- Home
- LP - By metro50 (11/2024)
Earth shields
In 1994, the Channel Tunnel was put into operation. The Eurotunnel symbolically linked the UK to continental Europe. From a purely civil engineering point of view, however, it confirmed that mechanized tunnelling was a highly suitable method of tunnelling for long-distance tunnels, both technologically, time-wise, and economically. This method is characterized by the use of full-bore tunnelling machines, which underwent powerful development at the turn of the millennium. It was, therefore, only a matter of time before this method and the tunnelling machines were deployed in the Czech Republic. A favourable moment came with the extension of Line A of the Prague metro from Dejvická station to Motol Hospital.
Here, two earth shields manufactured by the German company Herrenknecht with the serial numbers S-609 and S-610 were deployed. The shields consisted of an 8.5 metre-long shield part and a 93 metre-long suspended part. All the excavation cycle operations were carried out in the shield section, such as breaking the rock with the cutting head, supporting the face with treated spoil, spoil removal, and securing the excavation with a segmented lining. The suspension part, consisting of seven suspended cars, contained all the technology required for the shield's operation and control, equipment for handling the segments, for grouting the inter-ring between the excavation as well as the segments, and for pumping mine water, ventilation, conveyor belt extensions, facilities for the work crew, and so on. The shields were equipped with a number of computer units and monitoring sensors that collected various data on the state of the machine, but also indirectly on the behaviour of the rock environment or the amount of fill grouting outside the lining. The machine was operated from the cockpit, which contained a control console and four screens displaying operational data, navigation, spoil consistency, etc. In line with the fact that mechanised tunnelling has evolved in the early 21st century into a complex system in which shields are only one element of a whole set of technologies, it is worth mentioning all the accompanying technologies that were used on Line A of the metro.
These included a production plant for prefabricated reinforced concrete segments, designed specifically for the tunnelling machine, and a production plant for filling mortar including the pumps for transporting it to the tunnelling machine. The material logistics was provided by multi-purpose wheeled platforms (MWP), and a conveyor belt system was used for the extraction of muck. Everything had to be designed with sufficient capacity so that none of the sub-technologies was the proverbial weakest link in the chain and neither of the two tunnelling machines was limited by any of the technologies.
Facts in numbers
- 102 mLength of earth shield
- 4,4 MNmNominal cutting head torque
- 6,1 mCutting head diameter
The first of the machines, the Herrenknecht S-609, was assembled during the first quarter of 2011, and in April of that year it started excavating from the shaft from Vypich to Petřiny station. The second machine, the S-610, commenced excavation from the same location three months later. Both machines headed for Petřiny station which was under construction, were pulled through the station, and at the end of the station they went on to excavate the following intermediate section. This scenario was repeated at Nádraží Veleslavín and Bořislavka stations. After completing about half of the four-kilometre section, all the accompanying technologies were moved from the Na Vypichu construction site to the construction site on Evropská Street. Since the second machine was able to benefit from the experience gained from the excavation by the first one, the S-610 earth shield managed to excavate faster than the S-609, thus gradually reducing the original three-month time gap. This enabled the two machines to run together in the section to Dejvická station, allowing for a spectacular joint breakthrough into the area of the station's turntable tracks. The first deployment of the earth shield technology in Prague's geological conditions was successful. The ability to minimize the subsidence of objects in the surface impact zone was demonstrated by both shields. The required construction speed of both single-track tunnels was also achieved. The implementation team delivered many improvements and innovations both during the actual excavation (one of the solutions was patented) and, especially, during the passes of both machines through the stations, their starts, or disassembly.
This text was compiled from the following sources:
Magazine Tunel 1/2023, Štíty na ražbách pražského metra