Ring method

It was the ring method, which was nicknamed the "Prague method" in the literature, that became the dominant method on the Prague underground from the 1960s to the first half of the 1990s. Not only the line tunnels, but also the station and escalator tunnels were constructed using it.

A prerequisite for the use of the ring method is a geological environment in which the stability of the free excavation for the length of the advance is sufficient to allow the lining to be installed in time. Prague´s geology offered just such an environment. The advance - usually to a length of 1 m - was carried out by blasting. After the broken rock was removed, a partial circular reinforced concrete or cast iron lining was installed in the open space. The individual parts, once assembled, formed rings, which gave the name to the whole method. In order for the ring lining to be structurally functional in the rock excavation, it had to be activated with the surrounding geological environment by grouting.

The ring lining is the final lining and must be sufficiently load-bearing and watertight. The segments are bolted together within the rings and spatially arranged in a bond. The watertightness of the joints is ensured by inserted elements. The line tunnel segments are not lined in any way and the engine drivers are thus given a stark view of the tunnel as it was built more than half a century ago.

The characteristic tunnelling machine was the 'erector'. This was a mobile platform from which the boring work on the face was carried out. After drilling, the rock was broken to a length of about 1 m by means of explosives. The blasted rock was loaded by a rail loader into the narrow-track groove carriages. A ring of segments was built into the drilled area using a stacker. Behind the erector platform was a grouting platform from which the space between the rock and the ring lining was grouted. The purpose of the grouting was to fix the tunnel lining rings in the excavation. 

There were several types of erectors - station, track, and escalator. This is because each of the tunnels had different dimensions. All were circular in cross-section, but the track tunnel had an excavation diameter of 5.8 m, the station tunnel 7.5 - 9.5 m and the escalator tunnel 7 m. The first erectors were imported from the Soviet Union, but gradually began to be produced by local companies - as early as the end of the 1960s by the national enterprise Výstavba kamenouhelných doly Kladno, and later by Metrostav, which developed an erector of its own design.