Historic structure in Stendal

The Uchte canal is one of the oldest parts of Stendal’s subterranean infrastructure. As early as 1887, the River Uchte, which used to flow openly through the Stendal town centre, was ‘tamed’ by turning the water course into a canal. For this purpose a dome profile sewer, with a crown height of 1.74 m and bottom width of 1.5 m, was built in the present-day alignment of Bruchstrasse.

The sewer was constructed in tamped concrete on a brick foundation – a technology applied to many structures at that time. However, after 130 years of continuous use, these structures are fraught with the same problems. The integrity of the concrete is weakened, the walls are becoming porous and no longer watertight, their surface is disintegrating and they are becoming unstable.

Unusual structural design

Due to the traffic load, and a minimum coverage of only 15 cm in some places, it was threatened with imminent collapse. Immediate action was called for; especially because Bruchstrasse is a main street in Stendal’s town centre.

Having recognised this danger, the owners of the Uchte canal instructed Magdeburg-based engineering office MUTING GmbH to prepare a rehabilitation concept. An open-cut method to renew the old structure was quickly excluded, as it would have been highly labour intensive and costly due to the unstable subsurface conditions and constant high groundwater table.

Furthermore, reconstruction would have taken much longer – an impossible scenario in the centre of Stendal’s business district, particularly since Bruchstrasse is currently undergoing a full facelift as part of the town centre redevelopment. In addition, Bruchstrasse is immediately adjacent to Marienkirche and crosses a century-old burial ground, making it an archaeologically sensitive site.

Trenchless solution called for

A key consideration in opting for the trenchless rehabilitation method was the reliable restoration of the necessary stability of the old structure. After a careful technical analysis the authorities decided to use the SPR spiral-wound pipe lining method over short pipe relining.

This option was offered by the Brehna branch of KMG Pipe Technologies GmbH, a subsidiary of Sekisui SPR, Schieder-Schwalenberg. The deciding factor in Stendal was that the spiral-wound pipe lining could be installed using the existing manholes – without the excavations usually needed for laying short pipes. The road resurfacing work was able to be performed parallel to the sewer rehabilitation because of trenchless installation.

Description of the procedure

The SPR method is a cutting-edge version of spiral-wound pipe lining technology. The basic principle of SPR spiral-wound pipe rehabilitation is that the decayed sewer section is repaired using an endless PVC-U or HDPE profile strip (spiral-wound pipe).

The endless profile strip, wound on a drum, is fed into the sewer through an open manhole and joined by a spiral-winding machine installed in the sewer to form a pipe by a tongue and groove technique.

Liners of practically any length can be created: as soon as one of the above-ground profile drums is empty, a new coil is joined onto it by the heated tool butt welding method. Apart from this short interruption, the SPR method permits continuous working.

Flexibility as a guarantee for success

A major advantage of the spiral-winding method is its extreme flexibility in irregular cross-sections, and furthermore open cut construction can be avoided. The SPR spiral-wound method can be used on special profiles of every conceivable type and dimension up to DN 5,000 mm, as well as the standard-sized circular and egg-shaped profiles.

For a new situation, as was the case with the special dome profile of the Uchte canal, the winding frame only needed to be adjusted to the dimensions and cross-section of the structure being rehabilitated. This sewer data had been precisely determined in the structure beforehand using a 3D laser scan to ensure unhindered passage of the spiral-winding machine.

The special feature of SPR technology is the unique winding frame crawls along the full length of the structure, forming the spiral-wound pipe from the profile strips trailing behind it. The machine does not remain in the starting shaft, pushing the spiral-wound material into the sewer ahead of it.

Being waterproof and stable are not the only structural benefits of the spiral-wound liner as a rehabilitation system. The spiral-winding process intentionally leaves a defined annular space between the SPR liner and the old pipe, into which a high-strength mortar is injected in a subsequent work stage. In addition, the annular space is walled off and cells filled at regular intervals.

Ultimately, the SPR liner serves only as formwork for a new concrete inner shell of the old sewer, and long-term it provides a reliable protective layer to shield the concrete structure from the effects of sewage and sewer atmosphere. Logically, the annular space is not minimised at the planning stage, but consciously defined according to static requirements.

The dimensions of the SPR liner are measured in ‘as much annular space as (statically) required’ and not ‘as little annular space as possible’. Whether the resulting reduced cross-section is acceptable compared to the old pipe is a question of the hydraulic analysis necessary for each individual case in the course of planning. The reduction in diameter is already partially compensated for with the extremely good friction co-efficients of the new, smooth SPR liner.

Intakes open during winding

Several feed conduits enter the canal in the course of the section to be rehabilitated – these had to be reconnected after rehabilitation work. For this purpose the liner was opened at the precisely measured and colour-marked position of the conduits, and a PVC pipe of the appropriate diameter inserted into the intake. Spiral-winding technology enables the inflow from the feed conduit to be maintained.

The entry points were closed off with deep plugs, which were removed after the sealant had hardened, to prevent material from obstructing the junctions when the annular space was subsequently sealed. In a final stage the intake pipes were milled off flush and cleanly connected by GRP hand laminate to the inside wall of the new sewer.

Second generation material

In view of the obvious advantages of the system, municipal network operators have overcome their historic reservations in respect of the material PVC – particularly pleasing from the point of view of KMG Pipe Technologies.

In recent years, the nationwide bans, some of them instigated in the 1970s, have been lifted following an updated ecological evaluation of the material, giving further rise to confidence that the certified SPR spiral-wound pipe lining method will consolidate its position in the sewer rehabilitation market in the near future.

At an open viewing of the construction site in Stendal’s Bruchstrasse, held from 17–19 May 2011, sewage experts from authorities and water boards including Kiel, Magdeburg and Erfurt were able to inform themselves of the possible uses of the SPR method in difficult situations.

Among them were employees of the Berlin water utilities responsible for sewer rehabilitation. All of them took a critical look at the spiral-wound dome profile liner below ground. The project was completed successfully.