QUANTIFYING THERMALLY INDUCED ROCK FLEXURE AS A POTENTIAL ROCK-FALL TRIGGER

Among rock fall triggering mechanisms, thermally induced flexure is likely the least understood. The mechanism, where solar radiation and temperature variation drives deformation of partially detached rock flakes, has been postulated for some rock falls, but has yet to be adequately quantified. For example, in Yosemite Valley, California, rock falls have occurred on hot summer days in the absence of any meteorologic, seismic, or other recognized trigger, but the late afternoon timing of the majority of events, when rock temperatures are maximum suggest that thermally driven rock flexure may be the cause. However, little is known about the magnitude of rock deformation during thermal stressing or whether this is sufficient for crack propagation. To address these questions, we are monitoring the deformation of a near-vertical granitic rock flake in Yosemite Valley. The flake, 14 m tall, 4 m wide and 12 cm thick, faces south and receives direct sunlight. Whereas the flake is attached to the cliff face at its bottom and top, the sides are detached from the cliff by a 10 cm wide crack on one side, tapering to a 1 cm wide crack on the opposite side. Instrumentation consists of three custom-designed crackmeters placed between the flake and the adjacent cliff face, three air temperature sensors located behind the flake, and three dual air temperature-solar radiation sensors located on the outside surface of the flake.

Five-minute interval data from summer 2010 indicates the flake undergoes maximum deformation at mid-span between attachment points and that it deforms from both diurnal and climatic temperature fluctuations. Recorded maximum deformations are 1 cm diurnally and nearly 1.5 cm (including diurnal effect) over a 5-day period of cooler temperatures. Diurnal fluctuations reach peak contraction (crack closing) in mid-morning, synchronous with low solar radiation and air temperature, and peak expansion (crack opening) in late-afternoon when temperatures are maximum. These measurements demonstrate that thermally driven rock flexure is capable of deforming large rock slabs. Cumulative outward deformation with moment-inducing tensile stresses and crack tip propagation may also be occurring in this and other partially detached rock flakes in Yosemite Valley, thereby providing a potential trigger for many rock falls.