# Geotechnical Study of Raspadalica Cliff Rockfall, Croatia
## 4 Results
### Stability Analysis
Analyzing planar sliding and direct toppling stability of the block in the Raspadalica Cliff, the geometrics and acting forces of these two mechanisms included in the analyses are schematically presented in Figure 11 (according to [90]). General geometric conditions in order for planar failure to occur in the identified blocks at the Raspadalica Cliff face are satisfied [90], except that the dip of sliding plane (in this case bedding plane) must be greater than the angle of friction of this plane:  
\[\Psi_{p}>\Phi \tag{2}\]  
where \(\Psi_{p}\) is dip of sliding plane and \(\Phi\) is the angle of friction of bedding plane. According to the remote sensing survey and analyses (Table 2) and traditional geological and geotechnical surveys (Table 3), the maximal dips of the bedding plane in all cliff zones are lower than 25 degrees. If a conservative value of basic angle of friction, \(\Phi_{b}\), is 30 degrees for limestone, that is mostly significantly lower than a real angle of friction at bedding planes, \(\Phi\), which was adopted as the relevant value; therefore, it is clear that the sliding failure along the bedding plane cannot occur. Additionally, there are several other parameters, except of basic angle of friction, that influence the strength of discontinuities: undulation or roughness expressed with the joint roughness coefficient, JRC, and joint wall compressive strength, JCS [100]. There are several other parameters that are usually not directly included in determination of the strength of discontinuities, such as separation, infilling, weathering grade of joint walls, and persistence. These parameters have an impact on the joint roughness coefficient and joint wall compressive strength. Smaller separation, together with undulation and infilling, directly influenced the strength of discontinuities: smaller separation impact on rise of the strength as well as hard filling or no filling, while the weak filling (clayey material) can significantly decrease the strength. Weathering grade of joint walls is related to joint wall compressive strength, where lower degree of weathering influences higher joint wall compressive strength. Longer persistence has no direct influence on the strength of discontinuities but has an unfavorable impact on instability mechanisms for larger volumes of rock mass. In case of the Raspadalica Cliff and the determined characteristics of rock mass summarized in Table 4, it can be concluded that the separation of bedding and joint sets is relatively large in all cliff zones and, despite the absence of infilling and presence of hard  
Figure 11: Geometrics and forces of (**a**) planar sliding failure; (**b**) direct toppling failure (Reprinted with permission [90] 2004, Spoon Press). filling, has a negative influence on the strength of discontinuities. Measured values of the joint roughness coefficient for bedding and joint sets in all cliff zones point to smooth (JRC = 4-8) to undulating (JRC = 8-10) bedding and joint sets that just slightly influence the rise of the strength of discontinuities. At present, weathering has a very low degree of significant impact on bedding and joint set walls that is mostly slightly (SW) to moderate weathered (MW), implicating higher values of joint wall compressive strength and consequently higher values of the strength of discontinuities. In general, the conditions of bedding and joint sets in the Raspadalica Cliff can be accepted as positive in terms of bedding and joint friction increasing, and it is reasonable that for considerations of general stability, higher values of basic angle of friction for limestone (\(\Phi_{b}\), = 31-37 degrees [100]) can be used. In particular, analyses of all three considered mechanisms (planar sliding, direct toppling, and flexural toppling) are necessary to determine the strength of discontinuities, taking into account all individual influences and the particularly crucial influence of normal stresses, as well as the possible impact of ground water.