A site consists of 3 m thick loose sand with an in-situ void ratio of 0.75. The laboratory tests show that the maximum and minimum void ratios of the sand are 0.85 and 0.5, respectively. Based on the field SPT value of 5, this soils will be liquefiable at an earthquake magnitude of 6.5. Vibro-compaction method with granular backfill is proposed to densify this soil. Assume the diameter of the granular columns is 0.75 m. To minimize liquefaction potential, the densified soil should have an SPT N value of 20, which corresponds to the relative density if 70%. Assume the ground settles 40 mm after the densification. Find the required spacing of the granular columns.