BUILDING REPORT FOR ANKLAM GAS PIPELINE
(Salzburg / Kalbach, in October 2013)
The Baltic Sea Pipeline Link (OPAL) transports natural gas from Lubmin (Mecklenburg-Vorpommern) to Olbernhau in Saxony and is part of the connection to the Czech Republic. The pipeline was welded together from 26,000 single pipes (22.3 mm wall thickness and 1.4 m diameter), each 18 m long and with a weight of 15 t. They were installed in a trench with 1 m soil covering. The annual capacity of the pipeline is 35 billion cubic meters. The pipeline has a length of approx. 470 km, of which 270 km runs through Brandenburg and 100 kilometres through both Mecklenburg-Western Pomerania and Saxony. Special technical requirements had to be observed in the areas where the rivers Peene, Spree and Elbe are crossed. The pipeline was completed in 2011.
Creating a micro tunnel for the gas pipeline
When creating a micro tunnel for the gas pipeline during the course of it's construction, around 20 meters had to be created under the Peene using a tunnel procedure, near the nature reserve in Wismar. The geological structure included on one hand a sandy gravel floor and on the other hand a layer of solidified peat close to the surface. Despite the dry ground, water was found under the riverbed of the Peene. A slurry machine with a 3 m outside diameter was chosen to be appropriate for the cohesive ground.
Specifics during transit
After the first half of the tunneling route posed no problems, a boulder was identified in front of the machine. A plan was made to destroy the stone by driving through it. Since the cutting tool of the machine, however, was designed for soft floors, this could not be achieved despite several attempts. The strong forces placed on the stone during the attempts to destroy it resulted in cavities appearing around the boulder and the machine. This had to be balanced out by a greater amount of supporting fluid with higher pressure at the same time. The ground coverage over the microtunnelling machine could not, however, withstand the pressure of more than 2.5 bar of supporting pressure, therefore the stone had to be blown up. All attempts to fill the resulting crater by using liquid soil and then solidify it thus failed: the support pressure could not be started up again. The filled crater was neither pressure-tight nor was it able to hold the supporting slurry fluid. It had to be sealed and solidfied using an additional measure on the ground in front of the tunnel face. A particular challenge in finding solutions was the lack of logistics and the requirement to use neither large appliances nor environmentally unfriendly injectables. The tight time frame for the completion of the work also had to be taken into account.
Sealing by ram lances
TPH, a company involved in the project, suggested stabilizing and sealing the backfill soil by injection. In addition, an injection by the tunneling machine should fill in and compress the blasted boulder which was checked previously. As no machine movement was allowed in this nature reserve, conventional methods such as cement injection or freezing were out of question. The injection by means of a Desoi ram lance was therefore selected. The ram lances are of modular design and can be adapted with different technical properties according to on-site conditions. In this case, at up to 17.5 m, they were rammed into the ground with a light pile hammer and could be placed very close over the machine.
Procedure of injection work
Over the lances acrylate gel had been injected by the DESOI pump PN-1412-3K and so the soil was consolidated. The whole equipment could be transported without machinery and the acrylate gel was approved for use by the local water authority on the basis of the DIBt certification. A total of 56 ram lances were used and approx. 2,400 l gel was introduced. The basically dense backfill soil was consistently permeated and solidified only by the use of very low viscose acrylate gels, where a combination of solidification and sealing had to be achieved. The ram lances enabled the required targeted injection.
The cavities which had been created in the ground by the boulder blast were filled with a silicate foam by a further injection device, PN-2036-2K, out of the tunneling machine. The benefit of silicate foam is that it provides a very good adhesion to siliceous substrates due to the high water glass content, but it also passes through easily from the machine.
Within 3 days the problem was solved. The slurry pressure could be increased once again and the machine could continue its work.