Instability of casalabate village (lecce, italy): analisys of risk conditions

Abstract

Casalabate is an outlying ward of Lecce located on the Adriatic Sea shore, over the time this small village has undergone remarkable hydrogeologic disturbance resulting in the collapse of some dwellings and the condemn of many others. In order to find out the causes, an accurate multidisciplinary investigation planning has been implemented. This includes a geological survey with 23 boreholes, a georadar investigation, as well as chemical-physical analyses, thermo-saline logging and groundwater flow velocity measurements. The above led to the determination that lithotypes in most territory are related to fossil Dune deposits, formed by irregularly cemented calcarenites which can be found up to 4-9 m below sea level whose physical-mechanical quality is basically poor and varies from place to place. Beneath fossil dune deposits, pure marine deposits of abundantly fossiliferous and irregularly cemented whitish biocalcarenites and calcirudites are encoutered. No cavity and/or channel associated with parakarstic dissolution phenomena has been found, but grounds underlying the foundations of buildings greatly vary in their lithologic structure both vertically and horizontally.Groundwater, whose hydraulic heads range from 30 cm and 80 cm a.s.l., flows unimpeded to the sea and shows the typical saline stratification of coastal aquifers with salinity values which, in the space of 5-6 m, increase from a few mg to 3-4 g/l. Moreover, the exclusively horizontal outflow to the sea takes place only through some dune levels whose groundwater flow velocity, Vf , is about 250 cm /g .As a whole, investigations proved, contrary to the assumptions, that static disturbance phenomena cannot be due to the collapse of the underground cavities vault, but because of the incidence and combination of the following factors: poor, and in some cases, very poor geotechnical characteristics of foundation grounds; the lack of a preliminary geotechnical characterization of buildings leading to the choice of improper practice in laying down foundations; the letting of rainwater into the subsoil which caused the leaching and the removal of the fine fraction of the foundation grounds.