The evolution of the Prague Metro21 October 2013
Vaclav Soukup and Ermin Stehlik of Metrostav presented the keynote talk ‘Prague Metro construction – past, present and future’ at UC Prague 2013. Here, they detail the construction of the Prague Metro from its beginnings in 1966 to the present, and discuss future plans.
Prague Metro construction began in 1966 with the building of an underground tramway system for a stretch of the C Line, close to the main railway station. The time from the start of construction up to 1989 was affected by the political situation, with the methods and experience from metro construction in Soviet cities being applied on the Prague Metro.
All things considered, the construction of tunnels and stations in difficult geological conditions, in densely built-up areas of the historical city, and with a minimum impact on the surface, was successful. During the construction process a number of in-house methods and design solutions were developed and the result was a civil engineering project that was presented as a case study of the construction industry.
In hindsight, it could be stated that the arrangement of automatically awarding the projects to the general consultant and general contractor for the metro construction had a great advantage for the continuity of works for both subjects. There was no need to conduct tender processes and a lot of the effort could be devoted to the new solutions and technologies. Of course, such efforts were often a substitute, due to the limited capability to import modern technology and equipment. The mined metro tunnel construction throughout this time period was oriented to the application of segmental linings.
The application of, for example, shotcrete, was not easy to introduce, although there was constant effort to do it. From today's perspective, the waterproofing of running and station tunnels was a weak point. With the cast iron lining, the joint insulation with caulked lead was very time consuming, but the resulting watertightness was good. Nevertheless, for the stations, protective sheeting collecting the seepage water had to be installed. As for the watertightness, the use and quality of two-stage grouting applied behind the segmental lining was closely related.
One significant problem was being unable to use bigger equipment for transport, loading and drilling. Constrained spaces in erectors, combined with rail-bound transport, necessitated the use of small loaders, although later continual loaders were introduced. Another weak point was drilling; basically all the drilling at tunnel faces was done by hand with negative impacts on production and also the health of tunnelling staff.
Another weak point has been in lagging of drifts and other temporary excavations, which has been done mostly by concrete or steel sheeting boards with backfill. Only towards the end of this time period was shotcrete used. The absence of active support by bolts and shotcrete caused occasional overbreaks, and even sinkholes on the surface.
The change of political system, and therefore the possibility of importing modern technologies, was a turning point in Prague's metro construction. The application of the new Austrian tunnelling method (NATM) gained pace, and was used for stations, escalators and running tunnels. For drilling, jumbos were employed, and for the primary support bolting and shotcrete with lattice girders were used. The lining of running and station tunnels was double-pass. Primary lining application was followed by installation of a waterproofing layer, and in the end the secondary (final) lining was cast into the formwork.
Unlike earlier projects, the application of precast segmental linings was abandoned. For subsequent running tunnels, however, this construction was considered not quite suited to the situation. The domination of NATM for construction of running tunnels ended in 2010, when Metrostav purchased two EPBMs for construction of metro extension VA.
A significant achievement on the Prague Metro has been the underpassing of Vltava River on the IVC extension. Applied technology is one of the biggest contributions made by Czech tunnelling to the global industry.
It was only matter of time before the ring method with segmental lining would gradually be replaced by the NATM. At the start of 1989, the first experimental application of NATM took place for the excavation of a gallery for special purposes on Florenc (formerly Sokolovská) station. As a part of the experiment, it was necessary to verify and evaluate the geological conditions, prepare the design documentation, evaluate new support elements, create monitoring documentation and prepare testing of shotcrete.
After this first approach, a few drifts and parts of station tunnels in primary lining were executed at stations Vysocanská and Hloubetín on metro line IVB, and already more than 2km of running tunnels with double-lining and intermediate waterproofing were built on the same line.
The first double track metro tunnel built by NATM, between 1995 and 1997, was also on the same project (IVB). For its construction, the machinery and equipment customarily used for NATM abroad, was for the first time, used on site. For four escalator tunnels on this line, three of them have been constructed by NATM.
Another turning point in NATM application was the construction of the first single vault station on the line IVC 7. The first attempts to build single vault stations happened in the 1980s and it was another dream of a whole generation of tunnelling experts, being fulfilled in 2002, when the excavation of Kobylisy station was performed by Metrostav.
