Abstract:  When two overlapped tunnels crossing with small distance and angle, the underpassing tunnel construction has a significant impact on the stability of the existing operating tunnel. In this paper, a case study is conducted on Qingdao North stationCang′an Road station of Qingdao metro line 8 obliquely crossing underneath adjacent existing tunnel of Qingdao metro line 3, and the ABAQUS is used to establish a finite element calculation model to analyze the vertical and horizontal displacement and stress evolution laws of the existing tunnel in the process of new tunnel excavation. The settlement displacement of the existing upper tunnel structure is monitored in real time during construction, and the difference between monitoring data and simulation results is compared and analyzed. The results show the following: (1) The most dangerous crosssection of the existing tunnel is located at the intersection of the upper and lower tunnels, and the excavation range of the crosssection of the existing tunnel with most adverse condition is 20 m ahead of and behind the crosssection of the new tunnel at intersection. (2) In a same tunnel crosssection, the underpass tunnel construction will cause displacement of the arch and the left waist and the arch bottom and the right waist in the existing tunnel in an opposite direction, and the existing tunnel structure crosssection has a tendency of torsional deformation. (3) The excavation of new tunnel has little influence on the maximum principal tensile stress and compressive stress of existing tunnel, and the tunnel bottom plate has a larger stress.

Key words: metro tunnel, closespace undercrossing, obliquely undercrossing with small angle, numerical stimulation, surrounding rock stability