Hazard connected to railway tunnel construction in karstic area: applied geomorphological and hydrogeological surveys

Abstract

Abstract. In a mature karstic system, the realisation of galleries using the methodology of railway tunnel boring machine (TBM) involves particular problems due to the high risk of interference with groundwater (often subject to remarkable level variations) and with cavities and/or thick fill deposits. In order to define groundwater features it is necessary to investigate both hydrodynamic and karstification. To define and quantify the karst phenomenon in the epikarst of the Trieste Karst (Italy), an applied geomorphological approach has been experimented with surface and cavity surveys. The surface surveys have contributed to determining the potential karst versus the different outcropping lithologies and to define the structural setting of the rocky mass also through the realisation of geostructural stations and the survey of the main lines thanks to photo-interpretation. Moreover, all the dolines and the cavities present in the area interested by the gallery have been studied by analysing the probable extension of caves and/or of the secondary fill deposits and by evaluating the different genetic models. In an area 900m large and 27km long, which has been studied because of the underground karst, there are 41 dolines having diameters superior to 100m and 93 dolines whose diameters range between 100 and 50m; the dolines whose diameters are inferior to 50m are 282. The entrances of known and registered cavities in the cadastre records are 520. The hypogeal surveys have shown 5 typologies in which it has been possible to group all the cavities present in a hypothetical intersection with the excavation. The comparison between surface and hypogeal structural data and the direction of development of cavities has allowed for the definition of highly karstified discontinuity families, thus having a higher risk. The comparison of the collected data has enabled to identify the lithologies and areas having major risk and thus to quantify the probability of intersection with the different cavity typologies for each area. To make an example, out of 27000m of studied gallery 3930 are the metres expected to be at very high "karst risk". Out of these, as a whole 310 are risky because of the probable presence of gallery cavities, 2170 because of the probable presence of pits and sinkholes diffusely present under the dolines, and along 1450m karst is particularly intense. Moreover, 2200 should be the metres in which the rocky mass will be particularly divided because of tectonic causes. From a hydrogeological point of view a monitoring of water level has started to quantify water excursion, due to closeness of the railway tunnel to the mean water level. First results related to galleries intersection are here presented.