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Soil deposits crack self-seal landfill system

A SELF-REPAIRING landfill barrier capping system has been developed by German contractor Bilfinger + Berger.

The system paradoxically relies on drainage, since it requires soil particles from the clay mineral sealing layer to be transported to fill any cracks that might occur within it.

German surface sealing systems normally consist of a 500mm thick clay sealing layer with a plastic geomembrane sheet placed directly above. But Dr Rdiger Wunsch of B+B claims that there are problems with the specifications set down by the

two main regulating bodies, TA Abfall (technical instruction for waste) and TA Siedlungsabfall (technical instruction for residential waste).

He says shrinkage cracks often form in the clay layer because the plastic prevents water reaching it. Cracks are also caused as the result of deformation of the clay layer through differential settlement of the landfill waste.

And because the plastic sheeting has low shearing resis- tance, only shallow slopes can be built, which reduces the volume of waste that can be dumped.

As regulations allow alternative systems with comparable performance to the recom- mended designs, B+B decided to seek an alter- native solution. This has led to the development of its self-repairing mineral surface sealing system.

The system comprises a clay layer sandwiched between a drainage layer above and a geotextile separator below. This

is then covered with a revegetation layer. Its self-repairing capability derives from soil particles in suspension in water flowing through the drainage layer. These are deposited in any cracks that form, effectively sealing them.

B+B tested a variety of systems for their ability to self-repair. This was found to be mainly dependent on the amount of water within the revegetation layer, the composition of the sealing material and any deformation that has occurred.

The intensity and direction of water flow over the clay layer depends on the water balance of the whole system but particularly the physical characteristics of the soil in the revegetation layer.

Two main series of experiments were carried out to determine the amount of mineral particles carried in water flowing over the clay layer and how the rate of discharge through any cracks was reduced by deposition of the particles.

B+B claims that it can now calculate the mass of transported soil particles with water flow intensity and pattern for different sealing materials. It can also work out the reduction discharge rate with the mass of particles and any deformations.

Where large deformations are predicted or calculated from research, clay with specific grain-size distribution and low plasticity index is suitable.

In the case of smaller deformations, a mixed-grained sealing material with fine-grained additives (for example bentonite) is more suitable, because self-repair can be supported by swelling.

Adding material directly on to the clay, such as washing sludge from gravel production, further improves self-repairing ability, especially when the clay has high plasticity. This material must have a very low plasticity index - around 10% - and it only serves to provide soil particles for self-repairing.

Large-scale field trials have confirmed that permeability of the system after self-repair meets specified levels.

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