| 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. |