An area of former parkland near Kiel, the capital of the Schleswig- Holstein region of northern Germany, was heavily contaminated with PCBs and heavy metals resulting from a variety of illegal activities. These included repairing cars, and scrap metal recovery from stripping down and burning transformers and other debris such as wire cables.
The original plan was to carry out conventional excavation and thermal treatment of 600t of contaminated soil and to seal off the area with an HDPE liner. However, main contractor Sonnichsen & Gortz proposed a method that it considered more environmentally friendly and ultimately cheaper alternative.
The chosen remediation method involved mixing the top 750mm of soil with specially selected cement to fix the pollutants and prevent further contamination migration. This was carried out by German manufacturer Telerob's high power Rhino earth tiller.
Because the method had not been used before, Sonnichsen & Gortz first had to convince the local environmental agency, Umweltschutzamt der Landes- hauptstadt Kiel, that it would clean the site to acceptable limits. Laboratory tests on soil samples showed that it would be effective in immobilising the contamination.
Further tests were then carried out over a trial area on site. The binding agent, a special mix of Heidelberger cement that cures and binds the soil by its natural moisture content was mixed in with the Rhino machine, followed by heavy compaction of the treated soil.
The cement permanently fixes pollutants by reducing the soil's permeability and by reducing the mobility of the heavy metals by changing the pH value to create a higher alkaline environment.
The trial on the 2,500m2 area, which is underlain by sandy boulder clay and glacial marl underneath a thin layer of backfill, was carried out in September 1997.
Telerob's Mike Naismith explains that as the results were acceptable, it meant that the trial area could be incorporated into the main work, which took place in April this year.
As the entire area had to be fenced in for safety reasons, the cement was pumped via tubes directly from delivery trucks into a distribution vehicle which then spread out the cement to the required thickness of 750mm. The work area was split into several sections. After the cement was spread, the Rhino machine mixed it in, creating an homogenous soil and cement mixture of the contaminated soil. Compaction was to follow on immediately, but Naismith says this was not always possible due to heavy rain.
Fortunately, it was discovered that a delay in compacting the soil did not have an effect on the final performance of the method. This was confirmed after main works finished, when undisturbed soil samples taken from different areas and different depths across the site were tested.
These were left for four weeks to allow the cement to bind with the soil and then their permeability and density measured. An eluate test was also carried to assess the mixture's ability to bind the heavy metals. 'Results showed that in spite of the difficult weather conditions, specified values were met,' Naismith confirms.
The entire area was then covered by a composite drain beneath an 0.8m thick topsoil layer, which allows rainwater to drain off site and prevents the immobilised layer from drying out.
'As no results on the long term behaviour of this type of immobilised layer existed,' Naismith says, 'decisions had to be based on experience in concrete construction, where there is a wealth of knowledge on long term stability behaviour.' Using this information, it was assumed that the immobilised soil offered long term protection to the surrounding environment.
Work showed that the method made significant savings in terms of both cost and time over conventional remediation such as dig and dump, with less risk to the environment. This is the result of treating the soil insitu, reducing the cost and time of transporting and dumping the material in landfill.
Naismith explains that the method was made possible because the Rhino earth tiller can work soil down to 750mm, while other machines can only turn to depths of 300mm to 400mm.
The track-mounted machine can be fitted with three tiller units: forest, mixing and demining. The forest unit can be used for a variety of forestry work such as soil preparation, fire prevention and fire fighting. The mixing unit is suitable for all sorts of soil remediation work where it can also be used to mix in other additives including chalk (for fertilisation), bacteria and fungi (for reduction of pollutants) and clay (to reduce permeability). The demining unit is used for clearing minefields.
The machine can also be remote controlled in dangerous areas such as minefields and heav- ily contaminated ground using a special video camera and television.