1,720,970 research outputs found
Nonlinear strain effects induced by thermal forcing on jointed rock masses
No full textThe 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 valuespotentially related to a progressive damaging process and acceleration toward failure of the structurewere 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
Multimethodological Study of Non-linear Strain Effects Induced by Thermal Stresses on Jointed Rock Masses
No full textA multimethodological method based on environmental, stress–strain, microseismic, and ambient seismic noise monitoring is here presented, with a view to identifying non-linearity of thermally-induced deformation of jointed rock masses at different dimensional scales. Rock masses experience non-negligible deformation cycles due to the continuous fluctuations of their surficial temperatures. However, the interpretation of such strain effects, in terms of the ratio between elastic and inelastic percentages, is still debated. In particular, the relation between microseismic emissions, considered as primary indicators of crack-growth related energy release, and resonant frequencies fluctuations of rock structures, witnesses of the thermally-induced effect at the macro- or structure-scale, have not been yet studied within a coupled framework. The combination of different approaches able to investigate the behavior of rock masses from micro- to macro-scale, then from fracture-scale to joint-isolated rock blocks up to rock structures, could provide new insights and perspectives on the effects related to shallow thermal stresses fluctuations. This paper presents the preliminary outcomes from two case studies, the Acuto experimental test-site (Italy) and the Wied Il-Mielaħ sea arch (Malta), where multiparametric monitoring surveys were conducted and are still ongoing, aiming at the assessment of the cause-to-effect relation between near-surface thermal stresses and induced strains. Data analysis was carried out following different approaches, with a particular emphasis on the Acuto test-site dataset recorded so far, allowing to establish a well-constrained correlation among temperature fluctuations and rock mass deformation both at the daily and seasonal scale
3D Thermal Monitoring of Jointed Rock Masses through Infrared Thermography and Photogrammetry
Full text linkThe study of strain effects in thermally-forced rock masses has gathered growing interest from engineering geology researchers in the last decade. In this framework, digital photogrammetry and infrared thermography have become two of the most exploited remote surveying techniques in engineering geology applications because they can provide useful information concerning geomechanical and thermal conditions of these complex natural systems where the mechanical role of joints cannot be neglected. In this paper, a methodology is proposed for generating point clouds of rock masses prone to failure, combining the high geometric accuracy of RGB optical images and the thermal information derived by infrared thermography surveys. Multiple 3D thermal point clouds and a high-resolution RGB point cloud were separately generated and co-registered by acquiring thermograms at different times of the day and in different seasons using commercial software for Structure from Motion and point cloud analysis. Temperature attributes of thermal point clouds were merged with the reference high-resolution optical point cloud to obtain a composite 3D model storing accurate geometric information and multitemporal surface temperature distributions. The quality of merged point clouds was evaluated by comparing temperature distributions derived by 2D thermograms and 3D thermal models, with a view to estimating their accuracy in describing surface thermal fields. Moreover, a preliminary attempt was made to test the feasibility of this approach in investigating the thermal behavior of complex natural systems such as jointed rock masses by analyzing the spatial distribution and temporal evolution of surface temperature ranges under different climatic conditions. The obtained results show that despite the low resolution of the IR sensor, the geometric accuracy and the correspondence between 2D and 3D temperature measurements are high enough to consider 3D thermal point clouds suitable to describe surface temperature distributions and adequate for monitoring purposes of jointed rock mass
Insolation Cycles Control the Timing and Pattern of Resonance Frequency Drifts at a Natural Rock Tower, Utah, USA
Full text linkResonance frequency monitoring can detect structural changes during progressive rock slope failure; however, reversible environmentally-driven frequency drifts may inhibit identification of permanent changes. Frequency drifts are commonly correlated with air temperature, lagging temperature changes by zero to 35–60 days. Here we report observations from two years of monitoring at a rock tower in Utah, USA where annual resonance frequency changes appear to precede air temperature cycles by ~35 days. Using correlations with meteorological data supplemented by numerical modeling, we identify changes in insolation as the primary driver of annual frequency drifts, giving rise to the negative lag time. Sparse in-situ insolation data show that the daily frequency increase lags sunrise by several hours, while frequencies decrease at sunset, responses we attribute to the west facing aspect of the tower. Modeled daily insolation patterns match frequency data for months when in-situ measurements are not available. Numerical models offer the advantage of predicting insolation patterns for different aspects of the rock tower, such as the west facing cliff where measurements would be challenging. Our study highlights the value of long-term datasets in identifying mechanisms driving environmentally associated frequency drifts, understanding that is crucial to facilitate detection of permanent changes during progressive failure
Impact of environmental stressors on jointed rock cliffs by acoustic emission sensing. Preliminary findings from the Acuto Field Laboratory (Central Italy)
No full textSeasonal variations in natural environmental stressors represent preparatory factors of rock block deformation driving to failure on slopes as well as on underground contexts. To better assess the role of the environmental stressor regarding the rock slope stability, the present study focused on monitoring an unstable rock block cropping out in a quarry wall at the Acuto field laboratory (Italy), through a methodology for a high-resolution detection of acoustic emission (AE) signals, their characterization and correlation with the concomitant environmental stressors has been developed. Through the spectral analysis of the AE waveforms, the seasonal distribution of average frequencies varies significantly, and the main frequency band was 0-200 kHz. The two-step clustering method and frequency characteristics were used to initially identify group "I" as microcracks-related AE events. The identification results using the MLP neural network showed that microcracks-related AE events accounted for only 23.2% of the total monitoring cycle. This percentage rises to 51.1% and 55.2% for the periods May to December and December to February 2024, respectively, as the rainy season tapers off. The power-law index of AE event energy is variable in the considered time windows in which the monitoring was carried out. Results of a comparative statistic indicate a joining between high AE rates and cooling-relaxation processes, as well as a weak correlation among the waveform attributes and meteorological stressors. The power-law index of AE event energy decreases during the cooler months, particularly from July to December, correlating with the overall drop in temperature. During this period, the few high-energy events can be considered potential indicators of geological hazards.An identification model for acoustic emission events based on two-step clustering and neural network is established.A link between high acoustic emission rates and cooling-relaxation processes was found.Seasonal variations in rock block stability were judged based on power-law indexes
Identifying fracture-controlled resonance modes for structural health monitoring. Insights from Hunter Canyon Arch (Utah, USA)
No full textProgressive fracturing contributes to structural degradation of natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value, where non-invasive approaches are essential. This study aims to determine how fractures affect the dynamic properties of rock arches, focusing on resonance modes as indicators of structural health conditions. We hypothesize that damage resulting from fracture propagation may influence specific resonance modes that can be identified through ambient vibration modal analysis. We characterized the dynamic properties (i.e., resonance frequencies, damping ratios, and mode shapes) of Hunter Canyon Arch, Utah (USA), using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz. Experimental data were then compared to numerical models with homogeneous and heterogeneous compositions, the latter implementing weak mechanical zones in areas of mapped fractures. All numerical solutions replicated the first two resonance modes of the arch, indicating these modes are insensitive to structural complexity derived from fractures. Meanwhile, heterogenous models with discrete fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to the presence of fractures and thus most likely to respond to structural change from fracture propagation. An evolutionary crack damage model was then applied to simulate fracture propagation, confirming that only this higher mode is sensitive to structural damage resulting from fracture growth. While examination of fundamental modes is common practice in structural health monitoring studies, our results suggest that analysis of higher-order resonance modes can be more informative for characterizing fracture-driven structural damage
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Preliminary results from multitemporal infrared thermography surveys at the Wied il-Mielah rock arch (island of Gozo)
No full textRock 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
