The Mrázovka tunnel in Prague before completion

Abstract

Prague, the capital of the Czech Republic, lies in the heart of Europe. It is exceptional not only by its beauty, but also due to the fact that the planned system of main urban roads has not been completed yet. Currently the most topical issue is the Mrazovka tunnel being in the process of completing the works, with the scheduled inauguration in August 2004. The Mrazovka tunnel is 1300 m long in total. It consists of two tubes with one-way traffic (the ETT and WTT). Three-lane mined tunnels form the major part of the tunnel structure. In terms of length and cross sections, the Mrazovka tunnel tubes are the largest and technically most difficult project implemented till now in the Czech Republic using the New Austrian Tunnelling Method. The difficulty followed from the longitudinal profile of the tunnel route, the geology encountered and existence of residential buildings above the tunnels. The most difficult sections of the drive were the three-lane tubes at the portal North under Mrazovka Hill, the excavation under the buildings existing at locations with shallow rock cover, and the excavation of bifurcation chambers with a cross section up to 324 m2. The tunnel excavation and primary lining were completed in April 2002. Casting of final lining started in April 2001, the work was finished in August 2003. In terms of the approved traffic study, various profiles of the mined tunnels, i.e., profiles for three-lane and two-lane tunnels, bifurcation chambers, tunnel ramps and cross passages had to be carried out at the Mrazovka tunnel. The structural design of the final liner was significantly affected by the longitudinal profile of the tunnel too. It called for application of a closed waterproofing system and for the liner proportioning counting with a full hydrostatic head from groundwater level found up to 30 m above the tunnel crown. The waterproofing is provided along the entire circumference of all mined profiles. In terms of structural analysis the most interesting were the design of final lining of the three-lane tunnel with the slab under the roadway participating in a composite action with the liner as a tension member, and the design of final lining for a bifurcation chamber having an internal width and height of 20.5 and 11.4 m, respectively. The original design of equipment for the Mrazovka tunnel was elaborated in compliance with recommendations and conclusions of the PIARC conferences held in Marrakechi in 1991 and in Montreal in 1995. At the time when the design had already been completed, the contractor selected, and civil and mining work on the Mrazovka tunnel started, the opinion on the active and passive operational safety in road tunnels changed substantially all over the world. This change took place in the nineties, after several conflagrations in tunnels with catastrophic consequences. For the above reason the already designed Mrazovka tunnel equipment had to be redesigned. The new system designed was based on the requirement for intensive evacuation of smoke from the fire vicinity, within a length of about 400 m. The PIARC recommended suction performance for a three-lane tunnel was 120 m3/s in 1999. Functionality of the newly designed system was verified by mathematical and physical modelling. A study on operational safety in tunnelled sections of the City Ring Road in Prague was developed in 2002, using acknowledged world-wide parameters. It proved that the operating Strahov tunnel, Zlichov tunnel, the Mrazovka tunnel being currently completed, and the other three tunnels on the City Ring Road being designed, satisfy the operational safety requirements. Currently the Mrazovka tunnels are in the phase of equipment installation. The required term of bringing the Mrazovka tunnel into service is August 2004. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.