It must be possible to inspect underground installation channels in sewage treatment plants.
To protect soil and water, from January 2029, no sewage sludge may be applied to agricultural land as fertiliser. In addition, from this point on, the rare raw material phosphorus contained in sewage sludge and sewage sludge ash must be recovered using physical and chemical processes. For this reason, the city of Munich is building a new plant for the thermal utilisation of the sewage sludge produced on the site of the Gut Großlappen sewage treatment plant – one of two municipal sewage treatment plants. This plant will replace the plant that has been in operation since 1998.
As part of the construction of the new plant, the capacities of the pipeline routes have to be expanded. These pipelines are laid in underground installation channels that connect the individual plant areas in the sewage treatment plant to ensure efficient and reliable access, control and maintenance options, as well as quick accessibility in the event of an accident. The new installation channel is 115 metres long, around 6.5 metres deep and has a clear dimension of 4 x 4 metres.
High groundwater levels require a watertight construction pit
To create the up to 9-metre-deep construction pit, PORR's special civil engineering team constructed around 200 metres of cross-section length of intersecting bored pile walls. As the groundwater level was 1 to 2 metres above the construction pit floor, the shoring was designed to be watertight. A total of around 3,300 metres of drilling was carried out. The piles, with diameters of up to 880 millimetres, were up to 13.5 metres long. As the deeper bored piles partially encountered a water-bearing sand layer, particular care had to be taken. Drilling was only possible under water load to prevent the drill pipe sole from breaking open.
In some areas, the shoring was temporarily secured with 4-strand anchors and braced with I-beams and pipe stiffeners. Additional specialised civil engineering works were required in the areas connecting to the existing buildings. In addition to jet grouting underpinnings and sealing columns, pipe injection columns were installed locally to seal various connections in the area of subsidence-sensitive underground installations.
Wolfgang Wiesnet, Regional Manager for PORR Special Civil Engineering, on the challenges of the project: ‘Due to subsequent construction sites and the fixed commissioning date, the schedule had to be meticulously adhered to. The fact that we succeeded in doing so despite various logistical stumbling blocks during the European Football Championship and the requirements for environmental protection and occupational safety is a testament to the high level of commitment and expertise of the entire team.’
