ABSTRACT:

Acoustic emission (AE) and ultrasonic velocity studies were undertaken during the excavation of tunnels by tunnel boring machine (TBM) and drill and blast methods. A much greater level of AE activity was associated with the drill and blast tunnel than the TBM tunnel. In both cases the majority of AE events were confined within one metre of the tunnel perimeter. Velocity studies showing little excavation-induced change indicated that the microcrack damage was not extensive around either tunnel.

RESUME:

Les etudes des emissions acoustiques et des vitesses ultrasoniques ont ete entreprises pendant Ie creusage de tunnels exaves soit par TBM soit par forage et charges explosives. II a ete observe une activite bien plus intense d'emissions acoustiques dans Ie tunnel avec methode forage/charge explosives plutot que dans Ie tunel avec TBM. Dans les deux cas la majorite des evènments sismo-acoustiques ont ete localise dans une region ne depassant pas un mètre autour du tunnel. Les etudes de la vitesse de propagation montrant de faibles variation induites par l'excavation ont inique que les micro fissures n'etaient pas etendues à l'ensemble des tunnels environnants.

ZUSAMMENFASSUNG:

Studien von SE und USG wurden wahrend Tunnelgrabungen durchgefuehrt um die gestörte zone von einer Tunnelbohrmaschine (TBM) und von der Bohr- und Sprengmethode (BSM) zu vergleichen. Mit dem BSM Tunnel war ein höheres Niveau anfallender Schallesmission verbunden als mit dem TBM Tunnel. In beiden Fallen war die Mehrzahl der auftretenden SE auf einen Meter des Tunnelperimeters beschrankt. Schallgeschwindigkeitsmessungen, die wenig Änderung bei Wechsel del' Grabungsmethode ausweisen, zeigten, daß der Schaden in Form von Mikrorissen in keinem der Tunnel betrachtlich war.

1. INTRODUCTION

The ZEDEX project took place at the Swedish Nuclear Fuel Waste Company (SKB) Hard Rock Laboratory (HRL) on Äspö Island in South Eastern Sweden. One of the primary objectives of the ZEDEX project was to gain an understanding of the nature and development of the excavation-disturbed zone (EDZ) associated with an underground excavation. The EDZ phenomenon is of great interest in the field of radioactive waste management as it may provide a pathway for ground water flow and potentially radionuclide transport around an underground repository for radioactive waste. Two five-metre-diameter tunnels were excavated using different excavation techniques, with the objective of comparing the dependence of EDZ development on the excavation method. The near-by tunnel sections were of similar geometry and orientation, and were excavated under similar initial conditions. Acoustic emission (AE) activity in rocks results primarily from the sudden release of strain energy associated with cracks and microcracks. This may take the form of crack opening, closing, extension, shear slip, or some combination thereof. Monitoring AE activity provides an ideal way of studying brittle damage as it occurs. In this study we concentrated on the spatial and temporal distribution of AE activity around the tunnels. Ultrasonic velocity studies were undertaken for two reasons. Firstly, an understanding of the velocity field is a necessity for AE source location studies. Secondly, ultrasonic velocity studies provide a complementary method of studying the microcrack population in the rocks. The presence, distribution, orientation and saturation of cracks and microcracks are among the dominant factors affecting seismic velocity in crystalline rock (Hudson, 1981). The first of the two sections of tunnel discussed in this paper was excavated by a tunnel-boring machine (TBM) and the second by a "smooth blasting" drill and blast technique using electronic detonators to improve control of detonation times, low-shock perimeter explosives and a blast design optimized to reduce blast damage. A third section of tunnel was excavated using a blast design more similar to the production blasting at the Äspö HRL. Due to limitations on space, those results will not be discussed in this paper.

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