WWTP Liestal: A complex hydraulic engineering project

The ERNE Building Construction team built a new combined sewer overflow basin in Liestal for the Canton of Baselland. Its purpose is to collect wastewater from the sewer system as well as rainwater, in order to protect the sewage treatment plant from becoming overloaded, for example following heavy rainfall when large volumes need to be treated all at once.

Before excavation could begin, sheet pile profiles were installed in the ground to prevent the soil from slipping. Filter wells were installed within the sheet pile walls. This procedure was essential as the work took place in the groundwater zone and the excavation pit was below the water table. In parallel with the excavation work, steel beams were installed at right angles and welded together, along with temporary injection anchors. These supported the sheet piling against the soil behind it. Due to these safety measures, there was very little space available on the construction site, which required flexibility from the team.

This was followed by the foundations and the installation of the floor slab, which had to be anchored in the ground at a depth of around ten metres using bored piles. This was necessary to prevent the future 3,000-cubic-metre basin from floating up. Now the actual new construction began. The basin was formworked and concreted, and constructed as a watertight tank. Filling tests were carried out to check for leaks. The area between the structure and the ground was then backfilled. Only now could the steel girders, sections and sheet piling be removed piece by piece. As the mixed water treatment plant is to be located underground, the basin was sealed with a concrete slab and covered with soil after commissioning.