Introduction
The chemical industry, with its products, offers an indispensable basis for many other sectors, such as the automotive, food, plastics, pharmaceutical and construction materials industries, as well as agriculture. As such, it is a key driver behind our modern-day achievements. The development of the chemical industry has also brought about advances in process engineering, which have significantly boosted the efficiency and resource-friendliness of contemporary production. This was not always the case: at one time, for instance, it was standard practice to tip unusable waste materials and by-products from production at company-owned landfill sites. With the growth of the environmental movement, public attention has also come to focus on these sites, thereby triggering large-scale remediation projects.
The remediation of contaminated industrial sites necessitates detailed preliminary investigations. In addition to soil surveys, these should focus on the nature of the deposited materials, the potential pollution, the volume and depth of the landfill body, the overlay as well as possible follow-up uses. These investigations are often hampered by the incomplete documentation of old polluted deposits, due to the lack of statutory guidelines at the time of their construction. The German Waste Disposal Act (Abfallbeseitigungsgesetz) came into effect in 1972. In Austria, the first Hazardous Waste Act (Sonderabfallgesetz) appeared in 1983. The preliminary investigations can then serve as the basis for the development of a remediation plan that meets the requirements imposed by the contaminated site and all other interest groups. The investigation results may even lead to the adoption of different remediation plans for adjoining sites. Here, the Kesslergrube landfill in Grenzach-Wyhlen, Germany is a case in point: while BASF has opted for containment by encapsulating its zone, Hoffmann-La Roche is excavating and thermally treating the contaminated soil in its section.
History of the K20 Contaminated Site Remediation Project
The K20 contaminated site, a former company landfill, lies roughly one kilometre south of the Austrian town of Brückl in the lower part of the Gurktal valley. It is divided into two zones and, between 1926 and 1981, was used for the disposal of carbide lime, chlorinated hydrocarbons (CHC) and mercury-polluted waste. The deposited CHCs mainly comprise tetrachloroethylene, trichloroethylene, hexachlorobutadiene, hexachloroethane and hexachlorobenzene. The total quantity of CHCs was estimated at some 100 – 1,000 t. An aerial photo of the K20 site is shown in Figure 1.
As of 1995, various remediation works were performed in tandem with the ongoing evidence-collecting measures. Following a risk assessment conducted by the Environmental Agency Austria (Umweltbundesamt) in 2000, the former landfill was entered as contaminated site K20 in the Austrian Contaminated Sites Atlas. In 2003, in the wake of further investigations, K20 was classed as a priority 1 site.
Figure 1. K20 contaminated site; Zone I, over most of which a cover ining has been installed, is at the front while Zone II lies to the rear. (As 28.3.2017)
In December 2009, a notice was issued by the Governor of the Austrian Federal State of Carinthia for remediation of the K20 site through the continuous and complete clearance of all landfill materials. Depending on their pollutant content, these were to be recycled, treated or disposed of. In 2012, with the necessary preparations completed, the clearance works commenced and approx. 150,000 t material, including some 100,000 t lime sludge, was removed.
In November 2014, when hexachlorobenzene was detected – among other things, in locally produced food – near the cement plant entrusted with recycling the polluted lime sludge, clearance of the contaminated site was discontinued.
The conclusion reached following a new pan-European call for tenders for transportation and treatment of the polluted lime sludge was that "no project involving continued site clearance could offer legal, technical, time or cost certainty".
Cover Lining
Containment of the contamination had already been considered as a remediation option in the first analysis performed by GUT (Gruppe Umwelt + Technik GmbH), dated 1.9.2008. The disadvantage of this procedure is that it involves a permanent monitoring and maintenance regime for the containment, and that the potential pollution is left in place. It is nonetheless the next-best alternative to clearance with recycling and disposal. However, the developments described above necessitated reconsideration of the containment option for the contaminated site. A more recent assessment of the options (by GUT) paved the way for an updated containment concept issued on 4.7.2016 by GWU (Geologie-Wasser-Umwelt – Geology–Water- Environment).
In addition to measures below the water table (cut-off wall to enclose contamination), the containment contracts awarded by the authorities include an innovative, multifunctional cover lining system comprising a 11 kg/m² calcium bentonite mat, an LDPE membrane with integral CHC-proof aluminium layer, a drainage element and a 2 kg/m² activated carbon mat. This system constitutes a practically impenetrable barrier for gaseous CHC emissions. The activated carbon mat is installed below the membrane in order to reduce the CHC concentrations acting on the membrane and slow down the associated diffusion momentum. Tektoseal Active AC unlocks whole new areas of application for activated carbon – a widely and successfully deployed high-performance adsorption agent – as part of active geocomposite solutions. The geotextiles in Tektoseal Active guarantee the mechanical stability of the active layer. This allows simple and rapid installation of the product wherever it is needed. At the same time, the active layer is fully protected against any erosion caused by water or inclinations. The lining system also incorporates horizontal suction pipes on two levels, above and below the sealing. Soil air is continuously exhausted from the lower extract layer and passed through a cleaning unit. The upper extract layer is used for monitoring purposes, though can also be exhausted if necessary. Figure 2 shows the on-site installation of the main lining components. Installation of the cover lining started in November 2016.
Figure 2. Installation of cover lining. The 11 kg/m² NaBento RL-C calcium bentonite mat (black) can be seen in the foreground. Behind this is the 2 kg/m² Tektoseal Active AC activated carbon mat (white). Also pictured is the LDPE membrane (grey and on the roll) with integral CHC-proof aluminium layer.
