The sewer network of Magdeburg, Germany includes a 600 m long, DN700 cast-iron free-flow pressure pipeline. The functionality of this pressure pipeline, built in 1928, required rehabilitation and was restored using a trenchless reduction method without heat, Swagelining. This was carried out by sewer specialists Frisch & Faust Tiefbau GmbH on behalf of Abwassergesellschaft Magdeburg mbH (AGM) for Städtische Werke Magdeburg GmbH & Co. KG (SWM).
Background
With a length of approximately 1,100 km and with 90 pumping stations, the sewer network of Magdeburg transports the wastewater of approximately 240,000 inhabitants of Magdeburg and more than 100 business and industrial enterprises as well as a number of neighbouring communities to the Magdeburg/Gerwisch wastewater treatment plant. The state capital Magdeburg has granted the concession for sewage disposal to AGM. In the context of continuous rehabilitation and renewal measures, AGM commissioned Frisch & Faust Tiefbau GmbH (Frisch & Faust) with the rehabilitation of an almost 100-year-old pressure line. Planning and execution were undertaken by the sewer rehabilitation department of the company, which was founded in 1991, headed by civil engineer Timo Heidbrink.
The pressure pipeline being rehabilitated is of critical importance for the Brückfeld, Cracau and Prester drainage areas east of the Elbe and the only drainage connection to the Cracauer Anger main pumping station. The circular profile of the cast-iron DN700 pipe is 600 m long, and it runs through a residential area between one of the largest pumping stations in Magdeburg and a pressure pipe end chamber at a busy crossroads. In addition, old trees stand in the immediate vicinity of the pressure line. Accordingly, traffic and environmental aspects had to be taken into account and inconveniences for residents minimised during the project.
Initial assessment
To select and plan suitable measures, Frisch & Faust thoroughly inspected the old pipe. Prior to the inspection, a bypass had been built to divert the waste water. The CCTV inspection showed incrustation and graphitic corrosion, a material specific type of corrosion. The damage was already fairly advanced, as the corrosive degradation had been going on for a long time. The pipe material was dissolving layer by layer and being carried away by the wastewater. The original shape of the old pipe had been retained, but the integrity of the pipe was reduced due to the selective dissolution of the iron content. This caused potential weak spots and an increased risk of breaking. The old pipe was no longer mechanically resistant.
Determination of the profile
To ensure the best possible execution of the planned rehabilitation, the actual profile of the pressure pipeline was determined over its entire length. To do so, an IBAK ORPHEUS 2 pan and tilt camera was used for continuous, laser-assisted determination of the profile. Frisch & Faust then used the measurement results as a basis for planning the rehabilitation. Swagelining is a suitable method for trenchless rehabilitation of pressure pipelines which have been damaged by corrosion. The old pipe only serves to route the new pipe line and has no static function. The old pressure pipeline therefore remains in the ground and serves only as a sleeve for the new pipe. As the extent to which the diameter of the new pipe has to be reduced significantly depends on the inner diameter of the old pipe, exact knowledge of the actual profile along the section was important for planning rehabilitation.
The lasers integrated in the fully rotatable camera head enabled determination of the profile over the entire pipe length. For this purpose, the IBAK T 86 tractor moved at an even speed of approximately 50 to 70 mm/second. The laser points registered in this manner were analysed with the IKAS evolution software and evaluated by Frisch & Faust. The laser scan allowed for the collection of data and thus findings which exceeded those of a purely visual inspection. Among other things, the analysis of the measurement data showed that the minimum diameter measured was 692 mm. The PE pipe was then selected based on the maximum and minimum diameters determined over the entire length of the pressure line.
Pipe run measurement
The position of sewers can theoretically be determined based on the manholes. Between the manholes, a straight run of the section is assumed. Between the inspection openings, the pressure pipeline to be rehabilitated does not run straight. The exact curvature was not known, but so significant that it had to be considered in the planning of the rehabilitation operation. Because of this, the exact run of the pressure pipeline concerned was therefore determined using x, y and z coordinates. This was done by means of an IBAK 3D GeoSense pipe run survey.
