The aging Horgen and Thalwil wastewater treatment plants, situated near Zürich, Switzerland, require extensive renovation to enhance their cleaning efficiency. The two plants are therefore to be merged. The new, modern Zimmerberg wastewater treatment plant (ARA) will treat the wastewater of up to 78,000 inhabitants and industry from 2027.

In the event of an extreme flood of the Sihl a river near Zurich, building damage in the city of Zurich alone could exceed 6 billion Swiss francs. A spillway tunnel from Langnau am Albis to Thalwil will make it possible to transfer flood peaks from the Sihl into Lake Zurich from 2026. It will thus eliminate by far the greatest flood risk in the canton of Zurich.

For both structures the Waste water treatment plant (WWTP) and the outlet of the spillway tunnel a deep excavation pit is being constructed very close and beneath to one of the most important railway lines in Switzerland, on steep terrain and directly adjacent to existing buildings.

An intensive monitoring of the challenging excavation retaining walls is implemented with total stations (motorised theodolites), a total of 50 anchor load cells and 11 boreholes equipped with in-place-inclinometers. The pre-anchor load cells manufactured by Huggenberger, Switzerland and the MD-profile type in-place-inclinometers from Sisgeo are connected one the WWTP-site to Lora based nodes to have a wireless data transmission to the gateway. The in-place-inclinometers and the Data are then managed on an internet platform, specifically configured for the project. The platform allows an easy access to the information. In addition, it is possible to configure alarms if the deformations or /and anchor loads exceed the established threshold.

Monitoring of the anchor load cells and the in-place-inclinometers show so far, that the readings and results are still within the threshold values and clearly demonstrate the stability of the excavation pit and the adjacent railway tracks.

Thanks to Daniel Naterop – Huggenberger PM