• Sprinkling intensity in the upper part: In March 2009, the sprinkling intensity in the upper part of the slope was large enough to establish perched groundwater.
• Tree roots: The strength of the tree roots was more important for the rst experiment because trees located on the test site were cut only a few weeks before the rst experiment. Thus the eect of the roots was more pronounced during the rst experiment. Additionally, before the second experiment roots around the test site were cut with a motor saw up to 0.5 m depth.
This experiment showed the complexity of dierent hydrogeological processes. Phenomena like bedrock in ltration and ex ltration are very local. Therefore it is not possible to extrapolate the results of the experiment on the entire slope between Buchberg and the River Rhine. Nev- ertheless, it is possible to de ne hydrogeological processes that should be taken into account by analysing the landslide susceptibility and slope stability in the area:
• The bedrock and unconsolidated sediment are locally drained by large vertical joints. Bedrock in ltration is the dominant process in the area of the sprinkling experiment. • Well cemented sandstone layers and marlstone act as impeding layers if there are no vertical
joints. Thus, locally positive pore water pressure can be formed in the unconsolidated sediment above the low permeable bedrock.
• Uncemented sandstone layers are permeable. At locations where no vertical joints are draining those layers, aquifers can be formed in those permeable layers. Spring horizons, for example observed in the Upper Marine Molasse above the test site, indicate the location of such permeable sandstone layers.
5.7 Triggering mechanisms
Two dierent situations have to be distinguished to classify the case of Rüdlingen. These are natural shallow landslides and the triggering experiment with arti cial rainfall (see Figure 5.17).
Figure 5.17: The hydrogeological classi cation applied for natural landslides and the arti cially triggered landslide in Rüdlingen. The unconsolidated sediment and the bedrock are permeable. Both were initially unsaturated. During the rst experiment, saturation was reached only in the unconsolidated sediment. During the second experiment the bedrock was most likely also saturated.
Potential triggering mechanisms for the arti cial landslide are (according to the hydrogeological classi cation):
5.7. TRIGGERING MECHANISMS
• Rise in pore water pressure: During the second experiment, positive pore water pressure was build up rapidly in the unconsolidated sediment. This decreased the shear strength of the material and led to seepage. Positive pore water pressure was probably not the most dominant triggering mechanism, because prior to the failure, the pore water pressure decreased abruptly. Porewater pressure fall prior to landslide movement (especially rapid landslides) is interpreted to occur due to draining ssures that opened prior to the failure (Harp et al., 1990). Another explanation for the drop in the porewater pressure prior to failure is dilation of the soil (and subsequent increase of the pore volume) at the shear zone (Askarinejad et al., 2010a).
• Seepage forces: Slope parallel seepage occurred along the bedrock surface. Local ex ltration from permeable bedrock layers (as observed below the head scarp) most likely caused horizontal or even upward oriented seepage, which in uenced the triggering.
• Seepage erosion: Seepage erosion was most likely an important triggering mechanism. This was also observed in triggering experiments in dierent permeable sandy soils performed by Harp et al. (1990). They described that the pore water pressure dropped 5 - 50 min previously to failure. This drop in pore water pressure occurred before visible cracks formed, like in Rüdlingen. Harp et al. (1990) observed water ow rates at cut slope faces. They found out that ow through fractures and macropores was predominant and that the water ow path and permeability were changing. And as the slope failed, water poured out of the slope along the failure surface, like it was the case in the upper left corner in the Rüdlingen experiment. The ow of this muddy water shows that water could accumulated locally in fractures. Thus preferential ow and seepage erosion played an important role for the triggering. When ne grained soil fraction is removed along a potential slip surface, the apparent cohesion of the soil is reduced which decreases the shear strength.
• Upward seepage and liquefaction: No liquefaction was observed during the triggering. After the landslide was triggered, very small slips and ows of saturated soil was observed in the head scarp area.
• Eect of water on clay minerals: The plasticity index of the unconsolidated sediment in Rüdlingen is 6-12% which is rather low. Even though little swelling of the soil could be observed, it is assumed that the clay minerals played a minor role by the triggering. For the natural landslide triggering in the area of the distal Molasse, it is possible that local con ned aquifers may play an important role. The springs and spring horizons observed above the test site give evidence for the occurrence of groundwater in more permeable sandstone layers. This would additionally lead to ex ltration and overpressure from below the landslide.
The example of the Rüdlingen triggering experiment shows how dierent triggering mechanisms may play together. Like it can be seen on Figure 8.9 in Chapter 8, the case of the experiment and the natural landslides are both in the "high criticality" zone.