Publication Date:
2011-12-13
Description:
Progressive rock-fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high-resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 1.2-year-long sequence of at least fourteen rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock-fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 740 m 3 progressed upward along a sheeting joint behind the overhanging slab. Over the next three weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m 3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m 3 , radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions driving fracture propagation along sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how a sheeting joint can propagate to lengths many times greater than its depth behind a cliff face. We posit that as a region of failure spreads across a cliff face, stress concentrations associated with previous failures will likely tend to spread with it, inciting further crack propagation and rock falls. Copyright © 2011 John Wiley & Sons, Ltd.
Print ISSN:
0197-9337
Electronic ISSN:
1096-9837
Topics:
Geography
,
Geosciences
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