# Geotechnical Study of Raspadalica Cliff Rockfall, Croatia
## 3 Materials and Methods
### In Situ and Remote Sensing Surveys  
The traditional geological and geotechnical field survey was carried out to understand the main lithology, geological formations, discontinuity sets, faults, and geotechnical geological model of the Raspadalica Cliff. The geological and geotechnical surveys were carried out at a scale of 1:500 developed for the wider zone of the cliff area. The field investigations were combined with the study at the orthophoto (at 1:5000 scale) and 3D HRPC to better understand the geological and geomorphological features at the study area. Of particular importance was the delineation of the contact between the limestone cliff structure and the flysch rock mass in the foot. A traditional geotechnical survey was conducted to determine the characteristics of the main discontinuity sets at the cliff, as well as to carry out a rock mass classification using geomechanical classification (rock mass rating, RMR) [34] and the geological strength index (GSI) [35]. Although the values of both classifications cannot play an important role in determining the rockfall process, the classifications' parameters would give a good insight into the geotechnical cliff model. As highlighted earlier, the geological and geotechnical field survey was limited to a narrow accessible zone of the rock cliff at the foot of the cliff. Field measurements of the discontinuity features and rock mass classifications were carried out at 34 accessible points at the foot of the cliff. The precision of the measurements taken was achieved within certain limits defined by the ISRM's Suggested methods for the quantitative description of discontinuities in rock masses [36]. After the establishment of the 3D HRPC model described previously, the model was analyzed to identify the main characteristics of the rock mass structure, such as the detection and mapping of the discontinuities and discontinuity sets, orientation and dip of discontinuities, spacing of discontinuities, persistence of discontinuities, and roughness of discontinues, as well as determination of rock block volumes. Due to the increasing use of UAV photogrammetry and the _S/M_ technique to create 3D HRPC models, various automatic and semi-automatic techniques and methods have been developed to detect and map the discontinuities and discontinuity sets [20; 22; 23; 24; 25; 26; 28; 31; 37; 38; 39; 40; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59], orientation and dip direction of discontinuities [20; 22; 23; 26; 28; 29; 31; 37; 38; 40; 42; 43; 44; 45; 48; 49; 54; 55; 56; 57; 59], spacing of discontinuities [20; 23; 27; 24; 46; 49; 53; 63; 65; 70; 72; 75; 76; 77; 78; 79; 80; 81; 82; 83], persistence of discontinuities [21; 22; 31; 44; 47; 48; 62; 73; 76; 77; 80; 83; 84; 85], and roughness of discontinuities [22; 26; 37; 71; 86; 87]. In this study, a combination of traditional geological and geotechnical field surveys [86; 88] and remote sensing techniques were employed to detect and mapping of the discontinuities and discontinuity sets, orientation and dip of discontinuities, and other discontinuity features necessary for the analyses of rockfall occurrences and their consequences. Based on the identified discontinuities and discontinuity sets and their orientations and spacing, the representative block volumes were determined using the mean discontinuity spacing values for each of the discontinuity sets according to Equation (1) proposed by [PERSON] [83]:  
\[\mathrm{V_{b}=S_{1}\times S_{2}\times S_{3}\times(sin\gamma_{1}\times sin\gamma _{2}\times sin\gamma_{3})}, \tag{1}\]  
where \(\mathrm{S_{i}}\) is the normal spacing and \(\gamma_{1}\) is the angle between the discontinuity sets. While the shape of the blocks is mainly perpendicular, Equation (1) gives good data on the volumes of the blocks present in the cliff.