Long-Term Permeability Characteristics of Soil-Geosynthetic Combination Used in Landfill Covers

Multilayered cover system (MLCS) forms an integral part of engineered landfills nearing closure. Geosynthetic materials find multiple applications in MLCS as hydraulic barrier, drainage, separation, and filtration layers. Performance of geosynthetics used as filters needs to be well understood to ascertain the stability of MLCS. The main objective of this study is to investigate the compatibility of the soil-geosynthetic combination by long-term permeability tests. The compatibility of five different types of geosynthetics was evaluated with respect to two different gradations of red soil (RS) and RS–fly ash (FA) 50:50 mix. The results of long-term permeability and clogging characteristics were used to judge the effect of soil type, soil gradation, and type of geosynthetics on soil-geosynthetic compatibility. The efficacy of geosynthetics as filters was established based on (1) permeability improvement factor, which is defined as the ratio of the permeability of soil-geosynthetic composition to the permeability of the soil, and (2) stabilization time, i.e., the time required by the soil-geosynthetic system to achieve hydraulic equilibrium. It was noted from this study that the soil type and gradation had significant effect on the long-term permeability variation. The effect of the geosynthetic on the soil-geosynthetic combination is governed by its pore-size characteristics and was found to be marginal in this study. The stabilization time of a soil-geosynthetic combination increases with increase in soil fines content. The cyclic variation in permeability with time for a particular soil was found to be similar when different geosynthetics were used. Irrespective of soil type, higher permeability improvement factor was observed in the case of a woven multifilament geosynthetic and less in the case of a geocomposite. The class F FA in conjunction with RS was found to be compatible with the geosynthetics used in this study.