Reconnaissance of the July 10, 2000, Payatas Landfill Failure

Transcription

Reconnaissance of the July 10, 2000,
Payatas Landfill Failure
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Scott M. Merry, M.ASCE1; Edward Kavazanjian Jr., M.ASCE2; and Wolfgang U. Fritz, A.M.ASCE3
Abstract: Following ten days of extremely heavy rains from two typhoons, a fast moving slope failure of municipal solid waste was
triggered at the Payatas Landfill, Quezon City, Philippines. The wasteslide buried more than 330 persons. Only 58 persons were rescued
while, after weeks of recovery efforts, 278 bodies were recovered. This paper documents the events and circumstances that led to the
failure. Beyond stating the known facts leading up to the failure, only a brief discussion of the suspected reasons for the failure is
presented in this paper.
DOI: 10.1061/共ASCE兲0887-3828共2005兲19:2共100兲
CE Database subject headings: Landfills; Failures; Slopes; Municipal wastes; Philippine Islands; Landslides.
Introduction
On the morning of July 10, 2000, a wasteslide was triggered at
the Payatas Landfill in Quezon City, Philippines. Initial news reports indicated that approximately 100 people had been killed by
a very fast moving debris flow of municipal solid waste 共MSW兲
but that many more people were missing. These news reports also
stated that the landfill had initially been 18 to 30 m 共60 to 100 ft兲
high with very steep side slopes and had been subjected to torrential downpour from two consecutive typhoons. Because of
conflicting reports regarding the details of the failure and that this
appeared to be a potentially important landfill slope failure, the
two primary writers traveled to the affected area and performed
field reconnaissance approximately four weeks after the
wasteslide. This reconnaissance effort included observations of
the wasteslide area both by helicopter and by ground, personal
interviews of affected residents and managing city officials, an
extensive search of news records including other eyewitness accounts, collection of precipitation records, and a review of pertinent engineering information including local agricultural and topographical maps and historic engineering reports. This paper
describes the results of this reconnaissance effort.
Characteristics of the Payatas Landfill
The Payatas Landfill is located in the northeast corner of Quezon
City, which is on the island of Luzon in the Philippines 共Fig. 1兲.
1
Senior Project Engineer, Golder Associates Inc., 4730 N. Oracle Rd.,
Ste. 210, Tuscon, AZ 85705. E-mail: smerry@golder.com
2
Associate Professor, Dept. of Civil and Environmental Engineering,
Ira A. Fulton School of Engineering, Arizona State University, Tempe,
AZ 85287–5306. E-mail: edkavy@asu.edu
3
Geotechnical Staff, NCS Consultants, 1860 E. River Rd., Suite 300,
Tucson, AZ 85718. E-mail: wufritz@msn.com.
Note. Discussion open until October 1, 2005. Separate discussions
must be submitted for individual papers. To extend the closing date by
one month, a written request must be filed with the ASCE Managing
Editor. The manuscript for this paper was submitted for review and possible publication on August 26, 2003; approved on April 6, 2004. This
paper is part of the Journal of Performance of Constructed Facilities,
Vol. 19, No. 2, May 1, 2005. ©ASCE, ISSN 0887-3828/2005/2-100–107/
$25.00.
Quezon City, which is the largest of the six cities in the Metro
Manila area, covers an area of about 16,100 hectares and has a
population of about 2.3 million people. The heart of the city lies
immediately northeast of Manila and straddles the northern extension of the Guadalupe plateau of the Philippine Islands; it is an
area of moderate slopes and the most common soil type is a hard
fine-grained loam, or adobe 共moderate plasticity silty clay兲, which
was often used in construction during the city’s history. The city,
which is divided into four congressional districts, has a total of
142 barangays, which are akin to smaller cities or towns within
the larger city. Barangays have formal to semi-formal governments within them. One of these barangays, located in the northeast corner of Quezon City, is known as Lupang Pangako 共the
Promised Land兲. The housing development called Payatas is located within Lupang Pangako.
