Performance and design of cappings for contaminated sites and landfills

Cappings for contaminated sites and landfills have a variety of tasks and usually are complex multilayered systems. The paper presents an overview of the results of an in-situ-study on the performance of various types of cappings in the Georgswerder landfill in Hamburg, Germany, where different types of landfill cappings have been studied in-situ by collecting the leakage on areas ranging from 100 m² to 500 m² since 1987. Composite caps with geomembranes performed best, showing no leakage. An extended capillary barrier also performed well. Performance of compacted soil cappings, however, have deteriorated severely within five years due to desiccation, shrinkage and plant root penetration (leakage now ranging from 150 mm/a to 200 mm/a). This has the consequence that about 50 % of the water that reaches the surface of the cap is leaking through. The maximum leakage rates have increased from 2 × l0⁻¹⁰ m³ m⁻² s⁻¹ to 4 × l0⁻⁸ m³ m⁻² s⁻¹. Two types of geosynthetic clay liners (GCL) have been tested as capping for two years now with disappointing results. The GCL desiccated during the first dry summer of the study. High percolation rates through the GCL were measured during the following winter (45 mm to 63 mm in four months). Wetting of the GCL did not significantly reduce the percolation rates. Additional experiments show promising results for capillary barriers and polymer-modified sand-bentonite-barriers, which are much less sensitive to desiccation and shrinkage than the traditional cohesive soil barriers. With respect to the EU Landfill Directive traditional cohesive soil cappings are not recommended for most European countries with rather dry climatic conditions. The design of a landfill capping should be site specific and incorporate innovative barrier systems.

• 1 TYPES AND TASKS OF CAPPING SYSTEMS

• 2 PERFORMANCE OF CAPPINGS

• 3 CONCLUSIONS

• 4 REFERENCES