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From quarry to landfill site – planning and quality control in the context of steep wall sealing at the Wirmsthal landfill site

Introduction

At the end of the 1980s, a new landfill site was needed in the district of Bad Kissingen, as the Arnshausen landfill site that had been in operation until then was almost completely filled. A quarry in the immediate vicinity of the old landfill site, where shell limestone had been mined for gravel since 1965, was the obvious choice for the new site. Measured from the top of the site, the quarry has a depth of approximately 60 metres. During the extraction of the rock, berms were created, dividing the almost vertical quarry walls vertically into several sections, each approximately 20 metres high.

Fig. 1: Location of the Wirmsthal landfill site with the old Arnshausen landfill site to the south-east

The use of the old quarry as a landfill site promised a number of advantages (appropriate subsequent use of a site that had already been extensively remodelled, very large storage volume on a small footprint, closure of a "wound" in the landscape through backfilling and subsequent recultivation, etc.). However, the structural design of the landfill had to be completely different from that of a conventional landfill, which is ideally set up as a hill landfill on a largely flat area with the optimal subsoil properties of a geological barrier. In addition to the facilities for collecting and draining landfill leachate, technical solutions had to be found, particularly for sealing the steep rock faces, which would satisfy both the applicable technical regulations and the high demands of the landfill operator.

Data on the Wirmsthal landfill

General information

The Wirmsthal landfill is being constructed in a 16-hectare quarry in the municipality of Euerdorf, approximately 4 km south of Bad Kissingen (Bavaria). The landfill area is located in the central part of the quarry, whose already completely sealed base area covers approximately 7 hectares. By incorporating rock berms, etc., the landfill area in the overlying backfill areas gradually increases in size up to the upper edge of the quarry. The landfill surface to be recultivated covers an area of approximately 10 hectares.

Since 1990, waste with a volume of approx. 2.2 million m³ has already been stored. When the landfill is completely filled, the fill volume will be 4.2 million m³. In addition to waste incineration plant slag and treated residues from flue gas cleaning, domestic waste and domestic waste-like waste was also incorporated until 2005. In accordance with the requirements of TA Si and the Landfill Ordinance, only waste that meets the classification criteria for DK II landfills (organic content ≤ 5%, etc.) has been deposited since then.

The landfill site was developed as a pit landfill. An approx. 240 m long inspection gallery runs across the quarry floor, into which both the surface water collected in the storage area and the landfill leachate are discharged. A 320 m long drainage tunnel was excavated in the extension of the inspection gallery. It ends east of the landfill in the Gösselgraben valley. There are leachate storage basins with a volume of 1,700 m³ and a leachate treatment plant, as well as a rainwater retention basin for surface water with a total volume of 2,300 m³.

Fig. 2: Cross-section through the landfill

To minimise the formation of new leachate, VA 1 to VA 3, which were established between 1989 and 1994, were covered north of the inspection walkway. For economic and landfill technical reasons, it was decided not to cover further backfill sections. In the meantime, the roofing has already been largely dismantled. Currently, only a small part of VA 3 is still covered. As long as further landfill expansion allows, this area will continue to be used as an interim storage area and for similar waste management purposes in the coming years.

The first backfill level directly above the sealed quarry floor was divided into six backfill sections. Mostly household waste and household-like waste was stored in backfill sections VA 1 – 5. The landfill gas produced in this area is collected in a total of 35 gas wells and fed via gas control stations and transport lines to a degassing plant in the entrance area of the old Arnshausen landfill.

Base sealing

As the site does not have a sufficient geological barrier, a complex, controllable base sealing consisting of several fine-grained mineral sealing layers was installed. Between the 2-layer lower control sealing with a thickness of 50 cm and the 4-layer, 100 cm thick upper sealing, there is a 50 cm thick drainage layer of basalt gravel with a grain size of 16/32. In the most recently constructed VA 6, a 2.5 mm thick plastic sealing membrane was also installed to further improve the safety standard.

The sealed quarry floor was divided into individual fields with a length of up to 140 m and a width of approx. 20 m. The fields have a cross slope of 3% to 4% towards the seepage water drainage pipes located at the lowest points and a longitudinal slope of 2% to 3% towards the inspection walkway in the middle of the landfill base.