The Payatas housing development was started in the early
1970s as a 30-hectare upscale housing development. When the
development first started, the Payatas area did not appear to be the
future home for a landfill as it had well-developed streets with
relatively large homes along them. Records indicate that wastes
were first placed at the Payatas Landfill area in 1973 as general
fill for a depressed area. For more than a decade, it remained as a
small landfill only for the use of the Payatas housing development. In 1988, the Smokey Mountain Landfill in northwest Manila was closed and, consequently, the rate of landfilling at the
Payatas landfill increased significantly. Hence, many of the
Smokey Mountain squatters relocated from that area to the Payatas area. Since 1996, the metro-Manila area generated an average
of a little more than 6,000 tons of trash per day. About
1,500 to 1,800 tons per day of this trash were placed at the 18
-hectare Payatas Landfill. The landfill was scheduled to be officially closed in 1998. However, the Quezon City government
asked the Metro Manila Development Authority 共MMDA兲 to
postpone closing the landfill because it could not afford the additional cost of using the landfill in San Mateo, Rizal 共Fig. 1兲,
which is the next closest landfill to Quezon City, about 40 km
away. According to a representative of the MMDA, the Quezon
City Government continued to request the postponement of the
closure including as late as one month prior to the failure.
The Payatas Landfill formed into two separate waste areas
共Fig. 2兲. The reservoir seen in the background of Fig. 2 is the
100 / JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © ASCE / MAY 2005
J. Perform. Constr. Facil. 2005.19:100-107.
Fig. 1. Location map of the Payatas housing development and
landfill in the Metro Manila area
Novaliches Watershed Reservation and, while it is separated from
the landfill by only 200– 300 m, it is on the other side of a hill
from the landfill. In fact, the Novaliches Watershed Reservation is
in a completely different drainage basin than the landfill. The
landfill area is underlain by clayey material mixed with occasional layers of coarser material to a depth of more than 50 m
共Zarco personal communication, 2003兲. Based on its proximity to
Manila, the soil is likely similar to the widespread adobe-type soil
described earlier. The area is sloped at a grade of approximately
20 horizontal to 1 vertical 共20H:1V兲 from the west 共the area adjacent to the reservoir兲 to east 共right to left in Fig. 2兲. These
fine-grained soils, together with the reasonably sloping terrain,
dictate that the recharge of groundwater is low and, hence, runoff
from nonlandfill areas is dominated by surface precipitation rather
than groundwater drainage.
There is no physical barrier between the landfills and the adjacent residential areas. Many of the squatters that lived on and
around the landfill relied completely on the landfill for sustenance. The daily work of these scavengers consisted of sifting
through the waste that was delivered to the landfill to find anything of value including building materials for shanty homes, materials that could be resold in nearby open markets 共cardboard,
plastic, copper wire, and glass bottles兲, and any items deemed
edible. Typical wages earned by these people were reported to be
Fig. 2. View of Payatas’s two landfills, a smaller 5.3-hectare landfill
to the left and the main 12.7-hectare landfill to the right. The slide
failure can be seen along the front of the larger landfill. Fires, as
indicated by the smoke, did exist at both landfills; no attempt to
extinguish them was being made.
about 200 Pesos, or a little less than $4.00 per day. When waste
was delivered to the Payatas Landfill, it appears that there was
little formal compaction, although there is good evidence that at
least one bulldozer was on site. The waste, dumped in a large
heap, would be picked through completely by the local scavengers. Due to this scavenging, the waste would ultimately be distributed into a very thin lift. It is likely that in addition to trampling by human feet, only limited compaction by equipment was
completed. Scavenging also changed the makeup of the waste.
Items, such as wood or metal building materials, cardboard, and
intact bottles, were quickly segregated and reused or recycled,
leaving a waste stream that was predominantly organic matter.
