When the Ovcˇiarsko Tunnel in northern Slovakia got underway in July 2014, no-one quite expected the scope of the task that lay ahead for the engineers.
The project, which forms part of a 427 million euro plan to build a new, 11 km section of the D1 highway as a southside bypass of Žilina, seemed a fairly straightforward assignment.
Consisting of two tubes (2 367 m and 2 372 m long) and with first class contractors and equipment, the excavation got off to a good start to meet the January 2018 completion date.
At first, working from portals at the eastern and western ends simultaneously, things went relatively smoothly. Then, after about one month, the eastern portal came to an abrupt halt.
Igor Jurik, Project Geologist at Ovcˇiarsko, explains why. “This has turned out to be one of the most difficult tunnel construction projects ever undertaken in Slovakia,” he says. “In this part of the country it is extremely mountainous and there are large areas of weak, soft rock formations, in particular a type of clay called flysh.
“In the Ovcˇiarsko tunnel we have two types of rock formations to deal with, an older section which is relatively stable and not too bad to work with, and a younger, flysh-type formation which, frankly, is a nightmare.”
Jurik explains that the softer formations are not only weak but also highly porous causing them to disintegrate and swell in contact with water. According to the Austrian standard, it is classified as poor, between 4-XF and 5-XF. For the engineers on the ground, this translates into a major challenge, particularly as the geology varies substantially and frequently throughout the tunnel alignment.
The entire construction project, which also includes 11 bridges and numerous access roads, is being handled by a consortium of top contractors made up of Doprastav, STRABAG, Váhostav, and Metrostav. Doprastav is responsible for the tunneling and has subcontracted the work to Uranpres with equipment supplied by sister company and construction management specialist EKOFIN.
When M&C visited the site in March, work at the eastern portal had just been restarted while the western end of the tunnel had been advanced more than 500 m.
The first 300 m consisted of the older, harder formation and was solid enough to excavate. But beyond that, the geology changed every few meters, forcing the engineers to repeatedly switch methods back and forth between drill and blast and rock breaking with a breaker and excavator.
Advance hard to estimate
In the more favorable formations, two Boomer E2 C drill rigs from Atlas Copco, equipped with COP 2238 rock drills, are used, while the poorer ground is tackled by two Atlas Copco MB1700 hydraulic breakers mounted on excavators. And with each changeover of equipment, there is naturally also a changeover of crews.
Because of this, the rate of advance is difficult to estimate and varies from 0.5 m to 2.5 m in a single production cycle.
Jozef Valko, Senior Construction Manager, says: “It’s just not possible to use one method throughout the whole tunnel. The right technology has to be used according to the nature of the geology so we have to adapt to what we find. This not only involves difficult technical challenges but also places high financial demands on the project.”
Naturally the company was prepared to tackle these demanding conditions. As Valko explains: “Of course we were aware of the tough geological conditions as indicated by the pre-studies and we knew it would be a challenge. But the fact is, we are doing better than expected. From the beginning we thought we would only be able to use drill and blast for about 20 to 25 percent of the tunnel, but judging from our progress so far we think this figure will rise to about 50–60 percent.”
The crews work two, 12-hour shifts, seven days a week, and in each shift they attempt to complete a full cycle of drilling and blasting the top and bottom of the face. This is not always possible due to the ground conditions, so utilization of the rigs is only around 30% or 8 hours per day. However, as the rigs are also used for rock reinforcement, they are not often idle.
Reinforcement and ventilation
A variety of bolt types are used including grouted, hydraulic, fibreglass and self-drilling anchors. The number of bolts installed per shift depends on the geology but in good conditions about 20 hydraulic bolts are installed in two shifts.
When installing self-drilling anchors – a classic solution for unstable ground conditions such as sand, gravel, silt, and clays – two Atlas Copco M400NT Mai pumps are used. In addition, two Potenza wet mix concrete spraying units from Atlas Copco Meyco are used for shotcreting.
Besides this, there are two Atlas Copco Liftec UV2 lifting trucks on hand for general service and support work such as the installation of wire mesh, ventilation ducting, lighting and other applications.
Ventilation is provided by two high pressure Atlas Copco Serpent fans (formerly Swedvent) together with 2.4 km of heavy duty, PVC-coated ducting. Serpent is a
complete ventilation system that controls the flow of air with great efficiency, taking fresh air in and extracting the fumes from blasts and exhausts.
First rate service
All of the equipment at the site is supported by a service contract and supervised by Atlas Copco’s local distributor, ISOP. Headed by Jozef Parobok, ISOP has represented Atlas Copco in Slovakia for 23 years. “We look after all of the Atlas Copco equipment here, 90 percent of which is new,” he says.
In line with the contract, service and maintenance is provided with a reaction time of two hours. In addition, ISOP has a workshop at each portal and onsite containers stocked with rock drilling tools and parts. Another container serves as an office.
After more than 20 years of working in international tunnel construction projects, Valko is well acquainted with Atlas Copco equipment and its competitors. “The reason we use Atlas Copco is the high quality of the drill rigs and, of course, the service which is first rate,” he says. “For underground work, I think Atlas Copco equipment is the best on the market.”
