Multifunctional sealing at the Ihlenberg landfill

Situation

The Ihlenberg landfill site is located in the north-west of the federal state of Mecklenburg-Western Pomerania in the district of North-West Mecklenburg. The objectives of the construction project (MFA) are:

• Structural separation of landfill section DA 1, which is in the decommissioning phase, from the current filling area DA 7
• Construction of the final surface seal (OFA) for sections of DA 1, which is currently in the decommissioning phase
• Reinforcement of the base seal for a section of the active DA 7

In the first construction phase, a multi-functional seal (MFA) will be constructed over an area of approx. 3.0 ha. The second construction phase covers an area of approx. 7 ha. The landfill body beneath the MFA consists of older, partly inhomogeneous waste deposits. This poses a risk that uneven settlement could cause incompatible deformation in the mineral component of the MFA, meaning that the required water permeability in the mineral seal cannot be maintained.

Solution

To secure the MFA, geosynthetic reinforcement using Fortrac geogrids, in accordance with the structural requirements, is planned. The geogrid’s properties support the stabilising effect of the GTA.

The MFA essentially consists of the following components:

• 100 cm gas, load-bearing and levelling layer (GTA)
• Fortrac geogrids within the GTA
• 50 cm mineral seal
• 2.5 mm HDPE plastic membrane including a membrane inspection system
• Protective layer
• 30 / 50 cm mineral drainage layer

The required geogrid was recommended by HUESKER’s application engineering department in accordance with DIN 1054 and EBGEO for ground subsidence. Key parameters for the calculation were:

• Sinkhole diameter 1.5 m
• Longitudinal and transverse elongation of the geogrid < 3%
• Cover height approx. 35 m
• pH value of > 9

A maximum permissible strain of 3% for the mineral seal was adopted as the critical deformation criterion for the design. In accordance with EBGEO, the verification was carried out using the RAFAEL method for uniaxial bridging of an area at risk of sinkholes for the Fortrac geogrid, which is approved by the BAM. Fortrac is a flexible, extremely resilient geogrid. It is manufactured from high-modulus, low-elongation and low-creep synthetic raw materials, which are surrounded by a protective polymer sheath.
The project-specific anchoring and overlap lengths required for the verified geogrid type were then determined. The geogrid is laid at a depth of approx. 30 cm and subsequently covered with a further 70 cm of backfill. This reinforced base forms the foundation for the subsequent, layer-by-layer installation of a mineral clay seal, HDPE plastic sealing membrane, seal inspection system, protective layer and drainage layer.

Advantages

• High-tensile geogrids limit the expansion of the clay seal to < 3%
• The clay seal is protected against tensile cracks and exhibits consistently minimal water permeability
• Installation is optimised in accordance with the laying plans
• Geogrid lengths are supplied to match the plans, minimising waste
• BAM approval simplifies the approval process for the construction method