Two Herrenknecht AVND micromachines handled the difficult drives with a total of 5,860 m with bravura.

As well as pipe jacking, with extraordinarily large diameters of 3 m, the extreme winter conditions and a high groundwater level, demanded top performance by everyone involved. With performances of up to 36 m a day, the Herrenknecht machines blazed the trail for an efficient new wastewater system in the Polish capital fast and efficiently.

Wastewater systems in many large cities have already reached their limits requiring modernisation or expansion in order to avoid overflowing into nearby rivers. For this reason, the city of Warsaw is investing in the construction of a state-of-the-art wastewater treatment system. This will treat the wastewater produced by 80 per cent of Warsaw’s inhabitants, thereby improving the quality of water and life along the banks of the Vistula over the long term. Hydrobudowa 9 S.A., PRG Metro Sp.z.o.o. and KWG (PBG Group) were commissioned to connect the plant with the underground wastewater network.

The Polish contractors selected the pipe jacking method for the tunnel project and relied on the two AVND2000AB (with Extension Kit, Ø 3,025 mm) and AVND2400AB (Ø 3,025 mm) hydroshields from Herrenknecht. This type of machine is used globally in the construction of supply and disposal tunnels; the machines are able to tunnel through groundwater and can penetrate a wide range of heterogeneous soils. In Warsaw, the machines tunnelled at depths of 10 to 14 m reaching their final targets after boring through 5,860 m of soil comprising coarse and fine sand, clay, loam, gravel and boulders.

Project challenges

The high groundwater level, which in some places reached up to 8 m above the top of the pipes, was a significant challenge on the project. The site teams were obliged to dewater and seal the requisite construction shafts prior to tunnelling. The Herrenknecht AVND pipe jacking machines proved its worth during this tunnelling process.

Where required, these machines secure the tunnel face using a support fluid, such as bentonite and water mixture, and an additional compressed air cushion in the split excavation chamber, allowing safe tunnelling, under water pressure. The separation of solids of the excavated material from fluids was realised with the Herrenknecht Separation Plant HSP500, providing a capacity of 400 to 500 m³/h depending on the particle size distribution.

During the particularly cold winter months of 2010, the site team continued tunnelling work despite some extremely low temperatures and a lengthy period of frost. The micromachines completed a total of 16 drives, some with short distances starting at 80 m, but also including long-distance sections of 800 m and more.

The longest individual drive was 930 m. The hydroshields safely under-passed a six-lane highway without having any negative impact on the flow of traffic. Hydraulic intermediate jacking stations serve to reduce the friction forces prevailing in such long-distance tunnelling projects. Herrenknecht had already adapted these interjack stations exactly to the thin walls of the HOBAS GRP pipes installed in Warsaw.

The longest section revealed yet another particular feature: two underground curves with a radius of 450 m and 900 m had to be realised. Thanks to the SLS RV navigation system the cutterhead is guided exactly with millimetre precision, the machine operators also mastered this task in a precise and safe manner.

The last section was completed on 14 December 2010, representing a significant milestone for the wastewater project. During the construction project, the equipment achieved impressive advance rates of up to 36 m a day, on a double shift, and an average of 20 m per day.

To complete the entire sewer system and connect it with Czajka wastewater treatment plant (WWTP), PRG Metro Sp.z.o.o. and Hydrobudowa 9 S.A. are also completing the 1,300 m long Vistula River Crossing tunnel with a Herrenknecht large diameter tunnel boring machine. The Mixshield S-642 with a diameter of 5.35 m was successfully accepted in February 2011 in the Schwanau/Germany plant of Herrenknecht. Once this tunnel is completed the Czajka WWTP will go into operation.