Compressibility of municipal solid waste and its implications

This paper deals with the compressibility of municipal solid waste (MSW) with the purpose of highlighting the engineering implications related to the waste subsidence during disposal, closure and quiescence. A parametric analysis of settlement is carried out for a 40 m high MSW column, by considering two biological degradation models, EPA and SWANA, known in the technical literature. The biodegradation models are implemented inside a one-dimensional numerical model of waste column, increasing in height with a prefixed law. The model takes into account the mass loss resulting from the biodegradation process and the fluid transport along the vertical direction, from the most saturated areas towards the less saturated. The effect of pore pressure, resulting from both the production of biogas or from the raising of the leachate level, is neglected in the hypothesis that the plant remains efficient in time. It is assumed that the leachate is continuously recirculated from the top to the base of the waste column. The analyses have the purpose of investigating the magnitude of the three components of settlement, immediate, creep and biodegradation, at the end of the disposal phase, after the construction of the coverage barrier and, finally, after thirty years from the closure. Bio-mechanical parameters are in part adapted from the technical literature [GOURC et al., 2010] and in part obtained from large diameter oedometric tests carried out at the ICEA Department of the University of Padua [COSSU et al., 2015]. In the light of engineering applications, the main results achieved are: - The two biodegradation models, EPA and SWANA, do not have a decisive influence on the waste settlements for disposal rates in the range of the usual practice; however, as showed by the data of Tab. VII, the choice of the model may be helpful in order to calculate the mean degree of maturation of biodegradable matter. - The filling coefficient, that is the ratio between the reached MSW height and the planned one, is strongly dependent on the disposal rate. The engineering goal is of assessing the optimal disposal rate to maximize the landfill volume with the lower operating costs. - The differential settlements due to creep and biodegradation can induce failure in the covering barrier. Such settlements are accentuated by a fast disposal rate; a slower rate minimizes this hazard because part of the settlement occurs during the disposal itself. For slower disposal rate, the creep settlement can become more crucial than the biological one. - A careful assessment of the creep parameter is recommended, also in the light of the European policies on MSW, according to which the organic component must be minimized and stabilized by pre-Treatment. - The immediate settlement of MSW and the relative law of evolution represent a very critical point for the subsidence problems occurring during disposal and closing; in this respect, it should be taken into greater consideration the extra-situ pretreatments and those in-situ (compaction, preloading), in order to stabilize the medium. As a final practical indication, it is suggested to run numerical simulations of the landfilling program in order to ascertain the final landfill profile and the coverage stability; on the basis of such analyses it will be possible to redefine the cultivation plan to minimize the costs of management and postclosure. Concluding, it is clear how geotechnics can bring a basic contribution in the design of MSW landfills; anyway, the need of a multidisciplinary approach is also evident, in order to avoid drawbacks in a complex field not yet fully consolidated.