Answer:
the correct answer is option A
Explanation:
given,
Load = 6000 lbf      Â
depth = 3.5 ft                   Â
distance from the load = 4 ft         Â
vertical stress = [tex]\dfrac{3Pz^3}{2\pi (r^2+z^2)^{\frac{5}{2}}}[/tex]
            = [tex]\dfrac{3\times 6000\times 3.5^3}{2\pi (4^2+3.5^2)^{\frac{5}{2}}}[/tex]
            = 28.96 lbf/ft²           Â
            = 29 lbf/ft²       Â
hence, the correct answer is option A