The Waste Isolation Pilot Plant (W[PP), a United States Department of Energy underground nuclear waste disposal facility, commenced disposal operations over one year ago. The radioactivity of most of the waste will be low enough to allow storage in steel drumstacked in the underground openings at WIPP. Some of the waste will have higher radioactivity and will require storage in horizontal boreholes drilled in the walls of the excavations. This paper investigates the geomechanical effects of the horizontal boreholes on the stability of the WIPP underground excavations.
INTRODUCTION
An underground excavation is considered physically stable if all activities for which the opening was intended can safely be completed in that opening without requiting measures beyond standard ground control and maintenance. A room may remain usable, even though rock conditions have significantly deteriorated around it, if adequate ground support has been installed and maintained. For a waste disposal panel, the time that the excavations in the panel must remain stable is the time between mining and panel closure. During this time the rooms are closely monitored to continually assesstability and ensure safe working conditions. After a panel is closed, stability is no longer a concern because no personnel shall enter the panel. This paper investigates the effeet of remotely handled (RH) waste disposal boreholes on short-term (zero to 7.5 year) room stability, and determines whether the presence of the RH boreholes and waste could cause the disposal rooms to become unstable before panel closure. The work presented here was completed for the United States Department of Energy (DOE), which was authorized by Public Law 96-164 to provide a research and development facility for demonstrating the safe permanent disposal of transuranic (TRU) wastes f?om national defense activities and programs of the United States exempted f?om regulations by the U.S. Nuclear Regulatory Commission (NRC). The Waste Isolation Pilot Plant (WIPP), located in southeastern New Mexico near Carlsbad, was construeted to determine the efficacy of an underground repository for disposal of TRU wastes located 650 meters deep in a bedded halite formation. The WIPP will dispose of waste in eight panels of seven rooms each (Figure 1). The rooms are about 90 meters long, 10 meters wide, and four meters high. They are separated by 30 meter wide pillars. The panels are separated by 60 meter wide barrier pillars. Figure 1. WIPP disposal area layout. The proposed RH borehole layout consists of 0.76-meter diameter boreholes drilled horizontally into the ribs about 1.6 meters above the floor. The boreholes are spaced 2.44 meters apart center to center and are 5.2 meters deep.