In 1992, the landfill was considered to be nearing its final
height and, hence, an initial closure design was prepared by VBB
VIAK Engineers, Stockholm, Sweden 共Seman and Rydergren
1992兲. Topographical contour maps shown in the design report
indicate that the waste deposit area affected by the failure was
inactive, at least at that time. Additionally, these maps show that
the landfill was much lower in height than what was observed
following the failure. Hence, it is likely that most of the waste
that was involved in the failure was relatively young and had been
placed in the eight years preceding the failure. A July 31, 2000
aerial view of the waste slide area is shown in Fig. 3. A cross
section taken through the landfill for the closure design happens
to coincide very close to where the actual failure took place 共Fig.
4兲. In this cross section, it is seen that maximum side slopes of
3H:1V were specified. The design report also gives warning to the
steep landfill slopes being created at different areas around the
landfill:
“From a geotechnical point of view, the most sensitive
portion of the construction work is the risk for slides in
the waste along the creek. The slope is far too steep and in
combination with the risk that waste is washed away at
times with high water in the creek, local slides may well
occur in the future.”
The slopes being discussed in this statement were at 1.5H:1V.
Fig. 5 shows the side slope of the landfill just beyond the far side
of the failure and from this viewpoint, the slope appears to be
very close to true length and orientation. A line along the surface
of the side slope has been drawn and measured digitally. Based on
this measurement, the side slopes of the landfill adjacent to the
failure area are estimated to be 1.5H:1V, the same slope inclination that the VBB VIAK report had warned about eight years
previous.
In late 1999, REM Transport, the company managing the landfill, began to push waste placed in the middle area of the landfill
to the outer edges. This practice had two effects. First, it tended to
make the side slopes steeper. Second, it made a large depressed
area on the top of the landfill, akin to a catch basin. When this
catch basin filled with water during the heavy rains preceding the
failure, the managing personnel made a trench through the waste
at the outside edges of the basin so that the water could drain
down the side of the landfill, which led to erosion of the side of
the landfill. On the larger waste heap that failed, only limited
evidence of this practice was preserved at the time of the reconnaissance. However, in the smaller heap at the site, evidence of
the practice of pushing waste off the top and down the slope can
be readily seen, as evidenced by the excavated drainage trench
共Fig. 6兲. At the time of the reconnaissance, at least one of the
slopes on the larger heap was being reworked with heavy equipment to add benching 共Fig. 7兲. This slope was much shorter in
JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © ASCE / MAY 2005 / 101
Fig. 3. North to south aerial view of waste slide. A creek, formed from seeping leachate, is at the toe of the slope.
height than the slope where the failure occurred and it is unknown
why this slope was being reworked.
The manner in which the scavengers live was a factor in the
large number of people buried by this failure. The scavengers live
in shanty homes, many of them as small as 1.3⫻ 1.6 m 共4
⫻ 5 ft兲, built right next to each other without yard space. While
these homes exist throughout the Payatas area 共Figs. 2 and 3兲, the
more desirable locations for these homes were considered to be
those actually on the side slopes of the landfill, as they offered
both a hillside view and a shorter commute to work at the landfill.
Building materials for the scavenger’s homes include discarded
wood planks and corrugated metal recovered from the landfill.
One woman’s home, which was about a block away from the
failure, was observed to be the bed of an old Datsun pickup that
had wooden walls built onto it with a corrugated metal roof above
it. These homes were located very close together with only small
paths or alleys to walk to and from their homes. When the failure
happened, the congested locations of their homes slowed any
chance of escape and created a deadly situation for the large number of people in the path of the rapidly moving waste.
Fig. 4. Cross section made through final closure design. This section
was made very near to where the failure took place 共modified after
Seman and Rydergren 1992兲.
Fig. 5. Picture of failure at an oblique angle showing remaining
slope beyond the failure. The remaining slope line is shown with a
black line, which is very nearly 1.5H:1V.
