In situ assessment of rock mass fracturing using infrared thermography

The reliable assessment of the fracturing state of rock masses is a fundamental step towards the evaluation of their geomechanical quality and the quantification of their hydraulic and mechanical properties. Traditional field discontinuity mapping techniques remain fundamental to collect statistical populations of discontinuity attributes and characterize rock mass structure and quality. However, point-like field surveys are strongly biased by scale and orientation. The development of 3D surveys allowed to partly overcome this problem by providing high-resolution point clouds. These allow a robust characterization of fracture geometry but require significant mapping efforts. Here we proposed a quantitative contactless approach to rock mass fracturing assessment by the use of Infrared Thermography (IRT).

IRT is increasingly used in rock-mechanics to characterize rock porosity/fracturing and to monitor rock mass stability, by measuring the thermal response of rock materials to heating or cooling. However, existing IRT applications to the geomechanical study of rock masses are mostly qualitative and lacking sound theoretical and experimental foundations. Starting from the laboratory scale, studying the thermal behaviour of rock samples with different fracture degrees, we propose a quantitative approach to quantify rock mass fracturing, that combines IRT rock temperature monitoring during cooling with the quantification of different descriptors of fracturing state suitable for different analysis scales (laboratory vs in situ).

As a field laboratory we used the Mount Gorsa porphyry quarry (Trento, North Italy), characterized by a homogeneous rock type but strongly variable fracturing states related by complex structurally-controlled and progressive slope damage processes.

We performed a field campaign on quarry front making a) Geometrical UAV surveys and b) field Geological Strength Index (GSI) evaluation on typical spots, c) we carried out IRT monitoring during night cooling using FLIRT1020 thermal camera.

During data processing d) thermal data acquired were corrected by environmental effects (blue sky radiation, slope inclination etc.) adopting original and ad hoc calibrated filters to skim the thermal response from geometrical and external biases. Finally we try to find a correlation between the thermal response of rock-mass outcrop to their quality index.

Our results support the possibility to upscale the analysis to field conditions in order to account for the radiative characteristics of natural environments, the limitations of the technique and upscaling issues typical of fractured rock-mass, taking into account that fracturing metrics (used in laboratory phase) at rock-mass scale, should influence block size distributions, which is fundamental in the evaluation of quality indices, e.g the Geological Strength Index (GSI) widely used in engineering applications.

Emphasising all these issues, the goal of our work is to investigate the relationship between the thermal response of rock mass quality index, through an experimental method developed at laboratory scale and upscaled to in situ conditions.