AREA A/B ENGINEERING REPORT - Waste Management

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generated, the concentration of degradable leachate organics will continue to decline or remain
steady. This attenuating condition is also conducive to the continued immobilization of heavy
metals and other persistent constituents that may be present. 5
1.4.4 Physical and Biogenic Settlement
Settlement of a MSW landfill results from the complex interaction of many processes, including
mechanical settlement due to self-weight, superimposed loads (i.e., additional waste layers and
cover soil), physiochemical changes such as corrosion and oxidation, and biochemical
decomposition under aerobic and anaerobic conditions. The rate of settlement is important in
determining the nature and timing for beneficial end uses of the landfill property. Several factors
affect waste settlement processes, 6 including:
• Method and rate of waste placement;
• Landfill thickness, method and rate of compaction, and waste density;
• Waste composition and organics content; and
• Temperature, moisture availability, and other operational factors.
Waste settlement behavior in many ways is similar to that of other highly organic materials such
as peat, and is generally considered to occur in three stages:
• Immediate settlement is due to the application of a superimposed load, under which
voids and particles in the waste are compacted, and is often considerable. Landfill
operators take advantage of immediate settlement to maximize use of landfill airspace
by compacting waste at the working face.
• Thereafter, primary settlement is principally due to dissipation of pore water and gas
present in waste voids at placement. It has been estimated that primary settlement is
usually completed within 30 days and accounts for up to 10 percent of total settlement.
• Finally, although both inorganic and organic wastes settle due to decomposition (inorganic
waste decomposes through corrosion, oxidation, or other means), secondary settlement of
MSW under landfill conditions occurs primarily due to biodegradation of the organic
components of MSW. Depending on the thickness of the waste mass, mechanical
settlement may occur concurrently with processes that cause secondary settlement.
Secondary settlement may result in a reduction of up to 25 percent of the total original
landfill thickness. The most significant control on long-term settlement is the availability of
moisture within the waste mass (which effectively controls the rate of biodegradation).
5 This phenomenon is well reviewed and elaborated on in some detail in EREF (2006). See also Appendix A.
6 Waste settlement in landfills is discussed in numerous seminal studies, including Sowers (1973), Wardwell & Nelson
(1981), Edil, et al. (1990), Gordon, et al. (1986), Morris & Woods (1990), Watts & Charles (1990 and 1999), Wall
& Zeiss (1995), Park & Lee (1997 and 2002), and Leonard, et al. (2000).
MD10186.doc 25 29 March 2009