Gozo (Malta), led us to classify the susceptibility of the cliff surfaces to instability mechanisms, recognizing the wedge failure as the highest hazardous one.
CORRELATION BETWEEN EROSION PATTERNS AND ROCKFALL HAZARD SUSCEPTIBILITY IN HILLTOP FORTIFICATIONS BY TERRESTRIAL LASER SCANNING AND DIAGNOSTIC INVESTIGATIONS Deodato Tapete, Giovanni Gigli, Francesco Mugnai, Pietro Vannocci, Elena Pecchioni, Stefano Morelli, Riccardo Fanti, Nicola Casagli Department of Earth Sciences, University of Firenze, Via G. La Pira 4, 50121 Firenze, Italy ABSTRACT A holistic methodology combining conventional diagnostic investigations and kinematic analysis performed on 3D laser scanning survey is here proposed for rockfall hazard assessment and erosion patterns study, to map critical sectors and evaluate potential impacts on the conservation of cultural heritage sites built on unstable rock masses. Experiments carried out on the fortifications of Citadel, Gozo (Malta), led us to classify the susceptibility of the cliff surfaces to instability mechanisms, recognizing the wedge failure as the highest hazardous one. Observations on thin section of the rock textural properties and measurements of the resistance to abrasion completed the laser-based analysis, clarifying the intrinsic weakness of the outcropping limestones. Levels of conservation criticality were assigned to the rock mass sectors located underneath the historical buildings, and on site monitoring system was installed to follow the evolution of the crack patterns. Index Terms— Terrestrial laser scanning, kinematic analysis, hazard assessment, deterioration study, cultural heritage 1. INTRODUCTION To effectively preserve stone architectural heritage of historic towns and military fortifications built on unstable rock masses, both the anthropogenic structures and underlying geologic substratum should be analysed as parts of a same system. This is particularly fundamental whenever their constituting materials share common properties of durability and are consequently exposed to similar physicalchemical alteration. The employment of a dual approach considering the natural and artificial components of the system seems a feasible strategy [1], rather than the execution of masonry repairs without a consolidation of the rock mass or vice versa (as it indeed frequently happens). With this perspective, we set up a multidisciplinary methodology based on the integration of terrestrial laser
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scanning (TLS) survey and diagnostic investigations (i.e. minero-petrographic and geotechnical tests), to assess the rockfall hazard, also in relation to rock/stone susceptibility to erosion and fracturing. TLS is currently widely exploited to retrieve high detailed 3D models of huge scenes of view, thanks to its properties of wide spatial coverage and reduced time-consumption during data acquisition. Extraction of rock mass discontinuities from TLS point clouds/models allows overall stability analysis to be performed, also including inaccessible and impervious areas, thereby overcoming limits of conventional scanline methods [2, 3]. Nevertheless, information about the inner structure and composition of the rocks (e.g., texture, grain size, mineralogy) cannot be retrieved without the execution of diagnostic tests on collected samples, as well as detailed geological reconstruction based on borehole findings. Benefitting of the mutual complementarities between the selected remote sensing technique and ground investigations, we tested the applicability of this integrated approach on the instability mechanisms threatening the preservation of the fortifications of Citadel, in Gozo (Malta). This site-specific analysis is aimed to demonstrate how such combined information can lead to find a correlation between exterior deterioration patterns and inner factors of structural weakness, in the context of rockfall hazard assessment. 2. METHODOLOGY The main phases of the proposed methodology consist in: 1. processing of 3D TLS models covering the entire rock mass and the overlying built heritage, to calculate the “Kinematic Hazard Indexes” (KHI) for different instability mechanisms (e.g., wedge failure, block toppling, etc.) as defined by [4], retrieving the respective values of occurrence probability for each cell of the 3D surface; 2. characterization of the mineralogical and geotechnical properties of the geological strata, the outcropping surfaces of which show differential erosion patterns and morphological configuration which create conditions for rockfall triggering.
