The change of political system, and therefore the possibility of importing modern technologies, was a turning point in Prague’s metro construction. The application of NATM gained pace, and was used for stations, escalators and running tunnels. For drilling, jumbos were employed and for the primary support bolting and shotcrete with lattice girders were used. The lining of both running and station tunnels was double-pass. After the primary lining application, the installation of a waterproofing layer would follow, and in the end the secondary (final) lining was cast into the formwork. Contrasting with the earlier projects, the application of precast segmental linings was completely abandoned. For subsequent running tunnels however, this construction was considered not quite suited to the situation. This was often championed by the noted Czech consultant Zdenek Eisenstein. He was a keen promoter of TBM use for running tunnels in Prague and it is regrettable that he is no longer here to see them on metro extension V.A. The domination of NATM for construction of running tunnels ended in 2010, when Metrostav purchased two EPBMs for construction of metro extension V.A.

A very significant achievement on the Prague Metro has been the underpassing of Vltava River on the IV.C extension. Applied technology is one of the biggest contributions made by Czech tunnelling to the global industry.

NATM APPLICATION It was only matter of time before the ring method with segmental lining would gradually be replaced by NATM, which was a dream of many tunnelling experts during the 1980s. Introduction of the method was done with help of the State Research Project, which was approved in 1989. The main coordinator was Metrostav and the research started as planned in 1990 and continued through 1991, financing of the research besides the state was by Metrostav, Vojenské Stavby, Subterra and Železnicní Stavitelství. In 1991 the research ended due to the changes in financing, but the results gained during two years were used and NATM had started on site.

At the start of 1989 the first experimental application of NATM took place for the excavation of a gallery for special purposes on Florenc (formerly Sokolovská) Station. As a part of the experiment it was required to verify and evaluate the geological conditions, prepare the design documentation, evaluate new support elements, create monitoring documentation and prepare testing of shotcrete. After this first approach, a few drifts and parts of station tunnels in primary lining were executed at stations Vysocanská and Hloubetín on metro line IV.B, and already more than 2km of running tunnels with double-lining and intermediate waterproofing were built on the same line.

The first double track metro tunnel built by NATM, between 1995 and 1997, was also on the same project (IV.B). For its construction, the machinery and equipment customarily used for NATM abroad, was for the first time used on site. For four escalators tunnels on this line, three of them have been constructed by NATM.

Another turning point in NATM application was the construction of the first single vault station on the line IV.C [7]. The first attempts to build single vault stations happened in the 1980s and it was another dream of a whole generation of tunnelling experts, being fulfilled in 2002, when the excavation of station Kobylisy was performed by Metrostav.

The cross sectional area of the station was 228 m2, the length of the station was 148m. The primary lining thickness with lattice girders was 400mm, and the _ nal lining with intermediate waterproofing has a thickness of 600mm. The excavation was with both vertical and horizontal subdivision of the tunnel face, using both mechanical excavation by tunnel excavator and drill and blast. Permanent monitoring was contracted to Stavební Geologie-Geotechnika Prague, the maximum measured settlement above the station was 45mm and the effects on buildings were minimal.

UNDERPASSING THE VLTAVA RIVER ON THE LINE IV.C EXTENSION

Underpassing the Vltava River by the so-called immersed tube ‘launching’ method was, and still is, one the most important contributions of the Czech tunnelling community to the wider industry [8]. The chosen method was based on the construction of a complete tunnel tube in the dry dock, which in later stages served as an open pit for construction of running tunnel structures on the Trója river bank. The trench for the first tube was dug in the riverbed, the trench for the second one was dug after complete placement of the first tube. The dry dock was separated from the river by steel sheet pile wall, part of which was opened after completion and fitting out of the tube and flooding of the dry dock. Fit out included the installation of internal water balance tanks and provided both faces with a temporary steel cap. Two towing systems were fixed to the cast-in anchor elements in the ceiling slab. A horizontal system (towing and braking) fixed the tube position during launching and, a vertical system, in the form of suspension cables, set the height of tube above the trench bottom. The rear part of tube was shifted on the prepared track in the dry dock.

The tube was moved by help of two towing cables anchored on the opposite Holešovice river bank. The rear brake suspension prestressed the cable system and enabled braking of the movement. Vertical suspension was connected to the pontoon, which carried the weight of the tube in the front part, reduced by the buoyancy. The rear sliding part of the tube carried a majority of the weight and secured the stability of the whole body.

The forces required to move the tube were relatively small, the weight in the water was only one per cent of actual weight. After the support structure, built in a cofferdam on the Holešovice bank, was reached, the tube was supported on both banks and was stabilised. The support to the river bed followed, by concreting in the regular intervals and anchoring the tube. The weight of the tube was 6,700t, the length 168m, outside dimensions 6.48m by 6.48m, the thickness of the invert and ceiling 700mm and thickness of walls 730mm. The horizontal curvature was 750m in the right track and 670m in the left track, vertical curvature 3,800m. The construction method was completely new and unusual and therefore during planning and construction there was a need to solve important technical problems, some of them requiring large rate of innovative approaches:

  • Watertightness of concrete structure – the tunnel has no insulation and therefore it was necessary to solve sequence of casting, joint sealing and reinforcement in a way to avoid creation of cracks;
  • The tube balancing – the curved shape of tube is the cause of its instability in the water. The reliable establishment of tube reactions was an essential factor for maintaining sufficient structural stability during manipulation. The weight calculation might be encumbered with a high degree of uncertainties, therefore the experimental verification of actual weight and its distribution both in transversal and longitudinal direction was required. This was achieved by help of hydraulic cylinders placed under the tube, which after the dock flooding lifted the tube and from pressures in cylinders the actual weight and reactions were recalculated and the final loading by water tanks designed;
  • Additional tube loading system – the tube dimensions were chosen to allow floating, but due to the instability caused by the curved shape this was undesirable and therefore the water tanks were filled in such way that the tube was not floating, but at the same time its weight was adequate with respect to the bending stresses and stability (approximately one per cent of actual weight);
  • Foundation strip – the tunnel in the dry dock was cast on specially treated sub-base, which allowed _ owing of water under the tube, providing the required buoyancy for manipulation. Concrete foundation strips reached under the tube only to the distance of 0.5m and the remaining part of the space was filled by gravel, and plastic pipes served for water supply;
  • Tube launching – the movement had to be fluent, which superimposed considerable demands on hydraulic equipment, and also accurate, because the tube passed through a narrow opening in the sheet pile wall. Geodetic monitoring also allowed for steering of the tube when it was submerged.

Execution of the running tunnels, by the exceptional technology of launching into the Vltava River, was very successful thanks to all involved, namely IDS Praha, Metroprojekt Praha, Zakládání Staveb, VSL Systems CZ, Peri, VUT Brno, Professor. Tichý (risk analysis), Professor Šmerda (calculations). Division Six of Metrostav was responsible for execution; the author of the construction method was Professor J. L. Vítek from Metrostav’s head office team. The designed method brought savings in time and budget when compared with alternative methods.

The original project contributed to the reliable and high quality construction of metro tunnels and received prestigious awards and recognition from the International Federation for Structural Concrete (FIB) and the Engineering Academy of the Czech Republic