Prediction of room closure and stability of panel 1 in the Waste Isolation Pilot Plant

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. Both surface and underground facilities, including one waste panel, were excavated during the period from 1982 to 1988. The decision to use the repository for disposal has not vet been made. The objective of this paper was to predict room closures and ground conditions for Panel 1 based on an analysis of extensive deformation data collected by Westinghouse Engineering from 119 instrumented locations throughout the WIPP facility. This study was important for both maintaining the safety of personnel and assuring the continuity of waste emplacement operations, given that the anticipated service life of the Panel 1 has nearly tripled from the original design life of 5 years. The technical approach consisted of (1) characterization of 11 mining, bolting, and geologic variables at individual instrumented locations (119 clusters), (2) development of a mathematical model (multilinear regression analysis using stepwise inclusion of independent variables), (3) examination of the results, and (4) prediction of entry closures and ground conditions for the anticipated life of the facility given the specific geologic, mining, and support conditions at selected waste panels and access entries. Based on an examination of standardized regression coefficients, several significant variables were identified that influence entry closure and long-term stability. These were excavation ratio, entry width, roof beam thickness, entry height, and age. Interestingly, bolting variables did not help explain variability in the data and thus were not included in the mathematical model. Considering the present roof conditions, anticipated future deformation, and mining experience with variety of proven roof support systems, it was concluded that only portions of Panel 1 may be used for waste emplacement with a reasonable degree of confidence.