Lecture Notes CT 4860 Structural Pavement Design Design of ...

Figure 35b: Influence of Poisson’s ratio on the vertical stress at the top of the bottom layer.
By means of the figures available we now can estimate the stresses and strains in two layer
pavements. This will be illustrated by means of the following example.
Assume we have a two layer structure consisting of a 150 mm thick asphalt layer on top of a
sand subgrade. The elastic modulus of the asphalt layer is 5000 MPa while the modulus of the
sand layer is 100 MPa. A 50 kN wheel load is applied on the pavement. The contact pressure 700
kPa which results in a radius of the circular contact area of 150 mm. Poisson’s ratio for both the
asphalt and the sand layer equals 0.35. We want to know the stresses and strains in the locations
indicated below.
Asphalt
h = 150 mm
E = 5000 MPa
ν = 0.35
Sand
E = 100 MPa
ν = 0.35
Pavement surface, z = 0
Bottom of asphalt layer, z = 150 mm
Top of sand layer, z = 150 mm
Figure 36: Two layer pavement example problem.
Let us start with the calculation of the stresses and strains at the bottom of the asphalt layer.
Since both layers have a Poisson ratio of 0.35, we have to use figure 35 and interpolate between
the lines for ν = 0.25 and ν = 0.5. Since E1 / E2 = 50 and a / h = 1 we read from the graphs
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