# A Case Study of a Large Unstable Mass Stabilization:
## 3 Results  
### Field Investigations and Satellite Surveys  
In situ (SPT) and laboratory tests conducted on the different investigations carried out identified six geotechnical units in the area under study. Table 1 lists the average parameters of those units related to SUCS classification, Afterberg limits, unit weight, water content and mechanical parameters (cohesion and friction angle for soil materials and uniaxial compression strength for rocks). Rock mass of the area consisted of variably calcareous slates with calcite filled veins (Geotechnical unit \"Slate rock\") belonging to the \"Facies Culm\" from the Carboniferous period. This rock mass presented an average RQD of 60% and the RMR reached an average value of 60. According to the geological-geotechnical investigations conducted, rocky formation crops out only in the northern area, where no instabilities were detected. The rest of the slope surface is covered by quaternary deposits (Figure 4d), which include colluvial deposits, green clayey sands, black clayey sands and fragmented calcareous rocks and slates. Additionally, fault breccia materials were found in an old fault detected, though currently inactive (yet some water was seen circulating through it). Except for colluvial deposits, quaternary materials come from a roto-translational, with unidirectional movement, landslide that occurred in the area in the past. Materials appeared unstructured and remo \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Geotechnical Unit** & **Depth [m]** & **SUCS** & **W\({}_{\text{P}}\)** & **PI** & \(\boldsymbol{\gamma}\) [kN/m\({}^{3}\)] & **W [\%]** & **c [kPa]** & \(\boldsymbol{\varphi}\) [\({}^{\circ}\)] & **UCS [MPa]** \\ \hline Colluvial deposits & 2–10 & GM-GC & 30.5 & 8.4 & 21.0 & 7.0 & – & 29.0 & – \\ \hline Green sand-clay & 2–6 & SC & 32.1 & 10.4 & 20.4 & 14.0 & – & 18.0 & – \\ \hline Black sand-clay & 2–6 & SC-GC & 30.9 & 10.6 & 22.2 & 8.1 & – & 25.0 & – \\ \hline Fragmented calcareous rocks and slates & 2–6 & GC-SC & 33.6 & 12.9 & 21.7 & 6.7 & – & 38.0 & – \\ \hline Fault breccia & – & – & – & – & 23.0 & 6.0 & 20.0 & 25.0 & – \\ \hline Slate rock & – & – & – & – & 27.7 & – & – & – & 12.5 \\ \hline \end{tabular}  
* Notation: SUCS: Soil Unified Classification System ([PERSON]’s classification); \(W_{\text{P}}\): plastic limit; PI: plasticity index; \(\gamma\): unit weight; W: water content; c cohesion (residual); \(\boldsymbol{\varphi}\): friction angle (residual); UCS: uniaxial compression strength.  
\end{table} 
Table 1: Geotechnical parameters of the materials found in the area under study. Data from boreholes, inclinometers and surface movements led to defining a failure surface located at depths between 5 m and 19 m (see Figure 5), with a fairly planar shape and with greater curvature in the slope toe area. The failure surface daylights on the slope and is observed on the ground surface (at both the slope head and toe) as seen in Figure 6. Surface movements in the E-SE direction, topographically recorded during the previous geotechnical investigations [15], showed small movements on the head of the slope as well as on the excavated slope. Those movements ranged between 1 cm and 3 cm, with movements in the northern area being slightly inferior to southern areas. Inclinometers revealed the development of the failure surface mainly through the sandy clay materials in contact with the fragmented calcareous rock. An example of inclinometer data obtained in the previous study [15] and new geotechnical investigations  
Figure 5: Geotechnical profile I-I (see Figure 2) of the area under study; failure surface detected is shown in red. Figure 6: Daylight of the failure surface: (**a**) stress crack; (**b**) warped plane morphology. is shown in Figure 7 (note that those inclinometers belong to profile I-I\({}^{\prime}\) shown in Figure 5; failure surface may be easily identified).