A site consists of 3 m thick loose sand with an insitu void ratio of 0.75. The laboratory tests show that the maximum and minimum void ratios of the sand are 0.85 and 0.50, respectively. Based on the field SPT value of 5, this soil will be liquefiable at an earthquake magnitude of 6.5. Vibrocompaction method with granular backfill is proposed to densify this soil. Assume the diameter of granular columns is 0.75 m. To minimize the liquefaction potential, the densified soil should have an SPT N value of 20, which corresponds to the relative density of 70%. Assume the ground settles 40 mm after the densification. Find the required spacing of granular columns.