# Geotechnical Study of Raspadalica Cliff Rockfall, Croatia
## 6 Conclusions  
This manuscript presents an analysis of the rock mass structure and the causes of rockfall occurrences at the Raspadalica Cliff, Istria, Croatia; a nearly vertical, 100 m-high limestone cliff with a railway line at its foot, known for numerous rockfall occurrences in the past. In situ traditional geological and geotechnical surveys, as well as remote sensing surveys, were used to establish a geotechnical model of the cliff and to determine the causes of numerous rockfall occurrences in the past that have threatened the railway line and caused significant damage and human injuries. The aim of this manuscript is to make scientists and practitioners aware that the investigation of rock mass cliffs and possible rockfall failures must not be based on usual methods without the critical review of the obtained results and consequences. The combined use of traditional geological and geotechnical methods and more used advanced remote sensing methods leads to better modelling, while the analysis of more associated failure modes can explain the triggers of a rockfall. The traditional in situ geological and geotechnical surveys were limited to the available positions at the foot of the cliff due to the steep cliff face. Data collected by remote sensing were limited because most of the discontinuity planes belonged to the joint set that are quasi-parallel to the cliff face (Joint Set 1), while the other two nearly perpendicular and rarely exposed joint sets (Joint Set 2 and bedding) were not clearly visible and made identification of the quasi-perpendicular planes impossible. The absence of these geotechnical elements made it difficult to construct a fully defined geotechnical model of the Raspadalica Cliff and to identify the causes that initiated rockfall in the study area. Although at first glance the geotechnical elements of the Raspadalica Cliff do not provide any clues to the causes of the frequent rockfall events in the study area, deeper analyses suggest reasonable causes for rockfall instabilities. The performed kinematic stability analyses using data obtained by in situ and remote sensing surveys indicated the joint influence of two instability mechanisms (sliding and toppling) in the rock blocks of the cliff face, probably supported by additional forces, were caused by rainfall infiltration and rise of horizontal forces. These two mechanisms are associated with other processes that have influenced the weakening of joint friction forces due to lateral stress relaxation, weathering, and the softening of joints infilling (coal interlayer) caused by freezing and thawing processes, rock mass weathering processes, and water and temperature effects. These processes will continue, so rockfall occurrences at the Raspadalica Cliff can be expected in the future. The next step will be to determine rockfall hazards and risks in order to select the appropriate protection from rockfalls for the railway line in the future. **Author Contributions:** Conceptualization, Z.A. ; methodology, Z.A. ; formal analysis, D.U. and Z.A. ; investigation, B.K., D.U. and Z.A. ; resources, D.U. ; writing--original draft preparation, D.U. and Z.A. ; writing--review and editing, Z.A. ; UAV survey and visualization, B.K. ; supervision, Z.A. ; project administration, Z.A. ; funding acquisition, Z.A. All authors have read and agreed to the published version of the manuscript. Part of this research was carried out in the frame of the UNIRi Project uniri-tehnic-18-276-1448 Research of Rockfall Processes and Rockfall Hazard Assessment, supported by University of Rijeka, Croatia, and the HRZZ IP-2019-04-9900 Methodology Development for Landslide Susceptibility Assessment for Land-use Planning Based on LiDAR Technology, funded by the Croatian Science Foundation. The received support is gratefully acknowledged. Not applicable. All data can be obtained by contacting the authors.