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. Only later in 1967 was the decision made to change construction to that of a classical deep metro. As of the end of 2012, there are some 59km of metro line in operation in the city, with 57 stations, some 24 of which are mined.

The first metro line: C, was constructed before the changeover to deep metro construction. It was built with shallow tunnels and stations, and as a result all the stations were open cut constructions. Mined stations have been built on subsequent Metro lines, where the tunnels and stations are situated in the Prague bedrock.

Running tunnels passing under the Vltava River are also situated in the bedrock. Thus the choice of technologies and equipment was infuenced by the tunnel locations.

From the start of Metro construction, the dominating firms, as their names indicate, were Metroprojekt as the general consultant and Metrostav as the general contractor. The political changes in 1989 brought tendering processes, but nevertheless these two organisations are still important partners of the City of Prague for design and construction of the Prague Metro. On the client’s side the firm Investor Dopravnich Staveb (later renamed to Inženýring Dopravních Staveb) has also been present from the beginning.

Prague geological conditions
The alignment of the metro lines, especially their depth, was affected by the landscape topography, the need to pass under the River Vltava, and the geological conditions.

The difference in the depth between, for example, Staromestské Námestí Station, and Petriny Station on Line A is around 200m. This shows the significant impact of the landscape configuration. The bedrock consists mainly of palaeozoic age rocks (mostly ordovic), and also to a lesser extent the rocks of Mesozoic era, as chalk period sediments were encountered during construction of Petriny Station on extension of Metro Line V.A. The bedrock outcrops are rather rare; bedrock is covered by weathered materials and alluvial and fluvial sediments.

Running tunnels
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.

The first running tunnel of Line C was bored by a non-mechanised open shield on the Pankrác, Štetkova Street site. The boring went mostly outside the built-up area, but it was necessary to go under the early baroque church of Saint Pancras (including the rotunda dating from the 12th century), which was achieved without any damage to this significant historical monument.

Geological conditions were sometimes complex, with some incidents of, for example, running sand lenses, which called for full lagging of the tunnel face.

In other sections of Line C the shield method was used in combination with segmental lining of 5.1m/5.5m diameter and ring width of 1m. But the standard method was the so-called ‘ring method’, with Soviet-made erectors used for assembling the lining.

On sections of the tunnel excavated by subcontractor Výstavba Kladenských Dolu (VKD), they used a special erector that created a finished lining diameter of 5.2m/5.6m and ring width of 0.75m.

For the ring method, the erector’s platforms were used for hand drilling the tunnel face and afterwards for blasting and mucking, mostly by PPN loader (a pneumatic overhead loader with shovel capacity of 0.4m3). The erector then moves to the face, and with the help of a turning arm erects the next ring [6]. Later, Metrostav developed special assembly platforms, used for shorter tunnel stretches.

The next line to be built was Line A, which, as a deep line, was located in the bedrock. Nevertheless, non-mechanised shields were still used in combination with blasting, but the main method remained the ring method with blasting and an erector for lining assembly.

Ahead of its time
Before the first Vltava River crossing, which was on Line A between Klárov and Old Town, there were significant doubts how the bedrock tunnel overburden under the river would react to the blasting. In the end it was decided to apply mechanical boring without blasting work, and for this two mechanised shields, of the type designated ‘TŠB-3’ were supplied from the Soviet Union [1], [2].

An interesting feature of these machines was the application of an extruded lining. In total three such prototypes were manufactured, two of them came to Prague for Metro construction. In this time period the with extruded lining was developed in Germany and Japan, and a bright future was expected for it. Though in the end it was likely ahead of its time and was ultimately not as successful as expected. The latest opinions of tunnelling experts suggest that this technology, owing to huge developments in materials, admixtures and equipment, might in the future have considerable usage.

For example, compared to segments there might be fewer problems arising from joints, settlement, and safety. In the future, the existing extruded lining under the Vltava River will be a good reference point. The extruded lining is, in respect of its watertightness, still in a good condition.

Before the first TBM bore, due to the existing uncertainty of boring under the Vltava River, the advance gallery was built, serving for geological surveys, with its concrete invert also providing support to the TBM. The tunnel overburden was improved by grout injected above by boat. The TBM completed both tunnels on the line I.A going under the river from Klárov to Staromestská, one tunnel from Staromestská to Mustek and, on the line I.B, the northern tunnel from Mustek station to Florenc station. The maximum monthly progress was 132m (on the I.B line). Compared to advance rates on the extension of line V.A this is a rather slow advance, but the stretches between the stations were shorter and therefore the learning curve had bigger impact on the rates. The main problem was a need to wait for the extruded concrete to achieve the required strength before formwork could be re-erected. But the main reason for the increase in efficiency is from the huge advancements in TBM technology made in last 20 years.

