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  The log-spiral torus geometry as shallow tunnel face collapse mechanism: a general-stability safety factor approach

  During soil tunnel excavation with a TBM, either Slurry-Shield or Earth Pressure Balance type, it is of utmost importance assess the pressure to be exerted on the face to ensure both the stability of the excavation and control surface settlements. In literature, there are reported mostly 2D or even 3D global stability proposals that do not accomplish kinematic constraints, that is, from a continuous failure surface. The present work aims to improve the face failure mechanism of a shallow tunnel of circular cross-section excavated in soil, through the torus geometry, which resembles a log-spiral curve in the space. In an attempt to improve from the safe side the accuracy of support pressure calculations compared to available theories, a particular method of limiting equilibrium was implemented on the aforementioned kinematically admissible mechanisms for both active and passive failure. Based on centrifuge test data, it was possible to validate the hypothesis about stress state at failure and also to establish TBM operational support pressure range which minimizes ground movements, by using the safety factor concept. Shallow tunnel, TBM, Face failure, Safety factor, Underground excavation, Logspiral.

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