## 5 Instrumental Monitoring
### Monitoring of Groundwater Level  
There are fluctuations in groundwater levels depending on seasonal climatic conditions. While the groundwater level rises in rainy periods, it decreases in hot and dry periods due to less recharge. Fluctuations in the groundwater level can affect the positive pore water pressure distribution and the negative suction profile in the vadose region, thus affecting the stability. While investigating the hydro-mechanical behavior of landslides, it is of great importance to examine the changes in the groundwater level.  
Figure 13 compares the groundwater levels in BH3 and BH1 with hourly rainfall intensity on various dates. Groundwater levels were monitored in BH3 between February 2021 and September 2021. Due to the collapse of the BH1, groundwater levels could only be measured in BH1 between February 2021 and June 2021. As shown in Figure 13, the groundwater level in BH3 was located at a 2.21-3.49 m depth below the ground surface. In BH1, it varied between 6.01 and 8.72 m. The reason why the groundwater level was measured deeper in BH1 can be due to BH1 being located at the foot of the landslide. It is possible that the groundwater level was measured at a deeper level due to the accumulation of soil at the foot. The highest groundwater levels in the BH1 and BH3 were 6.01 and 2.21 m, respectively, on April 2021. The deepest groundwater level was measured as 8.72 m in BH1 on May 2021 and 3.49 m in BH3 on August 2021. As shown in Figure 13, the groundwater levels tended to increase with the low-intensity, long-duration rainfalls observed in February, March and April. More fluctuation in the groundwater level was monitored in BH1. During these months, while the groundwater levels were rising, the water content of the soils in the upper parts was high, and the matric suction was low (see Figures 8-12). The groundwater level started to decrease after April. Decreased groundwater level had almost no effect on soil moisture and matric suction at a 1.63 m depth (see Figures 11-13). At this depth, the soil is in the capillary zone. After the high-intensity short-duration rainfalls in July and August (19.0 mm/h, 50.6 mm/h), no remarkable increase was observed in the groundwater levels, unlike those observed in the low-intensity long-duration rainfalls.  
Figure 12: Variation in matric suction at 1.63 m depth.  
Figure 13: Variations in groundwater level with rainfall intensity in BH1 and BH3.  
According to the measurements made in the field, it can be remarked that the first five months of the year are wet, and the others are dry for the region. The periodic variation in matric suction and groundwater levels can be considered according to the values given in Table 5. These values were proposed to obtain a general framework for understanding a representative landslide area's hydro-mechanical behavior and were used in the stability analyses. In Table 5, groundwater levels were taken according to measurements in BH3. The reason to consider BH3 for groundwater level is that (i) BH3 is right next to the other monitoring boreholes, and (ii) it is the least affected by landslide activity due to its location. Matric suctions were taken as 0 kPa for the wet period because in-situ volumetric water contents at those depths changed in saturated water content ranges according to lab tests, and matric suction sensors cannot meaningfully measure soil matric suction below 9 kPa, due to their capacities. Matric suctions for the dry period were based on the average measurements recorded during the dry period in the field.