Il ruolo delle indagini nella valutazione delle condizioni di sicurezza delle opere esistenti

THE ROLE OF INVESTIGATIONS IN THE EVALUATION OF ULTIMATE LIMIT STATE OF EXISTING STRUCTURES In the paper the influence of the reliability of geotechnical subsoil model on the safety of an existing work has been evaluated. A probabilistic analysis of ultimate limit state of a shallow foundation has been performed assuming the reliability level of subsoil model variable. This variation has been simulated by means of an increment of geotechnical data obtained through new investigations. The analysis points out that the probability of failure of a shallow foundation under constant loads decreases increasing the reliability level of subsoil model. This result is not dependent on the undrained strength data sets selected to perform calculations. A parametric study has been performed in order to highlight the dependency of the safety level on the foundation dimensions and on the level of applied loads. The probability of failure decreases as the foundation dimension increases and increases as the applied loads decrease. Those increments are considerably reduced for highly reliable subsoil models (e.g. high number of investigations performed). In the case study of a use exchange of an historical building, the increase of applied loads reduces the safety margin of the foundation; this reduction can be accepted if new geotechnical investigations have been performed, because the foundation limit state is assumed with a reduced probability of failure. In this perspective, the restoration interventions can be limited in order to preserve the material and historical identity of existing works.