Example $8$: A concrete pavement slab with modulus of elasticity of $27500$ Mpa $,$ Poisson's ratio of $0\mathrm{.}15$ and thickness of $20$ cm has been constructed on subgrade with modulus of subgrade reaction of $0\mathrm{.}69$ Mpa and subjected to $20$ kN wheel load has a radius of $100$ mm $.$ Calculate the critical stresses in the pavement when the load:

$1-$ At corner of slab.

$2-$ At the interior of slab.

$3-$ At edge of slab.