Record breaking GRP project installed in Munich

1. General information about the project

One of the most important sewers in Munich flows under the “Dachauer Straße” military area, in the area of the Spiridon-Louis-Ring and Ackermannstraße near to the previous Olympic site in the North of the Bavarian state capital of Munich (Figure 1).

The so-called Oberwiesenfeld ‘main collection sewer’ has to fulfill two tasks. On the one hand it has to transport the wastewater of the adjacent suburbs to the Gut Großlappen wastewater treatment plant and, on the other hand, it serves as a connection sewer between both wastewater treatment plants. If required, the incoming wastewater, which is usually transported to the Gut Mariehof wastewater treatment plant, is diverted to the Großlappen wastewater treatment plant via the Node 3 distribution structure 3 at the start of construction. The Oberwiesenfeld collector therefore fulfills an important regulating function in the sewer network of the Munich city drainage system.

Furthermore, the Oberwiesenfeld sewer, in conjunction with the Oberwiesenfeld rainwater retention basin, has a throttling function for discharging water into the sewer network during heavy rainfall incidents. For this purpose, the Oberwiesenfeld main collection sewer in the area of the rainwater retention basin comprises North and a South circulation sewer. Both sewers are combined to form the actual main collection sewer downstream of the rainwater retention basin.

2. Instigation

The Oberwiesenfeld Part II main collection sewer is a sewage sewer manufactured from C8/10 rammed concrete with a clinker lining, which was built in the period from 1906 to 1912. The main damage formation of the over 100-year-old sewer was primarily caused by longitudinal cracks on the berms and in the upper part of the sewer with partly massive groundwater inlets. In addition, there were sewer clinker spalling and longitudinal cracks in the crown area.

Extensive examinations were subsequently executed on the sewer in order to determine the causes of the damage, to forecast the further course of the damage and thereby gain knowledge regarding the structural conditions.

As far back as 2006, partial repairs had been executed in sections by utilizing insitu concrete shells in the base area of the most severely damaged areas. Since this refurbishment procedure seemed however unsuitable for additional refurbishment of the entire sewer section due to the constant risk of flooding of the sewer, alternative solutions were sought.

In November 2012, ACI Aquaprojekt Consult Ingenieurgesellschaft GmbH was commissioned by Münchner Stadtentwässerung to design a sewer refurbishment project for the two partial sections of the Oberwiesenfeld main collection sewer and to provide site supervision for this project during construction. After comparing the variants in the preliminary planning stage, the decision was made to use a GFP lining with large profiles for economic and technical reasons.

3. Scope of refurbishment

The planning executed by ACI Dresden comprises the refurbishment of the existing Oberwiesenfeld main collection sewer from the Dachauer Straße to Schleißheimer Straße with GFP full profile pipes of dimensions 3860mm/2935mm with a dry weather channel (DWC) of 1106mm/716mm to 3860mm/3055mm with DWC 1106mm/816mm (Figure 2) for a total length of 1,565 m including the new construction of 9 access shafts, 2 measuring shafts as well as refurbishing 4 existing shafts. In addition, 19 connecting pipes in the dimensions DN200 up to DN1000mm/DN1500mm were connected and four structures were adapted to the new base heights by means of a sewer clinker base lining.

The refurbishment was executed in two stages, including the implementation of the provisional pumping out concept. This therefore created the 1st refurbishment section for the southern circulation sewer, and as the 2nd section, the actual main collector.

Refurbishment lengths:

Profile: 3860mm/3055mm with DWC 1106mm/836mm for 621 m

Profile: 3860mm/2935mm with DWC 1106mm/716mm for 75 m

Profile: 3860mm/2970mm with DWC 1106mm/751mm for 869 m

The total construction period commencing from the first arrival of the colleagues and equipment on site until the complete evacuation of the site was 2 ½ years.

The surface areas made available by the City of Munich for the individual excavation trenches, the installation shafts, the construction site equipment, the pipe and material storage were kept to an absolute minimum, which represented a logistical challenge for all those involved in the construction work (Figure 3).

Due to the considerable importance of this sewer, a detailed sewer monitoring system with permanent level measurement and a thereby corresponding alarm plan were installed to protect the staff, the technology and the operation of the wastewater treatment plant.

The re-commissioning of the sewer section had to be guaranteed at all times within 30 minutes with the corresponding inflow volumes.

4. Calibration

The entire sewer section, which was to be refurbished, was recorded by means of a 3D laser scan after the contract was awarded. This therefore made it possible to execute an improved examination for planned new pipe cross-sections and adjust them to the actual status.

The objective of the calibration was:

1. To create a three-dimensional image of the old sewer with corresponding longitudinal and transverse sections.
2. To establish the exact position and dimensions of the existing inlet pipes.
3. To obtain a database for the preparation of the pipe laying plan (Figure 4) with pipe number sequence and for the determination of the different pipe lengths as well as for the planning/design of “trapezoidal elements” in the curve area of the Schleißheimer Straße.

5. Dewatering/pumping out

The dewatering concept included the diversion of the main sewer and the drainage of all inlets.

