Problem $1$: The figure below shows a pavement structure composed of the following three layers: $20cm$ HMA with an elastic modulus of $3600000$kPa, a $40cm$ granular base with an elastic modulus of $18000$kPa, and a subgrade with the elastic modulus of $9000$kPa. All layers are assumed to have a Poisson ratio of $0\mathrm{.}5.$ Calculate the maximum horizontal tensile strain at the bottom of HMA and the maximum vertical compressive strain on the top of subgrade under a wheel load of $P$ and contact pressure of $600$kPa, assuming that the contact area is a circle and $P$ is $68$ kN

Problem $2$: In problem $1,$ if the base and subgrade are combined as one layer, as shown in the below figure, what should be the equivalent elastic modulus of this combined layer so that the same tensile strain at the bottom of HMA can be obtained?

$P=20kN+48kN=68kN$