THE 2009 RHOMBUS WALL ROCK FALLS IN YOSEMITE VALLEY, CALIFORNIA: PROGRESSIVE FAILURE DUE TO FRACTURE PROPAGATION

Progressive rock-fall failures are common, yet their mechanics are not well understood. High-resolution photography, video, and laser scanning data reveal the mechanics behind a three-week-long series of rock falls from the Rhombus Wall in Yosemite Valley, California. The initial rock fall occurred at approximately 05:15 PDT on 26 August 2009, and took the form of a small (~3 m³) rock burst centered at the tip of an overhanging rock slab, a point of stress concentration. Several hours later, beginning at approximately 13:30 PDT, a series of three rock falls, separated in time by 20-40 seconds, detached from the upper northwest corner of the same area. The failures progressed upward along near-vertical sheeting joint behind the overhanging slab. Analysis of high definition video indicates that sudden shear failure occurred along the sheeting joint with associated tensile failure within the rock slab. At 14:09 PDT, a much larger rock fall occurred from the lower portion of the overhanging slab. Rock debris traveled to the base of the talus slope and damaged unoccupied vehicles in the Ahwahnee Hotel parking lot. Another smaller rock fall occurred from the upper portion of the overhanging slab at 15:35 PDT. The cumulative volume of the 26 August rock falls was 740 m³ as revealed by difference analysis of airborne and terrestrial LiDAR data. LiDAR analysis also revealed that between 380 and 1600 m³ of additional rock could be released in the event of subsequent failure of the remaining slabs located above the source area. Over the next three weeks, audible cracking sounds were heard on at least four occasions from the Rhombus Wall, suggesting punctuated incremental crack growth within or near these slabs. Visual monitoring of the rock-fall source area during this time indicated that the aperture of a new sheeting joint formed between the upper cliff surface and lower detachment surface widened, suggesting that the fracture was propagating upward behind the cliff face. At 18:12 PDT on 14 September 2009 a rock fall, whose location and volume (110 m³⁾ were defined by terrestrial LiDAR analysis, detached along this sheeting joint, concluding the rock-fall series. Detailed documentation and analysis of these rock falls provides valuable empirical evidence that furthers our understanding of progressive sheeting joint-type failures.