ABSTRACT:

Investigation of blasting damages in the contour of two rooms at the URL were performed on samples of boreholes. Different methods were used to study the microcracking and to determine the extension of the damaged zone: microscopy, P and S waves measurements, S-waves polarisation and isotropic compressive tests. A comparison of different results between the smooth blasting and traditional (normal) blasting methods indicates that the maximum extension of disturbed zone is about 50 cm and 80 cm respectively. Difficulty of this study resides in the important role played by the N 40° E microcracking.

RESUME:

L'etude de l'endommagement au voisinage de deux tunnels à I'URL a ete faite à partir d'echantillons provenant de sondages. Differentes methodes ont ete utilisees pour etudier la microfissuration et Pour determiner l'etendue de la zone endommagee: microscopie, mesures des vitesses des ondes P et S, polarisation des ondes S et essai de compression isotrope. Une comparaison des resultats indique une etendue maximale de zone perturbee dans Ie tunnel abattu par abattage adouci de 50 cm et de 80 cm avec la methode traditionnelle. La difficulte de cette etude reside dans Ie rôle determinant joue par la fissuration à N 40°E.

ZUSAMMENFASSUNG:

Die Untersuchung der URL ueber der Schadigung in der Nahe von zwei Tunnels ist mit Probestuecken aus Bohrungen durchgefuehrt. Um die Mikroklueftigkeit zu studieren und die Ausdehnung des Schadigten Gebietes zu bestimmen sind verschiedene Verfahren verwendet: Mikroscopie, Geschwindigkeitmessung der P und S Wellen, Polarisation der S Wellen und isotropischer Druckwiderstandversuch. Zwei Abbaumethoden sind genuetzt worden: ein uebliches und ein sanftes Verfahren. Die maximale Ausbreitung der gestörten Zone sinkt von 80 cm bis 50 cm. Die Schwierigkeit dieser Untersuchung liegt in der entscheidenden Rolle des Risses in N 40°W.

1 INTRODUCTION

Underground excavation induces stress redistribution In the surrounding rock mass which can be accompanied by fracturing and microcracking. This typical mechanism of fracture in brittle rocks is based on stable or unstable growths of cracks initiated from various pre-existing defects. The method of excavation induces a damage more or less high depending globally on the involved energy: tunnel boring machine, smooth blasting based on the application of low shock explosives and an optimized drilling pattern or normal blasting. This damage induced by excavation method combines with rock mass response to the stress redistribution (disturbance due to the cavity). Cracks and microcracks initiate and propagate in the near-field of the tunnel. This propagation occurs immediately after the excavation and extends with respect to time by the mechanism of sub-critical crack propagation (Homand et al. 1995). In situ characterization of this zone called generally, Excavation Disturbed Zone (EDZ), was performed in different underground laboratories. Specially in this issue, there is a description of the recent researches realised within the Aspö site. ANDRA was formerly involved in a smooth blasting experiment carried out in 1991 at the URL in Canada. This experiment located at the −240 level consisted to determine the extension of the damaged zone for different blast design. The excavation technique was low shock energy smooth blasting with different explosives and drilling pattern for each round. This paper presents laboratory study of samples from radial holes to examine crack damage and the data have been likened to those obtained with samples from a borehole drilled in a zone excavated with a normal blasting method.

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