Refined hybrid structural analysis shows plastic flexibility enhancement in NATM tunnel shell

Abstract

State-of-the-art monitoring equipment has been a key element of the New Austrian Tunneling Method (NATM) ever since its establishment and description in the 1960s. In particular so-called hybrid methods combining geodetic displacement measurements with material and structural mechanics modeling have provided access to the ground pressure distributions acting on the shotcrete tunnel shell and its utilization degree. Herein, we explore the effect of refined observation by five (instead of the classical three) geodetic laser reflectors positioned at so-called measurement points, as installed with the top heading of the tunnel Stein realized by the Austrian federal railways (ÖBB) in the south of Austria: An analytical model for the equilibrium and the deformations of aging viscoelastic cylindrical shells is specified for the displacements recorded at the measurement points, and utilization is assessed in terms of (plastic) moment-force interactions. The corresponding results widen the perception of the fundamental load-carrying characteristics of the NATM: Rather than monolithically, the tunnel shell, due to the formation of plastic hinges, may act as a literally flexible mechanical system, well adapting to the heterogeneous behavior of the surrounding ground.