Characteristics of the Failure
Based on compiled information, the following characteristics of
the failure have been established from interviews and newspaper
accounts. At approximately 4:30 a.m. Manila local time 共MLT兲 on
July 10, 2000, loud cracking sounds were heard from near the
northern flank of the landfill. As daybreak came, cracks leaking
water from out of the landfill were reported 共it is not known
exactly where these cracks were兲. Between 7:30 a.m. and 7:45
a.m. MLT, a large mass of waste mobilized and came down the
hillside in a manner similar to a debris flow, covering all of the
homes and people in its path. The mass of waste also toppled
Fig. 8. Drawing made by survivor of failure during interview
Fig. 6. View of smaller 5.3-hectare landfill showing deep trench
excavated from central area to the edge. Evidence of pushing the
waste over the top and allowing it to settle on its own down the edge
can also be seen to the left of the trench.
homes. Although heavy machinery was reworking the waste in
the area in which it was deposited by the wasteslide, a very steep
共vertical at the top兲 head scarp remained above this area.
several at least one 220-V electrical line. Whether from the electrical lines, from fired stoves in the buried huts, or from spontaneous combustion, the mass of waste then caught on fire. The
start of the fire was accompanied by a small explosion of unknown origin. The total volume of waste involved in the slide is
estimated to be 13,000 to 16,000 m3 共about 17,000 to 21,000
cubic yards兲. Using an in-place 共moist兲 density of 1,120 kg/ m3
共70 lb/ ft3兲, the total mass of the waste involved was 14.6
⫻ 106 to 17.9⫻ 106 kg 共15,800 to 19,400 tons兲. Kavazanjian
共2001兲 reports this waste density to be near the upper bound for
near surface 共⬍10 m兲 but below the lower bound for MSW at
depth.
Only 58 people were rescued while, after several weeks of
recovery efforts, 278 were confirmed dead and 80 to 350 people
were reported still missing. 共Because many of the inhabitants
were unregistered squatters, the exact number of people buried is
unknown.兲 Many of the recovered bodies were charred. Recovery
efforts were stopped in late August 2000, over 6 weeks after the
wasteslide occurred. As presented earlier, Fig. 3 shows the massive slide area and the extent of the movement of the waste. It is
noted that the area between the creek and the landfill, now covered with a deep layer of waste, had been filled with “shanty”
Eyewitness Accounts of the Failure
Mr. Jose Cabahutan 共age-40兲 and his son, Benjo, were two of the
rescued victims of the waste slide. On August 2, 2000, Mr.
Cabahutan was interviewed by Dr. Mark Zarco of the University
of Philippines-Diliman, and the primary writer. In this interview,
Mr. Cabahutan told an unnerving account of the tragedy. The
drawing shown in Fig. 8 was drawn by Mr. Cabahutan during the
interview. Key areas 共A–F兲 of his recollection are identified on
the “keyed” version of Fig. 8 presented in Fig. 9. The Payatas
Landfill is shown as “A.” Several homes are shown by the small
Xs at “B.” Mr. Cabahutan stated that these were very desirable
home sites, as they were higher on the hillside and allowed a
shorter trip to work at the landfill, as designated by the arrows
pointing towards the landfill. Other areas of homes and streets are
shown by the horizontal and vertical lines at “C.” For these
people to get to work at the landfill, they had to cross a creek 共a
recently excavated trench, designated “D” and shown later in Fig.
12兲 filled with leachate. To assist in traversing this creek, the
residents had excavated a set of steps into the side of the trench,
共“E”兲. Mr. Cabahutan noted that the working day of the landfill
scavengers was very long and started early in an effort to be at the
landfill when the sun came up, as many of the trucks deliver and
dump the waste at night. The following paragraph recounts Mr.
Cabahutan’s story.
Fig. 7. View of larger fill showing the slopes on the southwest corner
being reworked to add benching
Fig. 9. Keyed drawing
At approximately 4:30 a.m. MLT, a large noise was heard
throughout the area. Many men who were either already awake or
who were awakened by the noise began to gather and discuss
what the sound was and whether or not it was safe to go to work
that day. After considerable discussion, it was decided that the
storms that had produced torrential rains for the past ten days had
subsided and that it was a day that they should work. As a group,
they traveled down the steps to cross the creek. Suddenly, they
heard a very loud noise and when they looked up, they saw the
landfill coming at them very fast. Many turned to run away but
the steps were narrow and became clogged with people. Mr.
