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The principles of capillary barriers are well known from many\nlaboratory studies. A limited number of large-scale applications are\nknown, in particular for the sealing of past pollution and demolition\nwaste. This study investigates the suitability of a capillary barrier\nfor the sealing of a municipal landfill.\n\nLaboratory tests have shown the fundamental applicability of the\nselected materials. The sealing effect of the material combination as\na capillary barrier was examined in a large scale tipping trough under\nstep increasing hydraulic load and two differential slopes. The\nmaterials showed drain capacities suitable for an effective capillary\nbarrier.\n\nFor a long-term study under natural weather conditions, two\nexperimental fields with integrated large scale lysimeters and slopes\nof 10 % and 26 % have been constructed on site at the landfill.\nThe lysimeter data of three subsequent years were used for the\nanalysis. After the growth of a closed cover of vegetation the surface\nrunoff was negligible. Only in wet years it exceeded 2 % of the annual\nprecipitation. In 2000 and 2001 the evapotranspiration was between\n55 % and 65 %. In the very wet year 2002, however, it was\nsignificantly below 50 %. The predominant part of the lysimeter\nrunoff was drained by the capillary layer. Only heavy precipitation\ncaused a relevant drain into the capillary block. Except for the very\nwet year 2002 the effectiveness of the whole system exceeded 95 % of\nthe annual precipitation. As expected the efficiency of the steeper\nlysimeter clearly surpassed 95 %.\n\nIn order to verify the possibility to reduce the effort for the\nexperimental determination of the efficacy of the capillary barrier by\nnumerical simulations, it was tried to calculate the measured results\nby numerical models for the water balance layer and the capillary\nbarrier. The results show that in principle a realistic prediction is\npossible. However, a reliable prediction of the measured results can\nonly be achieved by using parameters that were calculated by inverse\nmodeling for the unsaturated hydraulic conductivity. Because of the\nhigh sensitivity of the model, calculations with parameters predicted\nby standard methods did not lead to plausible results, but\noverestimated the effectiveness of the capillary barrier. Therefore it\nis not advised to substitute large scale tipping experiments by\nnumerical modeling.\n\nAlthough the capillary barrier is not gas proof, the closed water\ncoating above the boundary to the capillary block largely restricts\nthe movement of gas. Laboratory experiments resulted in very low\npermeability for gas through an aqueous capillary barrier.\nMeasurements at the experimental fields showed that the capillary\nbarrier significantly reduces the emission of gas at the surface\ncompared to the non-sealed landfill surface. However, under adverse\nair pressure conditions low gas emissions are still measurable.\n\nConsidering the present investigations, the capillary barrier is a\ncompetitive and secure alternative especially for the fracture-prone\nclay liner. For demolition waste it can be used as single sealing.\nFor municipal waste, as in the underlying landfill of Bayreuth, the\ncombination with an additional sealing element, e. g. a synthetic\nliner is recommended.