This paper presents the collapse test results of single tunnel with different cover-to-diameter ratio (C/D) and of parallel tunnels with different C/D and distance-to-diameter ratio (d/D). The maximum settlement and width of settlement and width of settlement trough of parallel tunnels were more greater and wider than those of single tunnel. The settlement trough of parallel tunnels may be predicted by superposition of two Normal Probability Curves fitted by the single tunnel for the small values of overload factor. The collapse support pressure was well bounded by theoretcial lower bound and upper bound solutions. The proposed collapse mechanisms of parallel tunnels compare well with the observed ground movements. The changes of pore water pressures around tunnels may disclose the soil arching effects. The interaction effects increase rapidly as two tunnels become close.
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