An investigation into the impact of sequential filling on properties of emplaced refuse lifts and moisture stored in a municipal solid waste landfill

Abstract

The majority of investigations on municipal solid waste (MSW) landfills have been undertaken during the post-closure period, and therefore, the changes that occur in the properties of refuse layers placed during the period of infilling are often ignored. The impact of further tipping of refuse loads on the moisture content, hydraulic and geotechnical properties of emplaced refuse lifts, and the daily cover was examined in this study by undertaking field and laboratory tests on the refuse fill at White's Pit landfill, Poole, Dorset.

The field tests involved mainly pit tests and cone penetration tests. The porosity and field capacity of the refuse excavated from the pits were determined in 210 litre drums. In addition, factors that influence leachate production, which include the moisture stored in the topsoil and the runoff fromthe landfill were measured. The laboratory tests involved the determination of compression, porosity, and hydraulic conductivity of pulverised refuse samples with and without a cover soil, under increasing vertical loading. The data obtained from the tests were used in the simulation of moisture in refuse lifts at the site, using the Hydrologic Evaluation of Landfill Performance (HELP) model. The data were also used to formulate characteristic equations used for determining temporal changes in the physical properties of emplaced refuse lifts.

The results of the investigation show a reduction in porosity and hydraulic conductivity, and increase in the density of an emplaced refuse layer according to the quantity of further filling of refuse loads. The density of an emplaced refuse is further increased by the ravelling of the daily cover materials, but its permeability decrease as a result. Under an applied vertical load of 6 kPa, the hydraulic conductivity and density of refuse-only samples were 1.4 x 10^-3 m/s
and 291 kg/m³, while that for refuse with 7.5 % cover soil were 9.4 x l0-4 m/s and 353 kg/m³ respectively. The hydraulic conductivity of a refuse lift with a slightly clay/silt sand cover, however, appeared greater than its calculated value (10^-5 m/s) at low effective stresses.

The similarity between the results of refuse tested in experimental cells in present study and Beaven and Powrie's (1995) large-scale compression cell suggests that empirical models can be derived from the data obtained from cell tests to predict the behaviour of refuse with different densities. Furthermore, relatively small cells can be used in preliminary study of the behaviour of refuse if the particle sizes are reduced in proportion to the size of the test cell.

Apart from direct infiltration of water during waste placement, the volumetric moisture content and degree of saturation of a refuse lift increase during the fill period due to compression from overlying lifts. The saturation of the refuse fill is further enhanced by channelled water through the macropores in the cover soil system. The simulation technique used in this study may be used in evaluating alternative designs and plans of a MSW landfill. Large-scale testing of refuse with an intermediate cover soil is recommended.

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