Offenbach: Search in the chemical park – construction project stopped

• Offenbach sewer is to be carried out under the flood dam in the chemical park.
• But the construction work in Offenbach haven’t started yet.
• It is still unclear where two important power lines are located on the site.

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Offenbach – Anyone passing the former Clariant car park on Offenbacher Strasse does not see any construction activity at the moment – at most a wire fence. On foot, along the Kuhmühlgraben it looks a little different. There, the construction site for the final section of the canal, which is primarily intended to receive the wastewater from the new development areas of Bieber-Nord, Waldheim-Süd and Bürgel-Ost, has been cleared. While the underground work in Mühlheimer Straße (2017) and Kettelerstraße (2018) caused disabilities and a stir, this time it will be quieter – except for the residents along the trench.

Offenbach: Extensive search in the chemical park and on the parking lot

A search is currently underway on the locked parking lot and the former chemical park (now: Innovation Campus). Slots are dug in the ground on Offenbacher Straße in order to determine the exact location of a pressure line and – even more stressful – a power line. “We know that both of them are there, but the old plans are not always reliable,” says Melanie Gessner, head of the department Drainage of the city service. “Before we dig a pit here, we have to locate both supply lines precisely.”

The city service commissioned all work for the Main outlet on the bank at the level of the parking lot. This is the end of a new sewer, which relieves the mixed water sewer in heavy rain and directs heavily diluted mixed water directly into the Main. What is expected to flow in here from March 2022 in heavy rain comes from the rain overflow structure in Kettelerstraße. The overflow sill is designed so that the dirty water is mixed with a lot of rainwater when it pours into the river.

Construction project in Offenbach stagnates: power lines are still being sought

In preparation, blackberry hedges were mulched on the future building site and the area measured. When the exact location of the two old lines is known, the action begins: A double launch pit for a large drill head will be set up on the site, which will clear the way underground for the new pipes with a diameter of two meters. The canal ends in an outlet structure that extends into the Main. In order to build the outlet structure, an excavation pit made of sheet piles is being built in the Main, which is still protected from a possible impact by a ship by dolphins.

Sewer pipes in Offenbach are being laid in two directions at the same time

First, however, pipes are being laid on land from the parking lot in two directions: One is over 50 meters under Offenbacher Strasse to the Main, and the other is to the pipes 350 meters away from the sewer renewal, which was completed in Kettelerstrasse in 2019. There the excavation pit with the connection shaft, also in a parking lot on the former Clariant site, was left open for the connection. On this last, longer stretch, the companies use the pipe jacking method with a remote-controlled drill head.

The route runs in an arc from the Main to Kettelerstraße and therefore poses an additional challenge from an engineering point of view, which specialists can appreciate to overcome. Especially with a tighter curve radius, very high compressive stresses occur on the inside of the curve. “We commissioned a Swiss company to avoid damaging the polymer concrete pipe,” explains Nils Scheffer, the engineer responsible in the drainage department. “This has developed a patented process which, with the help of a so-called hydraulic joint, measures and distributes the propulsive force more evenly on the bearing surface of the pipe joints, even when pressing in the bend.”

Big challenge for engineers: shipping in Offenbach but not affected

Nevertheless, pressing through is the preferred option because the groundwater level is very high at this point and an open construction would be very complex and expensive. At the latest in the direction of the Main, this would be impossible anyway: there the main dam has to be crossed. To do this, some sheet piles in the (state) dyke are pulled up a little to lead the pipe underneath to the target pit on the Main. “Flood protection is guaranteed at all times,” emphasize Gessner and Scheffer.