A $4 \times 4$ group pile, with each pile $20 \: \mathrm{m}$ long and $500 \: \mathrm{mm}$ in diameter, is installed in a square pattern in a clayey soil, as shown in the figure. The average unconfined compressive strength of the soil is $100\: \mathrm{kN} / \mathrm{m}^{2}$, and the adhesion factor is $0.8$. Neglect the bearing at the tip of the piles. For a group efficiency factor of $1.0$, the centre to centre spacing ( $s$ ) of the piles (in $\mathrm{m}$) would be $\_\_\_\_\_\_\_\_\_\_\_\_\_$ (round off to two decimal places).
