When a 600m long section of 1100mm diameter gas pipeline was irreparably damaged during installation in August 2003, near Berlin, Germany, clients, E.ON Ruhrgas AG and Verbundnetz Gas AG knew they had a serious problem on their hands. With a gas delivery date contractually set for April 1st 2004, the timely start up of the complete 40km long Ferngaqsleitung (FGL) 306 pipeline between Kienbaum and Börnicke looked in doubt.
Although most of the gas pipeline had been laid in open trenches, the area where the pipe was damaged ran through a small creek, called Löcknitz, and then on under a 300m wide area of moorland. This critical section had been installed using directional drilling with the steel gas pipe pulled through later. A boulder was believed to have written off the pipe during this pull through.
Following an exhaustive technical assessment, it was decided to re-lay the length using pipe-jacking with a TBM to minimise the risk posed by more boulders possible along the planned alignment.
After having been contacted by the client on 23rd October 2003, German consultant, Babendererde Ingenieure GmbH presented and had accepted a design for a 1.6m i.d. pipe jacked tunnel with the gas pipeline installed by floatation.
Time was of the essence. The tunnel had to be handed over to the pipeline installation contractor by the beginning of February 2004 – just over three months later – to maintain the schedule for the start of gas delivery.
Planning the tunnel
The geology east of Berlin is dominated by its glacial history. The moor, with its groundwater level at surface, has a top layer of peat, 5m thick, over a thin layer of lacustrine clay overlying a bed of sand with imbedded stones. The presence of boulders larger then 0.3m was expected, as experienced during the HDD.
The Löcknitz creek crosses the tunnel alignment to the south of the moor. Beyond this, the terrain rises to a terrace some 10m above the moor level. Conversely, the area north of the moor is only marginally higher then the moor itself.
The pipeline’s alignment had already been defined and approved by an official court order. This meant the launch and reception shafts had to be placed along this alignment. A 40 year old 0.7m i.d. oil pipeline, located near surface, which crossed the planned gas pipeline at an angle of just 6° meant the launch shaft had to be constructed 150m to the south, on top of the terrace. The reception shaft was then placed at the northern boundary of the moor.
The alignment had to satisfy four requirements:
1. The launch and reception shafts had to be as close to surface as possible to reduce complications with groundwater. Furthermore, shallow shafts would make the connection of the tunnel to the existing open cut pipeline easier.
2. A minimum distance between the tunnel and the peat had to be maintained in case a compressed air intervention into the working chamber became necessary.
3. The alignment couldn’t be deeper than was absolutely necessary, due to the likelihood of glacial till occurring below the glacial sands. Experience has shown that a considerable number of boulders and stones are often found in the transitional area between these two layers.
4. The radius of the tunnel alignment could be no less than 1,300m, as this is the minimum allowable value for the gas pipeline installation.
Taking these requirements into account, the planner developed a 441m long tunnel alignment with three horizontal curves (figure 1). The steepest gradient of the tunnel is 12.7% and the maximum cover is 10m at the deepest point. The difference in the tunnel invert elevation is 18m from the highest to the deepest point.
Whilst tunnelling under the 300m wide moor, the removal of any obstructions in front of the TBM from the surface was considered impossible. Therefore, access to the tunnel face had to be possible from inside the TBM using compressed air. The smallest TBM available on the market equipped for this was designed for jacking concrete pipes with an i.d. of 1600mm. This machine was consequently chosen for use and included in Babendererde Ingenieure’s submitted tender.
Planning the gas pipeline construction
For pipe installation, Babendererde Ingenieure proposed the pipeline sections be welded together outside and then transported as a single unit inside the tunnel, due to the previously mentioned construction time constraints. Naturally, this had to be considered during the subsequent design and construction of the launch and reception shafts.
To reduce the forces needed to pull the pipeline through the tunnel, it would have to be completely flooded. The steel pipeline would also need to be equipped with a float to compensate for its weight and ensure it floated under water. Calculations showed that the 350kg/m weight of the pipe could be reduced to 30kg/m with an appropriate float. Since the required force to pull an object over ground combined with its coefficient of friction is directly proportional to its weight, a significant reduction of force was expected. Conservative calculations showed a maximum pulling force of 220kN for the 215 tonne package of pipe and float.
The complete filling of the tunnel with water would also be hindered by the 8m difference in level between the launch and reception shafts. Therefore, a special cap was made to seal the tunnel in the reception shaft. The cap and any connections had to be designed to withstand an 8m static water head whilst also allowing the pulling cable to pass through the tunnel. Additionally, systems to bleed and de-water the tunnel had to be installed.
Tender phase
Although the tunnel drive was scheduled to take place during the harsh winter month of January, the schedule did not allow for any delays.
Bad weather, especially frost, was not allowed to impact whatsoever on tunnel production, and it was stated that any claims for bad weather conditions would be dismissed.
For the bid preparation, the interested companies had just one week in early November, with the bid evaluation and negotiations taking place the following week. The construction start date was mandatory for the winning bidder in the week after the negotiations (figure 2).
The company Gildemeister Tief-, Stahlbeton- und Rohrleitungsbau GmbH & Co. KG from Berlin, Germany, was awarded the construction contract for the microtunnel. This included planning and implementation the tunnel drive.
The contract for the assembly and pulling through of the pipeline was subsequently awarded to Max Streicher GmbH & Co. KG a.A. from Deggendorf, Germany, who was already the contractor for the rest of the 40km pipeline.
Tunnel construction
With no time to waste, construction on site began on November 24th, 2003 to allow for the earliest possible TBM start. The hand over of the tunnel to the pipeline installation contractor was, after all, set for February 7th 2004.