Cabahutan says that he was one of the last in line to go to work
and so when they turned around to run, he and his son were near
the front of the line. Nevertheless, the waste overcame and buried
them. Fortunately, they were quickly rescued. He tells that the
slide was followed by a small explosion and fire, although Mr.
Cabahutan did not know what caused the explosion. At this point
in the interview, Mr. Cabahutan was visually shaken and had
difficulty continuing, but wrote the number 100 in the corner
共“F”兲 and then put a large X and circle around the steps 共“E”兲. He
explained that 100 bodies of men and children were later recovered at that location.
Other eyewitness accounts from local newspapers supplement
Mr. Cabahutan’s account of the events. Ms. Gloria Alano said she
was at the store and heard a loud sound from the direction of her
home. “It sounded like thunder and in an instant, our house was
gone,” she said 共Marinay and Andrade 2000兲. She hurriedly ran
back but found her house buried under a smoldering heap of
garbage. Her husband and three children were buried inside the
house. Mr. Armando Valenzuela, a student of the Hotel and Tourism Institute of the Philippines in Intramuros, tells that he was
awakened by what sounded like an explosion. He ran out of his
house to see the “piles of garbage collapsing.” Mr. Trinidad Cabil,
another rescued victim, tells of a rumbling sound like an airplane
crash before the mountain of garbage cascaded over the top of
them. As for the fire, the Manila Times reported that an 共unnamed兲 witness said the “pile 共of waste兲 broke in half and flames
leapt out of the crater” 共Marinary and Andrade 2000兲.
Weather Preceding the Time of the Failure
In the two weeks prior to the failure, the metro Manila area was
besieged by rain from two typhoons: Typhoon Kirogi 共known in
the Philippines as Typhoon Ditang兲 and Typhoon Kai-Tak 共known
in the Philippines as Typhoon Edeng兲. The typhoon that had the
largest impact to the Philippines was the second one, Typhoon
Kai-Tak. This typhoon started as a low-pressure area that brought
unsettled weather to the South China Sea, the body of water between the Philippines and China. On the morning of July 4, 2000,
this disturbance developed into a tropical storm 共designated
Tropical Storm 06W兲 just off the west coast of Luzon, the
northern-most island in the Philippines. All agencies were predicting that this storm would move northeast and follow the track that
the first and concurrent storm, Typhoon Kirogi, had made toward
Japan. However, Tropical Storm 06W meandered for days just
north of Luzon and, in doing so, continued to pull in significant
moisture to Luzon. By the morning of July 8, 2000, there were
more than 700,000 people homeless and 26 dead in northern
Luzon from the effects of the storm that had now been upgraded
and named Typhoon Kai-Tak. Fig. 10 shows Typhoon Kai-Tak
with a well-developed storm eye in the South China Sea and with
the counterclockwise circulating moisture just over the northwest
Fig. 10. Satellite photo of the Typhoon Kai-Tak at 0700 GMT,
July 8, 2000 共modified after Japan Meteorological Society 2000兲
corner of Luzon. Also seen in Fig. 10 is a flow of moisture extending to the upper right from the Philippines, which is the tail of
Typhoon Kirogi. The town of Laog, in northern Luzon, received
almost its entire average monthly rainfall average in only
48 hours. At the Quezon City weather station, which is about
9.5 km from the Payatas Landfill, a significant amount of rainfall
was also recorded 共see Fig. 11兲. After remaining virtually stationary or performing small cyclonic loops in the area northwest of
Luzon for a long period, Typhoon Kai-Tak suddenly moved north
to Taiwan where it continued to cause death and destruction.
Most accounts indicate that the rainfall in the Luzon area was
due strictly to Typhoon Kai-Tak. However, in observing the storm
track of Typhoon Kirogi 共http://mscweb.kishou.go.jp/general/
activities/aim/disaster/tគcyclone/animation.htm兲, it is seen that a
significant amount of moisture was pulled across Luzon by this
northeastern tracking typhoon. The storm track at the above referenced website shows the typhoon from 0600 GMT July 2, 2000
through 0600 GMT July 10, 2000. In the early portion of the
track, Luzon is the largest island seen on the far left side 共middle
vertically兲 of the picture, which then continually moves downward to the left. Later in the track, Luzon cannot be seen at all.
