ABSTRACT:

Microseismic (MS) and Acoustic Emission (AE) monitoring is being carried out as part of the Tunnel Sealing Experiment (TSX) at the 420m level of Atomic Energy of Canada's Underground Research Laboratory (URL). A 40m long tunnel was excavated in the direction of the maximum in situ stress, with approximate dimensions of 4.4m horizontally and 3.5m vertically. These two dimensions were chosen to reduce the magnitude of the high stress concentrations, which occur in the roof and floor of sub-horizontal tunnels at the URL. Two bulkheads have been constructed and keyed into the tunnel to create a chamber in granite, which can be pressurised to ambient pore pressure (4 MPa). This paper presents the results of the AE/MS monitoring of the excavation phase of the TSX and the velocity interferometry measurements taken to monitor the time-dependent development of the excavation disturbance zone (EDZ). The results support stress modelling predictions and show that induced microcracking is limited to within a 0-2m shell around the excavations.

THE TUNNEL SEALING EXPERIMENT

The TSX is designed to examine the performance of concrete and bentonite based tunnel bulkheads subjected to high hydraulic gradients in granite. Figure I shows a schematic of the TSX, while Figure 2 shows the main tunnel (room 425) as well as the other tunnels associated with the experiment which provide access for instrumentation and drainage boreholes. The chamber between the bulkheads will be pressurised to approximately 4 MPa which is representative of the ambient pore pressures at 420m depth in the rock. Instrumentation will monitor the seepage around and through each bulkhead. Additionally stresses and displacements in each bulkhead will be recorded. A second phase of the experiment will involve determining temperature effects by heating the water in the chamber to about 85°C.

Room 425 was excavated using a drill and blast technique between January and March 1997. The 40m long tunnel was excavated in the direction of the maximum principal stress, o?, and the cross- section geometry was elliptical, with approximate dimensions of 4.4m horizontally and 3.5m vertically. This tunnel shape was chosen to reduce the magnitude of the high compressive stress concentrations that occur in the roof and floor of subhorizontal tunnels at the URL. The circular Mine-by tunnel (room 415), which was excavated in the direction of 02, promoted the maximum stress- induced damage zone. This resulted in significant MS and AE activity (Young and Collins, 1997), the formation of a process zone (Read and Martin, 1996) and subsequent break-out notches in the roof and floor of the tunnel. The excavation of various elliptical shaped tunnels on the 420 Level (Read and Chandler, 1996) has shown that breakout notches can be stopped from forming.

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