Gildemeister had decided to use a company owned AVN 1600 T type Slurry TBM with an o.d. of 1970mm manufactured by Herrenknecht. The 9.4m long, 39.5 tonne TBM consisted of a front section with cutterhead, main drive and steering cylinders, a middle section holding mainly machinery and finally, the airlock section.
The AVN 1600 T had a peripheral drive mechanism with four motors. Access to the excavation chamber to clear obstacles from the face was possible through a 570mm diameter door. Additionally, on the T-type series of TBMs, it was possible to change worn cutterhead tools underground.
The TBM was equipped with a hard rock cutterhead fitted with disks and scrapers to deal with the sand and stoney geology. The available maximum torque was 474kNm at 0-4.45 r.p.m. and a maximum 281kNm at 0-7.5 r.p.m.
Integrated within the excavation chamber was the cone crusher – characteristic of an AVN-type TBM. Here, stones and boulders are crushed in the excavation chamber to a size which can be flushed to surface by the support fluid.
Personnel access to the excavation chamber was possible via the air lock under compressed air, provided by a station above ground. Prior to intervention, the support fluid in the excavation chamber would be displaced by the compressed air. The entire machine is then pressurised, as the airlock is located at the back end of the TBM.
Before tunnelling started, all details of a compressed air intervention were discussed with the responsible safety authorities. As a result the project was designated a compressed air construction site from the beginning.
Because of the maximum air pressure needed of 1.2 bar, the use of oxygen during decompression was required according to German regulations. Additionally a medical decompression chamber had to be on site in case of an emergency. The contractor provided licensed compressed air workers and supervisors on site, and all parts of the compressed air equipment had to pass through an acceptance procedure by the Technical Inspection Authority (TÜV).
The alignment did not permit the use of a fixed laser in the launching shaft. Therefore the contractor decided to use a dynamic traverse measurement system from VMT-GmbH from Bruchsal, Germany.
The required support pressure, the composition of the support fluid and of the pipe lubrication fluid were all determined according to sieve analysis and boring logs provided by the client.
For the treatment of the support fluid the contractor provided a closed loop separation plant with swing sieves and hydrocyclones. The separation plant was totally sealed with a tarpaulin and heated from the inside, because of the expected temperatures below freezing point.
For the actual tunnel, the contractor used sealed reinforced concrete pipes with an o.d. of 1940mm, a thickness of 170mm and a length of 3.5m. The intermediate jacking stations were equipped with redundant adjustable seals.
Machine to face
Tunnelling started on January 2nd 2004. Despite temperatures as low as –15°C, the 441m long tunnel, including four intermediate jacking stations, was finished in 31 x 12-hours shifts. The maximum advance rate was 48m in 24hrs. Through continuous adaptation of the tunnel lubrication and the use of a computerised grouting system the drive force averaged 2500kN. Most of the time the tunnel was driven only from the main jacking station.
In the early morning of January 19th 2004 the TBM reached the reception shaft as planned.
Experiences gained
During the 17 day tunnel drive, both the tunnel gradient and the working temperatures posed challenges for the contractor.
During operation of the intermediate jacking stations a section of the tunnel between two of the stations started to move downhill – just due to gravity. This made the contractor nervous, so he installed self-made spindles in the tunnel whilst removing the jacking stations.
Another problem arose when draining the process water from the tunnel. Initially the water had to be prevented from flowing downhill into the TBM, but after the machine had passed through the deepest tunnel point the water suddenly accumulated in the tunnel. This lead to an almost constant adaptation of the tunnel drainage system.
Even after the drive was completed, the gradient remained a challenge. Clearing the tunnel could not be done by hand. The steep slope and the concrete surface, lubricated with a mixture of water and slurry made it far too dangerous. The contractor ended up using a winch driven trolley to transport all unwanted materials out of the tunnel.
Even the hardest of jobs has its more comical moments though. In this case it was the regular disruption one day of the laser signal, leaving everyone on site helpless. For all of the complicated technical reasons considered, it turned out to be the unsecured TBM air lock door. Due to the rotation of the cutterhead the TBM started to tilt. When a certain angle was reached the door swung shut and blocked the laser. When the cutter head rotated in the other direction the process repeated, but the other way around, with the door opening again!
The tunnel was finally handed over to the pipeline contractor on February 4th – three days before the contractually agreed date.
Gas pipeline installation
The steel pipeline sections were welded together to create a continuous line outside the tunnel. Joint tests and steel prestressing were performed by exposing the line to a pressure of 150 bars. After successfully completing the tests, a pull adaptor was welded to the head of the pipe string and the float positioned inside the pipe. The tunnel was sealed with the cap and an HDD rig readied for use to pull the pipeline. The tunnel was then flooded.
Behind the former launch pit a ramp was excavated and the rear wall of the launch pit removed. The pit walls were designed for this so no modifications were required. The installation of roller brackets under the steel pipe completed the preparation work.
The pulling-in of the entire pipeline was then performed in one working shift. The longest time during this was used to attach skid rings around the pipe to protect the outside insulation of the pipe on its way through the tunnel. The force needed to pull the pipe was actually below the calculated 220kN.
Electrical checking of the insulation confirmed a perfect job. The pull through of a nearly weightless pipeline together with the skid rings had proved very effective.
Conclusions
The construction of this natural gas pipeline under the moor east of Berlin using pipe jacking techniques and an innovative pipeline installation method has been a success for all parties involved.
The project has shown it is possible to reduce risk by not only using appropriate techniques, but also by working with comprehensive tender documents which allow the bidding parties to evaluate and to account for those risks. It should be pointed out that the client did not award the contract to the cheapest bidder but to the one with the best combination of price and competence.
The specifications and the deadlines of the client were all met, showing that even today a construction project can be done successfully for a reasonable price.
Related Files
Figure 2
Figure 1
