Hydraulic Conductivity of MSW in Landfills

This paper presents a laboratory investigation of hydraulic conductivity of municipal solid waste (MSW) in landfills and provides a comparative assessment of measured hydraulic conductivity values with those reported in the literature based on laboratory and field studies. A series of laboratory tests was conducted using shredded fresh and landfilled MSW from the Orchard Hills landfill (Illinois, United States) using two different small-scale and large-scale rigid-wall permeameters and a small-scale triaxial permeameter. Fresh waste was collected from the working phase, while the landfilled waste was exhumed from a borehole in a landfill cell subjected to leachate recirculation for approximately 1.5 years. The hydraulic conductivity tests conducted on fresh MSW using small-scale rigid-wall permeameter resulted in a range of hydraulic conductivity $2.8\times {10}^{-3}–11.8\times {10}^{-3}\text{cm}/\text{s}$ with dry unit weight varied in a narrow range between $3.9–5.1\text{kN}/{\text{m}}^3$ . The landfilled MSW tested using the same permeameter produced results between $0.6\times {10}^{-3}$ – $3.0\times {10}^{-3}\text{cm}/\text{s}$ for $4.5–5.5\text{kN}/{\text{m}}^3$ dry unit weights. The hydraulic conductivity obtained from large-scale rigid-wall permeameter tests decreased with the increase in normal stress for both fresh and landfilled waste. The hydraulic conductivity for fresh MSW ranged from 0.2 cm/s for $4.1\text{kN}/{\text{m}}^3$ dry unit weight (under zero vertical stress) and then decreased to $4.9\times {10}^{-5}\text{cm}/\text{s}$ for $13.3\text{kN}/{\text{m}}^3$ dry unit weight (under the maximum applied normal stress of 276 kPa). The hydraulic conductivity of the landfilled MSW decreased from 0.2 cm/s to $7.8\times {10}^{-5}\text{cm}/\text{s}$ when the dry unit weight increased from 3.2 to $9.6\text{kN}/{\text{m}}^3$ . The results clearly demonstrated that the hydraulic conductivity of MSW can be significantly influenced by vertical stress and it is mainly attributed to the increase in density leading to low void ratio. In small-scale triaxial permeameter, when the confining pressure was increased from 69 to 276 kPa the hydraulic conductivity decreased from approximately ${10}^{-4}\text{to}{10}^{-6}\text{cm}/\text{s}$ , which is much lower than those determined from rigid-wall permeameter tests. The published field MSW hydraulic conductivities are found to be higher than the laboratory results. Landfilled MSW possesses lower hydraulic conductivity than fresh MSW due to increased finer particles resulting from degradation. The decreasing hydraulic conductivity with increasing dry unit weight is expressed by an exponential decay function.