In August 2005, a joint venture under the technical leadership of Porr Tunnelbau GmbH was awarded the contract for construction of the “Main District Heating Pipeline North West Contract Section 2” by the client, Wien Energie Fernwärme AG. The contract comprises construction of around 1500m of tunnels with four diaphragm wall shafts, as well as 160m of open-cut tunnelling, and the supply of all pipe engineering equipment.

During the period of 2003 to 2005 the same joint venture had already gained valuable tunnelling experience in the area whilst undertaking the works for Contract Section 1 of the same pipeline, which included a tunnel length of 1880m.

Project description

The project area runs through the 20th and 21st municipal districts of Vienna and augments the heating network that starts at the waste incineration plant at Spittelau (which has become well known for its architectural design by Friedensreich Hundertwasser), up into the area of Floridsdorf station and further into the residential areas in the northern fringes of Vienna.

Among other things, the scope of the project comprised two pipe jacks using an earth pressure balanced shield (EPBM) manufactured by Herrenknecht to install concrete pipes with an i.d. of 2.2m and a thickness of 250mm.

Ground freezing

Two shafts that had previously been constructed on Contract Section 1, with surface areas of around 100m2 and depths of approximately 23m, were used for the launch of the two shield drives, which were to be driven 614m and 909m (figure 1). As the groundwater level in the project area is about 5m below the surface, measures were required to drive through the reinforced concrete diaphragm walls, since excavating through these using the shield machine’s cutterhead was not possible.

The diaphragm walls had to be broken out in the launch area, then the sealing structure and pipe clamp assembled and the EPBM moved into the seal.

As the subsoil conditions in the launch area varied between cohesive silt and sand lenses with confined groundwater, different methods for stabilisation of the subsoil were investigated.

Conventionally, a sealing block is produced by means of jet blasting to stabilise the shield start-up. However, this method has drawbacks in densely packed cohesive soils.

It turned out, in this instance, that ground freezing was the most cost-effective method to guarantee the required factor of safety for launch.

However, ground freezing brings its own problems regarding obstruction of the shield drive by the steel freeze pipes, which are generally used to introduce liquid nitrogen into the soil at -196°C. If the steel elements obstruct the excavation cross-section there is a real risk that the cutterhead or system components of a TBM will be damaged when they encounter them.

Due to a good relationship with ground freezing equipment supplier MESSER Group GmbH, a new method was jointly developed to produce an effective ground freeze for the project, which would stabilise the start up area for the shield drives in a cost-efficient way.

The basic idea was to develop a method by which an ICE block of around 5m x 5m x 2m could be produced directly ahead of the diaphragm wall. This sealing block could then be driven through without impediment, using the shield.

A novel solution was developed using synthetic freezing lances instead of more conventional steel ones. The main hurdle that had to be overcome was developing a synthetic that was capable of withstanding temperatures of up to -96°C with sufficient stability, without turning brittle. Equally, a synthetic material had to be developed that would not have a negative effect on the dissipation of energy into the ground; that was compatible with the system components of the freezing equipment; and lastly that could be cut into appropriately small parts by the EPBM’s cutterhead so that damage could be prevented.

Different synthetics were tested and finally a patented composite material, derived from the electronics industry, was adopted. As their was no prior experience of ground freezing with synthetic lances, calculations and tests were performed in advance to ensure there would be no unforeseen problems, especially when considering the presence of groundwater at around 1.5 bar.

It became evident, following the trials, that the synthetic freezing lances complied with the project’s requirements and thus could be used.

Lances with a diameter of 2” were installed to achieve the ground freeze for the start-up of the second shield, and performed well.

As the period from conception to utilisation of the new system lasted several months, copper pipes were used during the ground freeze for the first EPB drive instead of the synthetic lances. During actual excavation these were cut by the cutterhead into roughly 700mm long sections. Although there was a risk that the metal parts could cause damage to the tunnel boring system, in this instance everything worked well and without incident.

For the second drive, following production of the ice body, with permanent electronic temperature control on several levels, the diaphragm wall was opened using plunge cuts with a wire saw. The concrete blocks were then removed and the seal with the pipe clamp was assembled. After the EPBM was installed it was moved into the seal and routine driving could begin.

During the second drive through the frozen area the synthetic lances were efficiently broken down into parts of about 150mm long by the cutterhead. This confirmed the suitability of the method, as any damage to the machine’s system components was ruled out.

Diaphragm wall shafts

Since the shafts already constructed on Contract Section 1 were available for Contract Section 2, they were used as the launch shafts. Therefore, two target shafts and two through-shafts, which will eventually serve as branches of the district heating pipeline, had to be constructed.

These shafts were built as diaphragm walls, with an 800mm width and depths of up to 35m. A system involving a “reinforcement-free prefabricated disc” that was patented by the building site was installed in the eye area of each shaft.

Tunnel drives

Tunnel driving was performed with an excavation cross-section of 2.7m. The pressure pipes DN 2200, with a thickness of 250mm and a length of 3.4m, were driven into the ground with a maximum force of pressure of 18,000kN. Intermediate jacking stations were set roughly every 80m.

Excavated material was transported to the respective start shafts by shuttle haulage using tippers. Transport from the shaft base to the surface was undertaken using a 50-ton portal crane.

The tunnelling and pipe jacking works were successfully completed in June 2006.

Lining of shafts

The internal finishing works of the shafts began immediately after the tunnel drives had commenced. All shafts are to be fitted out with an impervious inner lining, which is produced using a single-sided climbing formwork.

As described earlier, the contract for the construction of the district heating pipeline also includes all of the pipe-laying works (the laying of the flow and return pipeline to the dimension DN 600 including all fittings), which is currently being performed.

The completed structure is scheduled to be handed over to the client in May 2007, after construction of the shaft ceilings and re-cultivation of the affected surface areas has been completed.

As a result, the northern outskirts of Vienna will be provided with a reliable, environmentally friendly energy form for its district heating requirements; having used, for the first time, the new method of ground freezing to provide the start-up stabilisation required for the EPBM shield drive.

Innovation prize

In October 2006 an innovation prize for tunnelling was awarded for the first time by the “Austrian National ITA Committee” at the Austrian Tunnel Meeting in Salzburg, at which the best contribution of an innovative method with a practical importance to the tunnelling industry was awarded a prize.

“Development and Use of Synthetic Freezing Lances for Start-up Stabilisation of Shield Driving” was registered for this innovation prize by the author, and was subsequently awarded winner of the competition.


Long section of the project showing Contract 1 and 2 Figure 1 – Long section Herrenknecht EPBM on site Herrenknecht EPBM on site The break out area of the shaft that required the innovative freeze method for stabilisation Breakout area Pipe jacking gets underway in one of the start shafts previously constructed on Contract 1 Pipe jacking gets underway Cutterhead working through the freeze zone Excavating in freeze The difference in cut size of the freeze pipes showing the much smaller size of the synthetic lances Difference in freeze pipes