Since 1985 Metrostav used a shield with a roadheader boom for boring running tunnels. The first prototype was named RŠF-1, and it was the first Czechoslovakian shield equipped with a roadheader boom, the AM-100 produced by Austrian company Voest-Alpine [4]. Coordinated by Metrostav, more than 30 firms participated in developing the machine. On the Metro line II.B this equipment saw an average of 80m per month, the best monthly advance was 164m, which has been the highest rate on running tunnels achieved at that time. During the boring there were problems with keeping correct vertical alignment. There were plans to develop and produce more machines, but after 1989 the development followed a different path.

For running tunnels, during the first period, a cast iron lining was used, which was expensive and and not enough material was available. Therefore a combined lining was often used with the invert segment made of concrete, and the rest cast iron, and later a concrete segmental lining (from Hungary), which was gradually modified by having a one-sided or two-sided at invert segment. The grouting behind the lining was completed in two stages. After assembling the ring the front space behind the lining was provided with brick wall and upwards filling with low pressure (up to 3 bars) cement mortar grouting. With some distance behind the face the so-called sealing grouting with cement grout and higher pressures up to 10 bars was then applied.

The advantage of cast iron lining was, although with time consuming effort, the possibility to seal the joints with caulked lead. The concrete segment joints were sealed with expanding cement, later using chorda with expanding cement; this seal having its own weaknesses. Generally it could be stated that the sealing of the segmental linings was not optimal and Metrostav in the late ’80s started negotiation with Wayss & Freytag for the production of concrete segments with sealing rubber gaskets. The agreement was however not completed due to the events of 1989.

Stations and escalator tunnels
The first mined stations on the Line I.A were so called ‘pillar’ type, as used at Muzeum Station, or ‘column’ type, at Mustek Station. Both stations had cast iron segmental lining. Hradcanská and Námestí Míru stations had a concrete segmental lining, and since their construction there has been further development of mined stations with concrete precast lining. That particular lining was similar to the lining used in Kiev (at that time the USSR). Cross passages were created with the help of a in-situ cast reinforced concrete beam, which rested on pillars. These were built after the tunnels were excavated, starting with extraction of relevant segments, followed by reinforcement installation and in the end concreting the pillar width of 1.5m (width of two rings of segmental lining 7.8m/8.8m). This method, undertaken in the complicated and confined underground conditions, and requiring a very high strength of concrete to be achieved (mainly for the beam), called for a change. And such fundamental change happened during construction of station Jirího z Podebrad on Line II.A. The width of the pillar was reduced to the width of the ring, 0.75m. And a steel pillar consisting of two parts, the size of each being similar to the lining segment, was designed.

This enabled the installation of pillars by erectors during tunnel excavation. To be able to reduce the pillar cross dimension, the distance between the tracks was reduced from 23.4m to 21.0m. Another change was made by replacing the in-situ cast concrete by steel beam, installed after completing the tunnel, axially on the pillar; the width of cross passage was 3m, the same as in the original type. The steel beam was activated by steel fibre concrete, pressed between the steel beam upper ange and supporting rib of segmental lining.

The construction method used at station Jirího z Podebrad was successful and this type of station was further developed and improved. In the next phases the pillars with steel as a main bearing element were developed, compared to station Jirího z Podebrad, where the bearing element was also concrete. Further, the width of cross passage was increased to 3.75m, the size of the lower steel beam was decreased with the aim to save the steel volume. This station type was in the end standardised and named the ‘Prague Type’, [3]. On all lines, a total of 12 such stations with precast concrete linings were built, where the principle of pillar assembly during tunnel excavation, followed by installation of steel beams, was applied. All escalator tunnels have been built using cast iron segmental linings, most of them with a 7m internal diameter and using erectors during construction. When the layers of sediments with underground water had to be tunnelled through, the cement grouting with ‘tube a manchete’ method was performed by the specialised contractor Vodní Stavby.

Review of the period up to 1989
All things considered, the construction of tunnels and stations in difficult geological conditions, in densely built-up areas of historical city, and with a minimum impact on the surface, was successful. During the construction process a number of inhouse methods and design solutions were developed and the result was a civil engineering project, which 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 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 technologies 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 both of running and station tunnels was a weak point. With the cast iron lining, the joint insullation 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, in combination with rail bound transport, necessitated the use of small loaders, although later continual loaders were introduced to replace small PPN loaders. 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, sometimes even sinkholes on the surface. But media influence was not as it is today, and such events were not so publicised.