Investigation of the dynamic properties of coastal and other geomorphological features through the use of ambient vibrations

Abstract

The north western coastline of Malta features highly destabilised rock masses due to a layer of hard Upper Coralline Limestone on top of soft clay. This geological layering causes instability, which triggers the fracturing of the upper rock layers and large-scale mass detachments. These landslide processes stimulate lateral spreading phenomena. The Maltese coastline is a natural attraction but also poses geological risks especially to areas of cultural and national importance. Therefore, to mitigate hazards, it is important to understand the vulnerability and the dynamic properties of unstable blocks. The aim of this study was to find a correlation between the natural frequency obtained from experimental results and that generated from numerical modelling, on the behaviour of coastal detachment processes. Two sites around the Maltese coastline were chosen for this research: Selmun and Anchor Bay, both of which are located in the north western part of Malta. These locations consist of several examples of slab detachment at different stages. Ambient noise measurements which were recorded using the Micromed Tromino® were taken at these sites from previous studies. These measurements were taken on unstable blocks at both sites and were analysed using the Horizontal-To-Vertical Spectral Ratio to determine their natural frequency. Moreover, by using photogrammetric techniques, 3D aerial photos were created to examine the extent and dimensions of detached slabs near the cliff edge. 3D models of blocks were created with Autodesk Meshmixer and transferred onto a finite element code ANSYS, to study the vibrational behaviour of blocks detached at different depths of fractures. Using numerical modelling, the natural frequency of these models was determined and compared to the experimental data obtained using ambient noise. Furthermore, other geomorphological features such as isolated hills like Laferla Cross, were also considered to study the thickness of the underlying geology in relation to vibrational modes.