Sentvid Tunnel Nears Completion in SloveniaThe Sentvid tunnel, which is currently under construction in Ljubljana, Slovenia is a section of the future highway between the Karavanke tunnel and Obrezje and will be part of the link between the northwestern and southeastern parts of Slovenia. It connects Ljubljana's ring road to the highway to Austria (see map). The twin tube project consists of a main tunnel, which includes cut-and-cover sections, bored portions carrying one, two and three lanes, and two bifurcation caverns for intersection with two ramp tunnels. The two ramp tunnels connect the main tubes with a major regional road. The geological conditions are characterized by highly tectonized silt, sandstones and fault material, which are often encountered in chaotic distribution. Further challenges are an extremely small spacing of the tubes at the north portal and a fossil landslide area at the south portal.IntroductionThe Sentvid tunnel consists of three separate structures, whereof the first two are an already existing tunnel and a newly built cut-and-cover section. The third structure is the mined tunnel section, currently under construction. Although relatively short (1,060 m), the mined tunnel section offers a variety of very complex and demanding solutions that make it one of the major tunnelling projects in East Europe. The tunnel design began in 2003 in two stages. The first stage included the extension of the existing Sentvid tunnel under Celovska Road untill the foothill of Sentvid. This required the construction, now completed, of the first two parts of the tunnel, with a total length of 420 m (250 m of existing tunnel and 170 m of a new cut-and-cover section).In the second stage, the subsequent tunnel to the south portal had been foreseen as a twin-tube two-lane tunnel and had been designed accordingly. But when the building of new residential units in the area of Sentvid was announced, a traffic study was ordered. It showed that a full connection from Celovska Road to the new tunnel tubes via two ramp tunnels was necessary. Therefore, a solution with two bifurcation caverns at the intersection of the main tubes and the ramp tunnels was considered.To ensure the optimal location for the two caverns, an exploration bore, which runs in both tubes, was built in 2004. The pilot tunnel was finished after approximately 500 metres in January 2005. Based on the results of the pilot tunnel, a feasibility study of both caverns including 3D analysis of the cavern excavation was carried out. Furthermore, at the request of DARS, the Slovenian road administration, a risk analysis considering financial and time-related effects was prepared. Finally, the new solution with the ramp tunnels, the bifurcation caverns and three-lane tunnel tubes between the caverns and the south portal was adopted. Construction began in December 2004.Players of the projectThe contract for construction of the Sentvid tunnel was signed on 4th November, 2004 between DARS and SCT, the leading contractor, and its partner Primorje. Construction of the three-lane tunnel with connecting caverns and ramp tunnels is a big challenge to SCT, both in terms of building such an unique structure and excavating it in highly tectonized permo-carboniferous rocks. Visit www.dars.si, www.sct.si and www.primorje.siThe designers are IC Consulenten and its Slovenian branch Elea. Visit www.ic-vienna.at, www.ic-salzburg.at and www.elea.si. The geological and geotechnical follow-up during construction has been commissioned to IRGO. Visit www.irgo.si. The projet management has been awarded to the ZPC joint venture including ZIL Inzeniring, Projekt and Slovenska cestna podjetja. Visit www.zilinzeniring.si and www.scp.siDescription of the tunnel systemIn the north, the twin-tube tunnel starts at chainage 0.6+60 from Brod junction with a cut-and-cover stretch of a length of approximately 420 metres. The alignment enters the mined tunnel at chainage 1+80 and exits after 1,060 metres at the southern portal in Przanj at approximately chainage 2.1+40, which leads to a total length of the main tunnel tubes of approximately 1,480 m. In addition, two ramp tunnels, designed as single- and dual-lane tunnels, will connect Celovska Road with the main tubes. The exit ramp has a length of 360 metres and the entrance ramp 255 metres.At the intersection of the main tunnels and the exit and entrance ramps, two underground caverns are required. The caverns separate the two- and three-lane sections of the main tubes (north of cavern with two lanes; south of cavern with three lanes). Click here to view the tunnel layout (Fig. 1) and lengths of the various main tube sections and respective cross sections (Fig. 2).The cross sections are 75-100 sq m in the two-lane tunnel, 110-160 sq m in the three-lane tunnel, 180-360 sq m in the caverns and 70-95 sq m in the ramp tunnels.Other underground structures within the project are a cross passage for emergency vehicles and six cross passages for pedestrians. Five of these cross passages connect the main tubes and one is located between the main tube and the exit ramp. GeologyThe tunnel alignment passes through permo-carboniferous meta-sediments and is situated immediately to the south of the south alpine massif and between the Stricna and Idrija faults. To get a clearer picture of