Consider a cylindrical water tank on the ground surface. Given that the radius of the tank is $(R)=6ft$ and the depth of tank is $(D)=8ft,$ calculate the vertical stress increase $({}_z)$ at points $1\mathrm{.}5,3,6,$ and $9ft$ below the ground surface $($at the center of the circular area$)$ and $9ft$ below, and $3ft$ from the center. $($Hint: Use ${}_w$ to find pressure on soil caused by the tank.$)$