The Tunnel Boring Machine (TBM) excavating the Yucca Mountain Exploratory Studies Facility (ESF) encountered the Bow Ridge Fault and fracture zone February 1, causing a flow of material that brought work to a standstill.

The TBM ran into soft ground causing a 12-15 foot natural arch cavity to open above the tunnel. According to Department of Energy (DOE) reports, TBM operators observed that the conveyor belt had been overloading, and the cutterhead amperage had decreased. Work was stopped and an opening became apparent.

Within 24 hours, the opening was pumped with cement. During the downtime, the ESF Test Coordination Office, U.S. Geologic Survey, U.S. Bureau of Reclamation, and Sandia National Laboratories evaluated the fault and the adequacy of existing conditions and test controls. The construction team decided to apply stabilizing measures before proceeding with excavation. They concluded that patching the cavity would not pose a significant problem for scientific studies.

The DOE is bound by Administrative Procedures to report any significant geologic conditions that occur during the tunneling operations. If a condition is reportable, tunneling would have to stop, and an evaluation of the condition and how it might affect site characterization would be required. The DOE has stated that the incident did not constitute a significant geologic condition because personnel fully expected fault zones and associated structures during tunneling. In a report on the incident, DOE states that "off-normal" geologic conditions are expected as tunneling continues.

Carl Johnson, of the state's Nuclear Waste Project Office disagrees. "We've said it should have been reported as a significant geologic condition. In our view this is symptomatic of the project: they really have not characterized the sub-surface of Yucca Mountain well enough that they can predict what geologic conditions will be. The criteria are written so that nothing is reportable, anything is possible." In addition, Johnson believes DOE should have taken more time to investigate the cavity before it was patched.