Fig. 3: Base sealing of the Wirmsthal landfill

The leachate from the waste body is collected and drained by a surface drainage system made of 16/32 basalt gravel applied to the base sealing, through which the leachate is fed to PEHD drainage pipes DN 250. The collectors are connected to a transport pipe in the inspection tunnel, which leads to the seepage water storage basin outside the landfill body. Additional PEHD drainage pipes were laid to drain the inspection drainage system, which flow into separate transport pipes in the inspection tunnel. All drainage pipes can be flushed from the inspection tunnel and monitored by camera.

The complex sealing system used enables permanent monitoring of the base sealing via the camera-accessible control drainage pipes, which are accessible from the inspection gallery. These pipes can also be used to take samples of any water collected in the control drainage system under the upper sealing layer.

Steep wall waterproofing

Sealing the steep walls of the quarry in the usual way is not possible. Therefore, a steep wall sealing system was developed that was adapted to the conditions on site. It essentially consists of the following components:

• Support

The support for the waterproofing arranged on the rock walls consists of reinforced concrete filigree slabs that are anchored in the rock walls. The space between the rock walls and the filigree slabs is filled with water-permeable single-grain concrete so that any fissure water escaping there is drained away without water pressure building up behind the wall waterproofing. The collected water is drained via a drainage system at the foot of the wall into the surface drainage system in the inspection passage.

• Sealing

A mineral waterproofing layer is installed in horizontal layers in front of the filigree slabs and is gradually incorporated into the waste fill. The thickness of the waterproofing layer varies between 1.25 and 2 m, depending on the material used.

• Drainage

Immediately in front of the waterproofing, a seepage water drainage system consisting of stacked gabions with a cross-section of 1 x 1 m is installed. They are filled with coarse basalt gravel (grain size 80/200). This ensures high hydraulic performance of the drainage system with large safety reserves.

Fig. 4: Steep wall sealing of the Wirmsthal landfill

In sections VA1 to VA3, which were established before 1995, a 2.0 m thick fine-grained mineral sealing layer was installed. The same plastic clay was used for this as for the base sealing. Although this material has a very good sealing effect, its low load-bearing capacity and low modulus of rigidity cause considerable foundation engineering problems.

Particularly critical are the large settlements, which have already reached a magnitude of around 1.9 m at the upper edge of the wall after backfilling up to the first rock berm. These settlements can be compensated for before continuing with the construction of the wall sealing. However, further settlement of approx. 1.0 m may occur in the course of further backfilling and the associated increase in load. This settlement cannot be compensated for after backfilling the areas above the first rock berm. The waterproofing to be installed on the adjacent rock berm, on the other hand, will only settle to a very small extent, if at all. At the transition from the rock berm to the steep wall, there are therefore large differences in settlement over a short distance, which, taking into account the other soil mechanical properties of the clay, can lead to cracks or gaping joints in the waterproofing. This poses a risk of contaminated landfill leachate escaping into the adjacent subsoil.

For further expansion, the design of the wall sealing was changed to minimise the above-mentioned problems. Instead of highly plastic clay, a mixed-grain sealing made of bentonite gravel was used. This is a variant of the bentonite gravel already used several times in landfill construction, in which the normally used base material gravel has been replaced by crushed stone, which is easier to obtain locally.

Bentonite gravel is a mixture of grit, crushed sand, clay powder and bentonite. Its grain distribution has a steady progression close to the Fuller curve, resulting in a low pore content. The mixture has a fine grain content of approx. 15% (soil group GT according to DIN 19186). The components are mixed in a compulsory mixer so that a water content close to the optimum water content of 7% is achieved. Under the given boundary conditions, bentonite gravel has the following advantages over clay, which make it ideal for use in wall waterproofing:

• Due to its significantly higher shear parameters compared to loam and clay, bentonite gravel waterproofing can achieve a considerably higher load-bearing capacity.
• The sealing layer has a high modulus of rigidity due to its manufacture from predominantly non-cohesive raw materials. According to calculations, the settlements at the upper edge of the wall sealing of the first backfill horizon are less than 20 cm. These settlements can be reliably compensated for by design measures when designing the sealing in the area of the berm transition.
• The seal has a homogeneous and consistently high quality due to its manufacture in a compulsory mixer using defined raw materials with controllable properties. The effectiveness of the seal is thus guaranteed in every case.
• The installation of bentonite gravel sealing can be carried out more independently of weather conditions than the installation of clay. Since the wall sealing must be built up gradually depending on the progress of the waste dumping and there is therefore only a small margin for the installation time, this property is a great advantage.

