When designing twin tunnels, grasping interaction between tunnels accurately is essential. Although a spacing of at least 1.0D-4.0D (D is diameter of a tunnel) is recommended in current design, constraining factors that make the spacing requirements impossible exist; existence of important structures, cost and land expropriation difficulties. The ultimate goal of this research is to theorize the mechanical behavior of closely spaced twin tunnels. This paper will examine the influence of the spacing between tunnels and the overburden on the magnitude of interaction. Trapdoor tests were conducted to establish criterion for judging the dominant factors influencing severity of interaction effects.