The life-cycle development and cause analysis of large diameter shield tunnel convergence in soft soil area

摘要

The tunnel convergence is an important index of the construction quality and operation safety. In the past, most of researches were focused on the shield tunnel lateral convergence and its effect on the safety at operation period. In this paper, by employing the 3D laser scanning test method and convergence ellipse fitting identification technology, the shield tunnel full space convergence can be tested and divided into three typical modes: transverse ellipse, oblique ellipse and vertical ellipse. Based on series of field tests in three large-diameter tunnels under construction and operation, the spatial distribution and life-cycle development laws of above three typical convergence modes were firstly seriously studied, the changes of the long axis, short axis and deflection angle of above convergence modes are given. Meanwhile, the generation mechanism and main cause is studied by analysis of machine-structure interaction, soil pressure and back-fill grouting action during shield construction. It is found, in the soft soil areas, the formation of different convergence modes mainly depends on the initial load condition at the stage of segments leaving the shield tail, and the following action of back-fill grouting will have little effect on the structure convergence, but most on the surrounding soil deformation. Subsequently, the convergence will continue growth with the short axis deformation greater than the long axis, and gradually develop towards symmetry with soil consolidation. Besides that, the obvious coupling relationship between the convergence and the longitudinal settlement can also be found after long-term operation from the tests. The above findings are very helpful to fully understanding the stress and deformation interaction during the large diameter shield tunnel construction and operation.