ABSTRACT:

Ground vibration in blasting works for shallow tunnels may induce some effects on unstable blocks on the surface of jointed rock mass. This fact can be critical in cases such as Vall de Núria rack railway, where tunnelingworks are the main measure for rock fall risk mitigation, especiallywhen the service must remain fully operational and safe during the excavation phase. Newmark displacement theory can articulate a complete and useful method to take account of these effects during the design and tunneling process. Most relevant results obtained in both phases are presented and related considerations are discussed about data collection and processing, geological tracing observed or expected, ground effects and, in summary, the monitoring work capabilities for this kind of situations.

1 INTRODUCTION
1.1 Rockfall dynamics in Núria rack railway

The Vall de Núria Rack Railway is exploited by the Catalan Government Railways (FGC, Ferrocarrils de la Generalitat de Catalunya) and it is located in the eastern Pyrenees of Catalonia (Spain). It climbs a mountainous valley with the aid of the rack traction system that permits to remount 15% inclination railway. This track, reaching 2.000m high from 900m a.s.l., is subject to natural dynamics of mass movements on slopes, which supposes high risk to the unique communication corridor to access to the Vallde Núria mountain centre and ski resort. This fact was pointed out by the rock fall episodes occurred in 2003 with infrastructure damades. After intensive works focused to study the rock fall hazard (Janeras et al. 2004, Rendón 2004, Prat et al. 2009), analyze its risk and compare alternative mitigation measures (Janeras et al. 2009), it was decided to build a new tunnel to by-pass the most dangerous part of the railway track, the Roc del Dui section.

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