Mechanized shield

A significant technology, unique to this day, was used (not only) to build the Vltava riverbed underpass on the first section of Line A. It involved the construction of tunnels with a pressed lining - also known as definitive precast concrete lining. According to the available information, Czechoslovakia became the second country in the world to use this technology in metro construction.

The principle of using pressed concrete for tunnel lining was patented at the beginning of the 20th century in several countries - Germany (1910), France (1911) and Russia (1912). One of the first tunnels to be built in this way was the excavated collector tunnel under the Oise River in France, with a diameter of 2.63 m and lining thickness of 23 cm. The concrete mix was placed behind the formwork by hand.

The German company Halinger built several small tunnels in this way at the beginning of the 20th century. However, the method was not extended to larger traffic tunnels because it was not technically possible to mechanize it at the time. This was only achieved in the late 1960s in the Soviet Union with the use of rebuildable formwork. In 1972-1973, 825 m of the hydrotechnical tunnel of the Great Stavropol Canal were excavated by a tunnelling machine constructed analogously to the TŠčB-3 design.

In September 1973, a prototype of the TŠčB-3 excavation system was assembled at the diversion tunnel of the Alget reservoir in Georgia. To gain experience, a group of Metrostav employees took part in the assembly, as two identical machines were purchased for the Prague metro and were delivered a year later. The actual assembly of the two systems posed a major technical challenge, as they arrived as unmarked parts accompanied by very scant documentation. It was the finest hour for Ing. Hřebíček, who was very instrumental in putting the shields together. The assembly took place in the assembly chamber near Mánes Bridge, from where both machines excavated towards Staroměstská station.

Much attention was paid to the first Vltava underpass. A geological survey identified six fault zones. These were injected in two ways before the excavation by the mechanized shields - from a boat by injection borehole fans and from a pre-built geological "exploration" adit located in the profile of the right tunnel.  They were chemically injected with a urea-formaldehyde resin hardened with phosphoric acid to form an artificial resin - aminoplast.

The first shield, which passed below the Vltava river, broke through to the Staroměstská station. It was pulled through and continued on to the Můstek station, where it was dismantled. The second machine in the left tunnel tube was supposed to follow the same scenario, but for time reasons, with regard to the construction readiness of the line tunnels for laying the track superstructure, it was dismantled at Staroměstská station. During the first deployment of the mechanized shields, the builders gradually introduced new procedures, work organization and design changes to the shield complexes themselves. Thus, regular repair and maintenance shifts for the shields were introduced, which fell on Saturdays. For the concrete, which was initially lowered underground by a shaft in carts, boreholes were gradually drilled along the tunnel route behind the shield. To ensure the quality of the concrete, batched ingredients were brought in from the concrete plant, with water being replenished on site just before the mixture was lowered underground. The peripheral discs on the cutting head were decommissioned as their operation was not necessary even in the smallest directional curves of 350-400 m.

All the experience and modifications of the shield were used during the second re-deployment of the system, in the left tunnel of the IB route in the section between Můstek - Florenc stations with the passage through Náměstí Republiky station. It was 1980. Particular attention was paid to continuous operation, precise daily maintenance and planned preventive repairs to avoid unplanned downtime. The highest monthly progress of 90.5 m was achieved on this section.

Perhaps as if to live up to the motto that one should save the best until last, this excavation was also the proverbial swan song of the TŠčB-3 mechanised shields in the Prague metro. The pressed concrete tunnels are still in use today on Line A and B and visually they demonstrate satisfactory quality with no significant cracks, leaks, or other defects in the construction of the lining, which by the nature of its construction is not reinforced or insulated against groundwater seepage.