Lochkov Tunnel under Construction on Prague South Ring RoadLot 514 Slivenec-Lahovice of the Prague ring road project is currently under construction. This 6,013 m project, which is located in the southwestern part of the Czech capital, contains the Lochkov tunnel, also named Slivenec. The ring road alignment passes the river valley close to the confluence of the Berounka and Vtava rivers. The road descends relatively steeply from the hills skirting both riverbanks and crosses the rivers via a 2.4 km-long viaduct, then rises directly to the Lahovice eastern portal. The route climbs another 50 metres along the tunnel length of 1,662 metres (1,649 metres for the double-lane tunnel) to the Slivenec western portal, where it connects to another bridge which spans Lochkov Valley.Not only the tunnel was designed purposedly with respect to the selected route but also planners took into account the nature conservation areas in close proximity to the route. There are only alotted gardens with small houses and large but empty tanks above the tunnel. Several terraced and detached houses and a sewage treatment plant are above the route in the central part. The average overburden is 16 metres.A substantial part of the tunnel will be mined, namely 1,295 metres of the three-lane tunnel and 1,265 metres of the double-lane tunnel. The length of the cut-and-cover stretches is respectively 367 metres for the three-lane tunnel and 387 metres for the double-lane tunnel on the Slivenec side. Ten metres of three-lane tunnel and 20 metres of double-lane tunnel will also be built on the Lahovice side.The project owner is the Road and Motorway Directorate of the Czech Republic. Lot 514 is constructed by a consortium including Hochtief, Strabag, Max Boegl and Josef Krysl. The tunnel builder is a JV of Hochtief Construction (tunnelling division) and its Czech branch (traffic infrastructure division). The Road and Motorway Directorate of the Czech Republic employed D2 Consult as technical advisor and site supervisor for the pilot tunnel and now the tunnel construction. D2 Consult is responsible for continual detail documentation check, geotechnical monitoring check and adjustment, tunnel construction supervision, determination of NATM classes during tunnel construction, etc. Click here and here. Visit www.rsd.cz, www.hochtief-construction.com, www.hochtief.cz and www.d2-consult.czThe expected construction cost, according to the contractor's bid price, is about CZK5.03 billion. The tunnelling cost per metre amounts to EUR25,000. The tunnel construction started in October 2006, breakthrough is expected in January 2008 and full completion is scheduled in September 2009.Horizontal and vertical alignmentsThe tunnel tubes, independent for each traffic direction, follow a 600 m-radius left-hand curve (the direction from Radotin to Lochkov). The optimal distance between the tunnels is achieved by shifting the right tube from the route centre line by designing the radius slightly different from the radius of the route curve. There is a double-lane tube in the descending direction (from Lochkov), while the uphill tube (from Lahovice) has three traffic lanes. The third lane is added to enable heavy goods vehicles to climb the 4% gradient. The right traffic lane is 3.75 m wide, the left and central lanes are each 3.5 m wide. The three-lane tunnel carries 0.5 m-wide guide stripes. Therefore, the total roadway width is 11.75 m. The double-lane tunnel has guide stripes 0.5 and 0.25 m wide plus a 1 m-wide emergency lane. The total width of the roadway is nine metres. The walkways are one metre wide on both sides. The clearance profile is 4.8 m high.GeologyThe geological conditions were verified by an exploratory tunnel, driven throughout the length of the three-lane tunnel tube. The chosen setting and shape of the exploratory tunnel forms the first part of the tunnel top heading (an area of 25 square metres). Both tubes of the Lochkov tunnel are driven into Ordovician and Silurian rocks. The uphill excavation from the Lahovice portal passes through a difficult section formed by an up to 12 m-thick Quaternary cover consistinq of loams with gravel and rock debris. The route continues further through shale of the Lower Ordovician Kraluv Dvur formation and Upper Silurian Barrandiene formation. The downhill drive from the Slivenec portal should encounter Palaeozoic era's Silurian rocks of the Kopaniny and Motol formations, which are characterised mainly by the presence of shale. Underlying are predominantly laminated siliceous shales. The rock mass is tectonically faulted and broken by numerous faults and folds. The shales are locally finely crushed, slickensided then gradually replaced by a gravel sand-like material.The hydrological conditions are relatively favourable for the mining activities. The original water table, which is bound within a fissured aquifer existing in the weathered bedrock zones copying to some extent the ground surface, was lowered by the exploratory tunnel down to the tunnel level. The inflow rate at the excavation face within the water-bearing tectonic fault zones is anticipated at 12 l/s as a maximum. The maximum overburden height above the tunnel is 60 metres.Technical and structural designThe mined and cut-and-cover tunnels are designed to have unified internal cross sections, irrespective of the construction technique. All mined tunnel structures are similar in terms of design. They consist of the upper vault and strip footings, plus an invert where necessary. The climbing three-lane right-hand