Elsevier

Computers & Geosciences

Volume 45, August 2012, Pages 109-118
Computers & Geosciences

IRTROCK: A MATLAB toolbox for contactless recognition of surface and shallow weakness of a rock cliff by infrared thermography

https://doi.org/10.1016/j.cageo.2011.10.022Get rights and content

Abstract

Infrared thermography (IRT) can be used in remote recognition of potential weakening features of a rock cliff like shallow holes, high fracturing, moisture or material inhomogeneities, providing useful information for the corresponding rock mass geo-engineering characterization. A method aimed at such a recognition is proposed here together with its MATLAB implementation (IRTROCK package). It is based on the acquisition of a series of IRT images during the night-time cooling of a rock mass and on the search for possible anomalous thermal transients. The IRT alone is unable to completely characterize a rock mass; a meaningful interpretation of the results requires a geological on-contact survey or other on-contact or contactless techniques. Nevertheless, the results obtained in a portion of the cliff, where a detailed study with other techniques has been carried out, can be advantageously extended to the whole cliff. Moreover, the IRT measurements can be easily and safety repeated over time to evaluate possible changes that affect the studied rock mass. The effectiveness of the proposed approach has been verified in two test sites.

Highlights

► A series of thermal images allows the analysis of temperature pattern evolution. ► Weakened rock and sound rock have different thermal transfer efficiency. ► Surface weakness of a rock mass can be recognized with infrared thermography. ► A MATLAB toolbox able to perform this recognition has been developed. ► Cases study: Valstagna-Contrada Pieretti and Caprile (North-Eastern Italian Alps).

Introduction

The understanding of a complex phenomenon like the instability of a rock slope is very important because the induced hazard can reach a high level. General instability conditions sometimes lead to the sudden collapse of a large portion of the involved rock mass. This was the case of the 30×106 m3 Randa landslide, Switzerland (Sartori et al., 2003), or also of large rock avalanches triggered by earthquakes (Jibson et al., 2006). In many other cases, no generalized collapse is expected because only the shallow layers of the cliff are affected by rock weakness. Nevertheless, if a rock mass threatens a town, a railway or a national road, a sudden fall of stones or a relatively small rockfall can create high hazard for both human lives and properties (Corominas et al., 2005, Budetta, 2010). A suitable hazard assessment is therefore necessary.

Some observational techniques able to provide information about the possible weakness of the surface and/or shallow layers of a rock mass require contact with the studied system. Examples are the classical geological and geomechanical survey and ground penetrating radar (Deparis et al., 2007). These surveys are time consuming, cost expensive and potentially dangerous for the operators. Other techniques do not require a direct contact with the investigated surface and therefore allow safe surveys and an optimal returning time in the case of multitemporal observations. Examples are terrestrial laser scanner (Oppikofer et al., 2008), digital photogrammetry (Sturzenegger and Stead, 2009), satellite- or ground-based interferometric radar (Singhroy and Molch, 2004) and infrared thermography (Wu et al., 2005).

This paper focuses on infrared thermography (IRT), or thermal imaging. This nondestructive testing technique is widely used in civil engineering, e.g. to detect delamination, near surface honeycombing of subsurface cracks of a concrete structure (Aggelis et al., 2010), or near surface defects or features like the presence of water infiltration in a masonry structure (Meola, 2006). This suggests the use of IRT in detecting subsurface rock cracks or other features that could lead to hazardous conditions. This application of IRT is still not widely used, even if some interesting examples exist (Wu et al., 2005). The creation of a complete description of the state of a rock mass requires a series of surveys using several techniques together with numerical modeling. Nevertheless, the IRT can be profitably used as an ancillary technique and, thanks to the relatively fast measurement and data processing times, the observational sessions can be frequently repeated, allowing the recognition of possible changes of the hazard level.

In this paper, a method aimed at recognizing possible weakened areas of a rock cliff is proposed together with its MATLAB implementation. This method allows an evaluation of the corresponding rockfall-induced hazard starting from a series of IRT images acquired during the night cooling of the cliff. The method was applied to two test sites for validation purposes.

Section snippets

Basics

A blackbody is an idealized system that absorbs all incident electromagnetic radiation and completely re-emits it. The intensity spectrum of the emitted radiation depends exclusively on the blackbody temperature T, and is peaked at λM=a/T, where a=2.90×10−3 mK (Wien's displacement law). For T≈300 K (≈25 °C), it is λM≈10 μm, falling into the far infrared (FIR) region of the electromagnetic spectrum (the visible band ranges from 0.38 μm, violet, to 0.78 μm, red). Although the blackbody is an idealized

MATLAB implementation

The proposed method is implemented in the IRTROCK (InfraRed Thermography on a ROCK mass) MATLAB package. The input data are a series of files containing the thermal images exported from the software package provided by the IRT camera manufacturer. For example, in the case of a FLIR camera these files can be exported in a format compatible with IRTROCK (ASCII .txt; Excel .xls and MATLAB .mat) using the FLIR QuickReport freeware package (FLIR, 2011b). All the pre-processing operations, e.g.

IRTROCK application to two test sites

To evaluate the potentialities and limits of the proposed approach, it was applied to two rock cliffs located in Contrada Pieretti (along the Valsugana national road, Valstagna municipality, Vicenza province) and Caprile (Belluno province), respectively, both in the Veneto region, Northeastern Italian Alps (Fig. 3). In both cases, the IRT measurements were performed together with traditional geological surveys (also with climbing-based work) and terrestrial laser scanner acquisitions, aimed at

Discussion and conclusion

IRTROCK, a MATLAB toolbox intended for the recognition of surface or shallow inhomogeneities of a rock cliff, was developed and tested in two test sites. The toolbox requires a set of thermal images acquired at different times during the night-time cooling of a rock cliff and registered into the same reference frame, to allow a pixel-by-pixel comparison.

In order to validate the method by means of information provided by other techniques (terrestrial laser scanner and classical scan-line), it

Acknowledgements

This work was done within the scientific and geomechanical consulting agreement between DICAR Università di Trieste and Direzione Difesa del Suolo della Regione del Veneto and was funded within the Project no. 1381–277 MASSMOVE (INTERREG IV A Italia—Austria), by FERS, FDR and Regione del Veneto funds. G. Teza and G. Marcato were funded by Fondazione Cassa di Risparmio di Padova e Rovigo within the Research Project SMILAND. Moreover, the authors wish to thank Nuccio Bucceri (Land Technologies &

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