Construction of the interceptor in Dresden's old town centre. Linear Shoring: Making progress with large clearance

It is installing reinforced concrete pipes with a non-standard cross section of DN 1982/2200, adopting a trenchless procedure for a length of 1,100 m and a conventional trench for 250 m. To secure the up to 8 m deep trench, the contractor is using Linear Shoring from Emunds+Staudinger – a choice that has resulted in rapid progress. With Linear Shoring, the sewer construction experts are eating up the metres. According to everyone involved, the installation and extraction of the shoring elements is proceeding just as smoothly as the insertion of the 3 m long reinforced concrete pipes.

As soon as the base slab has been poured and the shoring system has sufficient ground support, the roller units can be raised in accordance with the manufacturer’s structural strength calculations. This way enough clearance is created for the lowering and trouble-free assembly of the roughly 16 t pipes with the so-called "Dresden cross section".

Measuring a total of 14 km in length, the interceptor in Dresden’s old town is one of Dresden’s longest sewers. "About 80 years after its completion, large parts of the structure are in a poor state of repair," explains Dipl.-Ing. Detlef Schwadtke of engineers p2m berlin GmbH.

The old pipes with their unusual cross section of DN 1531/1600 have been seriously attacked by concrete corrosion. And the exceptional flooding in 2002 had also left its traces. "Because of the poor state of the fabric and also because of the need to boost hydraulic performance, the responsible authorities decided to replace the system," Schwadtke continues. On completion, about 700 litres per second of sewage will be able to flow through the new interceptor. In heavy rain, the structure can even cope with up to 3,000 litres.

Worth mentioning is the fact that the new interceptor is not being laid along the old route under the riverside road, but through the Elbe meadows. The advantages of this are that sewage can continue to be discharged by the old sewer until construction work finishes. What’s more, the traffic on the busy road will not be disrupted. At the same time, disturbance of the landscape conservation area of the Elbe meadows must be kept to a minimum. Replacement and compensation measures on a large scale have to be taken for this reason.

About 250 m of the new interceptor will be built in an open trench. Contrary to the original invitation to tender, which envisaged securing the trench with sheet piling, the trench – as much as 8 m deep in places – is in fact being protected by Linear Shoring from Emunds+Staudinger.

Unlike sheet piling, the shoring is not installed in its entirety in advance, but is inserted during ongoing excavation as work progresses. Four to ten modules of the shoring system – for these depths consisting of inner and outer base and top panels, Linear Shoring soldiers and roller units – are being employed on this project for a continuous length of up to 38 m.

Special shape to aid cleaning
According to everyone involved, the installation and extraction of the elements is proceeding just as smoothly as the insertion of the 3 m long reinforced concrete pipes with the Dresden cross section.

The non-standard cross section measuring DN 1982/2200 is distinguished not only by its flat base, but also by the fact that its cross section precisely matches the dimensions of the sewer cleaning vehicle that is constantly in operation keeping sewers clean in Saxony’s capital.

"Cleaning is based on a simple principle," explains project manager Dipl.-Ing. Heiko Nytsch, Stadtentwässerung Dresden GmbH. "The cleaning vehicle pushes a shield ahead of it which is kept slightly above the floor of the pipe but otherwise rests against the walls of the pipe." If the cleaning vehicle is held up by congestion on the floor of the sewer, the water jet escaping through the gap washes the obstacle out of the way. "After this, the vehicle propelled solely by the flow of sewage can continue its journey," Nytsch adds.

Raise the roller unit, insert the pipe: That's all there is to it!
According to project manager Dipl.-Ing. (FH) Volker Preuss of Heinrich Lauber GmbH & Co. KG, a tightly meshed network of wells will lower the water table sufficiently. "The water table remains within the natural lower range and the trench is kept dry around the clock," Preuss explains.

After the pouring of the roughly 27 cm thick concrete slab to support the pipe, a mobile crane lifts the 16 t pipes into the trench. "Rapid progress overall is ensured by the size of module and by the fact that the position of the rigid roller units can be modified at certain stages of construction," explains Dipl.-Ing. Fritjof Heiland, consultant for Emunds+Staudinger GmbH. This is possible as soon as the concrete bed has achieved its minimum strength.

"In this situation, the excavator can raise the roller unit into the highest position that maintains the required structural strength," Heiland adds. In this way, sufficient space is created for the next tasks. When the reinforced concrete pipes are lowered in the trench, they can be shifted smoothly under the roller unit and joined together.

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