The approaching construction for two neighboring parallel tunnels in the numerous underground projects is the most normal and basic type in the development of underground space in city.

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.

In an excavating project of a small spacing parallel tunnel,the ground vibration acceleration data of the existed tunnel which is parallel to that being constructed were tested by modern testing means and these data thus obtained are of valuable in the excavation of small spacing parallel tunnel afterwords.