# Geotechnical Study of Raspadalica Cliff Rockfall, Croatia
## 1 Introduction  
Rockfalls, by definition, are phenomena that include the detachment, falling, rolling, and bouncing of rock fragments from a slope, singly or in clusters, and the fragments may be defragmented during impact [1]. Occurrences can range from small fragments to massive blocks of different volumes and shapes falling, rolling, and bouncing down a slope [2, 3]. Rockfall is one of the most frequent and dangerous types of landslides, which can cause numerous fatalities and high economic and social damage [4]. The high speed, mobility, and energy of falling rocks prevent fast responses by evacuation or protection [2, 3, 5, 6]. The Raspadalica Cliff is an almost vertical 100 m high limestone slope (Figure 1) located at the contact of two geomorphological units, namely, the hilly Paleogene Flybsch Basin and the elevated Cicarja Mountain Range in the northern part of Istrain Peninsula, Croatia [7, 8, 9]. The wider Raspadalica location is known for numerous instabilities occurring as rockfalls from the Raspadalica Cliff, as well as landslides [10], that have previously caused significant damage to the railway line located on the slope made of flysch succession at the foot of the cliff. In this paper we focus on rockfall occurrences from the Raspadalica Cliff, while the slides on the flysch slopes below the cliff, which also caused damage to railway facilities, are not mentioned in this manuscript. Although sliding of the flysch slopes have caused multiple interruptions of railway traffic as well as multiple slope stabilization measures,the rockfall occurrences in the past were many times more dangerous and demanding for remediation. The documented history of rockfalls from the Raspadalica Cliff dates back to 1951, when the railway technical office was faced with multiple traffic interruptions on the railway caused by several landslides and rockfalls that threatened the stability of the railway embankment and traffic safety. From 1951 to the present, numerous instabilities events have occurred followed by a series of field investigations, remediation designs and implementation of remedial measures, but the threat from rockfalls still exists. In recent years traditional (conventional) geological and geotechnical methods as well as more commonly used advanced remote sensing methods have been used to describe geotechnical rock cliff models and to derive the geotechnical elements used in stability analysis and potential failures of rock mass defined by the positions and orientations of the rock joint systems. The Raspadalica Cliff is an example of a cliff characterized by its geotechnical features that renders use of only traditional geological and geotechnical methods (unavailability of individual parts of the cliff) or remote sensing methods (lower exposure of individual joint sets) impossible and affects the necessary combination of both methods. Additionally, kinematic analyses did not indicate a high probability of rockfall occurrence, although rockfall phenomena from the cliff face were relatively frequent in the past. 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 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 triggering of rockfall. In order to determine the detailed geological structure of the cliff and the causes of rockfall occurrences at the Raspadalica Cliff, several campaigns of field investigations have been carried out over the last decade. The first, conducted for remedial measures and rockfall protection design, used the traditional methods of engineering geological surveying and mapping [11] and was limited to the rock mass at the foot of the cliff due to its height [12].