In Trondheim — Norway’s third largest city — or to be more precise in Gamle Jonsvannsheien, a drinking water transport pipeline ND 600 made of cast iron had to be renewed over a total length of 900 m.
The pipeline, originally installed in the late 1960s, showed definite signs of wear. Leaks caused by corrosion requiring elaborate repairs were re-occuring all the time. To assure the water supply for the city it was about time to renew the 40-year-old pipeline. In doing so, it was absolutely necessary to maintain the pipe’s cross section. Due to the corrosive soil conditions a HDPE pipe OD 710 x 52,2 mm (with coating) was chosen as the new pipe.
To avoid traffic chaos on the busy street in front of a kindergarten and a school and also to keep costs down to a minimum and thus minimize construction time a decision was made to replace the pipeline lying at a 3-m depth using the trenchless method instead of open-trenching. As an active member of the Scandinavian Society for Trenchless Technology (SSTT), the community of Trondheim is familiar with trenchless installations and replacement methods and wants to promote the use of these no-dig techniques in Norway with this project.
For this project, only the pipe bursting method was applicable. Just the right job for contractor Sandum AS from Geithus near Oslo. The civil engineering company won tender due to its long year experience, its know-how and the superior technical equipment. Costs were calculated with 12,000 crowns per meter for open trenching and only 8,000 crowns per meter for the trenchless renewal. These savings of more than 30 percent made the deal perfect.
The diameter of the drinking water pipe was a special challenge, requiring accurate planning in close cooperation with the engineering office Asplan Viak AS and machine manufacturer TRACTO-TECHNIK, as in Norway a ductile cast iron pipe ND 600 had never been renewed before using the pipe bursting technique. The question was over which installation length a PE can be pulled in as gently as possible to keep the tensile load affecting the pipe to a minimum. Thus, it was decided to use eco-friendly bentonite for lubricating the pipe string, which distinctively reduces the casing friction and eases pipe pulling.
The pipe bursting project started with a test section, with an option to continue the project. “Big guns” had to be brought up for this mammoth task. A GRUNDOBURST 2500G with 250-t pulling force and a special roller blade for cutting the ductile cast iron pipe was applied. An upsizing head Ø 830 mm was used to displace the split old pipe into the surrounding soil.
The test went well without any imperfection and the replacement of further sections of the pipeline could begin.
The total replacement length of 900 m was divided into seven sections with the shortest being 120 m and the longest being 170 m long. The single 18-m long segments of the PE pie OD 710, PE 100, SDR 13,6 were welded together to form a complete pipe length for the respective section, pressure tested and firmly connected using a pulling head.
The roller blade and expander head were attached to the bursting rods, which had been pushed through the old pipe in advance. When being pulled back, the old pipe is cut and the new pipe is pulled into the expanded void using bentonite to reduce the casing friction at the same time. The tensile load affecting the new pipe was constantly monitored and logged with the GRUNDOLOG measuring device. An average tensile load of 500 kN was measured, which is way below the permissible limit.
The total replacement project was successfully completed within four months, ending in August to the client’s satisfaction.
This article was submitted by TRACTO-TECHNIK GmbH & Co. KG, based in Lennestadt, Germany.