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To perform the 3D quantitative kinematic analysis, we propose the method developed by [5] and recently successfully applied to hilltop cultural heritage [6]. It enables surveyors to verify when a particular instability mechanism is kinematically probable, given certain geometry of the slope and rock mass discontinuities. The application of this method directly on TLS model allows a 3D hazard map to be produced, with immediate localization of the critical sectors over the rock mass. Precise positioning of the latter, associated with respective KHI, facilitates the estimation of the direct impacts on the overlying buildings, which are included in the holistic TLS model. As further element of novelty, this spatially referred hazard information is analyzed also in light of the textural properties of the rocks detectable at the meso/microscopic scale, which can explain the morphology and entity of the erosion patterns visible on the cliff surfaces. Thin section observations under polarized light microscope (PLM) are coupled with geotechnical tests (e.g., borehole examination, tilt test, etc.) and measurement of mechanical properties, such as the abrasion test according to the procedure by [7]. This methodology is particularly suitable for stability analysis of hilltop sites where the geological sequence is of sedimentary origin, and weathering processes are nonnegligible, as in the case of the heritage of Citadel.
Fig. 1. Hazard for the ancient walls of Citadel along the northern side of the rock mass is due to severe fracturing of the upper cap of the UCL outcrop, undermined by deep erosion niches. Fallen blocks testify the last rockfall event occurred in December 2001.
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3. CITADEL FORTIFICATIONS The bastion walls of Citadel, built with the Globigerina Limestone, are located along the edge of a relatively stiff and brittle limestone plate (Upper Coralline Limestone Formation – UCL) lying on a level of Greensand Formation (GS) and thick Blue Clay Formation (BC), which dominates the landscape of central Gozo Island. 3.1. Key areas of concern for the conservation As being fossiliferous coarse grained limestone, rich in shallow marine sediments consisting of calcareous red algae, corals and echinoderms [8], the Gozitan UCL is subjected to differential erosion processes whenever exposed to weathering, due to coactions of constantly blowing strong winds and percolating rainwater [9]. At Citadel, the upper cap is extensively fractured, with extended sub-vertical joints isolating huge rock blocks, the stability of which is further compromised by the progressive erosion of the underlying niches, located at the middle heights of the cliff [10]. Such sequence of ledges and niches contribute to trigger rockfalls, such as the last one that detached from the north-western part of the rock mass (Fig. 1). 3.2. 3D kinematic analysis As the orientation of the fractures appears to be likely related to local tectonic history, a kinematic analysis was performed, based on local slope orientation data derived from TLS survey of the rock mass (Fig. 2).
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Fig. 2. 3D maps of slope steepness (a) and aspect (b) for the Citadel rock mass, as input parameters for the kinematic analysis.
For this purpose we exploited a holistic 3D model processed by Consorzio Ferrara Ricerche of the University of Ferrara, made available by the Restoration Unit, Works Division, Maltese Ministry for Resources and Rural Affairs (see [11] for all technical details of the used Leica HDS ScanStation 2, acquisition plan and scanning accuracy).
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The input parameters of kinematic analysis were the slope dip (Fig. 2a) and dip direction (Fig. 2b), together with the discontinuity surface orientations. Classification of the rock mass identifying the overhanging sectors was performed as well, since the method by [5] employs the principles of kinematic analysis to overhanging slopes. The discontinuity shear strength was considered purely frictional, setting a mean peak friction angle of 48°, based on the geomechanical data from scanlines measured along the cliff surfaces as statistically representative according to [12]. Among the main instability mechanisms analyzed [6], the wedge failure [13] was found as the most hazardous one. The KHI reaches values up to 30%, with a spatial distribution (Fig. 4a) which finds strong agreement with the crack pattern survey and inventory of isolated blocks highly prone to detach carried out during the field inspections. Secondarily, the rock mass appears susceptible to flexural toppling, with KHI up to 17%. 3.3. Diagnostic investigations Taking benefit of the extended plan of ground investigations executed at Citadel, we undertook a geotechnical study of the cores recovered from vertical boreholes running through the entire stratigraphic column of the rock mass. Several levels of the UCL strata located at different heights (e.g., at the depth of 4-5, 8, 15-16, 18, 20-21 and 30-31 m from the top of the borehole BH B) were found from moderately to highly laminated, i.e. with a variable degree of shearing that produces “laminae”, which are frequently clearly visible within the cores (Fig. 3a-b). The latter perfectly reproduce the thinly layered texture characterizing the erosion patterns detected along the exposed rock surfaces at the correspondent heights (cf. Fig. 1 and 4b). Further insight was retrieved from the PLM observations on thin section (Fig. 3c). Abundant fossil remnants of Rodophiceae and corals, which originated the bioconstructed limestone, determine a preferentially oriented structure, with several planes of structural weakness along which the erosion processes develop. Such differences in sedimentation and related textural features are coupled with a different degree of cementation. For the same borehole, abrasion tests following the procedure by [7] interestingly highlighted lower values of resistance to abrasion for deeper rocks, with weight losses measured after the tests higher from 25% to 60% than those of the more superficial levels. It would suggest that more in depth the erosion proceeds and potentially weaker stones are progressively exposed, thereby favoring further erosion. 4. IMPLICATIONS FOR CONSERVATION Spatial correlation between the KHI distribution and erosion patterns facilitated the zoning of the critical sectors, associated to a point-wise evaluation of the potential threats for the architectural heritage.
