# A Case Study of a Large Unstable Mass Stabilization:
## 3 Results
### Drainage Evolution  
Water table lowering plays an essential role in the stability of the landslide studied. The approximate volume of the adjacent aquifer to the area under study was estimated at 0.86 \(\mathrm{Hm}^{3}\). However, based on the available data, the aquifer lateral supply was unknown, so this volume could be much higher. Although the effective porosity of the different materials traversed by the California drains was also unknown, adopting an estimated value of 5-7%, the volume of water to drain is probably between 100,000 \(\mathrm{m}^{3}\) and 200,000 \(\mathrm{m}^{3}\). Besides, the groundwater recharge of the aquifer should be considered and this does not stop, being especially significant due to the high rainfalls in the area. Recharge can be estimated at around 1900 \(\mathrm{mm}\) per year, widely distributed throughout the year except for the summer season.  
As a consequence, the water table lowering does not only depend on the drainage capacity and the efficiency of the Californian drain but also on the weather and the annual recharge (Figure 11). As a result, the drainage of the slope needs time. During that time, the slope is expected to have a safety factor lower than 1.0, which explains the movement of the slope recorded by the different investigations (in fact, the safety factor increased from the situation depicted in Figure 9a to the one given in Figure 9b). Thus, although the future solution in the long term will be stable, attaining this situation will need to wait for some time during which the slope will exhibit unstable behaviour.  
Figure 10: Stability analysis considering the projected wall alone.  
Although the tendency of the water table is the one explained, this evolution does not take place through a uniform decay. Figure 12 shows the evolution of the lateral displacements recorded by the inclinometers located at a depth of 5 m in boreholes B5 and B9 (Figure 7). Both boreholes were located close to one another and correspond to the previous (year 2005) and new (year 2016) geotechnical investigations, respectively.  
Figure 12 shows that, in 2005, the deformations were increasing and the slope was moving faster (A). Between 2005 and 2016 the deformations were greatly reduced, so the landslide was stopping (B). Finally, according to 2016 records, the slope continued to accelerate; from April to September the movement evolved from 5 to 20 mm, although from  
Figure 11: Expected evolution of the water table and its qualitative relationship with the safety factor.  
Figure 12: Evolution of the movements at a depth of 5 m. Long-term trend. The movements are represented in blue for borehole B5 (installed in year 2005) and brown for borehole B9 (installed in year 2016), Date given in Day/Month/Year. Letter A refers to the fast increment in deformations in 2005; B refers to the reduction of deformations between 2005 and 2016; C refers to the acceleration of deformations registered in 2016.  
September to November it seemed to stabilize (C). This acceleration in a given year was also observed in 2005 for the same period, April to September. That period corresponds to the spring season when precipitations normally occur in the area under study together with the snowmelt phenomenon [49]. Thus, these results confirm the influence of the water table in the landslide stability, the long-term tendency observed in the evolution of the slope movement and the long time needed to attain drainage of the ground mass (Supplementary Materials).