Well Casing Compressive Stress

[Kuhuh]

I am looking at ACP Design Data 20 on circular precast concrete manholes. They define the lateral compressive stress in the concrete cross section ( pounds per sf) as s'=pD/2t. p= total lateral earth and hydro force per sf and D as inside diameter of manhole and t as thickness of manhole. Why is it multiplied by D and not the circumference since the lateral force is applied to the entire circumference. I may be missing the fundamentals here but I'd like to apply this to a steel casing and I believe that this equation would not change since there is no inclusion of modulus of elasticity. Can you help me out. Thanks