ACOUSTIC-EMISSION AND ULTRASONIC-VELOCITY METHODS USED TO CHARACTERIZE THE EXCAVATION DISTURBANCE ASSOCIATED WITH DEEP TUNNELS IN HARD-ROCK

Acoustic emission (AE) and ultrasonic-velocity monitoring studies have been undertaken at both the Atomic Energy of Canada Limited (AECL) Un derground Research Laboratory (URL) and at the Swedish Nuclear Fuel Wa ste Management Company (SKB) Hard Rock Laboratory (HRL). At both locat ions the excavations were tunnels in granitic material at approximatel y 420 m depth. However, the stress regime was more severe at the URL M ine-by tunnel site than the HRL ZEDEX tunnel. Different parts of the Z EDEX tunnel were created using different excavation techniques. Using AE and ultrasonic techniques to study these tunnels we have been able to examine the nature of the excavation-disturbed zone around the tunn el, as well as examining the effects of different stress regimes and e xcavation techniques. Studies were undertaken both during and after th e Mine-by tunnel excavation and during excavation in the ZEDEX tunnel. AE monitoring in the wall of the Mine-by tunnel during excavation sho wed that some activity occurred in the sidewall regions, but the spati al density of AE hypocentres increased toward the regions in the floor and roof of the tunnel where breakout notches formed. This sidewall a ctivity was clustered primarily within 0.5 m of the tunnel wall. AE mo nitoring in the floor of the tunnel showed that small numbers of AE co ntinued to occur in the notch region in the floor of the tunnel over 2 years after excavation was completed. This activity became more acute as the rock was heated, imposing thermally induced stresses on the vo lume. Ultrasonic-velocity studies both in the Boor and the wall of the tunnel showed that the velocity is strongly anisotropic with the dire ction of slowest velocity orthogonal to the tunnel surface. The veloci ty increased with distance into the rock from the tunnel surface. In t he floor, this effect was seen up to 2 m from the tunnel surface. Most of the change occurred within the first 0.5 m from the tunnel perimet er. At the lower-stress HRL, most of the AE again occur very close to the tunnel surface. The occurrence of AE under relatively low stress c onditions suggests that the regions experiencing AE activity were dama ged during the excavation process, thereby reducing their strength. Th e section of tunnel excavated by a tunnel-boring machine had fewer eve nts, clustered much closer to the tunnel surface, than the sections ex cavated using drill and blast excavation techniques. P-wave velocity c hanges of only about 0.1% were experienced due to the tunnel excavatio n for ray paths within zero to 2 m from the tunnel surface indicating that crack damage was relatively low. (C) 1998 Elsevier Science B.V. A ll rights reserved.