The Contribution of Terrestrial Laser Scanning to the Analysis of Cliff Slope Stability in Sugano (Central Italy)
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remote sensing Article The Contribution of Terrestrial Laser Scanning to the Analysis of Cliff Slope Stability in Sugano (Central Italy) Paolo Mazzanti 1,2 , Luca Schilirò 1,* , Salvatore Martino 1 , Benedetta Antonielli 1, Elisa Brizi 2, Alessandro Brunetti 2, Claudio Margottini 3 and Gabriele Scarascia Mugnozza 1 1 Department of Earth Sciences, Sapienza University of Rome, Piazzale Aldo Moro, 5–00185 Rome, Italy; [email protected] (P.M.); [email protected] (S.M.); [email protected] (B.A.); [email protected] (G.S.M.) 2 NHAZCA S.r.l., Via Vittorio Bachelet, 12–00185 Rome, Italy; [email protected] (E.B.); [email protected] (A.B.) 3 ISPRA—Istituto Superiore per la Protezione e la Ricerca Ambientale, Via Vitaliano Brancati, 48–00144 Rome, Italy; [email protected] * Correspondence: [email protected] Received: 13 August 2018; Accepted: 14 September 2018; Published: 15 September 2018 Abstract: In this work, we describe a comprehensive approach aimed at assessing the slope stability conditions of a tuff cliff located below the village of Sugano (Central Italy) starting from remote geomechanical analysis on high-resolution 3D point clouds collected by terrestrial laser scanner (TLS) surveys. Firstly, the identification of the main joint systems has been made through both manual and automatic analyses on the 3D slope model resulting from the surveys. Afterwards, the identified joint sets were considered to evaluate the slope stability conditions by attributing safety factor (SF) values to the typical rock blocks whose kinematic was proved as compatible with tests for toppling under two independent triggering conditions: hydrostatic water pressure within the joints and seismic action. The results from the remote investigation of the cliff slope provide geometrical information of the blocks more susceptible to instability and pointed out that limit equilibrium condition can be achieved for potential triggering scenarios in the whole outcropping slope. Keywords: cliff slopes; geomechanical analysis; terrestrial laser scanning (TLS); rock toppling; slope stability analysis 1. Introduction Rock toppling is a type of slope movement affecting natural and artificial slopes characterized by the detachment of blocks or rock fragments, ranging in size from few dm3 to several hundred m3 [1,2]. The detachment of blocks from a rocky slope is mainly influenced by: (i) the structural conditions of rock mass, that are related with the joint set features (such as orientation, persistency, or roughness, according to the definitions provided by ISRM, 1978 [3]); (ii) the mechanical properties of both the rock mass and the joints; and (iii) external actions like pressured water within the discontinuities, the seismic shaking due to earthquakes, roots growth, and so on. Although the volume of the involved material can be limited, these events are characterized by a high temporal frequency of occurrence and, therefore, they can be highly hazardous for mankind and human activities [4]. Furthermore, the high speed and the long run out distances that blocks can attain typically result in extremely hazardous events even with small mobilized volumes. However, the morphological evolution of a rock cliff may represent a serious threat not only for the areas located downslope, but also for hilltop towns that, especially in Central Italy, can be characterized by a high historical-artistic value. Remote Sens. 2018, 10, 1475; doi:10.3390/rs10091475 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1475 2 of 17 InRemote this sense, Sens. 2018 well-known, 10, x FOR PEER examples REVIEW can be Civita di Bagnoregio and Calcata in Northern Latium2 of 17 [5 ,6] or the towns of Orvieto and Todi in Umbria [7,8]. In such cases, as in many others, rock toppling may also for hilltop towns that, especially in Central Italy, can be characterized by a high historical-artistic originate from remote cliff slopes that, therefore, could be difficult to reach. Thus, using conventional value. In this sense, well-known examples can be Civita di Bagnoregio and Calcata in Northern investigation techniques that require the direct contact with the outcrop can be challenging. For this Latium [5,6] or the towns of Orvieto and Todi in Umbria [7,8]. In such cases, as in many others, rock reason, techniques that can provide detailed point-based information through widespread remote toppling may originate from remote cliff slopes that, therefore, could be difficult to reach. Thus, using sensing solutions, such as terrestrial laser scanning (TLS) and digital photogrammetry, are getting an conventional investigation techniques that require the direct contact with