Page 44 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

(Section 7.1.1). Note that the transfer functions use the initial condition of the pavement to estimate the
terminal condition.
• For each load applied of each axle group (n j,i ), determine the proportion of damage done by that load using
Equation (3).
• Add the damage from each axle load and axle load group. When the total damage reaches one, the layer
has reached the end of its life (or phase).

n j,i
Damage = � ≤ 1 (3)
N j,max
where n j,i = Number of load applications of load j
N j,max = Repetitions to failure of load j, as determined by transfer function

This simple method is often performed using a specific time period, such as a 3 months season. The damage for all
the applications of each axle load group carried during that period is summed, and failure occurs when the damage
equals, or exceeds, one.

This Miner’s Law approach is linear in that the damage develops linearly between the initial and terminal condition,
as those are the only condition states. Each load application of an axle group is considered to do the same amount
of damage, regardless of the current condition of the pavement when the load is applied. This is a simplification of
how damage develops in pavements. This method is fairly widely used, but should be used with caution as the
failure criteria utilised were not developed and calibrated for such an application.

A full recursive pavement design method re-evaluates the condition of the pavement, and the changing stress state
of the materials, to determine the effect of damage on the pavement system from each application of each axle load
group. An overloaded truck will therefore not do the same amount of damage when applied at the beginning than at
near the end of the life of the layer. The revised SAMDM (SAPDM) (see Section 7.1) will incorporate recursive
analysis techniques.

4.1.4 E80s per Heavy Vehicle (E80/HV)

For many projects, the full axle load distribution is not available. In these cases, the concept of an E80 per heavy
vehicle (E80/HV) is used. An E80/HV is a factor that converts different truck loads to an equivalent number of
standard axles. The concept is illustrated in Figure 20, where the truck axle loading is first converted to an
equivalent standard axle (using Equation (1) in Section 4.1.3), and summed for all the axles to obtain the E80/HV for
that vehicle.

The E80/HV for each vehicle type is, however, not that useful, considering the many different vehicle types on any
one route. Therefore, the average E80/HV for a network or section of road is generally used. This average is a
weighted average of the E80/HV of the vehicle types and the number of the vehicles per day, and is specific to a
network route or project section. The value of the average E80/HV factor gives an indication of the loading on a
road. Generally, roads with a higher number of heavy vehicles, and with more axles, have a higher E80/HV. For
example, the N3, which carries large number of heavy trucks, typically assumes an E80/HV of 3.1 whereas the N2
between Port Elizabeth and Grahamstown uses 1.5. When the term E80/HV is used, it typically refers to the
weighted average of the E80/HV for the specific network. Changes to the legal axle load and the level of
enforcement generally affects the E80/HV for a network.

E80/HV
An E80/HV is a factor that
converts different truck loads
to an equivalent number of
standard axles.

When the term is used, it
typically refers to the
weighted average of the
E80/HV for the specific project
section.

Section 4: Design Traffic Estimation
Page 33