The history of Prague Metro construction started in 1966, and the first Metro Line C was opened in May 1974. Lines A, B, and their extensions were built over the years, and today the total length of metro operating lines is 59km with 57 stations.

Contractors are now working on the latest extension of Line A, known as V.A., which has a total length 6.1km (see Figure 1), three mined stations (Petriny, Veleslavin, Cerveny vrch) and one open cut station (Motol Station). The total length of running tunnels is 9.1km. Some 700m is twin track NATM tunnel and the remaining 8.4km is being constructed using two TBMs acquired by contractor Metrostav. Both machines are identical Herrenknecht EPBMs with the production numbers S-609 and S-610; shield diameter is 6m giving a finished segmental lining diameter of 5.3 to 5.8m.

In 2009, client Dopravni podnik hlavniho mesta Praha (Prague Public Transit Company) awarded the construction contract to a Metrostav/Hochtief CZ joint venture, with Metrostav as the JV leader. Hochtief CZ is building the open cut Motol Station, the adjacent twin track NATM tunnel and Cerveny vrch Station. Subterra, a subsidiary of Metrostav, is constructing Veleslavin Station. The consultant is Metroprojekt Praha. A JV consisting of Arcadis Geotechnicka and Inset is responsible for monitoring the project.

Of the three mined stations, two are single vault stations at Petriny and Cerveny vrch, whereas Veleslavin Station is a threevault station; all stations are being built using NATM. During various stages of construction the passage of the TBM has to be provided for. On top of this the logistic support for the TBM, namely the belt conveyor, segment transport system and other materials, have to remain in place for some considerable time. All of this puts pressure on contractors to keep the tight construction schedule. In May 2012 the TBM drives and construction of the stations were approximately half-way through, keeping on schedule for the 2014 opening. Side drifts allowing the passage of the TBM through the stations were driven first, followed later by excavation of top heading, bench and invert. The initial lining consists of shotcrete with steel mesh, lattice girders and bolting. The final lining will be cast after the waterproofing membrane is installed; in Veleslavin Station a sprayed waterproofing membrane is used, in all other sites, classic PE membrane with protective geotextile will be used.

Geology
The EPBMs have been designed to suit the prevailing geological conditions that have been predicted along the designed tunnel alignment. The geology is mainly characterised by clay shale of differing geological age, which makes up Prague’s bedrock and in which many kilometres of tunnel have already been built. Besides the shale, at the start of excavation there are mixed face conditions with, for example, a sandstone layer in the upper part of the face and siltstone and claystone materials at the bottom. Under such conditions boring was frequently accompanied by quite large inflows of water that had collected on the impervious clayey layer. To the end of their drives the TBMs will be challenged by stretches with deluvial deposits consisting of loamy-sandy soil, which are predicted along much of the tunnel alignment.

History of TBMs on the Prague Metro construction
The first use of TBMs on Metro construction goes back to the 1970s when two Soviet made TBMs bored running tunnels during construction of the first part of Line A. The machines were 5.8m diameter and featured an extruded concrete lining, where concrete was pumped from the rear of the TBM into a slip form to cast the lining. The extrusion was a popular method, being developed at the time in Japan and Germany. The machines bored the running tunnels under the Vltava River and also under the Prague Old Town, which has many significant historical buildings. There were no excessive settlements and therefore no damage to the old buildings.

Modern TBM application
After 1989 the prevailing technology for metro construction was NATM, used not only for stations, but also for running tunnels. It took nearly 40 years to reintroduce TBM technology for metro construction. This time it is the latest TBM technology; Metrostav acquired a pair of identical EPBMs manufactured by Herrenknecht after contract award.

The machines started their journey not far from the Motol Hospital, which is the largest hospital in the country, and has large children’s wards. Metrostav’s management and Prague mayor Bohuslav Svoboda arranged for the children from the children’s ward to name the two TBMs. The mayor made the choice of names during the visit of the hospital department. They named S-609 Tonda (equivalent to Tony) and S-610 Adela (Adelaide).

The EPBMs have a diameter of 6,050mm and are equipped with a 1,200kW hydraulic main drive. The TBM is 102m long and weighs approximately 680t. When tunnelling, 32 thrust cylinders (16 pairs) apply a nominal thrust force of about 39,000kN (340 bars). The cutting wheel is equipped for rotation in both directions. It is designed with an opening ratio of 30 per cent and fitted with 38 disc cutters (17-inch) of 100mm spacing, 64 cutting knives and eight buckets. Alternatively the cutter heads were fitted with ripper tools. Two hydraulic wear detection units are installed for the soft ground tools and buckets. Tail skin grouting used a two part system.