Analysis of Structural Stability
By virtue of its history, the K20 site has a highly irregular topography. While typical landfill slope inclinations of between 1:3 and 1:2 are found in many parts of the site, certain areas exhibit short breaks in the slope with inclinations of up to 70°. This necessitated extensive contouring works to flatten out these areas. While inclinations of 1:3 can normally be built with adequate stability against slip parallel to the slope through the specification of suitable mineral materials and geosynthetic products, steeper slopes of up to 1:2 or 1:1.5 require additional stabilisation.
Put simply, the risk of slip parallel to the slope always arises where the internal angle of friction in a potential slip plane (e.g. between protective nonwoven and smooth membrane) is similar to or smaller than the inclination angle of the slope itself. In such cases, with due allowance for all reduction factors, the maximum capacity of the system is exceeded and the system becomes theoretically unstable. The non-accommodated slip forces can be determined from the difference between the internal angle of friction and inclination angle, with due allowance for the length and thickness of the assembly and the specific weight of the incorporated soil. An element capable of resisting tensile forces, typically a geogrid, is then used to transfer the forces to the top of the slope and from there into an anchorage zone (Figure 3).
Figure 3 Structural system for slip parallel to slope.
The system remains stable as long as the acting force Ed is smaller than the restraining force Rd. Should the actions exceed the restraining friction forces, the difference RB can be accommodated and transferred by a geogrid.
The calculations for resistance to slip were performed for the maximum slope inclination of 1:2 prescribed by the authorities. The maximum slope lengths run to around 45 m.
The layer structure is shown in Figure 4. As can be seen, there are overall nine potential slip planes (Table 1), for some of which shear test results were available or project-specific shear tests were performed. To be on the safe side, only the contact friction angles d are applied in the calculations – in accordance with the GDA E2-7 recommendations issued by the GDA (German Geotechnics of Landfills and Contaminated Sites) working group – while cohesion (c) and adhesion (a) are disregarded.
Table 1. Possible slip planes
Drainage element vs. cover soil |
Drainage element, internal shear strength
|
Drainage element vs. 16/32 drainage gravel |
16/32 drainage gravel vs. 1,200 g/m² protective nonwoven |
1,200 g/m² protective nonwoven vs. LDPE membrane seal |
LDPE membrane seal vs. Tektoseal Active CHC |
Tektoseal Active CHC vs. NaBento RL-C |
NaBento RL-C, internal shear strength |
NaBento RL-C vs. base |
Figure 4. Structure of multifunctional cover lining
For most of the shear planes, the design values derived from the test results ranged from d=24° to d=37.2°. However, two potential and mutually independent slip planes with very low internal angles of friction were identified on either side of the gravel drainage layer. The internal shear strength of the drainage mat was found to be the lowest value (d=23°) above the gravel drainage layer. The critical slip surface below the gravel drainage layer is between the protective nonwoven and the LDPE membrane (d=10.5°). On account of their spatial separation, the two levels required separate consideration and suitable reinforcement.
The calculations were performed in accordance with the partial safety factor concept under EC-7, based on the Austrian version ÖN 1997-1. Allowance was made for both design situations BS-1 and BS-2, with a high consequences class rating of CC 3. The calculation procedure is based on Section 8 of the EBGEO ("Recommendations for Design and Analysis of Earth Structures using Geosynthetic Reinforcements", issued by the German Geotechnical Society, 2010) and the GDA (German Geotechnics of Landfills and Contaminated Sites) working group recommendation E2-7 (2015).
Required nominal strengths of up to 200 kN/m were determined for the top reinforcement layer and of up to 600 kN/m for the bottom reinforcement layer.
Given that the use of anchorage trenches is not technically feasible due to the need to prevent local emissions, the only means of tying back the geogrids is by flat anchorages. The poor internal angles of friction (in particular, d=10.5° at the nonwoven/membrane surface) necessitate very long anchorage lengths – of up to 66 m for 1 m cover. Yet, in places, the available space is limited to 25-32 m. The solution adopted due to these constraints and the site geometry provides for a saddle-shaped lining with a single continuous geogrid extending over opposite slopes. The anchorages on the opposite slopes thus have a counterbalancing effect. To avoid any imbalances, however, this solution necessitates the simultaneous, parallel placement of soil on the slopes as the works proceed. The placement plans were prepared accordingly (see Figure 5).
Figure 5. Placement plan for bottom reinforcement layer, zone
Summary
As there is no universal solution to the problems posed by contaminated sites, designers and engineers need a "toolkit" from which they can select the most suitable measures.
The multifunctional active cover lining described in this paper, for use in the containment of contamination, represents a new tool in this kit. Specific site factors can be readily accommodated through the variation of materials or their quantities. In this particular case, geogrids were used to install a durably stable cover lining within a limited space.
From an ecological perspective, there is currently no alternative to the containment solution for the K20 contaminated site. It eliminates the need for long-distance transportation, possibly to neighbouring countries, and prevents any local pollution through emissions during loading and movement. Thousands of truck journeys are now unnecessary. The cover lining will protect against both the ingress of precipitation and wind loads. The construction took place from winter 2016 and was completed in the winter of 2018.
2019 EcoForum Conference paper submitted by Kristof Thimm, HUESKER Synthetic GmbH & Ian Weir, HUESKER Australia Pty Ltd
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