Based on preliminary investigations, 100 smaller and 10 larger karst features were predicted along the karst section of the Second Track. The predictions proved to be fairly accurate—during construction, speleologists investigated, measured, and documented 99 karst caves using photo and video documentation.
The location and size of the caves were unpredictable, although areas with a higher likelihood of karst features were known in advance. This was primarily based on previous experience from motorway construction in the region, as well as geophysical surveys.
Predicted caves: 110
Actually discovered caves: 99
Deepest cave: 168 metres
Longest cave: 702 metres
Total length of discovered caves: 6,559 metres
The total length of the discovered caves amounts to 6.6 kilometres, which is almost as long as the longest tunnel on the route. For comparison: the length of the Postojna Cave exceeds 24 kilometres.
The construction of infrastructure is an intervention in the environment, but according to speleologists’ assessments, around 90% of the documented caves could be separated from the tunnel. In this way, parts of the caves outside the excavation area were preserved in a more or less natural state. The caves were not backfilled; hydrogeologically active cave passages remained connected after tunnel construction, allowing water to flow around the tunnel without retention and continue unobstructed toward karst springs, just as before the intervention.
How many karst features are expected during the construction of the parallel track?
The karst section of the route is practically already completed, as the surface excavation in Divača has also been finished for the left track, and both service tubes of the Lokev and Beka tunnels have been excavated. These are the only two tunnels along the route that pass through limestone; all tunnels to be built during the construction of the parallel left track will run through flysch. However, as observed during construction of the right track, additional karst caves may still be discovered during excavation for viaduct foundations.
