Nonlinear strain effects induced by thermal forcing on jointed rock masses

The study of the deformative response to cyclical thermal stresses of rock masses is considered crucial in geological risk mitigation relative to those instabilities that can configure high hazard slope instability scenarios due to their impulsiveness and high frequency of occurrence. Under specific climatic conditions, the superposition of heating and cooling cycles can influence the mechanical behavior of rock masses. Temperature fluctuations can exert slight yet repeated perturbations of stress fields resulting in a day-to-day cumulative effect, contributing to lead rock slopes toward prone-to-failure conditions over wide time scales. As a direct consequence of the thermal expansion-contraction cycles, the stress field of rock masses undergoes such perturbations capable of inducing both the genesis of new cracks and the growth of preexisting ones (i.e., subcritical crack–growth). These processes can induce inelastic deformations that can trigger shallow slope instabilities, such as rockfalls and rock topples. A multimethodological approach based on environmental, thermal, microseismic, and ambient seismic noise monitoring was designed for the purpose of identifying and characterizing nonlinearity of thermally-induced deformation on jointed rock masses at different dimensional scales. Two different case studies—a massive 10.000 m3 natural rock arch and a 20 m3 intensely jointed rock block—were selected to investigate the influence of repeated thermal cycles on their stability. In particular, their complex 3D geometries, different volume sizes and jointing conditions were considered to be of great interest to better comprehend the effectiveness of shallow thermal stresses interacting with different rock mass dimensional scales. Passive seismic monitoring techniques (i.e., ambient seismic noise and microseismic monitoring) allowed to obtain interesting insights on the interaction between the investigated rock masses and the periodic fluctuations of their temperature fields. The analysis of ambient seismic noise was aimed at investigating the possible wandering of resonance frequencies within short- to long-duration monitoring surveys, and highlighted the existence of thermally-driven, in-phase daily and seasonal fluctuations, but no irreversible modifications in their valuespotentially related to a progressive damaging process and acceleration toward failure of the structurewere observed. For what concerns the analysis of local microseismicity, a semi-automatic approach was implemented to identify possible irreversible clusters of fracture-related microseismic events over long-term monitoring windows. Based on the collected data, the here presented analyses highlighted not trivial insights on the role played by continuous near-surface temperature fluctuations and extreme thermal transients in influencing the stability of rock masses. In particular, the comparison of monitoring periods characterized by the most intense microseismic activity pointed out a peculiar distribution of microseismic events during heating and cooling phases of the rock mass in relation to different environmental conditions. These behaviors can be interpreted as the consequence of different driving mechanisms at the base of local failures. Along with the study of the seismic response of these jointed rock systems, Infrared thermography surveys were carried out at both sites for the characterization of their thermal behavior through different methodological approaches (i.e., 2D and 3D). The multitemporal acquisition of thermograms at Wied Il-Mielaħ allowed to achieve a preliminary characterization of the thermal behavior of the rock arch in response to the continuous fluctuation of near-surface temperatures at the daily and seasonal scale, highlighting the importance of considering the effect solar radiation and its interaction with complex morphological settings. Besides, a simplified method integrating Structure from Motion and Infrared thermography techniques was adopted at the Acuto field laboratory. The obtained results revealed that through the generation and co-registration of thermal and optical point clouds, the transfer of temperature attributes from low- to high-density point clouds can provide a detailed 3D representation of geometric features and surface temperature distributions and evolutions. The accurate reconstruction of 3D temperature fields will allow to obtain further insights for the assessment of the role played by thermal stresses in the concentration of elastic and plastic deformations in jointed rock masses, giving the possibility to weight the contribution of lighting and shadowing effects on entire slopes or isolated block volumes characterized by variable exposures and hence differentially heated by the solar radiation. The combination of different approaches can provide new insights on the effects related to near-surface thermal stresses fluctuations by allowing the investigation of the mechanical behavior of rock masses from fracture-scale to joint-isolated rock blocks, and the characterization of the spatio-temporal evolution of near-surface thermal fields.