the geological conditions, especially in the areas of the bifurcation caverns, the 500 m exploration tunnel was driven. Starting at the western side of the left tube the pilot bore first enters the right tube and follows the alignment for approximately 300 metres. Then it shifts to the left tube via a cross passage and continues along the alignment for approximately 150 metres. Since the area was subject to intense tectonic deformation during several phases, the distribution of the various materials appears often chaotic, with lack of lateral continuity of layers and beds due to displacements along low-angle and high-angle faults.The rock mass is classified into the following four rock mass types (RMT): RMT1: intercalations of meta-sandstone and meta-siltstone (meta-sandstone typically prevailing); RMT2: meta-siltstone, with meta-sandstone layers/lenses, locally with slate layers; RMT3: slate, locally with meta-sandstone and meta-siltstone layers; and RMT4: tectonic clay. ExcavationThere is about two metres to go in the right tube, the breakthrough is scheduled for 21st December, 2006. There is about 90 metres to go in the left tube. Breakthrough in the left tube is planned in about three and a half months. The ramp tunnels are not excavated yet.PortalsThe Sentvid tunnel has six attacks for the mined sections. The portals of the existing tunnel and the south portals in Przanj are the permanent portals of the running tubes. In Sentvid, four temporary portals are the start points for excavation of the running tubes and ramp tunnels.The Sentvid, north portal is characterized by a small distance between both tubes at the beginning of the mined section (see Fig. 3 and 4 here). The construction box is located on a very small area between steep slopes of the Sentvid hill. The hill is supported by up to 9 m-high prestressed anchored retaining walls just above the portal and under the very busy Celovska Road. The construction box was constructed with bored piles and concrete cover slabs as permanent structures. Excavation started at the left tube and after approximately 80 metres, construction also commenced at the right tube. To overcome the narrow spacing, the asymmetrically designed shotcrete shell of both tubes was bolted together with slightly prestressed bars. Additionally, grouting of the central pillar has been carried out to improve the stiffness and load capacity. A rigorous monitoring system of the retaining wall was adopted during all construction stages. The Przanj portal at the south is located in the area of a fossil landslide which, according to the geological survey, could be reactivated during excavation of the portal cuts. The idea was to move the temporary portals as far as possible to the south to avoid massive portal cuts. Along the area of the fossil landslide (approx. 150 m), bored piles were installed on both sides of each tunnel to act as reinforcing dowels and thus preventing lateral deformations.The temporary mined portal of the more than 150 sq m-wide three-lane profile was located in an area featuring an original overburden of 0-1.5 m only. The ground above the tunnel was therefore partly improved and raised to a minimum of three metres pouring a cement-stabilized soil cover. Excavation under the cover was done without pipe roofing, which was used later on. Mined tunnelsBetween the temporary north portal and the caverns, a two-lane tunnel with a roadway width of 2 x 3.5 m has been designed. Between the caverns and the south portal, the roadway carries three 3.5 m-wide lanes. The ramp tunnels are designed as single-lane tunnels with a traffic lane width of 5 m. The total width of the two-lane, three-lane and ramp tunnels including the shoulders and walkways are approx. 9.2 m, 12.7 m and 7.2 m respectively. The height of the clearance profile is 4.7 m.For all tunnels a drained final lining will be applied. In general no reinforcement is foreseen for most of the inner lining, except at intersections with niches, cross passages, and in areas with shallow overburden or sections with enlarged profiles, i.e. the caverns. The minimum thickness of the inner lining is 30 cm in the single-lane ramp tunnel and 40 cm elsewhere.To excavate the main tunnel, Liebherr excavators are used, models R932T and R934T. There are also two Atlas Copco two-boom jumbos, models: L2C and H352, used for drilling and installing rock bolts, forepiling pipes and pipe roof. The blasting method is used very rarely, only where geological conditions allow to use it.Support consists of IBO and IBI rock bolts supplied by Atlas Copco; pipes for SN rock bolts made in Slovenia or from abroad and completed by Marguc in Slovenia; forepoling pipes with Atlas Copco's IBO pipes; pipes for SN forepoling pipes made in Slovenia or from abroad and completed by Marguc in Slovenia; pipe roof with pipes made by Mittal and completed by Marguc; TH-21 steel arches in the two-lane tunnel and TH-29 steels arches in the three-lane tunnel and caverns, produced by Mittal; Q-189 welded wire mesh (two-lane tunnel) and Q-283 (three-lane tunnel and caverns), produced at Ironworks Jesenice, Slovenia; shotcrete with or without fibres, produced by SCT; shotcreting by a Meyco Potenza shotcreter; and Mapei 2500B admixture for shotcrete. For the final lining, SCT will installa a concrete and steel panels.The waterproofing consists of a waterproofing membrane supplied by Mapei or Sika, a protective layer and smoothening shotcrete. Mountain water will be drained away by drainage pipes, which are placed laterally on the abutments below the invert slab level. Visit www.mapei.com and www.sika.com/cc-waterproofingFor mucking-out, SCT is using Cat 950G and Volvo L120E loaders together with Volvo dumpers (A25 and A30). Visit www.cat.com and www.volvo.com/constructionequipmentBifurcation cavernsAt the intersection with the two ramp tunnels, which connect Celovska Road to the main tunnel tubes, two underground bifurcation caverns have been built. The right cavern links to the entrance ramp. It has a length of approx. 