As part of the Yucca Mountain site characterization studies, we conducted a suite of Plate Loading Tests (PLT) in the Exploratory Studies Facility, approximately 250 m below the crest of Yucca Mountain, Nevada. The objective of the in situ geo-mechanical testing was to define the deformability of the rock mass used for the design, performance assessment, and future performance confirmation of the monitored underground repository for high-level nuclear waste disposal. The uniqueness lies in the measurement of rock mass stiffness, for both ambient temperature and thermally perturbed rock masses, for the double acting system. The Plate Loading Test results suggest that the heated side of the PLT Niche, even early in the heating phase of the Drift Scale Test, has caused the fractures in the vicinity of the niche to close, thus presenting a stiffer rock mass for the PLT. The PLT results provide important information regarding the rock mass mechanical properties for the Yucca Mountain Project design and performance assessment programs. The estimations regarding the coupling of the thermal and mechanical response are critical inputs for estimates of rock mass stability, rock mass/ground support interactions, and the coupling of the thermal-mechanical and thermal-hydrologic processes necessary for performance assessments. Future PLT testing can further aid in understanding the complex T-M-H coupling necessary for determining the overall performance of the proposed Yucca Mountain repository.

Collaboration with M. Lee, R.E. Finley and M. Riggins