Deformation and stress characteristics of existing twin tunnels induced by close-distance EPBS under-crossing

This paper presented a case study on the deformation and stress characteristics of twin tunnels induced by close distance earth pressure balance shield (EPBS) under-crossing in sandy soil stratum, located in Changsha, China. The horizontal columns constructed by Metro Jet System (MJS) method were used to stabilize the sandy soil below the existing twin tunnels. The deformation and the stress of existing tunnels which affected by the construction of new tunnels were systematically monitored. The settlement development of the existing tunnels was found to experience four stages: (i) subsidence, (ii) heave, (iii) second subsidence, and (iv) steady state, respectively. The settlement profiles of the existing tunnels induced by the second tunnel under-crossing were found to be asymmetric with respect to the second tunnel centerline, and the location of the maximum settlement point deviated toward the twin tunnels center. The settlement caused by the second shield under-crossing was found significantly larger than that caused by the first tunnel under-crossing. After the two shields passing, the final settlement profile of the existing tunnels displayed a "U" shape. The induced hoop stress of the existing tunnels changed sharply during the shield under-crossing. The cross-section of the existing tunnels changed into a skewed oval shape. The final rotation direction of the existing left and right tunnels above the first and second tunnels centerline was opposite. Both the existing tunnels rotated toward the location of the large volume loss. The shifted Gaussian distribution curve was adopted to model the settlement profiles of the existing tunnels. The differences of the settlement profiles caused by the first and second tunnels excavation were also explained.