Pipe bursting in Norway: 900 m ductile cast iron pipe ND 600 replaced with HD-PE pipe OD 710

Feb 21, 2012

In Trondheim, Norway’s third largest City, or to be more precise in Gamle Jonsvannsheien, an ND 600 drinking water transport pipeline, made of ductile and grey cast iron, had to be renewed over a total length of 900 m. The pipeline, originally installed in the late 60s, showed definite signs of wear. Leaks, caused by corrosion and requiring elaborate repairs, were regularly occurred. To assure the water supply for the City it was decided that it was time to renew the 40 year old pipeline. In undertaking this replacement it was absolutely necessary to maintain the pipe’s original cross section. Due to the prevailing soil conditions an HDPE pipe of 710 OD x 52.2 mm wall thickness (with coating) was chosen as the new pipe.

With the cast iron pipe running beneath a busy street in front of a kindergarten and a school, a decision was made to replace the 3 m deep pipeline using a trenchless method instead of open trenching. This would ensure that traffic chaos was avoided and also that costs as well as construction time were minimised. As an active member of the Scandinavian Society for Trenchless Technology (SSTT) the community of Trondheim was familiar with trenchless installation and replacement methods and wanted to promote the use of these NO-DIG techniques in Norway with the project.
 
For the project, careful assessment of the site showed that only the pipe bursting method was applicable. This turned out to be just the right job for contractor Sandum AS of Geithus near Oslo. The civil engineering company won the tender due to its extensive experience, its know-how and the superior technical equipment it had at its disposal. Comparative costs for the work were calculated with the estimates showing values of 12,000 crowns per metre for open trenching and only 8,000 crowns per metre for the trenchless renewal option. These savings at more than 30% made the deal perfect for the client.
 
The diameter of the drinking water pipe was a special challenge, requiring accurate planning in close cooperation with the consulting company Asplan Viak AS and the machine manufacturer TRACTO-TECHNIK, as in Norway a ductile cast iron pipe of ND 600 had never previously been replaced using the pipe bursting technique. The question was over what installation length a PE could be pulled whilst keeping the tensile load on the pipe to a minimum. To this end it was decided to use an eco-friendly Bentonite fluid to lubricate the pipe string, which significantly reduces the casing friction between pipe and the surrounding ground and therefore eases the pipe pull-in.
 
The pipe bursting project started with a test section, with an option to continue the project. For such a demanding installation it was felt that a ‘Big gun’ had to be brought in for the mammoth task. Ultimately a GRUNDOBURST 2500G pipe bursting rig was chosen which offered a 250 t pulling force. The set-up also utilised a special roller blade for cutting the ductile cast iron pipe during the bursting operation. An upsizing head of 830 mm diameter was used to displace the split old pipe sections into the surrounding soil.
 
The test bursting section went well without any problems and the replacement of further sections of the pipeline could begin.
 
The total replacement length of 900 m was divided into 7 sections with the shortest being 120 m and the longest being 170 m long. The single 18 m long sticks of 710 OD, SDR 13,6 PE 100 pipe were welded together to form a complete pipe length for each respective section. These pipe lengths were then 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 was burst and the new pipe pulled into the expanded void using the Bentonite fluid to reduce the casing friction. The tensile load applied to the new pipe was constantly monitored and logged with a GRUNDOLOG measuring device. An average tensile load of 500 kN was measured, which is well below permissible limits.
 
The total replacement project was successfully completed within 4 months, ending in August 2011, to the client’s complete satisfaction.

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