Determination of Mechanical Behaviour and Performance of Forepole Support Elements used in Tunnelling

N. Vlachopoulos, J. Oke, B. Forbes

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Engineering Geology and Rock Mechanics

ABSTRACT: ion of Mechanical Behaviour and Performance of Forepole Support Elements used in Tunnelling Vlachopoulos, N., Oke, J. and Forbes, B. GeoEngineering Centre, Queen's-RMC, Kingston, Ontario, Canada ABSTRACT The mechanics associated with forepoling structures explicitly, has never been fully investigated in order to determine the associated support mechanics when installed in isolation and/or in groups as an umbrella arch. Further, these support structures cannot be modelled using industry standard, two-dimensional (2D) numerical software packages. In three-dimensional (3D) analysis codes, these support elements are commonly standardized using pile or rockbolt simplified noded elements that do not truly describe their behaviour when subjected to the true 3D stress conditions that result at face or near the face due to the tunnel or mining excavation process. As such, a deficiency exists with regards to prediction of the interaction of forepoles element within umbrella arch support systems for tunnelling practices within weak rock masses. Methods have been developed in order to predict the behaviour of radial support systems to include temporary support elements such as rock bolts, steel sets, and liners such as the convergence-confinement method. However, these tools do not have the ability to capture the influence of support systems installed longitudinally at the face of tunnel such as forepole, and core reinforcement elements. In an attempt to improve tunnel design strategies, this paper will focus on the mechanical response of the application of the forepole element as part of the umbrella arch method installed in deep and shallow excavations; As well, other issues associated with the use of forepoles and physical testing are also highlighted and discussed. RÉSUMÉ La mécanique associée à des structures forepoling explicitement, n'a jamais été entièrement étudiée afin de déterminer les mécanismes de support en connexe lorsqu'installé isolément ou en groupe avec une umbrella arch (UA). Ces support structures ne peuvent être modélisées en utilisant logiciel numérique de deux dimensions (2D). Dans le logiciel numérique tridimensionnels (3D), ces éléments sont généralement normalisés à l'aide de pieux de soutien ou d'éléments renflés instrument simplifié ne décrivent pas vraiment leur comportement lorsqu'ils sont soumis à des conditions de stress 3D vrai qui résultent au visage ou près de la face due au tunnel ou une excavation minière processus. Ainsi, une carence existe en ce qui concerne la prédiction de l'interaction de l'élément forepoles dans les systèmes de soutien arch parapluie pour le creusement des pratiques au sein des masses de roche molle. Méthodes ont été développées afin de prédire le comportement des systèmes d'appui radiales des éléments d'appui temporaires tels que boulons, ensembles en acier et doublures telles que la méthode convergence-confinement. Toutefois, ces outils n'ont pas la capacité de capturer l'influence des systèmes de soutien installé longitudinalement à la face du tunnel comme forepole et les éléments de renfort de base. Dans le but d'améliorer les stratégies de conception de tunnel, cet article se concentrera sur la réponse mécanique de l'application de l'élément forepole dans le cadre de la méthode de voûte parapluie installée dans les excavations profondes et peu profondes ; Ainsi, autres problèmes (influences) avec l'utilisation de forepoles sont également mis en évidence et discutés. 1 INTRODUCTION This paper describes concepts that are related to the forepole / support arch concept and the gaps which currently exist with their usage. The primary and specific purpose of this portion of the investigation was to capture the support-rock interaction / mechanics associated with the forepole temporary support elements (as seen in Figure 1 and Figure 2). The forepole umbrella creates a stable excavation environment. One postulated mechanism of support is the redirection of three dimensional (3D) stress flow around the tunnel (Gibbs, et al., 2007). This concept has also been labeled the arch effect by (Lunardi, 2000). In order to attempt to determine the true impact of a forepole umbrella on an excavation, 3D numerical models were developed within FLAC3D (Itasca, 2009). Selected factors of consideration when designing with forepole elements are: spacing between forepoles, positioning, and the size of the forepoles. It also introduces the concept of utilizing forepole temporary support elements according to analysis based on convergence-confinement theory (CCT), Ground Reaction Curve (GRC) from Carranze-Torres (2004), rock-support interaction from Hoek (2007), and the Longitudinal Displacement Profile (LDP) from Vlachopoulos and Diederichs (2009) within the context of improved geotechnical design of tunnels. Other factors that contribute to (but not limited to) the influence and performance of forepole elements is the way that they are simulated numerically, their influence at the design stage (i.e. influence on analytical, unsupported solutions) and their use in the reduction of surface settlements in shallow tunnel environments. These factors affecting the performance of the forepole support element, physical

Access this article:
Canadian Geotechnical Society members can access to this article, along with all other Canadian Geotechnical Conference proceedings, in the Member Area. Conference proceedings are also available in many libraries.

Cite this article:
N. Vlachopoulos; J. Oke; B. Forbes (2014) Determination of Mechanical Behaviour and Performance of Forepole Support Elements used in Tunnelling in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper234,author = N. Vlachopoulos; J. Oke; B. Forbes,title = Determination of Mechanical Behaviour and Performance of Forepole Support Elements used in Tunnelling,year = 2014}