However, during the time that it can be seen, a significant amount
of moisture is observed to come across Luzon as a direct result of
Typhoon Kirogi.
Fig. 11. Precipitation record at Quezon City Weather Station
共N14.6370° E121.0771°兲 for period of May 1, 2000 through July 31,
2000
Fig. 13. Two-dimensional cross section of slope evaluated in
stability analyses
Fig. 12. Picture showing deep excavation completed at toe of waste
slope 共N14.71798° E121.10622°; 08/02/2000兲
The combined effects of the two typhoons created the very
large quantities of precipitation encountered throughout Luzon
during early July 2000. Fig. 11 shows the observed precipitation
record at the Quezon City Weather Station 共approximately 9 km
from the landfill兲 for the period of May 1, 2000 through July 31,
2000. The date is shown on the X axis. On the left-hand Y axis,
the daily rainfall 共units of m兲 is shown. For the ten days immediately preceding the failure, a combined total of 0.746 m 共2.45 ft兲
of precipitation was recorded at the Quezon City Weather Station.
On the right-hand y axis of Fig. 11, the running total from May 1
through July 31 is shown. The total precipitation for this period is
1.793 m 共5.88 ft兲. Hence, approximately 42% of the total summer
precipitation fell in the ten days immediately preceding the failure.
During the reconnaissance, it was observed that there was a
considerable amount of leachate seeping from the waste into the
trench at the toe of the slope. This trench had been originally
excavated prior to the wasteslide and then cleaned out after the
failure 共Fig. 12兲. From this leachate came a continuous and well
spread out amount of gas bubbles. As there is not a source of
natural gas 共i.e., a pipeline兲 in the area, it is assumed that the gas
was landfill gas seeping out from below the leachate water level.
Although difficult to see in Fig. 12, the water almost appeared to
be boiling but, in fact, the leachate was not hot.
Summary of Failure Analyses
The hydrologic and stability analyses conducted by the writers to
investigate the wasteslide are complex and extensive and, hence,
only a brief summary of these analyses is provided. A more complete discussion of the hydrologic and stability analyses of the
wasteslide is provided in Fritz 共2003兲.
To evaluate the effects of the large quantity of precipitation at
the site, the Hydrologic Evaluation of Landfill Performance
共HELP兲 version 3.07 共Schroeder et al. 1994兲 model was used. The
clayey subsoil was modeled as low permeability clay and default
values for MSW were used in the initial iteration 共as discussed in
the slope stability summary, a parametric study was later conducted where the hydraulic conductivity of the MSW was varied兲.
The HELP model does not have built-in precipitation records for
Manila, and hence, the Tallahassee, Florida precipitation record
was modified so that it included the recorded data 共Fig. 11兲 for the
three months preceding the failure. Based on this analysis, a saturated depth of 15.0 m was predicted at the base of the landfill
immediately prior to the failure.
A representative cross section of the landfill was developed for
the slope stability analysis 共Fig. 13兲. The cross section is composed of an approximately 33.5 m thick layer of MSW overlying
the clayey native subsoil. The slope inclination was established as
1.5H:1V and the excavated trench at the head scarp shown in Fig.
