Abstract:According to the principle of the upper bound theorem of plastic limit analysis,through the analysis of the upper bound solutions of stability ratios which are obtained from four collapse mechanisms of a single circular tunnel,a collapse mechanism of two parallel circular tunnels with same diameters in cohesive soil is established. An analysis procedure of critical state of the two parallel circular tunnels between the stability and collapse is described;and an upper bound equation of stability ratio for two parallel circular tunnels with same diameters in cohesive soil is obtained. The equation expresses the stability characteristics of the two tunnels with the different pillar width between them,including two tunnels overlapping (a single tunnel),two tunnels touching,two tunnels influencing each other and two single tunnels. The upper bound solutions of stability ratio under the conditions of weightless soil and soil with self-weight are discussed from the equation. In the end,the correctness of the upper bound solutions is proved by the results of centrifugal model tests.