In a long high-pressure gas pipeline that has onshore and offshore parts the term "landfall" is used to describe the onshore segment of the offshore parts. Being actually at the shore, the landfall areas are characterized by various geohazards (e.g. slope instabilities, rockfalls, etc.) that are directly related to the local site conditions (e.g. topography, geomorphology, geology, etc.), while the interaction of the shore with the sea waves introduces the phenomenon of erosion, leading thus to the continuous changing of the topography and geomorphology. On the other hand, in areas characterized by moderate or high seismicity the design of high-pressure gas pipelines at the landfall areas may be a more demanding task since their integrity is directly related to the earthquake-related geohazards and the seismic vulnerability of the pipeline under consideration.
The current study focuses on the geohazards and geotechnics at the "grey zones" of landfall areas through the examination of two extreme case studies in the Mediterranean Sea. The first landfall is a mild area with low inclination that consists of saturated loose sandy materials that have the potential of soil liquefaction phenomena (vertical settlements and/or lateral spreading) under strong seismic excitation. In the alternative routing (i.e. the second landfall area) the pipeline follows the sloped terrain of the landfall, where the pipeline has to endure both the amplified ground motion due to the topography effects and the imposed permanent ground deformations due to a potential earthquake-triggered landslide. Based on the geotechnical data derived from sampling boreholes, the slope consists of a surficial layer of silt overlying a stratum of clay on top of the claystone bedrock.