Suffosion hazard for building and infrastructure in the High Lombardy plain (Northern Italy)

The suffosion is a geological hazard related to both hydrogeological and hydro-chemical features. Suffosional failures are well known to be dangerous for infrastructures and buildings both during their construction and life-time. In some cases, buildings and structures may be threatened. Suffosional failures of soils are well known in all of the world where mostly the soils are cemented by soluble material whose particles can be removed by subsurface water flow or chemicals- typical process in karst terrain. In the High Lombardy plain there are both dissolution of carbonate components and thin particles removal caused by groundwater flow: the reduction shrinkage of volume and the voids enlargement can cause a wide range of landforms including caves, subsidence hallows, collapse sinks, swallow holes. The aim of this work is a description and an interpretation of the surveys occurred during an important road designing. In order to know the spatial localization of the caves before the construction site, a huge number of surveys must be correlated not only parallel to the main road axis but also perpendicular to. It has been observed that surveys Standard Penetration Test, geoelectrical and mechanical surveys for calibration describe the spatial (x,y plane and z direction) complexity of the phenomenum. The results of different surveys cannot be used independently for a reliable interpretation, all information have to be correlated together because a typical range of SPT that could be associated with a presence of a caves has been within less 10 and the range of geoelectric surveys resistivity has been within less 300 Ωm. The interpretation of results leads to conclusion that the caves or pipes that individually can have a huge dimension, more than some cubic meter, are a system of void interlinked in the space where the water runs till a delivery which can be the aquifer or a spring or a superficial water body. Furthermore, the retrieval of this complex caves system is distinguished mostly for the high deepness (till 30 meters in some ways). The huge extension is so unpredictable that geological documents of the local Authority doesn’t concern about this problem, underestimating the hazard for the urban areas