ABSTRACT:

The excavation of a circular test tunnel at the Underground Research Laboratory was monitored with 16 triaxial accelerometers designed to provide optimal focal sphere coverage. During excavation of the tunnel, brittle failure was observed in the roof and floor. The excavation-induced seismicity, associated with the failure, initiated at the advancing tunnel face. The extent of the failed region can be defined by the stress at which crack initiation occurs.

RÉSUMÉ:

L'excavation d'une galerie d'essai circulaire au Laboratorie de recherches souterrain s'est faite sous le contrôle de 16 acceleromètres triaxiaux conçus pour assurer une couverture optimale de la sphère focale, Lars de l'excavation de la galerie, on a observe une rupture de la roche dans le plafond et le plancher. Cette sismicite induite par 1'excavation, associee à la rupture, a debute à la taille chassante de la galerie. L'etendue de la zone de rupture peut se definir par l'intensite des contraintes à laquelle s'est amorcee la fissuration.

ZUSAMMENFASSUNG:

Der Ausbruch eines kreisförmigen Testtunnels im Underground Research Laboratory wurde mit Hilfe von 16 Dreiaxial-Beschleunigungsmessern ueberwacht, die im Hinblick auf eine Optimierung des raumlichen Erfassungsbereichs konzipiert wurden. Beim Auffahren des Tunnels wurden in der Firste und der Sohle Sprödbrueche beobachtet. Die durch den Ausbruch bewirkte und mit dem Bruch verknuepfte Seismizitat hatte ihren Ursprung an der Ortsbrust. Der Umfang der Bruchzone kann anhand des Spannungswertes, bei dem der es zur Riβbildung kommt, beschrieben werden.

INTRODUCTION

Excavation of an opening at depth in rock commonly results in cracking in the zones of maximum compressive stress concentration around the opening. The general form of this cracking in brittle rocks, such as granite, is spalling and slabbing, which canlead to the formation of a v-shaped ‘notch’. The energy released by this cracking process is referred to as excavation induced seismicity. A unique experiment in Atomic Energy of Canada Limited's (AECL) Underground Research Laboratory (URL), called the Mine-by Experiment, was carried out to investigate the failure process in situ. A 3.5-m-diameter test tunnel was excavated, in massive granite, without the aid of explosives. An extensive microseismic monitoring network was used to source locate the excavation-induced seismicity during construction of the test tunnel. The location of the excavation-induced seismicity revealed that the process leading to failure starts ahead of the advancing tunnel face. This paper examines the effects of the near-face cracking on the failure process around the test tunnel and considers the limitations of two-dimensional plane-strain analyses for this class of problems.

SITE DESCRIPTION

The Underground Research Laboratory is located within the Lac du Bonnet granite batholith, which is considered to be representative of many granitic intrusions of the Canadian Shield (Everitt et al, 1990). Extensive characterization of the Underground Research Laboratory revealed that jointing essentially stops at a depth of about 220 m beneath the ground surface. The study, reported in this paper, was carried out at a depth of 420 m (420 Level, Figure 1), approximately 200 m below any regular joint patterns. Approximately 500 m of tunnel excavation at the 420 Level encountered only six fractures, each with a trace length less than 1.5 m. Also, boreholes drilled to depths of over 1000 m in the vicinity of the URL indicate the massive granite persists with depth. Hence, one can conclude that the study was carried out in massive granite. The in situ stresses at the URL have been investigated extensively (Martin 1990). This extensive characterization program has defined three distinct stress domains at the URL (Martin & Chandler 1993).

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