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 27 km 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 50 m; 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 of27 000m 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.
