Page 112 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Advantages of the Deflection Bowl Disadvantages of the Deflection Bowl
Parameter Method Parameter Method
• Very simple and easy to apply • Crude indication of remaining life.
• Calibrated for South African conditions, • Conservative: little remaining life is often
pavements and materials calculated when the visual condition of the
• Allows for different pavement types pavement is still good
• Allows adequate assessment of different zones in
the pavement
• Current: uses the preferred deflection device for
network and project level testing

7.6 Falling Weight Deflectometer (FWD) SN design method

The method was developed by Rohde (1994) for use in network level analyses, and uses FWD deflection
measurements to determine the AASHTO Structural Number. The method is rapid, does not need mechanistic
analysis tools and is generally used for characterising pavement strength in pavement management systems (PMS).
As such, it can be used in the initial assessment phase of rehabilitation projects. In South Africa, the method is not
frequently used, and when it is used it is generally only in PMSs, such as the Western Cape PMS.

The method uses the assumption that the surface deflection measured at an offset of 1.5 times the pavement
thickness originates entirely in the subgrade. By comparing this deflection with the peak deflection, an index
associated with the magnitude of deformation within the pavement structure is calculated using Equation (36).

SIP = D 0 – D 1.5Hp (36)
where SIP = Structural index of pavement (µm)
D 0 = Peak deflection measured under a standard 40 kN FWD load (µm)
D 1.5Hp = Surface deflection measured at offset of 1.5 times Hp, under standard 40 kN
FWD impulse load (µm)
Hp = Total pavement thickness (mm), which includes all imported pavement layers.
Where selected layers are used, they form part of the pavement thickness.

A relationship between FWD measured surface deflections and a pavement’s structural number was developed by
analysing a large number of pavements with layered-elastic theory, and shown in Equation (37). The structural
capacity is calculated using the AASHTO method.

k3
k2
SN = k 1 SIP Hp (37)
where SN = structural number (inches) as used in HDM-III
SIP = structural index of pavement (µm)
Hp = total pavement thickness (mm)
k 1 , k 2 , k 3 = coefficients
Surface Type k 1 k 2 k 3
Surface Seals 0.1165 -0.3248 0.8241
Asphalt Concrete 0.4728 -0.4810 0.7581

Using Surface Deflection Methods
Surface deflection methods are applicable to pavements where the subgrade is a problem, or where the
subgrade cover is insufficient.

If Benkelmen beam data are the only deflection data available, the TRRL or Asphalt Institute surface deflection
methods are appropriate, however the conditions for their use still apply.

When using the Benkelman beam, be careful of the loading used, as there are a few “standard” loads.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 101