Extending Prague’s metro line

The west end of Prague’s metro line ‘A’ is being extended in a construction project estimated to cost CZK 21.13 billion (expected total capital cost including indexing). The section known as Metro Line V.A is located in the northwest of Prague and will run from Dejvická Station via four new stations: Červený Vrch (Bořislavka), Veleslavín, Petřiny and Motol. After several appeals against the building of the section were addressed, construction began in January 2010. Operation of the new section is expected to start in late-2014.

However, the project’s capital cost had to be reduced and so the general designer was asked to submit cost-cutting measures which were mainly applied to the construction of Petřiny Station.

For the first time in Prague’s metro system, single-track tunnels have been bored using Tunnel Boring Machine (TBM) technology and benefiting from the latest tunnelling machines. Station tunnels and two-track running tunnels are being built using the New Austrian Tunnelling Method (NATM) using access tunnels to transport all material.

The project also includes construction of three bus terminals for urban lines, and zoning and construction permit applications have been submitted for a Park & Ride facility to meet the aim of terminating surface transport further from the city centre.

Three of the new metro stations (Červený Vrch, Veleslavín and Petřiny) are being built with the bored method, but Motol station is being constructed using the cut-and-cover method, including the adjacent track segment in the direction of the route’s end.

Červený Vrch Station

This single-span station has already been bored and includes an island platform at a depth of approximately 28m below the surface. Its eastern (main) underground vestibule and platform are connected to an already bored escalator tunnel linking to a pedestrian subway under Evropská Street with exits at surface public transit stops and adjacent sidewalks. The second exit has two high-capacity elevators that connect to another underpass that links individual parts of a housing development.

During the station’s construction, the boring method was changed due to geological conditions that were more difficult than expected. This caused delays during con – struction of the entire station due to the need to use a completely different method. These measures were adopted primarily to minimise the impact on the residential buildings above.

As a result of this delay, the station was not bored prior to the passage of the TBM, and concurrent operation with station bore completion and supporting technology for boring of the track segment between Červený Vrch and Dejvická had to be arranged. Any unresolved problems related to unexpected geology would result in an approximate threemonth delay in the entire new section being put into operation.

Veleslavín Station

This station will provide passengers with an important transfer hub between long-distance, suburban and urban transit modes in a relatively low-profile location away from the city centre. Its key transit benefit lies in the fact that it will be the site of a main bus terminal with a Park & Ride facility, has tram service connections, and is located by a station on the Praha–Kladno railway line that is to be modernised and augmented with a spur line to the airport. The metro station will be a three-span structure with an island platform and is approximately 20.5m below the surface. An underground vestibule connected to the western end of the platform with escalators and an elevator connects to pedestrian under – passes leading to the railway, the bus terminal, tram stops and surrounding buildings.

Due to necessary extensive layout changes in this station, delays occurred during the preparatory phase and a subsequent delay in original schedule dates have been caused by:

• Problems with timely signature of leases with owners of some properties and signature of relocation agreements with some utility owners (boring work delayed by three months)
• Unclear bus terminal concept (size and number of parking stalls), which depended on city funds allocation
• Changed boring technology due to deteri – orated geology and subsequent fears regarding safety and subsidence. The technical part of the station was relocated from its original planned location in the bored section to an excavated structure at the access ramp location.

Structural station changes during construction (optimisation) included:

● A change in the lengths of station tunnels due to unsuitable geology and time delays resulted in the use of TBM instead of NATM

● Relocation of the technical part of Veleslavín Station from the tunnel to the excavated part of the pit access ramp due to unsuitable geology

● A multi-storey bus terminal with a 600-stall Park & Ride facility.

Petřiny Station

This station’s primary aim is to improve city centre accessibility for the area. The completed bored single-span station is approximately 37m below surface and has an island platform. The station vestibule, connected to the platform with escalators, is on the surface close to the tram and bus stops. A second station exit at the south side of the platform has two highcapacity elevators. Turnabout tracks are located past the station.

Costs were reduced by replacing the original underground vestibule with an above – ground version and eliminating a pedestrian underpass under Na Petřinách Street. It was also necessary to move the top of the elevator shaft from Brunclíkova Street due to the impossibility of negotiating the construction permit with the owner of the adjacent property. These steps resulted in a new zoning decision and con – struction permit being issued, with resultant delays and cost overruns due to changes in work procedures. It was then necessary to split construction of the aboveground portion into phases so that at least some structures for which a new construction permit was not necessary could be built.

The entire station including the access shaft was built using NATM technology. TBM boring shields were simply pulled through pre-bored side tunnels without the need for secondary lining collars that were originally to be installed by the machine and then surmounted by NATM technology. The original plan was to use bored pilot tunnels from the TBM for access for station construction without the need for boring separate access shafts for the station itself. This showed itself to be disadvantageous.

Motol Station

This station is located right by Motol University Hospital and Na Homolce Hospital. These hospitals provide higher forms of healthcare for a substantial part of Prague and the Central Bohemia Region as well as some functions of national importance. The metro station will greatly improve the hospitals’ transit service which will include new bus transfer connections, especially to large areas of Prague such as Jihozápadní Město and Řepy. The related shortening of relevant lines will provide a beneficial reduction in bus traffic on roads leading to the city centre.

Motol Station is built into the slope by Kukulova Street directly in front of Motol University Hospital. It is embedded 6.7m-deep, has a side platform and its special architectural design lets in direct daylight through a glass roof. The main station vestibule connects to the hospital grounds and bus stops via a pedestrian subway under the adjacent road. A second station exit serves as an emergency escape route, and after expected additional con – struction on hospital grounds, will provide further access to the area. Since Motol is an end station crossed lay-by rails can be found after it.

The station itself is located in very difficult geological conditions at the foot of an unstable slope, requiring a massive under – ground retaining wall with permanent anchors to stabilise the slope above the station.

For reasons of safety (low rock cover) and cost reductions, it was decided cut-and-cover technology would be used instead of NATM to build approximately 180m of the total 400m from the station to the end of the line in the location of the lay-by rails.

TMB disassembly in existing Dejvická Station

Because of the impossibility of building construction site facilities by Dejvická Station and the very poor condition of utility networks and their immediate surroundings under Evropská Street, it was decided that the tunnel boring would end in Dejvická Station itself, where extensive demolition and shoring work is taking place on lay-by tracks while keeping metro line ‘A’ in operation. Some parts of the disassembled shields will then be transported along by metro to one of the metro depots.

Experience gained

It is beneficial to perform geological engin eering surveys during the project design phase for zoning proceedings, so that stations and associated structures can be moved to more suitable locations from a geological engineering perspective, thus eliminating cost overruns and delays. In Prague’s complicated geological engineering conditions, a greater financial contingency reserve for the project itself is necessary.

Prague’s metro system is an open one, meaning that options for its further extension past Motol Station have also been specified.