Geophysical and geotechnical investigations of coastal heritage sites : a case study of Maltese watchtowers

Abstract

As outlined by the UNESCO World Heritage Convention in 1972, cultural heritage sites have a universal value and are of outstanding interest to society. This study is an original attempt at assessing the geotechnical and geophysical properties of three coastal heritage sites. These are the Għajn Ħadid Watchtower, the Għajn Tuffieħa Watchtower and the San Ġiljan Watchtower. The aim is to determine the frequency response of the ground and observe the effect of the underlying geology at each particular site. A seismic site response was undertaken by recording ambient noise and applying the Nakamura method (H/V technique) in order to observe the dynamic behaviour of the coastal region. The results obtained show a correlation between the site response and the stratigraphy. A layer of Blue Clay is deposited on top of the Globigerina Limestone in the western region of the island and is responsible for H/V resonance peaks between 1 - 2 Hz. H/V curves illustrate more complex responses close to cliff edges due to stronger vibrations in the detached blocks. Such responses were not observed in the eastern part of the Island due to the variation in stratigraphy. A non-parametric analysis was carried out to determine the damping ratio of the Għajn Tuffieħa Watchtower. The geological thickness was also inferred from H/V results. Ultimately, the results have significant implications for the ongoing processes occurring in coastal environments as well as emphasizing the necessity of conserving cultural heritage sites.