Information and monitoring
The availability of actual data in real time is a basic requirement for optimal control of the tunnel driving process and risk minimisation. VMT supplied a modular and web-based multi-user IRIS (Integrated Risk and Information System). This system is combined with another VMT product, Tuis Navigation TBM Laser, and a segment documentation system (SDS) was used in segment production. To check the grouting procedure, a grout pressure sensor system (GPSS) for measuring the grout pressure behind the lining was used.

The system was vital during the TBM passage under sensitive areas, where strict settlement limits were set. In such cases, continuous information on settlement measured above the tunnel route was combined with all relevant operational data from the TBM to allow the development of sensitive tunnelling strategies and, in terms of risk avoidance, to continuously optimise the tunnel excavation.

Another database system used in the project is Barab, operated by Arcadis Geotechnika, in which mainly the settlements and extensometer measurements results are being shown.

Segments
The tunnel is lined with reinforced concrete segments. One 1.5m-long tunnel ring consists of five segments plus a key.

The geometrical design of the segments considers all loads including water pressure and soil conditions, as well as stresses arising from the construction of the tunnel.

The universal-ring, tapered on both sides, has an internal diameter of 5.3m and a thickness of 250mm. Herrenknecht Formwork supplied high-precision moulds. The segments are equipped with EPDMgaskets and are bolted during installation in the circumferential and longitudinal joints.

The segments were produced by Doprastav in the Prefa precast factory in Senec, where in total over 32,000 segments will be produced.

TBM launch and route
Both TBMs started the more than 4km long journey in the purpose built shaft at Vypich site. The shaft lining consists of secant drilled piles of 1,180mm diameter. The shaft internal diameter is 23.6m and depth is 32m. During construction the contractor, Zakladani staveb Praha (part of Metrostav Group), achieved extremely good results with a maximum deviation of 0.7 per cent, and with 70 per cent of piles drilled with deviation less than 0.3 per cent.

An incline tunnel was initially built for the construction of the twin track NATM tunnels constructed by Hochtief CZ, which are connnected to the shaft. Later, this incline tunnel was used for muck removal by conveyor belt for approximately half of the running tunnel excavation.

Once the shaft was connected to the twin track tunnel, two short starting chambers, 10m long were built using NATM. The construction of the launch chambers was followed by the installation of a concrete cradle, in which the shield and cutter head was assembled. The TBM was then launched from a heavy steel thrust frame.

In April 2011 the first machine, Tonda, started boring towards Petriny Station, making the first breakthrough to the partially excavated station (two side drifts were completed) in July, followed by Adela less than two months later. In the station the concrete cradles served for the passage of the TBMs and at the end of station two short 10m launching chambers and in-built thrust frame were constructed, allowing the re-launch of both machines towards the Veleslavin Station. Right behind the station the machines had to pass under the sensitive structure of a residential complex, where the maximum permissible settlement was restricted to just 6mm. The continuous monitoring system was installed in the residential complex, mainly in garages, allowing real time settlement values to be monitored on the TBM and in the site offices. The machines operated in closed EPBM mode for 150m of the route and the settlement limits were complied with, resulting in a maximum settlement of 3.3mm.

The relatively long stretch slightly over 1km between the stations allowed some high advance rates. The best day saw 25 rings, some 37.5m of tunnelling and the best month saw 630m of tunnelling.

The next breakthrough was into the Veleslavin Station, or rather to the open cut box used for station excavation. In the open cut the machines moved on a reusable steel support structure, and in the station concrete cradles were used again. With the same arrangement as in the first station the launching chambers were constructed and the machines sent on a very short 210m drive to the open cut box on the Site E2. Supported by piling, the box is 53m long and 32m wide, with a maximum depth of 26m. The machines arrived to the open cut box in February (Tonda) and March (Adela) 2012.

In the open cut box the movable steel support was used again and machines were dragged through the box and both restarted boring in March, Adela following Tonda after a three week interval. The supply of segments and muck transport by conveyor belts was still managed from the initial launch site at Vypich. This arrangement continued until Tonda broke through into the partially excavated Cerveny vrch Station on 18 April 2012. The tunnel boring in this part of the route was affected by an eight-storey-high building, as well as five 15-storey high-rise tower blocks — all of them above or close to the tunnel alignment. Moreover, this area was in the past used for mining iron ore. An additional geological investigation for verification of the old mining works did not find any of the old works in the tunnel alignment, nevertheless the machines worked in closed EPBM mode in this area. Monitoring of the buildings was again continuous, with real-time values transfered to the TBMs and required terminals. The settlements in the closed EPBM mode were 60-70 per cent of those in the open mode.

