In late 2009 the U.S. Geological Survey (USGS) and National Park Service (NPS) joined into a collaborative research project to investigate the deformation of large, vertically-oriented, granite slabs in response to thermal heating and solar radiation. The goal of the project is to quantify how partially detached rock slabs respond to these stresses and whether or not they might act as precursors to rock falls that are common in many areas of the United States. The field site, consisting of a 14 m tall granite slab partially detached by an 8-15 mm gap to a near-vertical cliff, is located in Yosemite National Park, California. Yosemite provides an ideal location to study rock falls because these events are relatively common, even during summertime in the absence of other, more well-known triggering events (i.e., rainfall, freeze-thaw, etc.)

To accommodate the challenge of measuring the gap between the slab and cliff, GEOKON designed a custom “scissors-jack” crackmeter, similar in appearance to a car jack, which was spring-loaded and fixtured with a modified 50 mm Model 4420 displacement transducer. The USGS and NPS approved the design and the instruments were installed in May 2010. The top and bottom pads were attached to the underside of the slab and mountainside face, respectively, then the transducer was set for initial readings and connected to a Model 8002 LC-2 Datalogger.

The gages have been monitored continuously since late spring 2010 and useful data has been gathered as the system now nears its first winter period. Data thus far indicates a direct connection between thermal heating of the slab and expansion of the gap separating slab from cliff, with cumulative deformation occuring over prolonged periods of heat during the summer months. USGS and NPS researchers are currently analyzing the data to see if a connection with rock falls can also be established.

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