A double plate gate valve (2 x 1.9m x 1.6m) was installed in the distribution structure 3 in the first refurbishment section, which thereby diverted the inflow volumes of more than 10 m³/s into the North circulation sewer by damming and therefore passed by the refurbishment section. In addition, a second double plate gate valve was installed at the end of the first refurbishment section to counteract back flow flooding. The existing gate valves of the Oberwiesenfeld rainwater retention basin were utilized for backwater in the second refurbishment stage. A double plate gate valve was also installed at the end of the refurbishment area in this refurbishment section to counteract back flow flooding.

The targeted objective for the dewatering was to maintain the discharge capacity as far as possible and to minimise the number of days lost due to flooding during heavy rain incidents.

6. Preparatory work

The preparatory work for this project mainly included the construction of the dewatering system as well as the construction of the seven excavation pits for inserting the GRP. The size of the excavation pits was approx. 6m x 3m and/or 6m x 4.5m with depths between 8m and 10m. The classical shoring with soldier piles was chosen as the shoring method. Closed groundwater dewatering measures in the form of submerged wells were necessary for the construction of the excavation pits. Corresponding pipework routed the groundwater to seepage wells (3x DN880, seepage rate 3 x 30l/s) on the grounds of the Olympic Park.

Furthermore, the 900/1500 egg-shaped inflow sewer, which drains the wastewater from the Olympic Park, had to be diverted into the North circulation sewer for the time of the refurbishment work. A DN600 bypass between the two sewers was utilized for this purpose.

Prior to the installation of the elements, it was necessary to execute cleaning, an inventory, repair defective positions and a very complex sealing of the cracks in the berms and dry weather areas against penetrating groundwater. These cracks had to be sealed every working day before the GRP elements were installed in order to rule out the risk of ground breakage in the sealing area. This presented the colleagues with great challenges, as leaks caused by penetrating groundwater in partial areas of up to 130 l/s over a length of 100m occurred.

In addition, mineral injection was utilized to stabilize the berm in an area of the canal section which indicated the strongest deformations and washout. The danger here was caused by sewer failure due to the loads applied by the installation carriage including the GRP pipe (total weight approx. 7.5 T) to the installation location.

7. Installing GRP modules

In conjunction with the Hobas Rohre GmbH pipe manufacturer, the final profile shape was determined on the basis of the calibration and taking the structural specifications into account. As a result of the refurbishment which was already executed with insitu in 2006, three different GRP profiles had to be produced to guarantee a certain constant sewer gradient. Each profile consisted of two individual profiles, the hood and the dry weather channel, which were precisely joined together during the manufacturing process. Only very small dimensional tolerances were permitted in order to avoid any problems with the connection of the individual elements in the subsequent installation process. In combination with a structural wall thickness of 68mm and the thereby associated element weights, the pipe manufacturer Hobas Rohre GmbH was faced with considerable challenges regarding production as well as for the logistics.

The profiles were inserted individually into the old sewer via an excavation pit during installation. This was executed with a tower crane due to the weights of approx. 3.8 tons per GRP element.

An installation carriage developed specially for this project was manufactured for the transport of the elements into the old sewer, which was able to transport and couple the elements due to its construction. It was also hereby necessary to ensure that this installation carriage could be recovered from the respective excavation pit within 30 minutes. Due to the profile shape, the collar formation could not be executed with the customary rubber sliding wedge seals, rather all elements had to be connected on site by using adhesive joints. This adhesive joint created the primary pipe connection. All pipe connections were provided with circumferential laminates as a secondary connection.

All the inlets were opened after installation of the GFP elements and then connected by means of a manually applied laminate. Due to the damage in the old canal substance, it was necessary to fill the annular space between the old sewer and the GFP elements installed immediately after installation up to the damaged berm area of the old sewer. A modified ring space filling material was utilised, which was specially modified for these requirements, with a strength of at least 5 N/mm² after 24 hours. The remaining annular space was then filled in layer by layer without voids.

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Another special feature of the pipe installation was the requirement for constant profile stiffening of the elements, from manufacturing up to installation of the annular space filling material. Supplementary supporting scaffolds were hereby utilized exclusively for the installation of the GRP elements on the installation carriage. After successful installation of the elements and fixation against buoyancy, site bracing was again installed which then remained in the sewer until installation of the end position after the annular space filling. A deformation of the GRP elements by own dead weight or other factors could therefore be prevented.

Key data:

• Installation of 787 GRP elements
• Installation of 2087 tons of annular space filling material
• Production of approx. 8600m internally-positioned laminate connections (four-layer), conveying and filtering away 887,000m³ groundwater

8. Conclusion

Based on a well-founded and very detailed planning of the measures and a professional execution in close cooperation with the Münchner Stadtentwässerung as client, and the supervising engineering office of ACI from Dresden, the construction measure could be finalized to the complete satisfaction of the client in the specified construction time. At no time were there any restrictions or objections on the part of residents, particularly with regard to the exposed and significant location of the construction project.

A positive conclusion can therefore be drawn up for all partners involved in the construction project, who have extended the limits of what is feasible in the field of sewer refurbishment by means of GRP single pipe lining through this construction project, which in our view have never been achieved before.