ABSTRACT:

In order to characterize the damaged zone in the near field of tunnels excavated within ZEDEX project, hydraulic tests, P-wave velocity and resonance measurements were undertaken in several short radial boreholes with different orientations. Cubic samples were taken from the boreholes cores and submitted to various laboratory tests. Different parameters showed similar trends. Damage was the more significant in the vertical boreholes, and less extensive in the horizontal boreholes. Damage was lower in boreholes drilled in the tunnel bored by machine compared to the blast-excavated drift. Differences in extent and strength of the damaged zone between smooth blasting and very smooth blasting were not clearly apparent.

RÉSUMÉ:

Des tests hydrauliques, des mesures de vitesse des ondes P et des mesures de resonance ont ete realisees dans plusieurs forages radiaux de differentes orientations dans Ie but de caracteriser la zone endommagee en champ proche de tunnels excaves par trois methodes. Des tests de laboratoire ont egalernent ete effectues sur des echantillons cubiques preleves sur les sondages. Les differents paramètres montrent les mêmes tendances. L'endommagement est Ie plus important dans les forages verticaux situes au mur, il est moins etendu dans les forages horizontaux. L'endommagement est plus limite dans les forages du tunnel excave par tunnelier que dans ceux creuses par abattage. Les differences d'etendue et d'intensite de la zone endommagee entre les tirs très adoucis et les tirs adoucis n'ont pas clairement ete mises en evidence.

ZUSAMMENFASSUNG:

Kleinraumige hydraulische Tests, P-Wellen Geschwindigkeitsmessungen und Bohrlochresonanzmessungen wurden in unterschiedlich orientierten kurzen radialen Bohrlöchern durchgefuehrt. Die Messungen dienten der Ermittlung von Ausdehnung und Starke del' Zerstörungszone im Nahbereich von Strecken, die mit drei unterschiedlichen Methoden aufgefahren wurden. Wirvfelförmige Proben aus den Bohrkernen wurden einer Reihe von Labortests unterzogen. Unterschiedliche Parameter aus den Messungen und Labortests zeigen gleichen Trend. Generall war die Zerstörung im Bereich der vertikalen Bohrlöcher starker als in den horizontalen Bohrungen. Die Zerstörungszone in der gebohrten Strecke war kleiner als in den Sprengtechnisch aufgefahrenen Teilen. Signifikante Unteschiede der Zerstörungszone bei den heiden Sprengtechniken wurde nicht festgestellt.

INTRODUCTION

The excavation of a tunnel will cause a disturbance to the rock mass surrounding the tunnel which is called Excavation Disturbed Zone (EDZ). The EDZ character and magnitude is a function of the existence of the air filled void represented by the tunnel, the method of excavation used, and the properties and the nature of the rock mass. Stress redistribution affects the rock mass in the far field is thought not to induce new fractures. Existing fractures may react by dilating, closing or shearing. In the near field, the process of excavation causes damage to the rock, changes its mechanical and hydraulic properties and produces new fractures. ANDRA (Agence Nationale pour la Gestion des Dechets Radioactifs, France), UK NIREX (United Kingdom NIREX Limited) and SKB (Swedish Nuclear fuel and Waste Management Co.) decided to perform a joint study of the disturbed zone effects. The relevance if this study in the frame of deep radioactive waste disposal projects is that the rockmass surrounding the seals of the access drifts must not provide preferential pathways for radioelements. The project named ZEDEX (Zone of Excavation Disturbed Experiment) comprised investigations before, during and after completion of two drifts at the Swedish Hard Rock Laboratory (Olsson et al. 1996) using three excavation methods under similar initial conditions: a tunnel boring machine (TBM) was used for excavation of one tunnel; the second tunnel was excavated with low shock energy smooth blasting (LSES) and normal smooth blasting (NS). As part of the ZEDEX project, an EDZ study in the near field was undertaken by performing various tests in short radial boreholes with different orientations. In this paper, results from in situ permeability, Pwave velocity and borehole resonance measurements are presented. Laboratory tests that were performed on cubic samples taken from the borehole cores are also discussed.

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