Langes Feld contaminated site
Securing the contaminated site
Investigation
To determine the depth of the diaphragm wall, 1,460 running metres of exploratory boreholes and 31 pumping tests were carried out. In addition, 1,600 running metres of shafts were constructed. Chemical analyses and landfill gas investigations were carried out alongside the exploratory boreholes to assess the risk.
Remediation
For decades, the site was used as a landfill, with uncontrolled dumping taking place. Everything that was no longer needed was disposed of: from construction waste and household rubbish to materials that would now require special treatment. Some of the deposited material was submerged in the groundwater or lay within the groundwater fluctuation zone – the risk to the groundwater was therefore extremely acute. Securing this contaminated site was essential. This was carried out using the ‘Vienna Diaphragm Wall Chamber System’, which offers the following benefits:
The chamber system allows each individual chamber to be checked for functionality. This is done by lowering or raising the water level in each chamber by a few metres relative to the groundwater level over a specific period. Any defects are thus identified and can be rectified at any time.
Long-term monitoring of the system also makes it possible to observe changes in the permeability of the chamber walls during construction and in the future. This provides reliable data on long-term durability.
Construction work
Cut-off walls
To minimise water retention in the enclosed area, cut-off walls were constructed using two methods.
Construction of a thin wall
A heavy steel girder is driven into the ground using a vibratory hammer. As the sheet is withdrawn, the remaining voids are grouted under pressure with a slurry consisting of bentonite, water, stone powder and cement. As this process takes place continuously and seamlessly, a joint-free wall is created. At Langen Feld, we succeeded in constructing slender walls on an industrial scale to a depth of 33 m.
Construction of diaphragm walls
These cut-off walls were constructed using the two-phase method, i.e. the construction of the diaphragm wall takes place in two stages. First, the soil is excavated by the excavation tool, supported by the diaphragm wall, using a pure bentonite slurry.
In the second stage, this support fluid is replaced with the actual sealing material. The dewatered bentonite is used to create the separation layer.
Drainage
It must be ensured that no leachate from the enclosed contaminated site area seeps into the surrounding groundwater table as well as into deeper aquifers. To this end, a potential difference of 50 cm from the outside to the inside or from the bottom to the top is constantly maintained by the drainage system. These necessary groundwater drawdowns inside the landfills and in the diaphragm wall chambers are achieved by a computer-controlled, fully automatic drainage system.
Water management is also carried out between the parallel cut-off walls. Setting the chamber water levels 20 cm above the landfill water level provides additional security against the escape of leachate. Defects in the cut-off walls can be detected early and repaired in the event of changes in the volumes to be pumped out, without any leachate escaping.
Once the contaminated site has been secured, the area is used as a landfill.
Technical data
Sealing wall area approx. 225,000 m², of which 186,000 m² are narrow walls up to 33 m deep and 39,000 m² are diaphragm walls up to 56 m deep. Enclosure length approx. 4,200 m, including a single-walled narrow wall on Markomannenweg. 42 diaphragm wall chambers based on the ‘Vienna diaphragm wall chamber system’ with a length of approx. 90 m and a width of 5 m. 104 chamber gauges.
The rehabilitation of an environmental engineering problem site in fast-forward
In 1990, environmental clean-up and Abraumdeponie Langes Feld GmbH were established for the purpose of rehabilitating the W15 contaminated site: Langes Feld. From 1992 onwards, the designated contaminated site was rehabilitated by the operating company and a landfill site was constructed on the site. The completion of the impermeable chamber system took place in 1997. In 2017, the contaminated site was designated as secured by the Federal Environment Agency.