Modelling of the permeation through a landfill cap system including a geosynthetic clay liner with the modified HELP model

摘要

Geosynthetic clay liners (GCLs) are currently used in different types of landfill cover systems. Generally a complete system include the cover soil layer of a approximately 1 m thickness, a drainage layer such as a geosynthetic drainage system (GDS) or coarse granular material and a GCL with approximately 5,000 g/m~2 sodium bentonite. Therefore several test sites have been built and investigated over some years. Test results show a different efficiency varying with a design of the whole system and the climatic conditions. All research projects published point out that the efficiency of every mineral sealing system - especially GCLs with encapsulated high active clay minerals - depends on the water balance between the sealing element and the surrounding soil layers. This paper describes on detail the shrinkage and swelling properties of bentonite. A comparison of comprehensive field tests to the structure healing effect of a needlepunched sodium GCL after ion exchange is done with laboratory flexible wall (triaxial) tests. It can be stated that all data correspond in an excellent manner if the permeability test conditions are the same in field and laboratory. Furthermore a modified HELP model is presented which allows a site specific modeling of the liner efficiency regarding desiccation and rehydration mechanism after ion exchange. A comparison between theoretical and field studies for a liner system with 0.75 m cover soil layer and 0.25 m mineral drainage layer show corresponding efficiency data of more than 98% after ion exchange.