Oval Profile Sewer Rehabbed Near Entrance to Vienna

In Vienna, a 1,550-m mixed water sewer is currently being rehabilitated using light-curing GFRP pipe liners. The project entails many challenges. Not only is the construction site next to a heavily trafficked street, but the dimensions of the old sewer, which was built in the 1930s and has an oval profile of DN 800/1200, vary significantly. This meant that the liner used for this project needed to have a particularly high level of expansibility.

In spring 2001, the rehabilitation work on the mixed water sewer near Bundestraße 44, at the west entrance to Vienna, began. The street through the Vienna forest lies in a valley and carries 15,000 vehicles every day. There is no available alternate route for the commuters and school buses. To reduce the burden for the residents and avoid obstructing traffic, the Wiental-Sammelkanal GmbH (WISAK) contracted to have the rehabilitation be carried out using pipe liners. The oval profile sewer is approximately 70 years old and is made of site-poured cement on ceramic slabs. The rehabilitation was necessary because of signs that the sewer was beginning to corrode.

Strabag AG was contracted to take on this project and its many challenges, not least of which was the challenge of the high traffic levels at the construction site. An oval profile sewer rehabilitation project of this size is always a special case,” says Stefan Schagerl, construction manager at Strabag. “The working conditions at the construction site were also very cramped, since the sewer shafts were only located over the pipes.”

This severely limited the freedom of movement of the workers when drawing in the liner and installing the packer. The construction methods employed 70 years ago are also responsible for the unusually high variations in the size of the old sewer. “Back then, instead of laying out finished cement pipes, the sewers were cemented using inflatable rubber hoses,” says Schagerl, explaining the variation in the dimensions of the sewer, which range from 320 to 335 cm. Because of the variation in size, the calibration for using pressurized air to set up the pipe liner had to be particularly precise.

Extensive Prep Work

The rehabilitation itself was preceded by considerable amounts of preparation. Since the sewer system included conduit lengths of up to 250 m, it was necessary to build seven new shafts. This was the only way to limit the individual construction sections to a maximum length of 110 m per section. “If you ask me,” says Schagerl, “we stretched the limits of what’s possible.” After these steps had been taken to prepare the 1.5-km long project, the individual stretches to be rehabilitated had an average length of 77.5 m. Four intersections with gas and electric lines, where the lines had been guided through the sewers, had to be removed in advance using an open cut. The project required a particularly involved water drainage system, since the sewer transported mixed water, a combination of waste water and rain water. When rain came, a wastewater flow level that measured 50 liters per second under normal conditions could suddenly increase to 250 liters per second. In preparation for these flow level increases, particularly large and powerful pumps were kept ready onsite. During heavy rain, the liner installation had to be interrupted; however, the drainage precautions ensured that during a small shower, work could continue. The water drainage system was set up in segments along the individual rehabilitation sections. After the completion of one section, the drainage was dismantled and reassembled at the next rehabilitation section. The heavy traffic in the area made it essential that the project be carried out with a high level of organization and carefully planned logistics.

Because of the importance of sparing the commuter traffic, work could only commence after 8:30 a.m. A permit request for a traffic signal light was not approved, so instead, the street traffic was regulated by two security employees during the construction work.

Before starting the actual rehabilitation, Strabag installed a test liner in a 60-m stretch of sewer in order to plan the rehabilitation in as much detail as possible. This allowed Strabag to precisely measure the time required for drawing in the liner and for the curing process. Calculations based on these results established that a curing speed of 40 cm per minute resulted in the optimum curing process. Dr. Sebastian, the technical specialist office charged with carrying out the elasticity tests, determined a modulus of elasticity of over 12,000 N/mm².

The rehabilitation of the entire sewer was subsequently planned and carried out in 20 installation stages. “We drew in four liners each week, so that installation alone took five weeks,” says Schagerl.

In order to keep the tight schedule, it was necessary to store the liners onsite in Vienna. Even though the stress analysis determined that the liners required a thickness of 11.2 mm, it was possible to carry out the rehabilitation without peroxide liners. The high transparency of the glass and resin surfaces combined with the special construction and carefully selected additives of the Brandenburger Liner made it possible to perform the entire curing process using UV light; an additional catalyst was not necessary. “What we saw in practice was that the expected material characteristics of the test liner were actually exceeded,” says Marc Albrecht, Brandenburger application engineer.

The ready-to-install pipe liners – weighing 64 kg per meter, leaving the weight of an entire rehabilitation section somewhere between 2.5 and 7.7 tons, depending on length – were delivered to the construction site in crates by a 10-ton forklift. A conveyor belt and a 10-ton winch then drew the GFRP pipe liner into the sewer system. At this point, the liner material first had to be folded to a size that would fit through the narrow shafts. Smaller DN 800 installation airlocks and special packers had to be used, since there were no existing chambers in the 2.5-m deep sewer shafts. When setting up the GFRP liner in the sewer, it was necessary to take into consideration the prevailing variations in size. For this task, the expansibility of the Brandenburger Liner, of 4.3 to 7.2 pro cent, proved to be a great advantage. The expandability of the liner combined with the careful calibration using pressurized air made it possible to completely compensate for the irregularities in the sewer system.

Schagerl, the construction manager, observed: “The GFRP liner lay completely flush with the pipe walls.” BLUETEC curing technology was used for the subsequent curing of the system. The curing time for approximately 100 m of sewer length lasted a little over four hours. The fast curing and the expansibility of the liner made it possible to quickly and seamlessly complete the rehabilitation work next to the main street in Vienna-Pressbaum.

This article was provided by Brandenburger Liner GmbH & Co. KG, Landau, Germany.
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