The vision for Kesslergrube: Germany’s ambitious remediation of a polluted area

Ambitious remediation of a polluted area instead of “out of sight, out of mind”. HPC AG is thoroughly remediating a pit filled with chemical and pharmaceutical waste, among other things, using state-of-the-art equipment for the Swiss-based Roche Holding AG. Excavating an average of 600 tonnes per day, and filling a total of 15,000 containers in a very confined space at a depth of 13 metres right next to a river – HPC AG, the general planner and construction manager, has shown that this is possible.

The Rhine River flows tranquilly past the Kesslergrube site in Grenzach-Wyhlen. For a long time, trees and bushes hid the former gravel pits, which were used as rubbish dumps. Shortly before the First World War, gravel began to be extracted there. From the 1950s until the mid-1970s the pits were refilled. Among the filling material was not only building rubble, household and commercial waste, but also waste from the chemical and pharmaceutical industries. It was precisely here that HPC AG planned and managed one of the largest and most ambitious remediation projects of a polluted area ever seen in Germany. Although no pollutants were found in the drinking water, they were certainly found in the groundwater. In addition, it was necessary to protect against potential long-term hazards, which could be considerable, especially from the neighbouring Rhine.

There are good reasons to clean up the Kesslergrube. The chemical company BASF and the pharmaceutical and diagnostics company Roche are responsible for separate parts of the pit. The two companies took quite different approaches. While BASF will enclose its part of the pit, for its side Roche completely excavated the landfill, totalling 365,000 tonnes – and incinerated the excavated the material following the highest safety precautions. As different as they are, both approaches meet the requirements for the remediation of a polluted area under German law. Swiss law is stricter. Roche applied this to meet its own requirements. Roche’s sustainability philosophy includes the goal of remediating contaminated sites within one generation if possible – in Grenzach-Wyhlen, not even ten years should go by.

Roche invested 240 million Euros in the remediation of the Kessler pit. HPC AG handled the risk assessment, approval and contract award planning, site management and technical construction management for Roche during the ambitious remediation project.

The team was led by Thomas Osberghaus, with civil engineer Arno Knöchel for geotechnics, Marcus Mumbach as site manager, Soledad Montesinos for quality assurance and Bernd Maier for occupational safety, along with hydro-geologist Dr. Adalbert Huss and hydro-chemist Dr. Stefan Hunger as well as other engineers and scientists of HPC AG who were also involved.

Some thick boards to drill

The planning faced many challenges. The excavation covered an area of 14,000 square metres with a depth of 13 metres directly on the Rhine and adjacent to two sewage treatment plants. A structurally and hydraulically secure solution had to be developed. In addition, it was anything but easy to keep the excavation pit dry, as the groundwater is already present at a depth of six metres and the pit had to be lowered seven metres further to the 13-metre-depth to reach the excavation base. The karstified, highly water-permeable shell limestone had to be mastered, which posed the risk of the groundwater surge forcing its way into the excavation pit from below via the excavation base. After extensive expert consideration for the geotechnical and hydrogeological characteristics of the area, the solution was found in the form of a retaining wall, which embedded into the shell limestone to a depth of 25 metres. There, the wall reached a hydraulically impermeable layer so that both the horizontal and the vertical groundwater flowing in from below could be shut off.

Another difficulty was that the area of the construction site was extremely tight. There was no way to stockpile excavated material, sample it and then decide according to the laboratory results which thermal plant the material should be taken. Therefore, it was necessary to pre-declare all of the landfill material in a small-scale grid using about 160 boreholes and thousands of individual analyses even before excavation. In addition, the spontaneous ignition potential of the hazardous waste had to be considered. Chemical reactions could lead to fires or even low-speed detonations or explosions when they came into contact with atmospheric oxygen, which is why special inspections and protective measures were necessary. Due to Roche’s particularly high requirements for occupational safety, immission control and neighbourhood protection, precautions were taken: an airtight and soundproof enclosure with permanent negative pressure maintenance and exhaust air purification was designed for the excavation site, covering the area of two football fields.
Geotechnics, hydrogeology, waste management, occupational safety and construction logistics, to name just a few of the many topics: HPC was able to master all of these with an interdisciplinary team of engineers and scientists.

Step by step

The complex remediation project began in 2014. HPC planned, tendered, managed and supervised the various project phases. To reach the project site, the beginning of an adjacent road was relocated and a neighbouring industrial building was modified. A 130-metre-long temporary jetty was built on the Rhine to enable the excavated material to be transported away in gas-tight and accident-proof containers in an environmentally friendly manner. This meant that thousands of trips with trucks through the Grenzach-Wyhlen municipality could be avoided. The jetty was also necessary to provide the appropriate space for the 1,100 square metre purification plant for the groundwater. In addition, not only a container village for the different remediation companies was built, but also a visitor centre.

Before the excavation began, 1,700 waste and soil samples were evaluated and allocated to the different thermal treatment plants in 100 cubic metre quadrants – depending on the type and level of pollution. In parallel, a retaining wall was constructed from 888 piles, each with a diameter of 1.2 metres. This served to provide not only structural support for the vertical pit walls and to seal off the excavation pit against groundwater, but also as the foundation for the massive enclosure of the entire excavation and logistics area in the form of a three-bay hall.

Headline

The 14,000 square metre excavation area was divided into three segments and kept dry with the help of pumps. The contaminated water that was pumped out was then cleaned directly on site and discharged into the Rhine. However, the HPC team worked not only with the soil and water, but also with the air. Large ventilation systems ensured a constant negative air pressure in the enclosure so that no pollutant particles could escape into the environment. The systems sucked in about 140,000 cubic metres of air per hour. This was then cleaned by a special exhaust air purification system. Excavation work could then begin in 2017 – 365,000 tonnes of waste were filled into the gas-tight and accident-proof special containers. According to German waste legislation, it would have been possible to transport a portion of the excavated material to a landfill.

Roche nevertheless opted for 100 percent thermal treatment in order to permanently eliminate all organic pollutants. The containers were transported to loading terminals by ship. From there, they continued their journey by rail to special facilities in Germany and the Netherlands.
After excavation and HPC confirmed the absence of contaminated material at the site, the excavation pit was successively refilled with uncontaminated soil material from regional construction projects. The retaining wall remained in the subsoil, but culverts were added in several places so that a natural groundwater flow could re-establish itself. The landing area for ships was dismantled and the Rhine riverbank was subsequently restored to its natural state.

Togetherness is the key

From our days in school we all know the difference between what’s necessary and what’s sufficient. Experienced experts, planners and resources are necessary, but not sufficient. Such a project can only be successful if there is cooperation amongst all those involved. This applies both to the client’s own team and to the attitude that the client adopts towards the contractors. The HPC team experienced on a daily basis how Roche maintained a consciously appreciative communication approach when dealing with partners and suppliers.

Professionalism, clear responsibilities, tight deadlines and very high quality results on the one hand and respectful personal interaction on the other are not contradictions. HPC is grateful to Roche for this trusting and collegial cooperation. The entire text was written in 2020 using the past tense for better readability, even though the project will not be fully completed until 2022.