Detection of rock bridges by infrared thermal imaging and modeling

Antoine Guerin,Greg M Stock,Martin Boesiger,Battista Matasci,Michel Jaboyedoff,Marc-Henri Derron,Caroline Lefeuvre,Yury Y Podladchikov,Brian D Collins

doi:10.1038/s41598-019-49336-1

Antoine Guerin, Greg M Stock + Show 7 more

Open Access

https://doi.org/10.1038/s41598-019-49336-1

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Abstract

Characterization of rock discontinuities and rock bridges is required to define stability conditions of fractured rock masses in both natural and engineered environments. Although remote sensing methods for mapping discontinuities have improved in recent years, remote detection of intact rock bridges on cliff faces remains challenging, with their existence typically confirmed only after failure. In steep exfoliating cliffs, such as El Capitan in Yosemite Valley (California, USA), rockfalls mainly occur along cliff-parallel exfoliation joints, with rock bridges playing a key role in the stability of partially detached exfoliation sheets. We employed infrared thermal imaging (i.e., thermography) as a new means of detecting intact rock bridges prior to failure. An infrared thermal panorama of El Capitan revealed cold thermal signatures for the surfaces of two granitic exfoliation sheets, consistent with the expectation that air circulation cools the back of the partially detached sheets. However, we also noted small areas of warm thermal anomalies on these same sheets, even during periods of nocturnal rock cooling. Rock attachment via rock bridges is the likely cause for the warm anomalies in the thermal data. 2-D model simulations of the thermal behavior of one of the monitored sheets reproduce the observed anomalies and explain the temperature differences detected in the rock bridge area. Based on combined thermal and ground-based lidar imaging, and using geometric and rock fracture mechanics analysis, we are able to quantify the stability of both sheets. Our analysis demonstrates that thermography can remotely detect intact rock bridges and thereby greatly improve rockfall hazard assessment.

Highlights

Erosion processes in rock masses depend on properties of the intact rock and the characteristics of brittle structures[1] such as joints and other discontinuities
The InfraRed Thermography (IRT) panorama of El Capitan (Fig. 4a) acquired on 8 October 2015 at 17:45 Pacific Standard Time (PST) shows calibrated temperatures ranging between 33.1 °C at the base of the cliff and 15.8 °C at the top
Given the average adiabatic lapse rate for a dry atmosphere is around 2.0–3.0 °C for every 300 m increase in elevation[107], this temperature range is two to three times higher than expected for neutral surroundings. This variance is due to the different incidence angles of infrared thermal radiation (Supplementary Fig. 1); the top of El Capitan reflects a portion of the colder sky and the base of the cliff reflects a portion of the warmer ground

Summary

Introduction

Erosion processes in rock masses depend on properties of the intact rock and the characteristics of brittle structures[1] such as joints and other discontinuities. The spatial resolution of the image and/or 3-D point cloud, the completeness of the 3-D point cloud (presence of shadow areas), as well as the quality of color contrasts used to distinguish fresh and pre-existing weathered fractures limit the ability of observers to provide reliable rock bridge extent percentages. These studies seek to identify former rock bridges, which are only exposed after rock detachment has occurred, whereas evaluating the contribution of rock bridges to rockfall hazard requires detection of rock bridges prior to detachment. Even though IRT is a fast, efficient and reproducible measurement technique, this study pointed out that IRT results provide only 2-D images, whereas a correct interpretation of thermal contrasts would require a 3-D model

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