tube has a theoretical excavated cross-sectional area of 103 square metres (without consideration of the pilot tunnel), whereas the downhill double-lane left-hand tunnel tube has a theoretical excavated profile area of 91 square metres.The primary lining consists of C20/25 sprayed concrete and mesh. The intermediate, unclosed waterproofing system uses a 25 mm-thick membrane, which diverts water to longitudinal drains laid along the strip footings. The vaulted cast-in-situ final lining is made of C25/30 concrete.The Lahovice portal is one of the difficult parts of the tunnel construction. lt is located in a steep slope above Radotin, where the thickness of superficial deposits exceeds 12 metres. The portal of the minedtunnel is in a location not very suitable for tunnelling. lt could not be shifted deeper to the rock massif where the tunnel bottom would already pass through the bedrock because it would have interfered with a biological protected corridor. The walls of the excavation trench and the portal are supported by piles anchored to the bedrock. The design of this section of the cut-and-cover tunnels contains an invert.The Slivenec construction trench for the portals of the mined tunnel and the long cut-and-cover tunnels has sloped sides stabilised by shotcrete and dowels. The C25/30 reinforced concrete vaults of the cut-and-cover tunnels are 600 mm thick. They rest on strip footings. The 3 mm-thick waterproofing membrane, which is applied to the outer surface of the vaults, is protected on both sides by non-woven geotextile.All final portals are designed as cast-in-situ reinforced concrete structures. The design features an elliptical crown with a "ribbed collar" which architecturally animates the portals and incorporates them into the surrounding landscape. The final terrain around the portals is formed by backfiIIing the adjacent cut-and-cover tunnels and parts of the portals. The eastern portal in Lahovice is formed by retaining walls, which are connected to the service building. A similar building is also in the area adjacent to the Slivenec portal. The final design of the entire tunnel is performed by Valbek, a designer appointed by Hochtief. Visit www.valbek.cz Construction and support methodsThe work in the mined tunnels is carried out downhill from the Slivenec portal, although this is not ideal for water drainage but it is not possible to excavate from the other portal due to traffic problems, which raised the issue of the muck transport route. In the three-lane tunnel, a pilot bore built two or three years ago by a different contractor is being enlarged. The New Austrian Tunnelling Method with a horizontal excavation sequence is applied to all mined tunnels by means of explosives in hard rock and excavator in softer ground. Limits will be set on the blasting operations carried out beneath existing buildings. A Sandvik Tamrock Axera T08 drill rig is used on each tunnel for drill and blast rounds. A maximum of 120 blast holes are drilled in the top heading, achieving advances of 2.5 metres. The explosive emulsions are provided by Westspreng. Swellex rockbolts secure the rock. Visit www.miningandconstruction.sandvik.com, www.westspreng.de and www.swellex.comTwo excavators, one Liebherr and one Terex Schaeff equipped either with a bucket or hydraulic hammer, are employed in softer ground in the three-lane and dual-lane tunnels respectively, after installation of an umbrella support system using Alwag steel pipes. Progress is about 100 metres per month. To date, progress in the two-lane tunnel is 610 metres of top heading and 410 metres of bench. In the three-lane tunnel, the achieved advance is 700 metres of top heading but no bench excavation so far. Visit www.liebherr.com, www.alwag.at and www.schaeff-terex.comThere is no really need for ventilation during construction in the three-lane tunnel because the pilot tunnel provides natural ventilation. A fan is placed at the Lahovice portal of the pilot tunnel at the opposite side to where excavation is ongoing at the moment. The two-lane tunnel is ventilated by Korfmann. Visit www.korfmann.comThe tunnel is supported with lattice girders, wire mesh and shotcrete. The lattice girders, supplied by Subterra, are placed just behind the face and in case of overbreak, they are filled with shotcrete. No face or support anchors are installed. If the ground is unstable, a first layer of shotcrete is sprayed, then lattice girders and wiremesh are placed, then a second shotcrete layer is poured. Two Meyco Potenza SA 180 shotcreters and two Normet lift platforms are used. View pictures here. Visit www.subterra.cz, www.meyco.basf.com/Meyco/EN and www.normet.fiThe rock and soft soil are mucked away by a Cat 966H wheel loader in the three-lane tunnel and a smaller 564 2plus2 Liebherr wheel loader in the two-lane tunnel. A fleet of four GHH dumptrucks stationed outside take the loads to the final dump site. Visit www.cat.com and www.ghh-fahrzeuge.deThe problems encountered are the stability of the face and overbreak. It is up to the contractor to choose shorter or longer cuttings. SG Geotechnika and Pudis monitor the face by ground mapping. There is water ingress in the three-lane tunnel, which is a consequence of the pilot tunnel boring. Combined with overbreaks, this has caused wells of residents to dry up, which compelled the contractor to pay a lump sum in compensation for the affected areas. Visit www.geotechnika.cz and www.pudis.czClick cz/22. Read E-News Weekly 15/2005. Click here to read more about safety aspects. 29/07.