Shallow Foundation Reliability Design

The increasing awareness that soils are materials that, even in a lithologic homogeneity, show pronounced variability of their physico-mechanical properties, has given a remarkable increase to the development of probabilistic computational models in geotechnics. Significant reflexes are present also in geotechnical regulations, in which (see Eurocode 7 for example) the research of characteristic values and the use of partial coefficients prove to be a significant step forward compared to the use of Global safety factors. In this note the problems of shallow foundations are taken into account, as far as the bearing capacity and settlements are concerned, remarking some of the most significant studies carried out in the last years on probabilistic basis. The outcoming frame is that there is still a lot to study as far as the statistic structure of input data is concerned, the models to use the verification in real magnitude of the results obtained from calculations. The more and more detailed knowledge of the mechanical behavior of soils and the interaction soil- structure have allowed to develop considerably the calculation procedures for the achievement of well defined safety levels in geotechnic field. There has already been a significant step forward from the classical Global safety factors, including indiscriminately all the possible uncertainties in the procedures of physical and mechanical characterization of soils and calculation to the partial ones able to better calibrate the entity of the penalizations of the Resistances and the amplifications of the Actions. A further step to the more aware safety has been for some time identified in the direction of the evaluation of the so called Reliability Index, connected to the probability that the unfavorable event, for example that the overcoming of the limit bearing capacity takes place. In order to obtain the previously wished results, even in classical situations as planning a foundation for geotechnical purposes, it is necessary to carry out a quantitatively and qualitatively significant characterization of soils and equally a calculation model . In this note a series of aspects will be pointed out connected to planning in terms of reliability as far as the bearing capacity and settlements of shallow foundations are concerned, reporting and commenting some results obtained from different Authors. 2 SAFETY FACTORS IN GEOTECHNICS In the recent past, in many regulations and codes, the safety of geotechnical works has been entrusted to an approach called WSD (Working Stress Design) based on the so called Global safety factor expressed as the ratio between the Resistances (indicated generally as R or C) and the Actions (indicated generally with Q or D), expressed both in terms of forces, moments or stresses considering obviously all the possible kinematics in the examined case. Normally this ratio was based, even if not appositely specified, on actions and resistances calculated through the mean values of the relating input parameters The understanding that, because of numerous uncertainties in play (between which the intrinsic variability of soils), those input parameters were not unambiguous but characterized by many times significant variability and consequently by specific frequency distributions has made understand that