Preliminary results from multitemporal infrared thermography surveys at the Wied il-Mielah rock arch (island of Gozo)

Rock slopes instabilities represent one of the most unpredictable natural hazards due to their impulsiveness and neglectable entity of propaedeutic deformations. Such instabilities are able to determine high-risk scenarios both when interacting with infrastructures in urbanized areas (e.g. railways, highways or aqueducts) and when they are involving highly frequented natural heritage sites. Natural rock slopes are often predisposed to instability in virtue of the presence of discontinuity sets which, due to their spatial density and orientation, can strongly influence strength and deformability parameters of jointed rock masses. Relatively to this predisposing factor of instability, the action of high-intensity transients, such as strong earthquakes, heavy rains or induced vibrations, can determine the exceedance of triggering threshold of rock mass instabilities in short- to very short time scales. On the contrary, on wider time scales preparatory factors, e.g. temperature fluctuations, act as primary agents in the evolution of rock masses by exerting a continuous and non-negligible action of progressive damaging. Over recent years, an increase in academic interest has been seen in relation to the analysis of thermally-induced strain and progressive mechanical weathering caused by daily and seasonal surficial temperature fluctuations of rock slopes, for the purpose of understanding how preparatory factors are able to influence rock mass mechanical behaviour. Under specific climatic conditions, where temperature ranges can exceed the tens of degrees, the effects related to the superimposition of heating-cooling cycles, negligible if considered in short to mid-term, can influence rock mass mechanical behaviour acting as a thermal fatigue process. It has already been observed that daily temperature fluctuations are able to exert slight, yet repeated, perturbations of stress fields, resulting in a day-to-day cumulative effect, contributing over wide time scales to lead rock-slopes to a prone-to-failure condition. In fact, the heat flux propagation within rock masses, directly deriving from periodical fluctuations of their surficial temperatures, determines the configuration of a temperature field whose variations, in terms of heating and cooling intensity, are an inverse function of the distance from the heat-exchange boundary. As a direct consequence of the superimposition of thermal expansion-contraction cycles, the rock mass stress field undergoes such perturbations that are capable to lead both to the genesis of new cracks and to the growth of preexisting ones. These processes can determine, over long-term, the reaching of anelastic deformations consistent with triggering thresholds of surficial slope instabilities, e.g. rock-falls and rock-topples. The comprehension of thermomechanical deformations on jointed rock masses thus requires the definition of both the amplitude of the thermal forcing and the spatial distribution of near-surface temperature fluctuations. This study presents preliminary results obtained through the execution of two InfraRed Thermography (IRT) daily surveys in different meteorological seasons (Autumn and Winter) at the coastal arch of Wied-Il-Mielah (Malta). Radiometric images allowed both to reconstruct the temporal evolution of surficial temperatures of the rock mass, together with the existing relations between air and rock temperature, and to evaluate the spatial variations of temperature fields due to morphological characteristics of the monitored surface. The here presented results highlight how IRT could represent a useful remote sensing technique for the characterization of the thermal behaviour of jointed rock masses.