The horizontal berms between the individual areas of the rock face are also sealed by installing bentonite gravel. However, this can lead to significant settlement differences over a short distance at the transition from the berm seal to the underlying wall seal, especially in areas where a clay wall seal has been installed.

If the wall sealing were connected to the berm sealing in the conventional manner, settlement cracks and gaping joints would occur in the transition area, so that there would no longer be a continuous sealing effect. However, due to the conditions at the berm transition, the occurrence of settlement differences cannot be effectively prevented. In the area of the berm transition, a material must therefore be installed that has the following properties:

• The material must be able to follow the settlement movements of the underlying wall waterproofing without forming settlement cracks or break joints. This property can only be achieved by using soil without cohesion (e.g. dry gravel).
• The material must have a good sealing effect, which is normally only achieved with soils with high cohesion (loam, clay or mixed-grain sealing materials such as the bentonite gravel used here).

These difficult-to-combine properties can best be achieved by using a material that was developed in the late 1980s under the name Dywidag mineral mixture. This material, also known as dry mixture (TGM), is an artificially produced mixture of the following components:

• Coarse grain: 50–70% gravel 16/32
• Medium grain: 20–35% sand 0/2
• Fine grain: 8–12% bentonite

The material composition is selected so that the finer-grained soil fits into the pores of the coarse-grained soil. An excess of medium and fine grain results in a "floating" coarse-grained structure. The mixture is installed practically dry with a very low water content of approx. 2% and has no cohesion in this state. In combination with the excess of medium and fine grain, this results in high mobility in the grain structure and thus a correspondingly high adaptability to a substructure with varying settlement behaviour.

When it comes into contact with water, the bentonite swells and fills the pore space of the dry mixture. Once saturated, the material behaves like a mixed-grain sealant and is therefore similar to the bentonite gravel otherwise used. However, saturation occurs very slowly because the bentonite first swells at the top of the installation area. This creates a thin layer with low water permeability, which prevents or at least significantly delays further water penetration. The still dry core of the layer still has no cohesion, so that its mobility and thus its adaptability to the settling wall sealant is retained.

Additional structural measures (installation of several overlapping plastic sealing membranes with geotextile protective layers) are intended to ensure the mobility of the dry mixture in the transition area in the long term. In addition, water ingress into the dry mixture is to be prevented or delayed so that the material remains dry for as long as possible. In terms of its effect, two phases can then be distinguished:

• Phase 1: Consolidation of the wall sealing

The wall waterproofing will settle in the course of the planned further backfilling. The settlements will slowly subside after completion of the backfilling. During this phase, which will last approximately 30 to 40 years, a sufficiently thick TGM layer must remain dry and thus in a cohesionless or deformable state.

• Phase 2: Final state

Once the settlement has subsided, there can be no further displacement in the area of the berm transition that could cause settlement cracks or gaping joints in the waterproofing. The dry mixture can thus gradually become saturated and then finally has the properties of a very good mixed-grain waterproofing.

The individual states in the area of the berm transition are shown below using the example of a particularly problematic situation with underlying clay waterproofing.

Fig. 5: Waterproofing of the rock berm – installation condition

Fig. 6: Sealing of the rock berm – condition after settlement has subsided

Construction of the steep wall waterproofing in backfill sections VA 3.1 and 3.2

Project participants

Client and builder: Municipal enterprise of the district of Bad Kissingen, Waste Management Division, Münchner Str. 1, 97688 Bad Kissingen

Competent authority: Bavarian State Office for the Environment, Bürgermeister-Ulrich-Str. 160, 86179 Augsburg

Planning and construction supervision: Dr. Blasy – Dr. Øverland Beratende Ingenieure GmbH & Co. KG, Moosstraße 3, 82279 Eching am Ammersee

External testing: Soil/geosynthetics: DBI-EWI GmbH Ingenieurgesellschaft für Umwelt, Wasser und Spezialbau, Halsbrücker Str. 34, 09599 Freiberg

Contractor: Burger Bau GmbH & Co. KG, Häuserschlag 3, 97688 Bad Kissingen,

Geosynthetics material supplier: NAUE GmbH & Co. KG, Gewerbestr. 2, 32339 Espelkamp-Fiestel

Geosynthetics installation companies: von Witzke GmbH & Co. KG, Joachimstraße 72, 45309 Essen

