Leak-tightening of 535m sewage pipe in Augsburg

Renewal of a 100 year old in-situ concrete sewerage pipe with a unique DN1500/2000 maul-profile.

The residents of Müllerstrasse no. 6 in Augsburg were last October a little astounded. The reason for this was a sewage pipe renewal project being carried out by Aarsleff Rohrsanierung. In a full stretch of approximately 104 meter long, we successfully inserted a felt liner into the more than one-hundred-year-old in-situ concrete sewer with a unique DN1500/2000 maul-profile. Stadtentw?sserung Augsburg, the Augsburg city drainage service, had already commissioned ngutis Ingenieurgesellschaft für Umwelttechnik und Infrastruktur mbH, Munich, a company that has been part of STEIN Ingenieure GmbH since 2020, with the planning and tendering of the renewal project.

 Image: The 'Augsburg support line' profile has a relatively pointed apex. In the transition area from the clinker invert to the vault section, the channel has a buckled edge. 

36.000 kilos of pure liner

When the heavy goods vehicle arrived in Müllerstrasse early in the morning with the ice-cooled felt liner weighing a good 36 tonne, the first renewal out of four sections began using our well proven No-Dig water relining method. While an minor upstream section could be repaired through manual renovation, the sewer had to be renovated by installing CIPP liners over a total length of 535 m in order to ensure reliable operation and the required leak-tightness.

Image: Thomas Moritz (left) and Klaus Diesenbacher of Stadtentw?sserung Augsburg standing in front of the full project scheme.

The old main sewerage stretch showed various points of leakage as well as transverse and longitudinal cracks, is one of the most important sewers in the university town and is used to discharge large amounts of the wastewater from the southern districts of the city as well as from the entire historic city centre. The combined sewerage system was built around 1911 using the in-situ concrete construction method and has a unique maul-cross-section with a clinker invert channel and a special geometric feature: "The cross-section is referred to as an 'Augsburg support line profile'," explains Dipl.-Ing. Klaus Diesenbacher, head of Stadtentw?sserung Augsburg. "This means that the sewer has a relatively pointed apex and almost a buckled edge in the abutment areas at the transition between the invert and the vault section." A further complicating factor for the execution of the construction work was that the sewer is located in the area of a two-lane main road running from north to south. The necessary installation pits were positioned in such a way that, the channel axis was as close as possible to the edge of the road and the pits could be used for the installation in both directions. This was the only way to safeguard two-way traffic and ensure continuous traffic flow during the renewal project.

 Image: Placement of the worksite was carefully planned to secure running traffic in both directions.

Sewage control through pumping power

Augsburg is known for its extensive artificial stream and watercourse system and its relatively high groundwater level. Since 2019, the historic water management of the city of Augsburg has been officially listed as a UNESCO World Heritage Site as an outstanding testimony to the history of water utilisation. For this reason, the groundwater situation in the Bavarian metropolis proved to be a special hydrological boundary condition that had to be taken into account during planning and construction. Therefore, pump sumps had to be provided especially in the areas of the future assembly pits in order to pump out accumulating groundwater where necessary. Another challenge was the considerable volumes of wastewater that had to be transferred during the rehabilitation project in order to maintain the wastewater drainage capability.

"During a heavy rainfall, up to 3,000 l/s of combined wastewater could have accumulated," as Diesenbacher explains the problem. Therefore, it was decided to divert a large part of the wastewater to another sewerage line through a gate valve in an upstream shaft construction to relieve the discharge situation. "A pumping station with three pumps was available for about 400 l/s for pumping the accumulating wastewater via a DN 400 pipeline laid above ground into a main sewer running parallel to this. A fourth pump served as a safety measure for the instance if one of the other pumps had failed," adds Dipl.-Ing. (FH) Thomas Moritz, site manager at Stadtentw?sserung Augsburg. This ensured that, for smaller to medium rainfall, the affected sections remained free of sewage water for the rehabilitation work. As an additional precaution, weather reports and the water levels in the sewerage system above were constantly monitored in order to allow the CIPP installation for the relevant construction section to be postponed to a later period in case of unfavorable weather forecasts. However, the liners with a thickness of 35 mm and a length of approx. 150 m should be installed within seven days after impregnation to ensure.

Image: The hydrostatic water pressure inverts the felt liner into the combined sewer undergoing rehabilitation. The available water is then used directly for hardening.

Inside becomes outside

Before the felt liner could be inverted into the sewer with water via the inversion tower, it was first necessary to remove drainage obstacles, seal off major water entry points and clean the sewer. "In order to prevent the resin with which the felt liner was impregnated from coming into contact with groundwater, we injected a preliner into the section beforehand by means of air pressure," says Dipl.-Ing. (FH) Jens Bechmann, site manager at Aarsleff. This two-layer PE-PP film tubing also protected the liner from damage during the inversion. Due to the exceptionally large dimensions of the liner, a high-performance conveyor belt was used to invert it directly from the truck to the inversion tower and from there into the sewer: "You can imagine this process as like turning a sock inside out," says Bechmann. "At a ring on the tower, we first turned one end of the liner inside out and tightly connected each of the seven total liner layers," Bechmann continues. By continuously filling water into the resulting rim, the synthetic fibre liner - including the heating hoses mounted at the end - is slowly turned upside down due to the hydrostatic pressure and thereby inverted into the sewer. The heating hoses are subsequently used to for the targeted feeding of the water that is heated in large heating systems into the sewer section so that it can circulate there. Temperature sensors at various points continuously monitor and record the hardening process.

Quality in-house production

The felt liner manufactured by Aarsleff with its own production facilities was factory-impregnated with sewage-resistant, thermosetting polyester resin four days prior to installation. A corrosion-resistant synthetic fibre manufactured with a plastic coating on one side is used as the liner carrier. "The liner was fabricated based on the mouth profile circumference using a calculated equivalent circular cross-section," says Dipl.-Ing. Jan Wozniak of Stein Ingenieure. Since the hardening reaction is triggered by heat, it is necessary to protect the liner from heat prior to installation and to transport it to the site packed in ice.

Image: The felt liner is completely packed in ice. This prevents the premature hardening of the resin.

The inversion of the liner was completed in the early afternoon. Immediately afterwards, the water was heated to initiate hardening. The entire hardening process took three and a half days. "The duration always depends on the wall thickness, length and cross-section of the liner," says Bechmann. And he adds: "Due to the special geometry of the support line profile and the static liner wall thickness of 35 mm, it was already clear to everyone involved in advance that the liner would not press entirely into the apex." Therefore, all parties involved in the project agreed on the decision to grout any existing cavities in the apex, in the abutment area and to some extent in in the vault section from the inside in four layers using highly flowable insulating material after the completion of the usual rehabilitation activities. "For the execution of the work, we took great care to adhere to the maximum permissible insulation pressure levels specified in the pipe static analysis submitted for this purpose in order to prevent excessive external static loads from being exerted on the liner," says Diesenbacher. Everyone involved in the project agrees on one thing: Liners in these dimensions and especially with such a challenging geometry are something special and anything but routine. Felt liners are the only method that covers a dimensional range from DN 100 to DN 2200. "We were very satisfied with the planning, the installation of the liner and also with the execution of the rehabilitation work," says Diesenbacher. And Moritz adds: "The entire team on site did an excellent job. We were able to work together effectively to tackle the additional complicating boundary conditions, such as the groundwater, maintaining the wastewater drainage capability and the traffic routing around the construction site."