15 maja 2015
As one of the five tunnels currently being built on the 40.3 km section of the Ljig-Preljina road in Serbia, the Šarani tunnel is extraordinary. This section of Corridor 11 connects Serbia’s northeastern border with Romania and its southwestern border with Montenegro on the Adriatic Sea.
In addition to the tunnels, it also involves the construction of 78 bridges. The five tunnels are Brdjani (length 455 m), Savnica (270 m) Veliki Kik (200 m), Ljig-Golubac (25 m) and Šarani, which at approximately 900 m is the longest.
“Two of the tunnels are being excavated using the cut-and-cover method while the other three, including Šarani, are classic tunnels”
Energoprojekt Niskograndna plc has been subcontracted to excavate the Šarani drive and is one of five Serbian compa-nies working in a consortium alongside AzVirt, the Azerbaijan construction com-pany, which is the main contractor for the civil works. The client is the state-owned Koridori Srbije (or Corridors of Serbia).
The project is financed by a €370 million loan from the government of Azerbaijan, of which €41 million has been allocated for the construction of the Šarani tunnel.
Šarani will be a twin-tube tunnel (883 m and 887 m). Construction began in March 2013 and is expected to be com-pleted before year-end 2015.
“We are currently on budget and on schedule and expect to remain so until the end of the project, despite the fact that advance has been difficult due to the geological conditions”
Excavation of the second tube is expected to be completed by June after which they will start to prepare the tunnel for use, working on the concrete lining, road construction, electricity installations and more.
Kovacˇevc´ explains that the geology here is difficult for any form of underground work as the ground is composed of Serbian Serpentine, a changeable type of rock that becomes soft on contact with water. “In practice it turns into mud,” he says, adding that the quality of the rock is not sufficient for the drill and blast technique all the time which would have made excavation much faster.
The fluctuation within the rock formation is naturally challenging.
“From one area with relatively good rock where we use drill and blast, to an area with very soft rock where we have to employ other methods, it can easily change within 20, 50 or 100 meters”
“We simply don’t know, so we have to continuously monitor the conditions to decide on the most practical and safe method of advancing the tunnel.” For this reason, geodetic surveys and geological mapping are performed on a daily basis.
When drilling and blasting permits, Atlas Copco Boomer L2 rigs are used, while in the poorer formations, hydraulic hammers HB 2500 and HB 7000 mounted on excava-tors are used. Pipe roofing is carried out using the Atlas Copco Symetrix system, radial MAI SDA self-drilling anchors and MAI M400NT pumps are used in rock reinforcement work.
“We also use steel arches and pre-cast concrete and two layers of wire mesh,” adds Kovacˇevc´, noting that in good rock, only one layer is required or no wire mesh is used at all. “If we’re lucky, our shotcrete only needs to be about 20 cm thick but often it can be over 30 cm,” he says, adding that this is proof of the poor quality of the ground.
Šarani also employs three Atlas Copco Unigrout grouting platforms, two of which are Unigrout Flex M E22. Ventilation in the tunnels is provided by a fan fitted with 1 400 m of Atlas Copco flexible ducting.