ABSTRACT:

The stability of underground openings is being evaluated for a potential high-level radioactive waste repository, subjected to in-situ, thermal, and seismic loads for at least 125 years following the initiation of waste eraplacement. Numerical analyses using continuum and discontinuumodels were conducted to examine the sensitivity of variations of rock mass properties and loading conditions on the drift behavior. Resuits show that the unsupported drift openings are in general stable under elevated temperatures and seismic motions, and the use of ground supports, such as rock bolts and steel sets, is warranted to prevent local loosening of rock blocks induced by thermal and seismic loads. These ground supports can be designed to sustain in-situ, thermal, and seismic loads anticipateduring the repository preclosure period.

INTRODUCTION

The long-term stability of underground openings is being examined for a potential high-level radioactive waste repository at Yucca Mountain, about 160 km northwest of Las Vegas, Nevada. The repository lies some 200 to 400 m deep in volcanic welded tuff and about 150 to 300 m above the regional groundwater level. The repository host horizon contains four lithostratigraphic units, the upper lithophysal (Tptpul), middle nonlithophysal (Tptpmn), lower lithophysal (Tptpll), and lower nonlithophysal (Tptpln). Among these rock units, the Tptpmn and Tptpln units are massively jointed, with an average joint spacing as small as 0.5 m. The other two lithophysal units, Tptpul and Tptpll, contains bubble-like voids, with a porosity of over 10 percent. Some 70 percent of emplacement drifts, with a diameter of 5.5 m and a center-to-center spacing of 81 m, will be excavated in the lower lithophysal unit, about 20 percent in the middle nonlithophysal unit, and the rest in the other units. These drifts, emplaced with nuclear waste packages that produce heat, will experience high radiation and temperature increases. In addition, the drifts must provide access for at least 125 years before the repository closure. Figure 1 shows a cross-section of emplacement drift to be excavated in the Tptpmn unit.

The study was to evaluate the long-term stability of emplacement drifts subjected to in-situ, thermal, and seismic loads, and the needs of ground control. Due to the uncertainty of design parameters, focus of the study was on the effects of variation of rock mass properties and loading conditions on the displacements and stresses near eraplacement drift openings and behavior of ground support components, such as rock bolts and steel sets. Numerical approaches using both continuum and discontinuum models were employed. Since the thermal and seismic loads considered would be time dependent, transient condition was analyzed.

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