Structural Design of Pavements PART VI Structural ... - TU Delft

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deflections. Normally the analysis is stopped when the difference between the measured and
calculated deflections is 2%.
As has been mentioned before, the iterative back calculation procedure can either be an
“automatic” or a “hand operated” one. In the “automatic” procedures the computer program
automatically performs the iterations while in the “hand operated procedure” it is the
experienced engineer who controls the iteration process. Both procedures have their
advantages. The automatic procedure is fast but might sometimes result in “funny” results. By
“funny” it is meant that the set of moduli that is back calculated results in a good fit between
the measured and calculated deflections but the moduli value themselves cannot be true
given the type and condition of the materials in the pavement, given the temperature
conditions etc.. Such results can occur because the back calculation procedure doesn’t
necessarily result in a unique answer. In such cases the hand operated procedure is more
powerful because the experienced engineer can adjust the moduli values to such levels which
are reasonable for the type and condition of the pavement materials present and still result in
a good fit between measured and calculated deflections.
Problems with back calculating layer moduli can occur when the top layer is thin (< 60 mm) or
when the base layer has a higher stiffness than the top layer.
A golden rule in the back calculation analyses is that one never must vary the moduli values
of all layers in the same time. This should be done in a step by step procedure. First of all one
should try to find a modulus value for the subgrade by finding a good fit between the
deflections measured and calculated at the largest distance to the load centre (see also figure
14). Then one tries to fit the deflections at intermediate distance from the load centre; this will
result in the moduli for the sub-base and base. Finally one should fit the deflections at the
shortest distance to the load centre and the maximum deflection; this results in the modulus
for the top layer.
Furthermore one should realise that a good fit of the measured SCI is of great importance
since this value gives a lot of information on the strain levels in the pavement. Sometimes the
measured deflection profiles are not easy to match. In such cases one should notice that a
good match of the SCI is to be preferred over a good match of the deflections measured at a
greater distance from the load centre.
6.3 Example:
The example that will be given here is taken from the OECD FORCE test pavements that
were built at the LCPC test facilities in Nantes, France. These pavements were tested by
means of the accelerated load testing device of the LCPC. The aim of the project was to
study pavement response and performance of three types of pavements under accelerated
loading. The results of the FWD data evaluation of two test pavements are discussed here [3,
4].
Figure 16 shows the two pavement sections analysed.
I
II
60 mm asphalt
120 mm asphalt
300 mm base
300 mm base
subgrade
Figure 16: Structures I and II of OECD’s FORCE project.