10% of the caves on the track of the second railway line have been discovered in the last two weeks
Excavation work in the tunnels started in September 2021, and after two and a half years, we are now witnessing the most massive discoveries of karst phenomena on the route. So far, 72 karst caves have been discovered and eight sites are currently active in the karst area of the Second Track route. The Lokev (T1) and Beka (T2) tunnels are twin-tube tunnels, with a service pipe next to the main one, and excavation is taking place on both sides.
The karst phenomena are discovered during excavation, after blasting, when the entrance to a new karst cave is opened. At that point, experts from the Karst Research Institute are mobilised to inspect, investigate and document the caves. All caves need to be inspected, both for the safety of workers and to protect the environment. After the discovery of a karst phenomenon, karstologists usually descend into the karst caves using rope technology, where they take all the necessary measurements, which are sent to the project planners, who propose certain solutions, which must be approved by the Institute for Nature Conservation. As a rule, the caves are separated from the tunnel pipe by formwork after documentation and are excluded from the tunnel volume.
The current abundance of karst phenomena is also the result of the crossing of the Beka-Ocisel cave system. Some of the caves that have recently been discovered also contain a lot of water. The rule is that water flows do not change.
Before excavation work started on the route of the second track, experts predicted that 100 minor and 10 major karst features would be discovered during construction. So far, the predictions seem to be fairly accurate.
The purpose of the geophysical surveys
Before construction contractors encounter a karst cave, they have a certain amount of information about it. This is because geophysical surveys can detect a possible karst feature on the construction route before it is excavated, thus reducing the unpredictability of karst features in the excavation area.
Horizontal boreholes 100 metres long shall be drilled into the face of the spoil along the entire length of the excavation of the Lokev and Beka tunnels. Georadar works by the transmitting radio antenna sending radar pulses into the hillside, which are reflected and returned to the receiving antenna when crossing an obstacle with different electrical characteristics. This detects anomalies in the structure of the hills that may indicate a possible karst phenomenon.
The surveys are used to determine the geometry of the karst features, their position in relation to the tunnel, their potential water-bearing capacity and other characteristics.
29. 1. 2023
Photo: Karst Research Institute