Geotechnical investigations in an old landslide situated in fine-grained Neogene sediments (Alkofen, Bavaria, Germany)

In 1988, a landslide occurred near the village Alkofen in the unfolded molasses of Lower Bavaria and was subsequently reactivated and reinforced in 1990. An area of ca. 8,250 m2 was affected. In the years 1990–1992 investigations were conducted to estimate the hazard potential and an inclinometer was installed in 2008 above the slide. This work focuses on the compilation of all previous collected data sets and the interpretation of the landslide. As a result, the main landslide body can be confirmed as a spoon-shaped and rotational slide induced by increased pore water pressure in consequence of high precipitation. The material of the slip surface has been proven to be water-saturated, unconsolidated and undrained sand at a depth of 8 m below surface. Additional, the existence of a second and deeper shear horizon has been approved. The material that turned out to be the sliding surface is composed of clayey and sandy silt with a soft consistency. Despite the high shear strength, the material is suited for a ssliding surface because confined ground water would cause an effect of upwelling in this layer. The analysis of the inclinometer data does not provide any decisive results concerning the activity and actual moving direction above the slide. It seems that there is only a movement of the surface layer down to a maximum depth of 5 m. .