Geotechnical News December 2011
37
WASTE GEOTECHNICS
the program and research topics will be
presented in this paper.
Geoenvironmental Hazards of
MSW Landfills and Root Causes
A MSW landfill, being a component
of a municipality, is generally located
close to the city. Improper management
of landfills usually results in three
kinds of geoenvironmental hazards to
the city (see Figure 1):
(1)
Massive landfill slides or debris
flow.
With limited land for waste dis-
posal, many Chinese cities are piling
MSW to greater and greater heights.
Many landfills in China have already
reached piling heights of 60 m and may
exceed heights of 100 m in the near
future. A landfill failure similar to the
Payatas wasteslide (see Figure 2) may
bury the surrounding area and cause
heavy casualties. A landfill failure is
usually accompanied with leachate
spill, resulting in the contamination of
ground surface. The slope failure at the
Xiaping Landfill in Shenzhen, China,
resulted in 60,000 tonnes of leachate
spill which travelled a distance of over
10 km.
(2)
Soil and groundwater contami-
nation
caused by leachate leakage and
diffusion. Devoid or defective landfill
barriers will result in the escape of
leachate into the underground environ-
ment. Field monitoring at the Beitian-
tang Landfill in Beijing indicated that
the groundwater contamination at the
landfill has extended to an area of sev-
eral square kilometers and a depth of
30 m. The remediation of underground
contamination is expensive and time-
consuming.
(3)
Air pollution, fires or explo-
sions
caused by landfill gas. Landfill
gas consists predominantly of methane
(CH
4
) and carbon dioxide (CO
2
), both
of which are greenhouse gases. The
emission of landfill gas will produce
odours and air pollution in the sur-
rounding regions. Methane is highly
explosive and may lead to fires and
explosion hazards. It is estimated that
the annual generation of landfill gas in
China is 13 billion m
3
and accounts for
5.8% of the nation’s total greenhouse
gas emission.
The intractable hazards of MSW
landfills are rooted in the biochemical
degradation behaviour of the MSW
with organic matter. Following waste
placement, the landfill becomes a form
of biochemical reactor in which the
wastes together with moisture react to
generate leachate, gases, heat and con-
taminants, resulting in the deterioration
of material properties and deformation
of the waste pile. All of these processes
contribute to the development of the
above-mentioned geoenvironmental
hazards. The biochemical process of
MSW is very complex, and it gener-
ally takes 30-50 years to stabilize.
When compared with the MSW gener-
ated in North America, Chinese MSW
contains much more organic content
(40-50%) and water content (40-60%
by wet mass), and hence its biochemi-
cal behavior is much more significant
(Zhan et al., 2011). One tonne of Chi-
nese MSW can generate 0.2-0.3 tonnes
of leachate with high mass loading
(e.g., Chemical Oxygen Demand
(COD): 40,000-
80,000
mg/L)
and 100-150 m
3
of gas. The degrada-
tion-induced compression of MSW is
up to 30% of the waste thickness. The
strong biochemical behavior of MSW
tends to result in high gas pressure, a
high leachate mound, high contami-
nant loading and large deformation of
the landfill, which may trigger vari-
ous geoenvironmental hazards. Figure
3 shows the blowout of a gas-leachate
mixture to a height of 5 m when a bore-
hole was drilled at a landfill (Chen et
al., 2010). The field investigation for
the failure of the Payatas Landfill indi-
cated that pore pressure increase due to
landfill gas – pore water interaction in
saturated or near saturated waste played
a significant role in triggering the slide
(Kavazanjian and Merry, 2005).
Sustainable Landfill Technology
Current landfill technology passively
controls environmental impacts by
designing landfills like dry tombs.
Controlled landfills are usually
equippedwith a bottombarrier, leachate
collection and drainage systems, cover
system, gas collection system and
ex-situ leachate and gas treatment
facilities. Under these conditions, the
landfill suffers from heavy hazard
loadings (gas pressure, leachate head,
contaminant loading, etc.), and the
loading will take several decades to
stabilize. Controlled landfills also face
challenges regarding the durability
of the control systems and materials
(barriers, cover system, leachate
drainage system, etc.), the need for
long-term safety and environmental
Figure 1. Geoenvironmental hazards of MSW landfills.
Figure 2. Flowslide of waste mass at the Payatas Landfill,
Quezon City, Philippines (Kavazanjian and Merry, 2005).
