ABSTRACT

In this paper, the strong interaction between the hardening shotcrete lining and the surrounding ground in NATM tunneling is investigated by means of axisymmetric analyses, accounting for the three-dimensional nature of the excavation process. The effect of the hydration of shotcrete on strength, stiffness, chemical shrinkage as well as microcracking and creep are accounted for. For the description of the mechanical behavior of the ground, a multi-surface plasticity model is employed. The performed analyses of the excavation process provide insight into the influence of each phenomenon accounted for in the material model for shotcrete on the response of the ground-shotcrete compound structure.

ZUSAMMENFASSUNG

In diesem Beitrag soll die Interaktion zwischen dem erhartenden Spritzbeton und dem Boden auf der Basis axisymmetrischer Berechnungen untersucht werden. Derartige Berechnungen ermöglichen die Beruecksichtigung des drei-dimensionalen Charakters des Vortriebprozesses. Die Auswirkung der Hydratation auf die Entwicklung der Festigkeit, Steifigkeit und der chemischen Schwindverzerrungen, sowie das Reißen und das Kriechen des Spritzbetons wurde beruecksichtigt. Das mechanische Verhalten des Bodens wurde mittels eines Mehrflachenplastizitatsmodells simuliert. Die durchgefuehrten Berechnungen des Vortriebprozesses geben Einblicke in die Auswirkungen der Spritzbetoneigenschaften auf das Tragverhalten des Boden-Spritzbeton-Komposits.

RÉSUMÉ

La stricte interaction entre le soutènement en beton projete durcissant et le terrain dans un tunnel creuse avec la NATM est etudiee par des analyses axisymetriques, compte tenu de la nature tridimensionnelle des processus de creusement. Les effets de l'hydratation du beton sur la resistance, la rigidite et la contraction chimique, ainsi que sur la micro-fissuration et le fluage, sont mis en compte. Pour la description du comportement mecanique du terrain on emploie un modèle de plasticite multi-surface. Les analyses conduites mettent en evidence l'influence de chaque phenomène considere dans le modèle constitutif du beton projete sur la reponse de la structure composite terrain-beton projete.

Introduction

The New Austrian Tunneling Method (NATM) is a widely known tunnel excavation system successfully applied even in difficult ground conditions (e.g., weak rock mass under high overburden stress). Shotcrete is the major support element for tunnels built in accordance with the principles of the NATM (Rabcewicz, 1948). It is applied onto the tunnel surface immediately after the excavation and forms a continuous and flexible ring, able to support a large ground stress release without structural damage. The confining action of shotcrete prevents the decay of mechanical properties of the ground, improving the stability of the tunnel.

NATM tunneling is characterized by the adaptation of the construction parameters, such as driving speed, shell thickness, numbers of rock bolts, etc., to the observed response of the already excavated part of the tunnel. On one hand, these adaptations are based on monitoring the deformation of the shotcrete shell (convergence measurements), surface settlements, and extensometer measurements in the surrounding rock mass. Numerical analyses as the basis for day-to-day adaptations of driving parameters in NATM tunneling, on the other hand, are a challenging task, because:

  • the tunnel excavation problem is a three-dimensional problem; since three-dimensional numerical analyses are rather time-consuming, simplified plane-strain analyses are performed in almost all cases.

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