The pipe run was recorded with the help of the 3D sensor integrated in the IBAK ORPHEUS 2 pan and tilt camera. The 3D GeoSense pipe run survey directly provided an accurate map of the pipe run, including width, length and height information. The geodetically exact layout survey completed the information about the pressure pipeline and was considered in the decision for the rehabilitation method. The map that was generated formed the basis for the decision on how many construction sections were needed to pull in the new pipe.
Reduction method
Based on these comprehensive preliminary surveys, AGM decided in cooperation with SWM and Frisch & Faust to use Swagelining for the rehabilitation operation in Magdeburg. The basic principle of this reduction method without heat is to evenly reduce the diameter of a PE pipe line by means of hydraulic tensioning until it can be pulled into the old pipe. This method utilises the viscoelasticity of PE. After the tension is relieved, the PE pipeline expands again due to the memory effect of PE until it rests evenly against the inside of the old pipe without any annulus gap.
Construction
The pipe was pulled into the pressure pipeline in two stages: first, a 240 m route section with three bends of up to 15° was pulled in, and then a 320 m route section with two bends. For this purpose, four concrete supports were cast with 14 m3 of concrete each, and one starting and two destination pits were excavated. The starting pit was used to insert the new PE pipelines; the destination pits held the pipe pulling system.
For the longer of the two route sections, 21 individual PE pipes with a weight of 1.7 t and a length of 15 m each were fused together on site to form a pipeline. The elastic PE pipeline was pulled through the swage rig, to which a lubricant had been applied, by means of a winch with a constant axial tensile force of approximately 50 t. In this process, the PE pipeline was elastically stretched and the cross-section reduced by up to 10% by cold deformation. After the PE pipeline had been pulled in completely, the tensile force was relieved.
Within 24 hours, the pipeline had re-expanded to its original outer diameter and was pressed smoothly against the inner wall of the old pipe. The entire length of 600 m with a nominal pipe diameter of DN700 was lined in this manner. The pipes were connected by means of joints. The final work, such as the connection to existing pipes, was only performed after the completion of the close-fit lining process.
The result in Magdeburg is a new, self-supporting pressure pipe which only reduces the original hydraulic capacity of the pipe line by the wall thickness of the PE pipe line of 41 mm. The new PE pipe assumes the static functions so that the old cast-iron pipe is no longer used in its original function. The normal operating pressure of the pressure pipe is 0.17 bar, the maximum operating pressure is 0.5 bar. After the rehabilitation of the pressure line, a pressure test with 1.5 bar was completed successfully.
Sustainable final outcome
For the rehabilitation project in Magdeburg, a geodetically exact pipe run survey and profile dimension determination were performed over the entire length of the pressure line.
Both measurements were made with the IBAK ORPHEUS 2 pan and tilt camera.
The findings far exceeded those of a purely visual inspection and helped to secure the best possible execution of the rehabilitation. Due to the comprehensive preliminary surveys, a suitable method and a PE pipe with an optimum fit could be selected.
Particularly in inner-city areas, as in this case, trenchless sewer network rehabilitation is advantageous. Only the construction pits had to be excavated for Swagelining. Extensive excavation works as well as damage to the environment and the trees along the route section of the pressure line was avoided. Disadvantages for the residents and traffic obstructions were reduced to a minimum. Compared to a reduction method with heat, the energy consumption was reduced because no heating was required.
The slight reduction of the cross-section by 41 mm only had a minor impact on the hydraulic capability of the pressure pipeline, as the new PE material results in a significantly better roughness coefficient of the pipe surface than that of the old pipe. In addition, a gap between the walls of the old pipe and the new pipe, which would have to be filled in a costly process, was avoided by using the closefit installation. Last but not least, Swagelining is more efficient than conventional rehabilitation methods. The new PE pipe guarantees a long service life in the affected drainage area east of the Elbe and is the result of a successfully completed rehabilitation project in Magdeburg.