56 m and is located between chainages 1.3+18.8 and 1.3+74.95. The left cavern links to the exit ramp. Its length is approx. 68 m and is located between chainages 1.4+49 and 1.5+17.15. The respective lengths of the sections for the right cavern are 20.4 m, 18 m and 17.75 m. For the left cavern, the lengths are 20.4 m, 24 m and 23.75 m. Both caverns are subdivided into three widening parts (cavern sections 1 to 3), with maximum cross sections between approx. 365 sq m (cavern section 1) and 220 sq m (cavern section 3).The excavation sequence is divided into a sidewall tunnel (part A) and a remaining part (part B) for the final profile. View Fig. 7 and 8 here. Both excavation steps are further subdivided into top heading, bench and invert excavation. The excavation of the remaining part of the profile (part B) will commence after full completion of the sidewall tunnel. Atlas Copco two-boom drill rigs have been used. Visit www.atlascopco.comThe designed support comprises of 50 cm of reinforced shotcrete, steel arches and up to 15 m-long IBI and 12 m-long IBO bolts from Alwag Techmo. The IBI bolts provide the opportunity for multiple regrouting under high pressure if required. For compensation of expected deformations, four lining stress controllers (LSC elements) will be installed in the final shotcrete shell. Visit www.alwag.atDue to the geotechnical conditions, the support of the two caverns is slightly different in terms of an unsymmetrical application of the IBI bolts.To control the stability of the cavern an extensive monitoring programme with a maximum of 19 measuring targets and a spacing of the measuring sections of max. 5 m has been applied. Additional measuring sections equipped with extensometers and measuring bolts will be carried out. Surface settlements in the area of the cavern are controlled by measurement profiles with a max. spacing of 5 m and a total number of 10 targets per profile. A drained, reinforced final lining will be applied in the cavern sections. The minimum thickness of the inner lining is 65 cm for cavern sections 1 and 2 and 50 cm for cavern section 3. The waterproofing system consists of a waterproofing membrane, a protective layer and smoothening shotcrete. Mountain water will be drained off by drainage pipes, which are placed laterally on the abutments below invert slab level.Support techniquesConstant adjustments of the construction method and support techniques have been needed due to complex geological conditions. It has been necessary to implement reinforced forepoling and pipe roofing to stabilize the working face systematically by means of a great number of rock bolts, even under a pipe roof. The number of excavation sequences in the top heading was increased. By the end of May 2006, round length in the invert was shortened for about 1.3 to 1.7 m compared to the round length in the tender documents. By the end of May also, lengths between the top heading and the invert were shortened in 96% of the excavated tunnel.Safety and maintenance facilitiesDuring the design period, major tunnelling safety standards were under revision and new measures were presented. The owner, DARS, wanted to comply with the new trends and made the decision to upgrade the tendered solution to the newest Austrian guidelines. View Fig. 10 here. The solution will make the Sentvid tunnel one of the first road tunnels in Eastern Europe to fully comply with the newest European standards.Conclusion For a major infrastructure project as the Sentvid tunnel, especially with a demanding geology and complex structures, a close cooperation of all the players in the design and construction is imperative.Extensive preliminary research including an exploration tunnel and the suitable use of the collected data proves to benefit the whole project.Short daily site meetings, where current technical topics (e.g. adaptation of support on the basis of geotechnical monitoring results) between the client, the contractor, the geologist and the designer are clarified, are an additional tool to a successful completion of the works.So far these control measures have proven to be effective, and certain flexibility from all the involved parties is required.Visit sl.drustvo-dpgk.si/sitedata/383/upload/File/Tema_Topic_3.pdf to read several papers on the tunnel presented at the 8th international conference on tunnel construction and underground structures held in Ljubljana in November 2006: Deformation monitoring and back analysis at Sentvid motorway tunnel - Lessons learned and Geodetic measurements in the exploration gallery of the Sentvid tunnel. Click si/16. 51-52/06.