[en] Geomechanical investigations comprise a significant portion of the site characterization program to be conducted at Yucca Mountain. The investigations will include a number of large-scale experiments that will be conducted in an exploratory shaft facility at the site. A program of prototype testing has been initiated to ensure the success of these expensive and complex experiments. The prototype testing program addresses three problems in rock mechanics: measurement of rock-mass strength, measurement of joint properties in situ, and measurement of rock-mass response to thermally-induced loading. Active areas of development in support of these tests include cutting deep, narrow slots in rock and fabricating high-pressure flatjacks. 7 refs., 3 figs