be influenced by growth trends.

Lane Configurations

The basis of a design traffic loading is passed over a single section of pavement (i.e. a single lane), where multiple lanes are used, consideration is often given to proportioning the traffic.

The magnitude in which this proportioning is undertaken is very much dependent on the nature of the section and the travel paths of the vehicles. It is, however, a common practice to adopt a reduced loading in the fast lane, particularly in a rural environment.

Heavy Vehicle Axle Configurations

The axle configuration of a heavy vehicle can affect the way in which the wheel loading is induced and how the pavement structure will perform.

The number of axles, location of the load and the payload can influence the design traffic loading and the number of ESAs/heavy vehicle.

Heavy Vehicle Growth

An aspect that can often result in premature pavement failure is the underestimation of heavy vehicle growth. To generate a sound design traffic loading, consideration should not only be given to historical trends but new development and growth in the area.

When incorporating these factors the design traffic can be calculated in terms of a design life in ESAs and SARs for input in to either empirical or mechanistic thickness design. The design life of flexible pavements can typically vary between 20 and 40 years depending on desired service life and local environmental factors.

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