Non-mechanized shield

Similarly, the Soviet KM34 tunnelling shield, which was deployed for the Prague Metro in 1969, was a steel protective envelope with a working platform inside. From this platform, workers used hand-held pneumatic jackhammers to break the rock from the face. The rubble was lifted by front-end loaders into mine cars and transported along a narrow-gauge railway line. Two platforms (an erector platform and a grouting platform) were attached behind the face, with the erector platform used primarily for mounting and assembling the tunnel lining segments. The ring lining was circular, with an outer diameter of 5.5 m and an inner diameter of 5.1 m. The first tunnels of the Prague Metro were excavated using cast iron segments. The space between the excavation and the segmental lining was filled from the grouting platform. The grouting was used to stabilize the segmental tunnel lining in the rock environment. For fans of technical descriptions, the erector platform was called TU 3 Gp and the grouting platform was TN 16 Gp.

In places where the face did not show sufficient stability, wooden reinforcements were used in the front of the shield, again using a technique similar in principle to that used by the "father" of this tunnelling method - Marc Brunel.   

The first shield system for the Prague metro, designated KM34, was purchased in 1968, and its assembly in the construction chamber near Štětkova Street also began that year. The complex was ready at the beginning of 1969, when construction of the 454 m long tunnel of Line C to Pražského povstání station began. The initial training of the crews was carried out in cooperation with Soviet workers.

The average monthly progress on the first tunnel was 54.5 m. After the completion of the tunnel, the shield was moved back to the construction chamber from which the second tunnel tube was excavated. Shield relocation was planned prior to the breakthrough, with full or partial dismantling and reassembly considered. However, Metrostav engineers came up with an idea to move the shield without dismantling it. They planned this unique solution, which resulted in significant time and financial savings, down to the smallest detail; for instance, Ing. Ermín Stehlík made a small-scale paper model, which he used for trying out various relocation scenarios at home.

On the second tunnel, an average monthly progress of 67.6 m was achieved. Both tunnels were broken through in close proximity to the Gothic St. Pankrác Church, which was not significantly affected by the excavation.

Another part of the first stage of the C route, which was excavated with a non-mechanized shield, was the Pankrác (then Mládežnická) – Budějovická intermediate section, where the shield was moved again without being dismantled. Here, the excavation workers faced the difficult technical task of reaching only 120 cm below the foundations of the five-storey building on Pacovská Street. This building was continuously monitored for seven days by a device developed specifically for the Prague metro. The subsidence averaged 7 mm in total and, apart from hairline cracks on the basement walls, there were no other signs of failure. The shield successfully reached Budějovická station and again managed to push the maximum values of monthly progress to 79 m (for the right tunnel) and 80 m (for the left tunnel).

A second excavation shield was purchased in the USSR for the construction of both line tunnels between the stations Hlavní nádraží and Florenc (then Sokolovská). The right-hand tunnel to the was excavated on the down gradient in the alluvium and gravel sands of the Maniny terrace with considerable inflows of groundwater, the level of which, before pumping, reached 3.5 to 4 m into the tunnel cross-section. In order to lower the groundwater level, 42 wells were dug in the trackway of the Masaryk railway station, which continuously pumped the groundwater and lowered its level by 2 m.

Despite this measure, the inflow at the tunnel head was between 7 and 9 litres per second. During the construction of this section, for the first time, the shield was not moved to excavate the adjacent tunnel tube but was rotated in the roofed chamber of Florenc station.

The era of non-mechanized shields continued from the first stage of Line C to the second stage, then to both Line A and B. It lasted until the early 1990s. During that time, over 18 km of line tunnels were bored by non-mechanized shields, which were put to a wide variety of tasks. Some interesting facts about non-mechanized shields include:

• the shield assembly and disassembly were carried out both in cut-and-cover pits and in excavated assembly chambers underground;

• the procedure of shield transportation without dismantling was successfully used to move the shields to the excavation sites of subsequent sections, with U-turns applied in some parts; authentic footage of one of the shield moves without dismantling was used in the film "Metro People";

• it was possible to closely (with a distance of only a few tens of decimetres) pass under various objects – residential buildings, industrial buildings, the D1 motorway – without restricting the traffic;

• during excavation works in Sinkulova Street, a residential building and business premises passed under were temporarily evacuated during the construction;

• the shields were used not only in soil but also in solid rock with the simultaneous use of blasting, which, according to an evaluation carried out, did not cause significant damage to the shield structure;

• various types of lining were used for tunnel reinforcement – initially, it was cast-iron ring lining, built-in due to the low overburden, dense development on the surface, and small directional radii during shielding; in later years, there was a significant transition to reinforced concrete parts, initially imported from Hungary, later produced in the Lužec and Hýskov prefab plants;

• the highest monthly output during excavation by non-mechanized shields was achieved in 1989: 117 m in August and up to 142 m in November.