Investigation into the Influence of Parallel Excavation of Twin Bored Tunnels within Weak Rock Masses

N. Vlachopoulos, P. Fortsakis, J. Oke

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Rock Engineering and Civil Infrastructures

ABSTRACT: Within the weak rock masses located in the extension of the Alpine Mountain system, highly deformed and altered sediments and low grade rock masses dominate the near surface environment creating a variety of technical challenges for transportation and utility tunnel excavation activities. Accurate equivalent rock mass performance predictions for tunnels in these materials is complicated by geomorphologic peculiarities such as mixed face conditions and variable orientation or rock masses and structure. In addition, portal stability problems, and geometrical issues such as the effects of simultaneous twin tunnel excavation on radial displacements of each bore are also key considerations. This paper investigates the influence of the twin bore excavation on the parallel tunnel based on the tunnelling sequence, support and the proximity of the twin bores to one-another within the framework of weak rockmasses. This work involves the use of 2D and 3D models of tunnel sequencing for numerical simulation of composite material behaviour and sequential tunnel deformation response. Validation data has been obtained from the field for tunnels that have been excavated as part of the Egantia Odos Motorway project in Northern Greece. The focus is on the Driskos tunnel, the longest of the project. The ultimate goal of the research is to provide guidance on the optimal spacing and sequencing of excavation of tunnels for design purposes within such environments while also taking into consideration the scale of the rockmass peculiarities.

RÉSUMÉ: Parmi les massifs de roches faibles situés dans l™extension des Alpes, les dépôts de sédiments très déformés et les masses rocheuses de faible qualité dominent l™environnement près de la surface, créant une gamme de défis techniques pour les excavations de tunnels de transport et de services. Les prédictions de la performance des massifs rocheux équivalents pour les tunnels dans ces matériaux, sont compliquées par les singularités géomorphologiques telles que les conditions de faces mixtes et l™orientation variable des massifs et des structures. De plus, les problèmes de stabilité des portiques et les enjeux géométriques tels que les effets de l™excavation simultanée de tunnels jumelés sur les déplacements radiaux de chaque ouverture sont aussi des considérations clés. Cet article étudie l™influence de l™excavation par forage jumelé sur le tunnel parallèle, sur la base de la séquence de creusement, des supports et de la proximité relative des trous jumeaux dans le contexte des massifs rocheux faibles. Ce travail implique des modèles 2D et 3D de l™avancement de tunnels pour l™analyse numérique du comportement de matériaux composites et la déformation des tunnels séquentiels. Des données de validation ont été obtenues de tunnels excavés pour le projet d™autoroute Egantia Odos au nord de la Grèce. L™emphase est sur le tunnel de Driskos, le plus long du projet. Le but ultime de cette recherche est de fournir des lignes directrices pour optimiser de l™espacement et la séquence d™excavation des tunnels pour soins de design dans de tels environnements tout en tenant compte de l™échelle des singularités des massifs rocheux. 1 INTRODUCTION Occasionally, the alignment of a road or rail communication route through mountainous terrain will involve the use of twin tunnels. This practice has come about in order to improve upon efficiencies associated with safety and route performance. In this way, one tunnel branch is constructed for each of the circulation directions. Another advantage of constructing twin tunnels is to reduce the size and thus, the geometrical and geomechanical challenges associated with constructing such large openings within weak rock masses and/or unfavourable geotechnical conditions; this is certainly the case with selected hydrological tunnels that have been constructed as twin tunnels rather than one large tunnel. Within a twin tunnel design, the method, type and rate of construction of one tunnel can, potentially, affect the behaviour and performance of the other tunnel during the construction process. As with any tunnel construction, the excavation process affects the overall stress field around the tunnel. Depending on the geological conditions, this could create significant plasticity zones around the tunnel opening as summarized by Vlachopoulos and Diederichs (2009). The extent of the plasticity zone and re-arrangement of the stress field around the first tunnel will affect the conditions through which the second tunnel will pass parallel to the first tunnel. The construction of the second branch within such conditions may lead to the development of additional convergence, loads or even failures of the support shell of the first branch. Such interaction of twin tunnels has been recorded in many cases, however, many of the current design

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