Investigating the thermal behavior of exfoliation sheets in granitic cliffs (Yosemite, USA) through laboratory experiments and numerical modeling

Yosemite National Park (California, USA) is characterized by high granitic rock walls affected by diffuse rock slope instabilities. These pose high rockfall hazards to roads and threaten the lives of millions of people that every year access the park to visit the natural beauties of Yosemite Valley, walk trails and climb iconic rock walls like El Capitan. Here, rockfalls are chiefly triggered by the progressive failure of portions of exfoliation sheets (“flakes”) bound to the cliff by rock bridges. In this context, identifying potentially unstable flakes is crucial for risk mitigation, yet the field characterization of such flakes remains difficult, highlighting the need for remote sensing mapping methods.

At the southeastern face of El Capitan, in situ time-lapse infrared thermographic (IRT) surveys, conducted in October 2024, revealed that exfoliation sheets cool faster than the surrounding rock mass heated by the same daily solar forcing. To lay foundations for a remote detection methodology, we carried out a combined laboratory and numerical study of the IRT signature of daily heating and cooling of exfoliation sheets and the underlying physical processes.

We conducted 37 laboratory experiments in a controlled setup, where the cooling of 20 cm by 20 cm granite slabs with variable thickness (1-6 cm) and opening of a simulated exfoliation joint (2-54 mm), oven-heated at 85°C, is monitored by contact thermocouples and a high resolution thermal camera. For each tested combination of slab thickness and joint aperture, we recorded detailed temperature time series and modelled cooling curves using the lumped capacitance solution of Newton’s law of cooling.

Experimental results show that, until a threshold value of the thickness/aperture ratio is reached, IRT can detect a dependence between the cooling rate of the external slab face and the aperture of the simulated exfoliation joint, with two contrasting trends. For very small aperture, cooling speed decreases with aperture. Beyond a certain aperture value, varying with slab thickness, the slab face cools faster as joint aperture increases.

To investigate the physical processes underlying this behaviour, we reproduced our experiments by 2D and 3D finite-element numerical simulations with the software Temp/W-Geostudio^TM, considering different conditions (i.e. initial temperature of the cliff rock behind the flake, conduction, and air convection parameters). Model results suggest that convective heat transport in the open simulated joint strongly controls the thermal energy dissipation within the cooling flake. For very small joint apertures or limited convective circulation, the insulating effect of air results in slower flake cooling. However, for increasing joint aperture and thus greater air convection, the results indicate more effective heat dissipation and associated faster cooling. Our study provides a quantitative framework towards the development of remote mapping of unstable rock features upon proper methodology upscaling to in situ conditions.