$FS=N_c\frac{s_u}{*D_f+q_s}$

$-$ if this formula is applied, why is qs divided by quarter of circle area?

A tank foundation $10m$ diameter is required to support a vertical centric load of $15,700kN$ on the surface of a deep deposit of a stiff clay with $s_u=80$kPa and ${}_{\text{s}\text{a}\text{t}}=20\mathrm{.}8k\frac{N}{m^3}.$ Groundwater is at the surface. A $75-$mm$-$thick compacted granular base will be placed on the clay surface prior to the construction of the tank foundation. Calculate the factor of safety.

$FS=N_c\frac{s_u}{*D_f+q_s}=6*\frac{80\text{k}\text{P}\text{a}}{(20\mathrm{.}8k\frac{N}{m^3}-(9\mathrm{.}8k\frac{N}{m^3}))*(0)+\frac{15700kN}{*\frac{({10}^2)m^2}{4}}}=2\mathrm{.}4$