Through laboratory investigation, it is shown that when a pile group is arranged in a cluster, the added mass (or inertial force) increases sharply after the spacing between piles falls below a certain critical value. Two configurations of pile group arrangement—circular and square shapes of approximately the same base diameter—were tested. The experimental results reveal that when the spacing between piles is smaller than two times the pile diameter, or the solidification ratio becomes larger than 0.5, the added mass of the pile group increases drastically. For pile groups with individual pile spacing less than this critical value, the volume enclosed, rather than the volume displaced, by the pile cluster should be used as bases for added mass computation.