Pz = Ib(Q/z^2),
Where, Pz= Pressure on soil at the depth z at a horizontal distance of r from the center line.
Ib = Boussinesq's constant = 3/2*pi[ 1+ 1/(1+ r^2/z^2) ]^(5/2)
pi = 3.14
For the various points on the line P1P2 Ib is constant. Using this equation you can find out the stress at a vertical distance of z and at a horizontal distance of r from center line.