A group of $25$ circular piles is arranged in $5 \times 5$ uniform pattern in a soft clay soil with equal spacing in both the directions. These are friction piles with negligible end bearing.
Consider the following details:
Diameter of each pile $=1 \mathrm{~m}$
Length of each pile $=15 \mathrm{~m}$
Cohesion of the soil $=20 \mathrm{~kN} / \mathrm{m}^{2}$
Unit weight of the soil $=16 \mathrm{~kN} / \mathrm{m}^{3}$
Adhesion factor $=0.75$
Considering the efficiency of the pile group as unity, the optimum value of the ratio of the pile spacing to pile diameter is $\_\_\_\_$ (rounded off to one decimal place).
