Deformation induced design for economical shield tunnel lining

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Conventional design method of shield tunnel is based on the principle that the segmental lining will bear entire pressure of earth and water to prevent from settling the ground surface. In order to strengthen the principle, practical efforts have been made in design and execution, such that closed face type shield tunnels have been extensively used, and execution efforts have been made to minimize the deformation by decreasing overbreaks, tail voids and time to bury tail voids by grouts. Hence the most of modern shield tunnels could avoid serious settlement problems, however in reality small deformations of the ground into the tunnel cavern exist and cause slight settlement on the surface. The NATM theory positively employs deformations of the ground and reduces support pressures as defined qualitatively by Fenner-Pacher curve. In order to integrate the design method between shield and NATM tunnel, NATM theory has been applied to the shield tunnel design. Key issue is a deformation, or can be explained by stress relief after excavation. Authors have developed simplified design procedures to unite both of shield and NATM tunnels by proposing the virtual segmental analysis. Deformation induced design could reduce the pressures on the segmental lining and was verified with the in-situ measured pressure data. The shield tunnel could obtain an innovative and slimmed-down design method by adapting the normal range of deformation in practice without causing serious settlement problems. Through defining the support pressures to the tunnel as being divided into the ground bearing pressures (GBP) and the mechanical support pressures (MSP), deformations could be related with mechanical support pressures to the segmental lining or ground bearing ratio (GBR) by using the virtual segmental analyses based on the elastic model of FEM. By setting the deformation to the tunnel as the design value through the empirical, practical and measured data, load pressures to the segmental lining can be obtained as MSP directly or through GBR. By comparing in-situ data of practical two shield tunnels with the load pressures calculated by FEM analysis, 22 and 34% of load pressures are recognized to be supported by the ground, and tunnel deformations were 2.0 and 3.8 cm respectively. By applying reduced load pressures to the segmental lining, slimmed down design can be carried out. In this sense, deformation induced design for shield tunnel can slim down the segmental lining. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.