The Alps are a tectonically young mountain range, where orogenic and erosional processes are still ongoing and active, affecting more or less every larger civil engineering structure. In order to plan and realise physical safeguarding measure or redevelopment, it is mandatory to understand the interactions and associated geotechnical mechanisms. A valuable instrument to visualize them is numerical modeling. Here we show two examples of landslides, for which numeric modeling was used as a tool to verify presumed failure mechanism and for the other to find it out.

1 Verification of Failure Mechanism

We start with a well known landslide in the north of Salzburg City, which threatened to block main transport routes and feed lines between the capital and the prosperous northern territories some years ago. Mountain Haunsberg is part of the geological flysch unit, which is a swiss term for "flowing" and consists of laminated layers of sandstone and claystone. Sliding movements are recorded continuously, but on decadal timescales great mass movements can occur transporting thousands of cubicmeters of clayey and silty debris flow deposits.

During the last large events between 1999–2004 an extensive investigation programme of about 50 exploration wells with depths between 20 to 50 m, numerous geotechnical laboratory tests, movement and displacement monitoring, geoelectric measurements, inclinometer and monitoring wells, pumping tests, rainfall and discharge measurements, etc. were performed developing a geological model based on these measured datasets (Figure 1).

While geologic features, hydrogeology and geotechnical parameters became known very well, the actual trigger of the sliding event remained in the dark. But aside from this, it was discovered by geological mapping, that debris flow sediment could be distinguished due to chronology. The old inactive landslide mass is located at hillside toe. Being drained and compacted it serves as the abutment for the active debris flow, which slid on it like viscous magma and then came to rest.

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