Ground subsidence is considered among the most frequent geological hazard that usually occurs as a consequence of a
number of phenomena, namely: natural compaction of unconsolidated fine - grained deposits, groundwater over -
exploitation, peat - oxidation and collapse of underground cavities. The main objective of the present study was to
investigate the spatio - temporal patterns of ground deformation in the wider area of Farkadona Municipality in the
north-eastern Trikala regional unit, Thessaly, Greece. A set of Synthetic Aperture Radar images, acquired in 1992–2003
by the European Space Agency satellites ERS1 and ERS2 and processed with the Persistent Scatterer Interferometry
technique, along with geotechnical data from nearby exploratory boreholes, were used in order to explain the observed
deformation. Increasing subsidence rates are observed at almost the entire area within the city complex of Farkadona,
which is covered by loose Quaternary sediments of considerable depth. In most cases, the increased deformation rates
have caused damages in the form of tensile cracks, mainly along the road network and adjacent buildings. The results of
the performed analysis concluded that the geotechnical conditions of the loose deposits occupying the site and also the
intensive exploitation of the aquifers are the two main causal factors of the land subsidence phenomena. Additionally, a
clear correlation between the subsidence deformation rate and the thickness of the loose deposits has been detected.
Besides the geological information, this study made clear that Persistent Scatterer Interferometry could be considered as
a valuable tool and cost-efficient method for validating subsidence mechanisms and could serve as an alternative to
The specification of the near surface ground conditions is highly important for the design of civil constructions. These conditions determine primarily the ability of the foundation formations to bear loads, the stress – strain relations and the corresponding settlements, as well as the soil amplification and corresponding peak ground motion in case of dynamic loading. The static and dynamic geotechnical parameters as well as the ground-type/soil-category can be determined by combining geotechnical and geophysical methods, such as engineering geological surface mapping, geotechnical drilling, in situ and laboratory testing and geophysical investigations. The above mentioned methods were combined, through the Thalis ″Geo-Characterization″ project, for the site characterization in selected sites of the Hellenic Accelerometric Network (HAN) in the area of Crete Island. The combination of the geotechnical and geophysical methods in thirteen (13) sites provided sufficient information about their limitations, setting up the minimum tests requirements in relation to the type of the geological formations. The reduced accuracy of the surface mapping in urban sites, the uncertainties introduced by the geophysical survey in sites with complex geology and the 1D data provided by the geotechnical drills are some of the causes affecting the right order and the quantity of the necessary investigation methods. Through this study the gradual improvement on the accuracy of site characterization data is going to be presented by providing characteristic examples from a total number of thirteen sites. Selected examples present sufficiently the ability, the limitations and the right order of the investigation methods.
For the seismic action estimation according to Eurocode (EC8) one has to characterize site conditions and suitably estimate soil amplification and corresponding peak ground motion for the site. For this reason, as specified, one has to define a design spectrum through the ground-type/soil-category (S), and the peak ground acceleration (PGA) of the reference return period (TNCR) for the corresponding seismic zone and for structural technical requirements chosen by the designer. Ground type is defined through geophysical/geotechnical parameters, i.e. (a) the average shear wave velocity up to 30 meters depth, (b) the Standard Penetration Test blow-count, and (c) the undrained shear strength of soil. Through the “GEO-CHARACTERIZATION” THALIS-PROJECT we combine different geophysical and geotechnical methods in order to more accurately define the ground conditions in selected sites of the Hellenic Accelerometric Network (HAN) in the area of Crete Island. More specifically in the present efforts, geological information shear wave velocity and attenuation model calculated from seismic surface geophysical measurements is used. Additionally we utilize the ground acceleration recorded through HAN from intermediate depth earthquakes in the broader area of South Aegean Sea. Using the recorded ground motion data and the procedure defined in EC8, the corresponding elastic response spectrum is calculated for selected sites. The resulting information are compared with the values defined for the corresponding EC8 spectrum for the seismic zone comprising the island of Crete. As a final outcome of this work we intend to propose regional normalized elastic spectra for seismic design of structures and urban development planning and compare them with Eurocode.