The objective of the project was to reduce overflow from the combined sewerage system into a nearby river, and prevent flooding of low-lying residential areas during heavy rain storms. Danish utility owner VCS Denmark used pipejacking with compressed air to install 750 m of 2.5 m and 1.6 m internal diameter pipes. This method is new in Denmark but will most likely be used much more in the future.

COWI was the consultant and Arkil the main contractor on the project, while all pipejacking work was executed by German-based subcontractor Meyer & John.

Under pressure

Pipejacking with compressed air involves bringing the working chamber with open excavation front under the same pressure as the actual groundwater. This allows soil to be excavated dry even if the ground contains sand or silt. In this project the groundwater table was 6–7 m above pipe crown. All pipes were to be installed in glacial till with water carrying sand and silt layers containing boulders of variable size. The air pressure in the working chamber was therefore kept 0.6–0.7 bar above atmospheric pressure. The actual soil conditions, in combination with high groundwater pressure, made pipejacking with compressed air very advantageous. Consequently, the soil could be excavated dry without risk of collapse of the excavation front causing influx of water and liquefied soil, which could generate settlements and damage to constructions at ground level. Furthermore, the open face allows easy access to the front for handling big boulders should they appear.

In order to make the soil impervious and solid, and bring the target point outside the pit wall, soil improvement was required where the boring machine penetrated the wall of the pits. This was achieved by high pressure injection of concrete through bore holes into the start and end points.

At the same time the displaced soil and water was pressed up through another bore hole. Concrete blocks of 5 m2 were established 8 m below ground, allowing the starting pit to be cut away and the initial boring to begin, including installation of the shield, without causing any infiltration of water into the pit.

When the boring machine travelled approximately 4.5 m into the concrete block, the air pressure system was put into operation and the remaining parts of the back-up equipment with air locks were installed. The total length of the boring machine and back-up equipment was 22 m. The front shield was equipped with a road header excavation tool and a belt conveyer for moving the muck into wagons. Behind the working chamber, which comprised a road header and a conveyer, were two air locks for passage of operators and the soil wagons.

The muck could have been conveyed wet from a full face slurry machine by pumping to a separation plant beside the entrance pit, but this option was rejected because of the noise problems it would have caused in the surrounding residential area.

Four jacks with a total pressing capacity of 1,200 tonnes were installed in the starting pit. These pushed the pipes into the soil through the pit wall at the same rate that the soil was excavated in the front. The daily progress of the 2.5 m diameter pipes was approximately 6 m. Three injection tubes for bentonite lubrication on the outside of the pipes were installed at every third pipe to reduce the friction. When the pipejacking drive had finished, a suspension of cement was injected through the same tubes to replace the bentonite lubricant in order to fill possible cavities and fix the pipe position.

Meanwhile, in the target pit a reception pot with a double seal was welded against the pit wall. When the boring machine reached the reception pot it was put under the same overpressure as the working chamber. When the front shield had cut through the pit wall and completely passed the seals in the reception pot, the air pressure in the working chamber was lowered to atmospheric pressure.

Hitting the target

The boring machine was guided by a laser steering system, in which the laser beam hit a target board in the front. Depending on the deviation of the laser beam from the target centre, the shield operator could extend or shrink the steering cylinders that were mounted in four positions behind the cutting shoe. The maximum deviation from the designed alignment was only several millimetres.

Safety precautions were highly prioritised through the entire project. Two of the drives had to be installed in soil polluted with vinyl chloride and other chlorinated compounds, which came from a dry cleaning shop previously located in the area. To manage this issue, the air quality was continually measured with a portable monitoring device. When air pollution was detected, operators inside the tunnel connected their face masks to a pipe system which supplied them with fresh air. Furthermore, the workers had weekly medical checks, while blood tests were conducted during periods when the pipejacking took place in polluted soil. The site was also supervised around the clock and stand-by equipment was available in case of failure or emergency.