ABSTRACT:

The Waste Isolation Pilot Plant (WIPP) is intended to be an underground repository for the permanent disposal of transuranic radioactive waste generated by defense activities. Project architects developed stmctural designs consisting of 30- by 90-m pillars in a seven-entry waste panel system Using 10-m-wide rooms. It was assumed that the pillars would experience creep deformation and enclose the waste packages gradually without significant fracturing. However, there has been a significant amount of fracturing in the mine roof and floor, and approximately 6 years after excavation, the pillars have begun to deviate from steady-state creep deformation. Based on a review of deformation data from 121 instrument clusters, multiple regression analyses, and observations of ground behavior since 1993, the authors identified factors influencing structural stability. These factors are (1) entry geometry, (2) extraction ratio within panels and in-panel pillar behavior, (3) sequence of excavation and load transfer between adjacent panels, (4) support density, and (5) age of excavations.

INTRODUCTION

The Waste Isolation Pilot Plant (WIPP) site is located about 40 km east of Carlsbad, New Mexico. Its purpose is to permanently dispose of both contact and remotely handled transuranic waste in underground workings (panels) located 655 m below the surface within the nearly 610-m-thick Salado Formation (U.S. Dept. of Energy [DOE] 1995).

Project architects developed structural designs for underground workings using experience from seven neighboring potash mines and applying both closed- form solutions and numerical models (DOE 1984). To account for differences in depth of cover between the WIPP site and the potash mines (320 m), a lower extraction ratio was used for the waste panels, and workings were developed gradually while completing geotechnical evaluations. These designs consisted of 30- by 90-m pillars in a seven-entry waste panel system using 10-m-wide rooms. Considering the lower extraction ratio and the large dimensions of the pillars, it was assumed that the salt pillars would experience creep deformation and enclose the waste packages gradually without significant fracturing.

Sandia National Laboratory and Westinghouse Electric Corp. completed detailed test mining and geotechnical monitoring in several experimental areas at different stratigraphic positions and with different entry configurations. The intent was to characterize the site and develop predictive techniques to assess repository performance. Sandia's work focused on development of site-specificonstitutive models for rock salt. These models were used in successful pre- dictions of room closure during the early stages of steady-state creep deformation, but did not reflect measured values when fractures formed in the mine roof. The lack of agreement between measured and calculated deformation is related to continuum finite- element formulations (Munson 1997). In view of the limitations for unambignous modeling of the complete strata deformation process (Maleki and Chaturvedi 1997), the authors incorporated the extensive defor- mation data into a statistical analysis to predict ground conditions at the site.

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