In-house soil inspector:

VA 3.1: Geotechnical Institute Prof. Biedermann

VA 3.2: pgu Ingenieurgesellschaft mbH

Work sequence

The steep wall sealing was carried out in several phases in accordance with the following descriptions and illustrations:

• Preparatory work on the berm (removal of plastic sheeting, stones)
• Installation of filter concrete at the foot of the berm and in the transition to the existing lower wall sealing. The protruding edges of the existing lower steep wall (filigree ceiling element) were levelled flush with the upper edge of the filter concrete (No. 7 in Figure 7).
• Installation of the drainage layer below the seal (No. 10 in Figure 8)
• Laying of the PET separating fleece, ≥ 300 g/m², GRK 5 between the drainage layer below the seal and the subsequent subgrade levelling layer (No. 11 in Figure 8)
• Installation of the subgrade levelling layer (No. 12 in Figure 8)
• Exposing the existing lower wall waterproofing (bentonite gravel – old stock and gabions) from backfill level 2 (No. 3 in Figure 7)

Fig. 7: Preparatory work

• Application of bitumen coating and bitumen membranes. The existing lower wall sealing was supplemented up to the upper edge of the first berm. The bituminous seal was newly applied along the upper wall sealing up to a height of approx. 150 cm (Nos. 8 and 9 in Figure 7).
• Erecting a row of gabions on top of the existing gabions in the lower wall (No. 2 in Figure 7)

Fig. 8: Seal support

• Installation of bentonite gravel between the lower wall and gabions up to a height of 120 cm below the transition point of the filter concrete lower wall (No. 14 in Figure 8)
• Attachment of the first layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, smooth/smooth, with BAM approval) to the top point of the lower wall using impact dowels at intervals of 100 cm by the BAM specialist company (No. 15 in Figure 8)
• Attachment of the 6th layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, smooth on both sides, with BAM approval) to the foot of the upper wall using dowelled aluminium rails
• Installation of the first layer of geotextile protective layer (on the subgrade levelling layer) with BAM approval, PP ≥ 2,000 g/m² (No. 16 in Figure 8)
• Installation of the second layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, smooth on both sides, with BAM approval) on the first layer of geotextile protective layer (No. 17 in Figure 8)
• Installation of the third layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, smooth on both sides, with BAM approval) as transition between berm and lower wall sealing (No. 18 in Figure 8)
• Installation of 2nd layer of geotextile protective layer with BAM approval, PP ≥ 2,000 g/m² (on the 3rd plastic sealing membrane)
• Installation of bentonite gravel between the lower wall and gabions up to the top of the lower wall sealing (installation in layers of approx. 20 cm with subsequent compaction (No.: 14 in Figure 8) • Carving out the trapezoidal filling area for the dry mixture

Fig. 9: Installation of berm sealing

• Installation of the dry mixture (removed with an excavator directly from the truck loading area) while ensuring that the material remains homogeneous (no segregation) and observing the maximum installation water content of 2.3% (No. 30 in Figure 9)
• Installation of PP ≥ 300 g/m² separation fleece, GRK 5, to cover the dry mixture
• Installation of the 4th layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, textured on both sides, with BAM approval) in the area of the installed dry mixture (No. 31 in Figure 10)
• Installation of the 5th layer of plastic sealing membrane (KDB made of PEHD, d = 2.5 mm, smooth on both sides, with BAM approval) over the entire berm and lower wall sealing (installation depth in the upper wall sealing made of bentonite gravel approx. 50 m,

No. 32 in Figure 10)

• Installation of the third layer of geotextile protective layer with BAM approval, PP ≥ 2,000 g/m² over the fifth plastic sealing membrane (No. 33 in Figure 10)

Fig. 10: Seepage water drainage above the berm sealing

• Installation of seepage water drainage (basalt gravel 8/16 mm) as a support wedge for upper gabion baskets
• Installation of the first layer of upper gabion baskets as a transition to the new wall sealing to be constructed (No. 37 in Figure 11)
• Installation of seepage water drainage (basalt gravel 8/16 mm) on the entire berm (between upper and lower gabion baskets, layer thickness ≥ 50 cm (No. 38 in Figure 11)
• Laying bentonite gravel in layers in the upper wall sealing between the gabions and the 6th layer of plastic sealing membrane (No. 39 in Figure 11)

Fig. 11: Start of wall sealing of the 2nd backfill horizon

During the installation of the bentonite gravel, pre-compaction was carried out using Ramax WACKER RT82SC2. The main compaction was carried out using the heavy vibrating plate WACKER DBU 100-70. Specific transitions and the surface were smoothed with the light vibrating plate. After installation of the respective layers, EP and FP carried out proper and professional sampling in accordance with QMP [ 1 ] and FP approved the superstructure.