5 was modeled as a tension crack. Stability calculations were
conducted using the computer program UTEXASED 共Wright
1996兲. The factor of safety 共FS兲 for the representative cross section was calculated using Spencer’s method 共Spencer 1967兲. As it
was believed that pore pressures played a significant role in the
failure, an effective stress analysis was conducted. The shear
strength parameters for the soil and waste materials that were
used in the effective stress analysis are provided in Table 1. The
MSW strength envelope was based upon GeoSyntec 共1998兲, who
reported that this strength envelope for MSW compared well with
shear strengths backcalculated from the failure of the Doña Juana
Landfill in Bogotá, Columbia. Kavazanjian 共2001兲 cites a similar
strength envelope for wet and bioreactor landfills based upon the
Doña Juana backanalyses and large diameter direct shear strength
tests on waste recovered from a bioreactor landfill. Based on values reported by Kavazanjian 共2001兲, the unit weights for MSW
reported in Table 1 are near the upper bound for near-surface
MSW in arid landfills. As the waste at the Payatas landfill has a
large ratio of waste to soil and was not well compacted 共if compacted at all兲, the unit weight values shown in Table 1 are thought
to be appropriate for this case study.
Using the cross section shown in Fig. 13 with the unit weight
of the fluid in the lower portion of the MSW equal to that of water
共9.8 kN/ m3兲, a FS equal to 1.2 was found, which indicates marginal stability of the slope, even with 15 m of saturated waste at
the base of the landfill. However, as the waste becomes saturated,
the degradation processes remain anaerobic and hence, landfill
gases continue to be created. As the waste is now saturated, the
landfill gases can no longer freely migrate from out of the waste
Table 1. Material Properties of Waste and Subgrade Used in Slope Stability Analysis
Material
Municipal solid
waste
Subgrade
Cohesion,
c
共kPa兲
Friction
angle,
␾⬘
共degrees兲
Unit wt.,
␥
共kN/ m3兲
Unit wt.,
␥sat
共kN/ m3兲
19.0
28
10.2
13.9
SU / ␴⬘v = 0.25
0
—
18.6
Fig. 14. Waste beginning to accumulate on a private lot adjacent to
the “GMA Kamuning” Metro Rail Transit station
and, hence, they may produce pore pressures in excess of that
predicted by fluid statics. To account for the excess pore pressure
that may be created due to the buildup of landfill gas within
saturated waste, the model derived by Merry et al. 共2005兲 was
employed. This model provides a linear distribution of pressure in
the saturated zone that is higher than would be created with normal weight water and, hence, is modeled in the analysis by specifying a unit weight of fluid that is proportionally greater than that
of water. As the unit weight of the fluid, ␥fluid,equivalent is raised, the
FS decreases. At a value of ␥fluid,equivalent equal to 20.9 kN/ m3, the
FS is lowered to 1.0. Excess pore pressure due to the formation of
landfill gas can readily create an equivalent fluid pressure of this
magnitude if the vertical saturated hydraulic conductivity of the
waste is on the order of 2 ⫻ 10−7 m / s 共Merry et al. 2005兲. This
relatively low value of hydraulic conductivity for the Payatas
waste is considered reasonable in that the highly organic waste
has been placed in relatively thin lifts. Additionally, this equivalent fluid unit weight creates a state of static liquefaction or hydraulic fracturing in the lower portion of the waste, a situation
that may allow the slide to be very fast moving, which is consistent with the slide behavior described by eyewitness accounts.
After Effects of Failure
Five days after the failure, Philippine President Joseph Estrada
declared the Payatas Landfill permanently closed. In the four
months following the closure of the Payatas Landfill, the metro
Manila areas suffered from a complete lack of refuse management. When a major refuse management area is suddenly closed,
large quantities of waste need to be disposed of elsewhere. In
Metro Manila, it was placed wherever there was room including
vacant lots, the sides of the roads, and private property 共Fig. 14兲.
Later in 2000, the San Mateo 共Rizal兲 Landfill had become filled
and was closed by the government of Rizal, thus compounding
the waste disposal problem. The “permanent” closure lasted four
months, about the time it took for the major publicity about the
failure to die down. On November 10, 2000, President Estrada
gave permission for the landfill to again accept waste. For the
scavengers, it was once again business as usual. The only change
that was that a 50 m 共150 ft兲 wide radius “danger zone” was
created at the toe of the waste mass and all homes within that
radius were demolished by the Quezon City government 共Bacalzo
2001兲.