Construction of Petriny and Veleslavin could not be completed while TBM supplies were passing through. After the Tonda breakthrough to Cerveny vrch Station, the second machine was stopped 230m in front of the station, before it reached the delicate geology under the high-rise buildings. With both TBMs in stable ground the supply system and muck conveyors were stripped out and rerouted through the intermediate shaft at site E2.

Both machines re-started boring in June this year. Tonda launched after being dragged through Cerveny vrch Station using the reusable steel support and a short starting tunnel with in-built steel thrust frame. The second machine followed the same procedure with a one month offset, after negotiating the short drive to Cerveny vrch Station.

The final interruption of the TBMs journey towards Dejvicka Station will be at site E1, approximately 650m from the final breakthrough. The machines will pass through short 37.5m tunnels perpendicular to the alignment which have relatively large cross sections of 120sqm and a low overburden around 7m.

The tunnel was built by NATM with extensive jet-grouting to improve the ground. Both machines will have to pass through this tunnel, and later, after the machines are dismantled at the Dejvicka Station, the tunnel will allow removal of the shield and back-up.

Logistics
H+E Logistik supplied the belt conveyor system consisting of 600mm-wide belts and including all supporting structures for 5.3km of tunnel: two vertical belt storage towers and main drive with installed power of two x 160kW.

The system was first installed on the Vypich site. Here an additional drive was added for an 800mm-wide, 300m-long belt that ran from the transfer station — situated in twin track tunnel — through the access tunnel and to the surface. Once the machines were behind the intermediate shaft at site E2, the whole system was transferred there. The belts at E2 are running directly to the surface, with a relatively high gradient of 17 degrees.

A multi service vehicle (MSV) system was chosen for transporting segments and other materials. Altogether, on the site there are four MSV with 18t capacity supplied by Herrenknecht and manufactured by TechniMetal Systemes.

Experience from the site
Metrostav hired TBM consultant John Foster from the UK, who was involved in negotiations concerning the technical details and both factory and site acceptance procedures. Professor M. Thewes is also consulting on specific problems related to the TBM and segmental lining.

Since the TBMs delivery, Herrenknecht personnel have been permanently on site, as well as supporting from their factory in Schwanau, Germany. The cooperation of all involved was crucial in solving the usual teething problems, such as segment production, TBM assembly and start/restart situations. The assistance of specialised suppliers such as BASF, Mapei Condat and Minova was relied upon from the start.

The initial problems with two component grouting were overcome and the opening of the first cross passage in May 2012 confirmed the results of grout fill random testing — the annular gap is fully filled. Thanks to Metrostav’s own improvements the cleaning procedures required were minimised.

In some parts of the tunnels segment cracking occurred, and extensive efforts were made to discover the reasons. The latest results show there were either several single sources, or a combination of factors leading to hairline cracks in the 250mm thick segments. One possibility is segments cracking in the tail skin under pressure from the thrust rams, mainly due to tail skin drift, especially when TBMs were boring in the 660m curve and downhill with a four per cent gradient. The solution has been to improve the driving technique of the TBMs and to fine-tune the VMT system to include the tail skin position factor for the choice of segment type. The machines do not have active articulation of the rear shield.

The cutter head of Tonda was fitted with 20 no. 17-inch disc cutters at the start of boring, while Adela’s cutter head was fitted with 17 rippers and four single-disc gauge cutters. In the first 500m of mixed face conditions, with clayey material and abrasive sandstone, the discs occasionally became stuck and damaged. After getting through such conditions, both cutter heads were fitted with disc cutters to bore through the shale.

The most complicated part of the construction process was, and still is, the frequent interruptions of TBM drives to pass the stations and open box. The site team made significant improvements by introducing steel structures for the TBM passage through partially built stations, which can be re-used for each particular TBM and also for differently-mined stations and open boxes.

With hindsight it could be stated that a different approach, allowing continuous boring with TBMs, would be more effective in terms of time and cost, and such a conclusion should be fully considered during preparation for the planned construction of Line D.

Status to the end of June 2012
By the end of June 2012 both TBMs had completed 4.6km of tunnels, which represents 55 per cent of the total running tunnels’ length. The belt conveyor system from the E2 site shall be operational by the middle of June 2012 when Adela starts boring the remaining 230m to Cerveny vrch Station. One-month later Tonda will start boring the remaining 1.7km of tunnel. The machines are expected to complete their respective drives in November and December 2012. Dismantling will be done in the chamber prepared at the end of the existing Dejvicka Station, where the preparatory works have been underway since last year.

The opening date for the Metro Extension VA is scheduled for 2014, which marks the 40th anniversary of opening the first Metro Line in Prague.