engleski

Rockfall Susceptibility Assessment at the Slope Scale

The aim of this paper is to present a method for rockfall susceptibility assessment at the slope scale, which is based on automated spatial kinematic analysis and calculation of the Kinematic Hazard Index. Rockfall susceptibility is defined as the likelihood of a rockfall occurring in an area based on local terrain conditions (BRABB, 1984). Investigation of rockfall susceptibility at the slope scale is based on the analysis of the relation between discontinuities and slope morphology (MATASCI et al., 2017), that can indicate if a certain instability mechanism is possible at a specific location on the rock slope. Method for rockfall susceptibility assessment at the slope scale presented in this paper is a follow-up of the researches presented in SEČANJ et al. (2017, 2018 and 2019). The method was tested at the locations of different size and complexity: rock cuts along the road in the Krka National Park, rock cuts above parking plateau in Matulji Municipality and slopes with complex morphology above the Town of Omiš. In order to gather input data for susceptibility assessment, remote sensing techniques were employed to rapidly obtain orientation of rock faces, including in the field inaccessible areas. Discontinuity data was obtained from the high-resolution point clouds and 3D models using different techniques and software. Possible rockfall sources can be identified by 3D spatial kinematic analysis that analyse kinematic possibility of specific instability mechanism defined by the geometry of the slope and discontinuities. The input parameters for the 3D spatial kinematic analysis are: 1) quasi-homogenous engineering geological zones (EGZ), defined by rock mass properties, representative discontinuity sets and the geometrical properties of the slope, 2) the orientation of the slope (dip angle and dip direction obtained from the digital surface model), 3) orientations of the discontinuities (obtained from the point cloud in the Split-FX and CloudCompare software), and 4) the average value of discontinuity friction angle. Spatial kinematic analysis was performed for each polygon of the digital surface model for all instability mechanisms (plane and wedge failure, flexural and block toppling). Kinematic Hazard Index (KHI) defined by CASAGLI & PINI (1993) is calculated for each model polygon where one or more modes of failures are possible. KHI is defined as relative ratio between number of discontinuities and number of intersections on which failure can occur in relation to the total number of discontinuities and intersections in one EGZ. Spatial kinematic analysis and the calculation of the KHI are automated with the in-house MATLAB scripts and functions using vector analysis for each polygon of high-resolution 3D digital model. Results are maximal values of Kinematic Hazard Index (MKHI) for each polygon. Interpolated values of MKHI represent rockfall susceptibility map or 3D model where the probability of instabilities is expressed by the colour scale, varying from green to red as the MKHI increases. Verification of the analysis was performed by comparison of areas with high susceptibility with realistic point cloud model depicting rock slope morphology and potentially unstable rock blocks. The method for rockfall susceptibility assessment, presented in the paper, has proven that it can be used for different types of rock slopes, ranging from single rock cuts to the area of complex morphology with multiple slopes of different orientation. Resulting rockfall susceptibility maps and 3D models are a useful tool in preliminary rock fall hazard assessment, because high susceptible areas indicate possible rockfall sources that require detailed engineering geological and geotechnical investigations. The applied method enables more objective selection of the slope parts for following up slope stability analysis and rockfall simulations (ARBANAS et al., 2019). Besides advantages related to fast assessment of rock slope instability, there are also some limitations in the presented approach. The analysis is based on heuristically determined average value of discontinuity friction angle which may led to overestimation or underestimation of the KHI values in certain parts of the slope. Despite this, rockfall susceptibility assessment provides useful input data for rockfall hazard assessment and deterministic rock slope stability analyses. Considering geological setting and extent of Dinarides in Croatia (MIHALIĆ ARBANAS et al., 2017) there is potentially large area of possible application of the presented method for rockfall susceptibility assessment, as a tool for risk reduction together with engineering measures of rock slope protection.

rockfall susceptibility ; spatial kinematic analysis ; Kinematic Hazard Index ; slope scale

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