decreased especially in the top layer in the upper part of the slope, where bedrock consists of sandstone (Figure 5.13.2). This indicates increasing saturation in the weathered bedrock and the unconsolidated sediment. It is possible that preferential lateral ow occurred in this layer.
Also in the bedrock the saturation increased locally in more permeable sandstone layers (Figure 5.13.3). It is possible that those layers were fractured and less cemented and that they drained the unconsolidated sediment.
5.5 Second experiment, March 2009
In this section, the data from the second experiment are described, interpreted and compared to the data from the rst experiment.
5.5.1 Sprinkling intensity
Total rain input for the second experiment was 150 mm in 15 hours.The mean rain intensity during the second experiment was 15, 12 and 8 mm/h in the upper, middle and lower cluster, respectively. Above the test site, where the upper six sprinklers were rearranged close to each other (see Figure 5.5), an intensity between 20 and 40 mm/h was estimated. The experiment started with a sprinkling duration of 2.5 h followed by a 40 min-sprinkling break. After 12 more hours of sprinkling, the slope failed. Overland ow was observed only in the upper part of the slope which rein ltrated in the lower part. Therefore no overland ow could be measured in the trench located at the toe of the slope.
At the three clusters, the rain intensity never reached the maximum of 45 mm/h from the rst experiment, but the intensity in the upper part was considerably higher compared to the rst experiment due to the rearrangement of the sprinklers.
5.5.2 Groundwater
Figure 5.14 shows the measured soil saturation, suction and groundwater table in the three clus-ters. In general, the sensors react similarly to the rst experiment. This means that the shallower sensors react rst but the deeper sensors react more pronounced. A dierence concerning the
rst experiment is the time lag which is shorter for all sensors in the upper cluster (see Fig-ure 5.12). That means that the wetting front arrived faster in the second experiment. On the contrary, the lowest cluster responds later than in October 2008 due to the smaller sprinkling intensity in the lower part of the slope. The time lag for the sensors at 60 - 150 cm depth in the lower cluster is about 6 h whereas in the rst experiment, the time lag was clearly higher for the deeper sensors. This could be because during the second experiment, the wetting front arrived at the lower cluster by lateral subsurface storm ow. Lateral ow occurred probably along the weathered sandstone.
The higher initial saturation of the soil in March 2009 can also be seen in the ERT data (Figure 5.13). Thus in March 2009, less water could be stored in the soil. In the upper part of the slope, where the sprinkling intensity was smaller during the rst experiment, 240 mm water could be stored. This is more than in the lower part of the slope, where only 150 mm was stored. It is likely that the same amount of water like in the rst experiment (400 mm) could be stored above the uppermost cluster (then in this area, the sprinklers were rearranged more densely for the second experiment). Thus an average value for the amount of stored water during the second experiment is assumed to be 300 mm.
5.5. SECOND EXPERIMENT, MARCH 2009
Figure 5.13: ERT pro les recorded before and during the rst experiment in October 2008 and the second experiment in March 2009 (Gambazzi and Suski, 2009).
5.5. SECOND EXPERIMENT, MARCH 2009
Figure 5.14: Soil water content, soil tension and groundwater level measured at the three clusters and rain intensity during the second experiment in March 2009. Data from Amin Askarinejad, Institute for Geotechnical Engineering, ETHZ.
5.5. SECOND EXPERIMENT, MARCH 2009
Piezometers 4 and 5 in the upper cluster showed a groundwater table of 0.6 and 1.6 m after 6 h of sprinkling. The high level of the groundwater led to the assumption that these piezometers react rather as wells and do not show the real groundwater table. After 3 h of sprinkling, piezometer 1 in the middle part, piezometer 3 in the lower part of the slope and piezometer 2 located outside of the sprinkling started to react. A groundwater table of 10-25 cm built up in four hours. At 18.25 h, piezometer 1 drained suddenly and the groundwater table in piezometer 3 rose for 20 cm. Then all three piezometer show an increasing water table up to 20 and 40 cm in the middle and lower part, respectively. 2 hours before failure, piezometer 1 drained again immediately for 15 cm and piezometers 2 and 5 drained suddenly 1 hour before failure for 15 cm even the sprinkling rate remained constant. In the piezometers installed in the 23 m-deep drill hole, no response to the sprinkling could be measured.
The reaction of piezometer 2 which is located outside of the sprinkling area is very interesting.
During the rst experiment, this piezometer remained dry but during the second experiment, it reacted after 2 hours of sprinkling. This leads to the interpretation that an important ground-water ow occurred through permeable subsurface hydraulic connections. This could be either lateral ow along the bedrock surface like observed in the dye in ltration tests, or ow in frac-tures and joints and along permeable layers in the bedrock. The fact that all three piezometer started reacting at the same time leads to the assumption that they are hydrologically connected by permeable layers or joints. The sudden drop in piezometer 1 could be due to movements in the slope or even the bedrock which opened new draining paths. The sudden rise in piezometer 3 around 18:00 h could be related to the draining of piezometer 1 or to the re-start of sprinkling.
The sudden drops of the groundwater level in piezometer 1, 2 and 5 around 2:00 h in the morning could be related with the failure, indicating pre-failure movements and subsequent opening of draining ow paths in the soil and bedrock. The reaction in piezometer 2 is surprising because it is located about 30 m away from the triggered landslide.
The piezometric data shows that lateral groundwater ow occurs probably at 3 - 4 m depth or deeper in a continuous perched aquifer. The faster and the more distinct reaction of the piezome-ters in contrast to the rst experiment lead to the interpretation that the bedrock was probably nearly saturated at the beginning of the second experiment. It is to say that in October 2008, hundred litres of water have been put in piezometer 1 and 3 and drained immediately. Even though the sprinklers were shifted towards the top of the test site, piezometer 2 and 3 at the toe of the slope reacted after 2 hours of sprinkling. The bedrock depression on the left side of the slope (as seen in ERT pro les and DPL, see Figure 5.6) probably favoured the formation of a groundwater table at this part of the slope.
Another observation which supports the assumption of groundwater ow through the bedrock was water that started owing out of large joins located below the test site (see photograph on Figure 5.4) a few hours before the triggering. This joints are at the same level as Piezometer 3, but they are located 100 m to the South of the test site. This water ow continued for a few hours after the experiment.
A higher initial saturation of the bedrock in March 2009 is also observed in the ERT data (see Figure 5.13.6).
A few hours after the triggering, an additional ERT pro le was recorded along the landslide slip surface (see Figure 5.13.10). Two layers with lower resistivity are indicated. These layers are in-terpreted as more permeable bedrock where the water probably in ltrated during the experiment.
5.5. SECOND EXPERIMENT, MARCH 2009
A photograph that was taken a few seconds after the triggering showed accumulated water in the upper part of the landslide before failure occurred along the slip surface (see Figure 5.15).
Water ex ltrating from a water patch in the upper left corner of the landslide and a spring in the middle part of the landslide was owing down slope. This two accumulations of groundwater could be related with the layers of low electrical resistivity indicated on the ERT pro le Figure 5.13.10. Water that had in ltrated during the experiment in high permeable weathered bedrock and permeable sandstone layers ex ltrated after the triggering. This permeable sandstone layers were also observed in the drill hole.
Figure 5.15: Photograph of the test site taken a few seconds after the landslide triggering. Water that accumulated in the upper left corner of the landslide just below the head scarp is owing down after the triggering. The spring marked in the bottom photograph continued to ow for several hours after the triggering. Photographs taken by Amin Askarinejad, Institute for Geotechnical Engineering, ETHZ.