Experimental investigations on failure mechanisms of DOT shield tunnel subjected to extreme surface surcharge

During the service lifespan of shield tunnels, the surrounding construction activities are easy to cause significant structural responses. To explore the structural robustness of double-o-tube (DOT) shield tunnels in the face of environmental disturbances and to provide a guidance for tunnel operation and maintenance, a failure test is designed and conducted on a prototype DOT shield tunnel structure. In this test, a quasi-static method is utilized to simulate the external loading conditions of the tunnel structures subjected to the extreme surface surcharge. Through an analysis of the evolution of deformation responses of the structures and longitudinal joints during the tests, the failure mechanisms of the DOT tunnel structures are revealed. The tests indicate that the failure process of the DOT tunnel subjected to the extreme surface surcharge can be divided into three typical stages: initial linear stage, rapid accumulation of plastic damage stage, and instability stage. As the convergence deformation of the long axis (from the left waist to the right waist) approaches approximately 25 mm, three negative moment joints (No. 6, No. 8, and No. 3) enter the elastoplastic state, and the structures simultaneously enter the second stage (rapid accumulation of plastic damage stage). Subsequently, with the extrados concrete of the No.7 positive moment joint being crushed and the intrados cast iron plate of that joint being fractured, the structures enter the third stage (instability stage). Ultimately, with the bending failure of the B7 block (segment), the structures completely lose their bearing capacity. The structural robustness evaluation indicates that the DOT tunnel, compared to the quasi-rectangular tunnel, better utilizes the good load-bearing performance of circular tunnels, resulting in a higher robustness performance. The relationships between the structural convergence deformation and the joint rotation angles as well as the joint opening deformations are also investigated. It is found that the structural deformation at the first stage mainly comes from three negative moment joints (No. 3, No. 6, and No. 8). Near the end of the second stage, with the redistribution of internal forces, the No.7 positive moment joint quickly reaches the linear limit and enters the plastic state, causing a rapid development of the structural convergence deformation. Based on these findings, the critical monitoring locations during the service lifespan of tunnels should include the structural convergence deformations of the long and short axes, as well as the crushing of intrados concrete of negative moment joints at the waist and the side of the small seagull block, and intrados opening of positive moment joints.