The International Information Center for Geotechnical Engineers

Biodegradation in Municipal Solid Waste landfills - Settlement in MSW Landfills

Settlement in MSW Landfills

MSW consists of multiphase media (gas, liquid, and solid) with each phase exhibiting spatial and temporal variations (El-Fadel and Khoury, 2000). Therefore, MSW settlement should depend on the contribution from all three phases.


Fig. 5 Phase Diagram for MSW (Hettiarachchi et al., 2008)

 Settlement in landfilled waste is commonly described in terms of the five mechanisms identified by Sowers (1973):

  • Mechanical: distortion, bending and crushing
  • Ravelling: erosion, sifting of fines
  • Physico-chemical: corrosion, oxidation and combustion
  • Bio-chemical decay: fermentation & decay, aerobic and anaerobic
  • Interactions, between the four other mechanisms.


Yen and Scanlon (1975) also identified five mechanisms of long-term settlement, which correspond closely to Sowers:

  • Movement of fines into large voids
  • Strength loss due to chemical & biological reactions
  • Material loss due to biodegradation and methane production
  • Creep processes
  • Consolidation processes.


The likely incidence of settlement mechanisms during the life of a landfill site is shown in   Figure 6.


Fig 6. Occurrence of settlement mechanisms and temporal classifications adopted by selected publications (McDougall, 2011)


Hence a combination of primary and secondary settlement conveniently classifies the mechanisms contributing to landfill settlement. Primary settlement occurs very quickly by comparison to the life of the landfill and can be adequately described by load-induced settlement models. Secondary settlement, driven by biodegradation effects, creep and physicochemical corrosion is a long-term phenomenon, which has for some time relied on time-dependent methods.


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