Buoyancy protection with pipeline cover

Project description

For the development of EVN Netz GmbH’s “West 4” western trunk gas pipeline between Auersthal / Gänserndorf and Langenschönbichl / Tulln, a consortium comprising Leyrer + Graf Bau GmbH and Nacap Rohrleitungsbau GmbH was contracted to build an approx. 60 km long pipeline section. The route includes a series of inverted syphons by means of which the (DN 800) pipeline crosses under the many intervening waterbodies and watercourses. To protect the pipeline against mechanical damage and buoyancy or uplift, the inverted syphons were encased in concrete.

Solution

The solution adopted to install the concrete encasement made use of concrete mattress system – in preference to the originally specified ballasting method by means of shuttered in-situ concrete. The concrete mattresses comprise two high-strength woven layers connected by synthetic spacer ties. The mattress thickness is determined by the length of these spacers. The mattress elements were filled with selfcompacting concrete (SCC) of class C 25/30 (minimum 28-day cylinder strength ≥ 30 N/mm2) to DIN 1045-2 (EN 206-1) so as to create a monolithic body of concrete within the textile formwork.

The first project phase, in May 2011, involved the trial construction and installation of a 15 m long inverted syphon. After the success of this “test” had convinced the contracting consortium that Incomat offered the best solution, it was duly adopted for all subsequent waterbody crossings. The average thickness of the concrete encasement on this project ran to around 0.11 m. Each mattress element constituted a separate concrete section with an approximate length of 4.90 m after pouring. A sequence of up to 13 adjacent elements was needed for the inverted syphon structures, depending on their size.

The preparations for concreting and the actual pouring operations were performed outside the waterbody. The inverted syphon sections of the pipeline were treated with a suitable polymer-based anti-corrosive coating. All necessary precautions were taken to prevent the Incomat concrete mattresses from damaging this protective finish. The integrity of the coating was additionally checked after the assembly had been lifted into its definitive position. It is particularly important for cathodic protection of the pipe that this type of encasement has no negative impact on electrical conductivity. The individual Incomat elements were custom-manufactured for the specific project at the HUESKER Synthetic GmbH plant, and fitted with filling inlets and a zip. The textile formwork elements were placed around the pipeline and zipped together lengthwise on site. The mattresses were then successively filled with concrete. At the end of the 28-day curing period, each inverted syphon assembly was moved to its place of installation and lifted into the waterbody by several construction machines.

Benefits

The Incomat Pipeline Cover (IPC) textile formwork helped to deliver a solidly built, easily installable, highperformance solution that fulfilled the necessary ballasting function for the inverted syphons.