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500 μm Fig. 3. Evidences of laminar fracturing and shearing observed at different heights within the vertical boreholes through the UCL layers (red lines and squares in a-b) find further explanation in the laminated texture visible on thin section (c; PLM, Nicols +, magn. 25x).
Due to the extent of the rockfall-prone areas and the need of harmonizing the mitigation measures with the landscape issues and sustainability of the restoration program, the installation of on site monitoring system was selected as the best conservation strategy, awaiting the design and execution of targeted interventions. Hence, the hazard classification of the rock mass constituted the diagnostic mapping product to identify the sites for the installation of biaxial inclinometers and crack gauges, respectively on the masonry structures of the walls susceptible to tilting and over the joints isolating the UCL blocks. Fig. 4 exemplifies the benefits of this procedure for a sector of the northwestern side of the rock mass, located few meters away from the area damaged by the 2001 rockfall. In light of the direct support provided to the conservators of Citadel in the decision-making process, further advances of this research are expected to be the implementation in other cultural heritage contexts and improvement, especially for the use of TLS acquisitions repeated over time as feasible and reliable monitoring tool.
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Fig. 4. a) 3D map from the kinematic analysis, with spatial distribution of the KHI related to the instability mechanism of wedge failure (WF) over the north-western side of the Citadel rock mass (legend is scaled according to the maximum measured values, 30%). Due to the high level of criticality, the entire sector (blue square in a) is included in the monitoring network (b), installing a biaxial inclinometer (c) on the stone-built masonry (green dot in b) and crack gauges (d) straddling the joint underneath the rock ledge (red dot in b).
6. ACKNOWLEDGMENTS This study was carried out in the framework of the service tender for the provision of geotechnical engineering consultancy CTD03, partly-financed by the European Regional Development Fund (ERDF) for Malta (20072013). The consultancy is being led by the private company Politecnica Ingegneria e Architettura for the Restoration Unit, Works Division, Floriana (Malta) under the Ministry for Resources and Rural Affairs. The authors thank E. Cantisani, J. Cassar, M. Fazzuoli, F. Fratini and E. Pandeli for their precious contribution during the research. 11. REFERENCES
[6] R. Fanti, G. Gigli, L. Lombardi, D. Tapete, P. Canuti, “Terrestrial laser scanning for rockfall stability analysis in the cultural heritage site of Pitigliano (Italy)”, Landslides, 2012, DOI: 10.1007/s10346-012-0329-5. [7] P. Tiano, C. Manganelli Del Fà, U. Matteoli, E. Pecchioni and F. Piacenti, “Nuove tecniche di valutazione del grado di coesione di superfici lapidee”, In: Atti del convegno di studi “Manutenzione e conservazione del costruito fra tradizione ed innovazione”, Bressanone 24-27 giugno 1986, Padova, Libreria Progetto ed., pp. 527-541, 1986. [8] H.M. Pedley, C.M. Hughes and P. Galea, Limestone isles in a crystal sea. The geology of the Maltese islands, Publisher Enterprises Group Ltd, 2002. [9] S. Scerri, IC-Citadel, Victoria, Gozo. Structural stability of the cliff margin, Geological report, 2003.
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