the outcrop can be increasingchallenging. importance For this reason, and interest techniques in the that scientific can provide community detailed [9 ].point In particular,-based information such methods through have beenwidespread successfully remote employed sensing in thesolutions, framework such of slopeas terrestrial stability analyseslaser scanning of high steep(TLS)slopes and [digital10,11] or cavephotogrammetry sections [12],, also are getting through an the incre deploymentasing importance of new and technologies interest in likethe scientific unmanned community aerial vehicles [9]. (UAV)In particular, [13,14]. such methods have been successfully employed in the framework of slope stability analysesThis paper of high describes steep slopes a combined [10,11] methodology or cave sections that exploits[12], also the through potential the of deployment TLS in the framework of new oft aechnologies slope stability like analysisunmanned of a aerial rocky vehicles cliff. Specifically, (UAV) [13,14] we present. the results of a detailed investigation carriedThis out onpaper a cliff describes frequently a combined affected bymethodology instability processes that exploits located the belowpotential the villageof TLS ofin Sugano the (closeframework to Orvieto, of a slope Central stability Italy). analysis Geomechanical of a rocky cliff. characterization Specifically, we of thepresent cliff the was results carried of a out detailed starting frominvestigation 3D high-resolution carried out point on a clouds cliff frequently collected byaffected TLS surveysby instability [15,16 ]processes with the located aim of assessingbelow the the overallvillage predisposition of Sugano (close to failure to Orvieto, of the Central cliff by accountingItaly). Geomechanical for different characterization triggering scenarios. of the Specifically,cliff was thecarried adopted out procedure starting from can 3D be high summarized-resolution in p fouroint levelsclouds of collected activity by (Figure TLS surveys1): [15,16] with the aim of assessing the overall predisposition to failure of the cliff by accounting for different triggering 1. scenarios.data acquisition, Specifically, including the adopted direct procedure geomechanical can be summarized scan lines onin four site andlevels TLS of activity surveys; (Figure 1): 2. data processing consisting in point cloud filtering and triangulation, stereograph projection and 1. data acquisition, including direct geomechanical scan lines on site and TLS surveys; geomechanical setting restitution; 2. data processing consisting in point cloud filtering and triangulation, stereograph projection and 3. datageomechanical analysis, including setting restitution; a kinematic compatibility analysis performed using the Markland 3. testsdata [ 17analysis,] for rock including toppling a ki andnematic a sensitivity compatibility analysis analysis for performed rock toppling using in the relation Markland to different tests potential[17] for rock triggering toppling factors; and a sensitivity and analysis for rock toppling in relation to different potential 4. deliverytriggering of finalfactors products; and consisting in hazard stability charts. 4. delivery of final products consisting in hazard stability charts. Figure 1. 1. FlowFlow chart chart of of the the applied applied me methodology,thodology, which which is based is based on data on datafrom fromdirect direct and Terrestrial and Terrestrial Laser Laser Scanning (TLS) surveys. Scanning (TLS) surveys. Remote Sens. 2018, 10, 1475 3 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 3 of 17 2. Geological and Geomorphological Setting Sugano is is a asmall small hamlet hamlet of ofOrvieto Orvieto Municipality Municipality in the in province the province of Terni of Terni(Umb (Umbria,ria, Central Central Italy), aboutItaly), 100 about km 100 north km of north Rome. of The Rome. village The is villagelocated is in located the geologic in the domain geologic of domain the Alfina of theplateau, Alfina a plateau,broad quaternary a broad quaternary volcanic plateau volcanic outcropping plateau outcropping in the northe in thern northern part of partthe ofLazio the LazioRegion Region [18]. [The18]. Theoutcropping outcropping rocks rocks are arenamely namely Pliocene Pliocene sedimentary sedimentary rocks, rocks, volcanic volcanic deposits deposits and and recent recent alluvial sediments (Figure 2 2).). Figure 2. Geological sketch of the study area. Elevation (in m a.s.l.) is also reported for specific points. Figure 2. Geological sketch of the study area. Elevation (in m a.s.l.) is also reported for specific points. The geological setting of the Sugano hill resulted from neotectonic and volcanic events occurred The geological setting of the Sugano hill resulted from neotectonic and volcanic