Pavel Jindrácˇek, Product Manager for Atlas Copco Central Europe, adds: “The Boomer E2 C, along with its sister machine Boomer L2 C, is a very popular choice in tunneling in the Central European region. They’ve been the rig of choice in civil engineering.”
In Serbia, which has its sights set on becoming a full member of the European Union, infrastructure development is also in full swing with construction of the Manajle and Predejane tunnels on Corridor 10 as key projects.
The new tunnels, located between the southern city of Nish and the Macedonian border, will play a major role in improving the flow of traffic in this area. At present, this section of Corridor 10 only has two lanes and is often overloaded with trucks heading south to Macedonia and Greece. Accidents are frequent, and especially in summer when the road is jammed with European holidaymakers. The tunnels will not only ease the congestion, they will also bring environmental and safety standards into line with those in the EU. Costing more than 50 million euros, the tunnels are being built by two Bulgarian companies, Euro Alliance Tunnels and Roads & Bridges Ltd., for the state-owned company Koridori Srbije.
“We have a long history in Serbia, mainly in the mining industry which is traditionally stronger here, but more and more construction projects are now being commissioned,” says Boris Loncar, Atlas Copco Product Manager, Central Europe, adding that Atlas Copco has been active in the country for more than 50 years.
Longest in Serbia
At 1.8 km, the Manajle tunnel will be the longest road tunnel in Serbia. It will have two tubes, one for each direction, with two lanes in each. Just 10 km further north is the Predejane tunnel with the same design but different lengths, 870 m and 1 050 m.
The work at both sites is being carried out continuously with a combined workforce of 170, working two 12-hour shifts per day.
According to Stojan Petrovski, General Project Manager for both tunnels, most of the engineers are from Bulgaria and about 90% of them are skilled. “It’s difficult to find skilled workers here so we brought our own,” he says. “They have a lot of experience of working on other similar projects.”
Preparation work started in September 2013 and the tunnels are scheduled for completiion in March 2016. “So far we have developed about 40 percent of the tunnels but the geological conditions at Manajle are more complex than we first anticipated,” explains Petrovski.
Good and bad
As in Slovakia, the tunnelers encounter competent rock as well as clay-like material. And again, the conditions demand frequent changes of technology; drill and blast in the good rock, hydraulic hammers and excavators in the poor ground.
More and more road construction projects such as this one in the south of the country are underway in Serbia. According to the original project study, the company was expecting to mostly employ drill and blast using the New Austrian Tunnelling Method (NATM).
“This would be much faster of course, but because of the conditions, we are forced to work this way, making progress slower,” Petrovski says. “The advance depends completely on the geology. In good rock, advance can be about 3 m in one shift. We don’t necessarily see this as a huge problem, but this is the situation we face and we have to deal with it.”
To help them do just that, Atlas Copco has provided two drill rigs, a Boomer E2 C and a Boomer L2 C, that are used in both tunnels. One of the rigs is a new unit featuring Atlas Copco’s latest Rig Control System RCS 5 for computerized positioning and high precision drilling. And when this rig arrived, Atlas Copco trained the operators to use it.
The blastholes are drilled to depths of 4 m at the rate of 2.5 m/minute. When working in the softer formations, three Atlas Copco HB1700 hydraulic breakers (mounted on excavators) are used to break the rock.
Pipe roofing required
Pipe roofing is also used to support the tunnel crown in the weaker zones, leading to less overbreak and ensuring safety for the operators. Here, the Boomer rigs are used to install 3 m pipes in an umbrella pattern to a hole depth of 15 m. About seven pipes are installed per shift and it takes 50–70 minutes to install one 15 m section.
“It’s fine, this is normal, we just know that we need two days to finish a whole pipe umbrella,” says Velin Mahov, Euro Alliance Construction Manager. Once the pipes are installed, four Atlas Copco MAI 400NT grout pumps are used for injection of selfdrilling anchors. The grout is provided by two Atlas Copco Unigrout grouting platforms, one for each tunnel.
Ventilation is by five different fans, one of which is a new Atlas Copco Serpent AVH 125 with a flow rate of 14–42 m3/sec.
All of this equipment is covered by a service agreement which includes the presence of an Atlas Copco service technician Monday to Friday. According to the agreement, Atlas Copco has to respond to any problems within 24 hours and there is also a container for parts and rock drilling tools.
“We have been working with Atlas Copco equipment for a long time,” says Project Manager Petrovski. “We have a good relationship and our operators are used to working with this equipment. I think they are good quality machines, but the most important factor for us when choosing equipment is the service support. This is especially important here in Serbia as the country is not yet in the EU and some processes, such as moving equipment and parts through Customs, can take a long time.”
“Other manufacturers whose equipment we are using in Serbia, do not have that kind of support, so if a problem arises, we have to call the sales office in the nearest country or even the manufacturer’s country of origin. Even then, it takes a long time for the parts to arrive and it is another long, complicated process through Customs.
“If we have a problem with a machine, I just call up Boris at the Atlas Copco office in Belgrade and I know it will be sorted out. That peace of mind is priceless.”
Epiroc operated under the trademark “Atlas Copco” prior to January 1, 2018.