Problemi di stabilità per le infrastrutture nelle aree interessate da cavità (occhi pollini) nei sedimenti pleistocenici della pianura lombarda

Underground cavities can develop in different geo-lithological settings. In fact, a huge number of experiences referred to rock cavities (i.e. in limestone, in evaporites or in gypsum) are reported worldwide. on the other hand, the need to understand and forecast the phenomenon is of chief importance in the upper Lombardy Alluvial Plain, where many infrastructures are under construction. The aim of this work is to underline some parameters or indicators which allow to forecast, and therefore prevent, the most important geo-lithological and geotechnical conditions that are likely to favour the formation of cavities in the alluvial deposits or in the upper and weathered bedrock layers. Indeed it is the first time that a good number of data are available in a wide area and not only in a little portion of a territory (the phenomenon was mainly known for individual buildings’ concerns). Moreover, the available technical data are systematic (both continuous and discrete data are available) and are finely spatially distributed. Actually, during design of a main road in the Lombardy Plain a huge number of data have been required in order to find a useful method to forecast and quantify the underground cavities’ occurrence in areas nearby the city of Milan. The importance of this work regards designing and building issues, occurring when the infrastructure actually intersects cavities. Their identification through geognostical surveys and the assessment of the stress-strain behaviour nearby the cavities in underground is essential. For this purpose, a statistical analysis was carried out over the geotechnical parameters, in order to deliver an experimental method capable of “occhi Pollini” hazard areas identification, in the designing phase of a big infrastructure. A sensitivity analysis was also done on the qualitative parameters: some lithological sequences show an increasing frequency of cavities with a superposition of alluvial carbonatical matrices and weathered conglomerates. Moreover, by evaluating geotechnical and geophysical parameters, a correlation was found between cavities occurrence and these parameters (SPT and nCPT surveys, Young modulus). The study shows the existing correlation between some typical geognostic parameters and the phenomenon under exam. The probability density function for the SPT parameter shows a very strong response with soil stress release whereas on the other hand, hydraulic conductivity was only poorly correlated while load modulus showed a moderate correlation.