Underpassing the Vltava River
Underpassing the Vltava River by the so-called immersed tube 'launching' method was, and still is, one the most important contributions of the Czech tunnelling community to the wider industry. The chosen method was based on the construction of a complete tunnel tube in the dry dock.
The rear part of tube was shifted on the prepared track in the dry dock. The tube was moved with the help of two towing cables anchored on the opposite Holešovice river bank. The rear brake suspension pre-stressed the cable system and enabled braking of the movement. Vertical suspension was connected to the pontoon, which carried the weight of the tube in the front part, reduced by the buoyancy. The rear sliding part of the tube carried a majority of the weight and secured the stability of the whole body. The forces required to move the tube were relatively small, the weight in the water was only 1% of actual weight. After the support structure, built in a cofferdam on the Holešovice bank, was reached, the tube was supported on both banks and was stabilised. The support to the river bed followed, by concreting in the regular intervals and anchoring the tube.
The construction method was new and unusual, and during planning and construction there was a need to solve important technical problems, some of them requiring a large rate of innovative approaches related to the watertightness of concrete structure, the tube balancing, the additional tube loading system, the foundation strip and tube launching.
The long-awaited turning point in construction of running tunnels came about in 2010, when Metrostav purchased two Herrenknecht tunnel-boring machines (TBMs) for the metro VA extension. From 2011, the TBMs bored more than 4km of running tunnels from the Vypich construction site to the existing metro station Dejvická.
Review from 1989 to present
Without any doubt, it can be said that Czech (and Slovak) tunnelling has reached the world standard, and not only on Prague Metro construction. The final missing link was the application of modern TBM technology, which although somewhat delayed, finally arrived. For NATM applications, state-of-the-art equipment is used. And for tunnelling crews the most striking difference is the absence of hand work, mainly drilling. The latest waterproofing systems are in use, and with increasing frequency even sprayed waterproofing is employed.
There was a development - in the authors' opinion an unwise one - in the design of primary shotcrete lining (with lattice girders and meshes). The main element of NATM is the use of the bearing capacity of the surrounding ground with the help of bolting, and the shotcrete with lattice girders and meshes is used to keep the integrity of the load-bearing ground arch. But the original idea of using relatively soft primary linings and letting the ground, activated by help of bolting, take the load, was somewhat lost along the way and we are getting primary lining thicknesses up to 500mm, which is basically against declared NATM principles.
But in some cases, the reason given is that tunnels are mined with very low overburden in built-up areas, making this a case of using reinforced shotcrete and not NATM. On the other hand, a step in the right direction is the application of unreinforced secondary linings, but this has not been the case in metro tunnels. A trend to attempt to calculate and model everything can be observed since the start of the 1990s, and is related to the development of hardware and software suitable for finite element modelling.
Today, it is possible, in a relatively short time, to give the clients coloured reports with calculation results, and it is forgotten that the results and their interpretation are heavily dependent on applied parameters, which are often only estimated, and usually involve safety factors, or rather protection of the worker himself.
There is still a difference in comparison with other advanced countries in respect to tunnelling practices, and it is also the case with the VA extension, namely, the absence of permanent supervision by the client. The rule should be that the supervisor must work the same hours as the contractor, 24 hours a day, seven days a week.
Future of metro construction
It would be helpful to summarise the latest experience from the contractor point of view. Both TBMs on the VA line had, during their 4km journey, to pass through three stations under construction and through two open pits (one of them is the construction pit in front of the Veleslavín station) and finally through the tunnel on the E1 site.
Except for the E1 site, there was always the need to restart the machines. And although the site engineers came up with innovative approaches, pulling through and the subsequent restarts required substantial effort as they are technically difficult, costly and represent time delays. Pulling the TBMs, eventual dismantling and any other movement should be minimised, if not avoided entirely.
The existing metro system is functional and very reliable, therefore the new metro Line D should be compatible with other metro lines and use their facilities. The diameter of the running tunnels shall stay the same, allowing interchange of trains between the lines. The design should, to maximum extent, respect the realistically required capacities, and the economic use of TBMs shall be a priority.
This article was first published in Urban Transport Agenda's sister publication, Tunnels & Tunnelling International.