Thermal imaging and rock slabs deformation due to daily solar radiation

In the Yosemite National Park, it has been shown that large granitic exfoliation sheets can be subject to spectacular daily deformations (with cracks opening and closing with a magnitude of up to 15 mm over 24 hours). These thermal deformations, observed during hot summer days, are known to contribute greatly to rock falls. However, it is questionable whether this kind of deformation only occurs with exfoliation flakes (which have very particular shapes), or if it can be observed on more common rock faces geometries.  Moreover, does this phenomenon only occur during hot summer days or also in other seasons?

To answer these questions, cracks and slabs in two sedimentary rock walls were monitored over 24-hour cycles, in summer and winter. The first site is in the massive limestones of the old quarry of St-Triphon (in the Swiss Prealps), the second one in the cliff of la Cornalle (near Lausanne), an intercalation of poorly consolidated sandstone and marls. Air and contact temperature loggers, a pyranometer to measure the incident solar radiation and crackmeters were used in situ. Thermal images were acquired every 20 minutes (surface thermocouples sensors and aluminum reflectors are used to constrain the surface emissivity and the environmental radiative temperature).

First it was shown, that during sunny days, the amplitude of the daily variation of the rock surface temperature is as large in winter than in summer (up to 30°C). As expected, this amplitude is larger in the detached slab than in the massive rock mass. In both sites, the deformation measured in the cracks reach about 0.2 mm. Depending on the slab geometry and its “attachment points” with the main rock mass, an increase of temperature can correspond to a closing or to an opening of the cracks. In conclusions, however these daily deformations are about two orders of magnitude smaller than those measured in the Yosemite big walls, they appear to occur also in “common” rock faces all around the year. On the long term, these deformations will contribute to the rock weakening at sub-surface conditions.