CT4860 STRUCTURAL DESIGN OF PAVEMENTS

Figure 7. Indicative relationship between ko and CBR for various types of
subgrade.
In principal figure 7 also gives the possibility to estimate the ko-value from the
dynamic modulus of elasticity Eo (that can be derived for instance from Falling
Weight Deflection measurements) by means of the following empirical
relationship:
Eo = 10 CBR (11)
where: Eo = dynamic modulus of elasticity (N/mm²) of the subgrade
CBR = CBR-value (%) of the subgrade
As already mentioned in chapter 2, generally a sub-base and/or a base are
constructed over the subgrade. In the case of a very weak subgrade
sometimes a lightweight fill material is applied between the subgrade and the
sub-base. The effect of these layers can be estimated by means of figure 8 or
equation 12.
The k-value at the top of a layer is found by means of figure 8 or equation 12
from the k-value at the top of the underlying layer and the thickness hf (mm)
and the dynamic modulus of elasticity Ef (N/mm²) of the layer under
consideration. This procedure has to be repeated for each layer, so at the end
the ‘modulus of substructure reaction’ k on top of the substructure, i.e. directly
beneath the concrete top layer, is found.
Table 5 gives indicative values for the dynamic modulus of elasticity Ef and
the minimum thickness of lightweight fill materials (5). If the lightweight fill
material has an Ef-value that is lower than the Eo-value of the subgrade, in the
calculations the lightweight fill material has to be considered as subgrade.
This is especially the case if EPS (Expanded Polystyrene foam) is used as a
lightweight fill material.
Table 6 gives indicative values of the dynamic modulus of elasticity Ef of
various (sub-)base materials (5). Table 6 includes the lightweight foam
concretes as they have an Ef-value that is substantially greater than the Efvalue
of the lightweight materials mentioned in table 5 and the Eo-value of the
subgrades mentioned in table 4.
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