ABSTRACT

INTRODUCTION

The study of the dynamic response of an underground opening in discontinuous rock is an essential component of any investigations concerning the short- and long-term stability of a deep radioactive waste disposal vault. Dynamic response, as defined in this study, is the behaviour of the cavern exposed to seismic waves generated by earthquakes as well as the behaviour of the rock during disruptive events such as rock bursts. Obviously, the potential of rock disruption is important in the design of the containers and the manner in which they are placed in the disposal vaults.

The study reported here constituted part of the Canadian Nuclear Fuel Waste Disposal Program, carried out under the auspices of the Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment. The current Canadian concept for an underground radioactive waste disposal vault calls for rooms excavated in granitic rock at a depth of about 1 000 m. Besides the shafts and accessways, the facility will consist mainly of long rooms with a maximum height or span of about 7 m in which the irradiated fuel (IF) or immobilized waste (IW) will be emplaced. Packages may be placed in backfill within the rooms or emplaced in boreholes drilled into the floor.

Even relatively small rock bursts or other associated dynamic failures would greatly interfere with the orderly handling of radioactive waste, and could pose a distinct danger to men and machinery. Relative displacements or offsets in boreholes may damage the waste containers. The likelihood of any of the events occurring will depend on the rock type, the thermal loading, the action of seismic waves and the in-situ stress

A state of high horizontal stress exists in many places in Ontario (Lee, 1978; Asmis and Lee, 1980). Although the stresses at the 1 000 m level at any arbitrary location in Ontario can only be approximated, using the available evidence (Herget, 1973), some values were chosen. The maximum principal stress (horizontal) was assumed to be 50 MPa and the vertical stress was taken to be 25 MPa.

Any rock in which a disposal vault would be located will probably not be completely free from discontinuities such as joints and minor shears. As such, it can be expected that defect-related failure will most likely be involved with dynamic failure modes, especially since these discontinuities will be under high shear stress and, perhaps, will be close to failure (Asmis, 1980).

FRICTION ALONG DISCONTINUITIES

Fault mechanics states that there cannot be seismic wave generation in the vicinity of a fault unless the resisting frictional stress decreases faster than the driving stress which is being relieved by the motion. This is clearly shown in Figure 1 which demonstrates the slider-block source model (Dieterich, 1979). The frictional stress t f must decrease faster than the driving stress t . The difference or stress drop (¿t) determines the acceleration of the block. As the block moves, is decreased, thus the best potential for the greatest stress drop is at the beginning of motion; or in the case of fault propagation, at the moving tip of the rupture. The greater the stress drop, the greater the peak acceleration.

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