Microtunneling to Exacting Standards in Holland

Jul 07, 2005

Not only were ground conditions difficult and the water table high, but also tunneling under an excavated canal with banks higher than the surrounding land required exceptional safety measures.

A water station in Nieuwegein, Holland, transports pre-treated surface water via dual pipelines to the dune purification plant of the Gemeente Amsterdam Waterleiding Bedrijf (City of Amsterdam Water Company) in Vogelenzang and the PWN Waterleidingbedrijf Noord Holland in Heemskerk. Important customers of pre-treated surface water are Corus Steel, Crown Van Gelder Papierfabrieken and industries in Amsterdam's western harbor area.
The WRK I and II pipelines between Utrecht and Abcoude run parallel with the A2 Freeway. During the next few years, Rijkswaterstaat (Directorate-General for Public Works and Water Management) is going to expand the A2 Freeway between Amsterdam and Utrecht from three to four lanes. For this reason, the pipeline to the east of the freeway must be moved some 60 m eastward over a distance of about 5 km.
Amsterdam Water carried out the preparatory survey work itself and had been able to decide the course the pipes needed to follow. After a pre-selection process, a building consortium, Visser & Smit Hanab, Papendrecht, Netherlands/Dura Vermeer Infra, Hoofddorp, Netherlands, was chosen as the most suitable partners. Good cooperation between Amsterdam Water and the chosen partners meant that a quick match between the owner's theoretical approach and the contractor's practical approach was possible and this saved time to produced a good, final, detailed plan.
The building consortium had little time for the drilling activities. The project involved a dual pipeline, with the final distance to be drilled of 9.5 km. Four tunnel boring machines, two supplied by Herrenknecht and two designed and manufactured by Dura Vermeer were used by two teams for seven days a week for eight months, four days on and four days off. The machines worked two-by-two toward each other using the slurry shield tunneling method. The route for the installation was divided into sections of a maximum of 900 m (twice 450 m from each direction). The drive pits were constructed at both ends of the section to be drilled (900 m apart) with a reception pit in the middle. The drive pits were specially designed to be used in two configurations in two directions. This was an attractive solution in terms of logistics and economics because the drive pits were larger than the reception pits, and more materials were needed, such as sheet piling, underwater concrete and reinforcement steel. Dual use meant less construction, lower costs and greater efficiency. After the first two parallel-drilled holes were ready, the machines were reversed and worked in the opposite direction.
In the soft peat soil, the boring machines could not be offset against the sheet piling as they would normally. The clay/water mixture does not exert much counter-pressure, so the piling would simply be pushed away unless additional measures were taken. For this reason, large profiles were filled in the middle of the concrete floor with frames behind it to offset the drilling frame and transfer the thrust to the surroundings through the concrete floor.
After the new pipeline had been laid and connected to the existing one, the section to be replaced was removed. This meant removing around 1,500 concrete pipe sections. Because the system could not be shut down completely and water transfer had to be maintained throughout, the pipelines were connected one at a time.
Soil Characteristics

The new part of the freeway is being built on an enormous sand bank whose weight compresses the peat soil layer. Everything else under the freeway can also be compressed and moved, including a pipeline. So, after examining the various options, Amsterdam Water chose to relocate the new pipeline as deep as economically possible. The traditional method of construction by excavating a trench was also discounted because it would have been very expensive and complex since the peat extends from the surface to a depth of 6 to 8 m. Instead Amsterdam Water chose to place the pipeline in the Pleistocene sand layer under the peat since the new freeway does not affect a pipeline at that depth. For the construction of a pipeline of this size (outside diameter of 1,200 mm), both concrete and steel can be used. The consortium chose steel for economic reasons.
During the preparation, it became apparent that the permit authority, Hoogheemraadschap Amstel Gooi en Vecht, would set exacting standards where the pipeline passed under the Geuzensloot. The Geuzensloot is an excavated canal at NAP level (Normal Amsterdam Water level). Because the banks of the canal are regarded as primary water-retaining structures, the Hoogheemraadschap standards are set for exceptional levels of construction stability and safety. For the crossings the new pipelines pass under the Geuzensloot in additional casing pipes with a diameter of 1,500 mm. This alteration to the plans meant more work and more time to get it done.
The entire project was carried out with minimal removal of groundwater. The removal of groundwater and its discharge are sensitive topics in the Netherlands — especially in view of the fact that the pipeline is being built at great depth through boring and a great deal of drainage would normally be required in peat soil to keep the pits dry. This meant that an alternative solution was required. The method chosen was more expensive than pumping out excess water, but did not have the discharge problem. The pits were designed to be waterproof and the pipe jacking was carried out without allowing water in flow. The pipes were jacked through a special rubber seal designed to withstand both the hydro-static pressure and the pipe movement to prevent ground water from entering the pit.
Amsterdam Water and the contractor’s consortium approached this project as a joint team from an early stage. The fruits are evident in the quality of the detailed plan, while short communication lines and quick responses for decisions saved time. The Visser & Smit Hanab/Dura Vermeer consortium was able to carry out the project in the most economical way and delivered the result that the customer wanted — an optimal win-win situation.

Where a technically complex project is concerned, a year passes by in no time, even if the twin pipeline to be built is only 5 km long. The pipeline was constructed entirely by micro-tunneling in Pleistocene sand at a depth of 10 m. Peat is located above this layer of sand, which is not the favorite kind of soil for pipeline owners or contractors. The performance of Visser & Smit Hanab/Dura Vermeer building consortium was unique in delivering this complex project on time, safely and to a very high quality.

Ton Timmermans is unit manager of trenchless technology at Dura Vermeer Ondergrondse Infra BV, Hoofddorp, Netherlands. Gerard  Jonkergouw is manager of special projects at Visser & Smit Hanab BV, Papendrecht, Netherland.The article was first published in the Trenchless Technology International, May 2005. 

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