The Application of Distributed Optical Sensing for Monitoring Temporary Support Schemes

B. Forbes, N. Vlachopoulos, J. Oke, A.J. Hyett

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

Session: Laboratory and Field Testing

ABSTRACT: Temporary support systems are commonly employed in order to mitigate and control tunnel deformation caused by excavating in weak ground. This temporary support regime comprises a combination of support including, but not limited to: steel-sets, shotcrete, rock bolts, forepoles, and other face stabilizing technologies that provide confinement for the ground and ultimately help the ground support itself. The research is focused specifically on spile and forepole support elements; two subcategories of an umbrella arch system. Within this context, an experimental application of distributed optical sensing used to monitor the strain profile along such elements is presented. The optical instrumentation was embedded into machined grooves along the length of the specimens using an epoxy resin. A variety of bending, axial, and shear loading arrangements were found to agree with theoretical predictions. RÉSUMÉ Systèmes de soutien temporaires sont couramment utilisés pour atténuer et contrôler la déformation du tunnel causés par l'excavation dans le sol faible. Ce régime de soutien temporaire comprend une combinaison de soutien, y compris, mais sans s'y limiter: acier-béton projeté, ensembles, des boulons d'ancrage, forepoles, et d'autres technologies de visage de stabilisation qui fournissent confinement pour le terrain et, finalement, aident le soutien au sol lui-même. La recherche se concentre spécifiquement sur chalumeau et éléments de soutien forepole; deux sous-catégories d'un système de voûte parapluie. Dans ce contexte, une application expérimentale de détection optique distribué utilisé pour surveiller le profil de déformation le long de ces éléments est présenté. L'instrumentation optique a été incorporé dans des rainures usinées sur toute la longueur des éprouvettes à l'aide d'une résine époxy. Une variété de flexion, axiales et de cisaillement arrangements de chargement ont été trouvés d'accord avec les prédictions théoriques 1 INTRODUCTION Observational methodologies, such as The New Austrian Tunneling Method (NATM), integrate the surrounding rock into the overall support structure (i.e. the supporting formations will themselves be a part of the supporting structure as the rock is able to support itself to a certain degree) (Romeo, 2002). Using the NATM, a controlled deformation of the rock is permitted (a limited strain of approximately 1%) allowing the strength within the excavation region to mobilize. This gives an opportunity for the stresses within the rock mass to be partly released resulting in less stiff and thus less expensive support systems being required (Kondogianni and Stiros, 2002). Within this framework of observational approach (i.e. well instrumented), optimization of tunnel support design (to include individual support elements: forepoles, rock bolts, shotcrete, etc.) can be achieved through the explicit determination of rock/soil and support element behaviour/interaction. The value of such observational knowledge becomes more apparent when considering the certain gap in knowledge in terms of the distinct performance of individual support elements in isolation and performance as part of a multi-component system. This research suggests a strategy in order to determine the continuous behaviour and performance of spile and forepole temporary support elements individually, and as part of the overall temporary support scheme. 2 BACKGROUND 2.1 Umbrella Arch: Spiles and Forepoles The umbrella arch (UA) is a tunnel pre-support system installed during the first pass of an excavation around and above the crown of the tunnel face. The UA can consist of a variety of support elements acting as longitudinal members; however, this research specifically investigated spile (i.e. rebar) and forepole elements. Spile temporary support elements are installed when the geological structure itself controls a possible local failure in the portion of the tunnel (Volkmann and Schubert, 2006). Forepole temporary support elements are installed when difficult and/or weak rock/soil is consistent for a large portion of the tunnel alignment and where there is a large potential for multiple geological structures of unfavourable orientation/structures contribute to a possible failure. By definition, a spile is a support element of an UA that is composed of metallic longitudinal members with a length less than the height of the tunnel excavation; conversely, a forepole is a support element of an UA that is composed of metallic longitudinal members with a length greater than the height of the tunnel (Oke et al., 2013). As discussed by Vlachopoulos and Diederichs (2009), the latter element should be long enough to exceed the plastic zone created due to the tunneling effect, thus adding stabilization ahead of the face. Longitudinal installation of such elements allows for the stable

RÉSUMÉ: pplication of Distributed Optical Sensing for Monitoring Temporary Support Schemes Forbes, B., Vlachopoulos, N., & Oke, J.

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B. Forbes; N. Vlachopoulos; J. Oke; A.J. Hyett (2014) The Application of Distributed Optical Sensing for Monitoring Temporary Support Schemes in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper259,author = B. Forbes; N. Vlachopoulos; J. Oke; A.J. Hyett,title = The Application of Distributed Optical Sensing for Monitoring Temporary Support Schemes,year = 2014}