All plastic-related work (especially with BAM-approved products) was carried out by the BAM-approved installation company "von Witzke GmbH".

Quality assurance for soil/geosynthetics

As part of quality control during construction work, DBI-EWI GmbH is responsible for external testing of geosynthetics and soil mechanics. The soil mechanics and plastics engineering work has been supervised and monitored by DBI-EWI GmbH since 2015 and VA 3.1 on the external testing side. VA 3.2 was produced in 2017/18 and VA 3.3 will begin in November 2018 (expected to last until July 2020). The work to be monitored relates to the berm and steep wall sealing.

Suitability tests for mineral and polymer building materials

4.1.1 Mineral building materials

The currently valid QMP for the construction project forms the basis for sampling, suitability tests and the resulting technical assessments. This quality assurance tool is based on the current requirements of the DepV (German Waste Management Ordinance) in addition to the currently valid guidelines and recommendations (e.g. GDA, BQS).

All of the following mineral building materials were examined for the construction project and found to be suitable by the competent authority.

4.1.2 Polymer building materials

Only BAM-approved products were to be used for the polymer materials of the wall and berm sealing system at the Wirmsthal landfill site. As part of the suitability verification process, the contractor and its geosynthetics supplier submitted the relevant suitability certificates, including the necessary engineering calculations, in a professional and competent manner.

Based on the suitability test, the following products were approved by the competent authority for delivery to the construction site and installation:

• The following plastic sealing membranes were used on the construction site:

In addition to the geosynthetic-based products, the external testing also verified the relevant technical certificates of the designated BAM-approved specialist installer. For the installation of the polymer materials, the competent authority confirmed the specialist installer (von Witzke GmbH) for the plastic engineering work on the berm sealing of the Wirmsthal landfill site on the basis of the external testing report.

Independent testing during the construction of the berm and wall sealing in the construction area

Since 2015, Burger Bau GmbH from Bad Kissingen has been installing the berm and wall sealing in backfill sections (VA) 3.1, 3.2 and 3.3. Thanks to the intensive cooperation between the construction company (including NAN for the geosynthetics), the quality assurance institutions (EP, FP) and the site management (öBÜ/BOL) as well as the client (AG), a high level of quality was achieved right from the start.

The testing services subject to internal and external inspection during the installation of the berm waterproofing and wall waterproofing were carried out in accordance with the specifications of the QMP. As part of the internal and external inspection (soil mechanics and geosynthetics), the individual system components were checked during the installation of the waterproofing system (in accordance with the construction process) and tested in accordance with the confirmed quality management plan.

The following mineral components were to be tested:

• Top edge of the berm
• Filter concrete layer
• Drainage layer below the seal (4/16 mm)
• Subgrade levelling layer
• Bentonite gravel (starting material; product after mixing; product after installation)
• Dry mix (raw materials: gravel, sand, bentonite; product after mixing; product after installation)
• Drainage layer above the seal (8/16 mm)

The following geosynthetic-based components were to be tested:

• Protective fleece (2,000 g/m²) with BAM approval
• Separation and filter geotextile (300 g/m²)
• 2.5 mm PE-HD plastic sealing membrane (textured/textured) with BAM approval
• 2.5 mm PE-HD plastic sealing membrane (smooth/smooth) with BAM approval

The corresponding samples for internal and external quality control testing were taken in accordance with QMP and tested accordingly. The individual mineral and geosynthetic-based components of the sealing system were obtained in a representative manner in accordance with the QMP specifications through internal and external testing in line with the construction process.

Based on regular checks of the internal test documentation and plausibility checks of the FP, it can be concluded that the work carried out during the construction project and the internal and external tests were performed in full in terms of quality and quantity in accordance with the QMP specifications.

The results of the external testing (FP) on site and in our own laboratory, as well as the comparison with the test results determined by the internal testing (EP), show that the mineral and polymer components comply with the quality parameters required by the QMP for the berm and wall waterproofing system.