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SANDWICH

SANDWICH
Contact:Dr.-Ing. R. Schuhmann
Project Group:IMK-TRO
Funding:BMBF

Overview

Hazardous waste which can not be recycled must be encapsulated in subsurface landfills. Therefore, both legal regulations and safety restrictions of the storage facilities must be obeyed. Mostly mines are used as subsurface repositories, where the waste must be encapsulated permanently and relialbly. The waste is mainly isolated in special containers and stored in caverns and mine shafts.
 
      
 It is the goal to isolate the hazardous waste from the biosphere by suitable geological and geotechnical barriers so that they can be finally left unattended. These barriers are the host rock on the one hand and mine shaft closure constructions on the other hand. The permanent operability of these closure constructions must be demonstrated. 
For the building of these constructions mostly mineral and geotechnical materials such as bentonite and other clays are used. The constructions are mainly two-fold consisting on a static counter bearing and a sealing core. It is important to make the sealing resistant against aggressive solutions from the host rock. Therefore the bentonites are installed in a mostly dry state (in form of pellets or bricks), because the tend to swell and build up pressure when in contact with water. Also the bentonites have enough plasticity so that a closure with enough capacity to the fill the cross-section can be build and possible deformations of the host rock can be compensated.

To guarantee the long-term stability of the closure construction a new layerd sealing system was developed and patented. The goal of the described project is to demonstrated the operability of the new sealing system.
 
  In contrast to conventional constructions of the sealing element, consisting of only one mineral material, the new system combines layers of different soil hydraulic properties. Layers with cohesive mineral materials (bentonites, other clays) with a sealing function are combined with layers of non-cohesive materials (permeability several ordres of magnitude higher than the cohesive materials) which compensate for the different hydraulic potential (equipotential element). Furthermore the non-cohesive layers can be equipped with suitable sensors for measuring soil-specific parametrs such as water content, temperature and pressure so that the sealing element is not disturbed.
       
Thus a long-term monitoring of the sealing element will be possible. The new sealing system shall prevent inhomogeneous moisture transport in the barriers. Penetrating water will be evenly distributed within the equipotential element and will build up a new homogeneous potential surface for the following clay layer. Damage of the sealing layer by thermic and pressure induced reaction coupling will thus be avoided.