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Two-arch tunnels require minimal spacing between the 2 tunnels and thus occupy small amount of land compared to parallel tunnels. But it is rather expensive. The parallel tunnel is not as expensive, but it requires more land than 2-arch tunnels. This may cause a problem when there is a land shortage. The new design is to connect these two types of tunnels by another pair of parallel tunnels where the separation distance is gradually increasing. The applicability of this new design to the cases where rock condition is not even fair has to be verified since the width of the pillar between the connecting tunnels can be quite narrow. Therefore both two and three dimensional numerical analyses were conducted and pillar stability was examined for rock classes IV and V in two different ways. Results showed that this new design is still effective for poor rock conditions if central pillar is properly reinforced by tension bolts as long as overburden is less than five times of the tunnel diameter.Key words 2-arch tunnel, Parallel tunnel, Pillar 2-arch varying section tunnel, Tension bolt  2-              .     2-            .           3   .                  .  4  5   2  3   2    .  ,          5    . 2- ,  , 2-arch  , ,

A Numerical Study on the Reinforcement Method of a Pillar Using Tension Bolts at the Connecting Part between 2-Arch Tunnels and Parallel Tunnels