To explore connections between rock strength and rock falls, we undertook a comprehensive rock mechanics testing program for six granitic rock types in Yosemite Valley (California, USA) where rock falls are a common geomorphic and sometimes hazardous process. We collected samples from boulders located at the base of cliffs, with the inherent assumption that the intact boulders should provide reasonable estimates of full-strength values. Our testing program included unconfined compressive strength tests, triaxial compressive strength tests, Brazilian tensile strength tests, and Mode I fracture toughness strength testing using two different types of samples – chevron bend (CB) and cracked chevron notched Brazilian disk (CCNBD). Our results, consisting of 88 individual tests, provide the most detailed evaluation of rock strength in Yosemite Valley to date. These results provide the data needed to evaluate the various failure modes (e.g., shear failure of wedge instabilities, tensile failure of overhangs) that might be expected for rock falls from cliffs in Yosemite. We expect that these data will provide an important resource for the evaluation of rock falls and other geomorphological studies in Yosemite National Park.


Rock falls are an intermittent and ongoing hazard in Yosemite National Park, California (Wieczorek et al., 1998; 1999; Stock et al., 2013), where steep cliffs up to 1 km in height attract millions of visitors each year (Fig. 1). The iconic scenery, composed of such famous landmarks as El Capitan and Half Dome, is composed of granitic rocks related to several intrusive suites that form the main bedrock geology in Yosemite (e.g., Peck, 2002; Putnam et al., 2015). Along the flanks of Yosemite Valley, a wide range of granitic rocks are exposed on the cliff faces (Putnam et al., 2014). The rock-fall hazards associated with these cliffs vary from place to place depending on a range of factors including topography, structural geology, triggering conditions, and presumably rock strength.

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