Die Wirksamkeit der Kapillarsperre als Deponieoberflächenabdichtung
The principles of capillary barriers are well known from many laboratory studies. A limited number of large-scale applications are known, in particular for the sealing of past pollution and demolition waste. This study investigates the suitability of a capillary barrier for the sealing of a municipal landfill. Laboratory tests have shown the fundamental applicability of the selected materials. The sealing effect of the material combination as a capillary barrier was examined in a large scale tipping trough under step increasing hydraulic load and two differential slopes. The materials showed drain capacities suitable for an effective capillary barrier. For a long-term study under natural weather conditions, two experimental fields with integrated large scale lysimeters and slopes of 10 % and 26 % have been constructed on site at the landfill. The lysimeter data of three subsequent years were used for the analysis. After the growth of a closed cover of vegetation the surface runoff was negligible. Only in wet years it exceeded 2 % of the annual precipitation. In 2000 and 2001 the evapotranspiration was between 55 % and 65 %. In the very wet year 2002, however, it was significantly below 50 %. The predominant part of the lysimeter runoff was drained by the capillary layer. Only heavy precipitation caused a relevant drain into the capillary block. Except for the very wet year 2002 the effectiveness of the whole system exceeded 95 % of the annual precipitation. As expected the efficiency of the steeper lysimeter clearly surpassed 95 %. In order to verify the possibility to reduce the effort for the experimental determination of the efficacy of the capillary barrier by numerical simulations, it was tried to calculate the measured results by numerical models for the water balance layer and the capillary barrier. The results show that in principle a realistic prediction is possible. However, a reliable prediction of the measured results can only be achieved by using parameters that were calculated by inverse modeling for the unsaturated hydraulic conductivity. Because of the high sensitivity of the model, calculations with parameters predicted by standard methods did not lead to plausible results, but overestimated the effectiveness of the capillary barrier. Therefore it is not advised to substitute large scale tipping experiments by numerical modeling. Although the capillary barrier is not gas proof, the closed water coating above the boundary to the capillary block largely restricts the movement of gas. Laboratory experiments resulted in very low permeability for gas through an aqueous capillary barrier. Measurements at the experimental fields showed that the capillary barrier significantly reduces the emission of gas at the surface compared to the non-sealed landfill surface. However, under adverse air pressure conditions low gas emissions are still measurable. Considering the present investigations, the capillary barrier is a competitive and secure alternative especially for the fracture-prone clay liner. For demolition waste it can be used as single sealing. For municipal waste, as in the underlying landfill of Bayreuth, the combination with an additional sealing element, e. g. a synthetic liner is recommended.
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