Coupling long-term GNSS monitoring and numerical modelling of lateral spreading for hazard assessment purposes

Lateral spreading is a complex geomorphological process occurring through the interplay of different factors. Due to their low rates of displacement, lateral spreads in rock are much less investigated than other landslide types even though sometimes they can evolve into faster and more hazardous movements such as topples. The lack of long-term monitoring data means that the deformation mechanisms of these landslides remain uncertain. Along the northwestern coast of the island of Malta (central Mediterranean Sea), the presence of a thick layer of clay underlying a brittle cap rock made of limestone has led to extensive rock spreading and associated block sliding. Two sites affected by such processes were monitored by GNSS (Global Navigation Satellite Systems) from 2005 to 2019. A network consisting of 17 benchmarks were surveyed twice per year, providing a 14-year displacement history. Coupling this exceptionally long monitoring dataset with Limit Equilibrium and Finite Difference slope stability modelling, the failure mechanisms of the landslides have been investigated to identify predisposing and driving instability factors. This research provides new knowledge on the kinematic behavior of extremely slow landslides and insights into landslide hazard assessments in areas extensively affected by lateral (rock) spreading.