As of August 2003, three years after the failure, the entire
Philippines remain in a garbage disposal crisis. Only two other
landfills in the metro Manila area are currently accepting waste,
both of which are unlined. The first landfill is in Rodriguez, a
community formerly known as Montalban and the second is in
Marikini 共see Fig. 1兲. On average, about 70% of the waste generated is collected in urban areas, and only 40% of generated
household waste is collected in rural areas. In poorer areas, the
percentages are typically less than the average. Although there is
an effort by the Philippine government to locate and construct
new landfills, the process has been slow, particularly due to local
opposition for new landfills 共Pulley 2003兲 and accusations of corrupt practices in the bidding process 共Sison 2001兲. In December
2002, MMDA Chairman Bayani Fernando publicly encouraged
all households that were hooked up to the sanitary sewer system
to begin disposing all biodegradable mass by flushing it through
toilets. In this manner, that mass of waste would not need to be
collected by sanitation trucks but would be collected at the sewage treatment plant.
On August 1, 2000, relatives of the victims of the failure filed
a 1-Billion Peso 共$22.7 Million兲 class action suit. The defendants
named were the Quezon City government, the Metro Manila Development Authority, TOFEMI Realty Corp. and Meteor Co. Inc.,
described as either owners of or claimants to the land used as a
dumpsite, and REN Transport Corp., the hauling firm contracted
to collect and dump garbage from Quezon City and some other
parts of Metro Manila. Included in the list of defendants were
Quezon City Mayor Ismael Mathay Jr. and MMDA Chairman
Jejomar Binay, personally. On August 9, 2002, the Office of the
Ombudsman in Quezon City dismissed the case and absolved
共now ex-Mayor兲 Mathay and 共now former MMDA Chairperson兲
Binay due to “procedural lapses.”
Summary
On July 10, 2000, at least 278 persons were killed when a debris
flow of waste came crashing down on the community of Payatas,
Philippines. The failure was preceded by extremely large quantities of precipitation, including 0.75 m 共2.45 ft兲 in the ten days
immediately prior to the failure. The landfill was formed from
waste that was placed without significant compaction. Following
placement, waste was pushed over the brink of the top slope so
that it would make a much steeper slope 共as high as 1.5H:1V兲
and, hence, make room for additional waste on the top of the
landfill. There is some evidence that, at the time of the reconnaissance, some of these slopes were beginning to be reworked by
heavy equipment. Based on the adjacent slopes not included in
the failure and the history of landfill development, it is likely that
the area where the failure took place involved relatively fresh
waste. There is also evidence that a large depressed area was
created by heavy equipment. When this area filled with precipitation, a trench was dug along the top to relieve the water. While
difficult to say for certain, it is possible that remnants of the
trench remained at the top of the head scarp after the wasteslide
共Figs. 2, 3, and 5兲. The digging of a trench parallel to the top of
the slope would effectively create a tension crack. In addition,
there is evidence that a 2 to 3 m deep trench was excavated at the
toe of the slope before the failure 共Fig. 12兲, though this trench
was not included in the analysis.
It is also evident that all parties involved in the management of
the Payatas Landfill ignored a prior engineering report that stated
that landfill side slopes on the order of 1.5H:1V were too steep
and should be have been reduced to no more than 3H:1V. The
consequences of not only failing to flatten the slope but also increasing its height was particularly devastating in that the steep
side slopes of the Payatas Landfill were constructed in immediate
proximity to residences, with no barrier 共such as a fence兲 to maintain a safe setback for the residences from the waste.
Specific analyses and discussions of the reasons for the failure
are presented in Merry et al. 共2005兲. However, one of the most
likely scenarios is that the failure was caused by elevated pore
pressures that lowered the effective stress along the failure plane.
The elevated pore pressures may have been caused by the buildup
of landfill gas, which could not escape due to the high levels of
saturation in the waste 共Fritz 2003; Merry et al. 2005兲. Slope
stability analyses that consider these elevated pore pressures predict a deep-seated base failure surface that starts at the bottom of
a tension crack 共created by the excavated trench兲 and exits
slightly beyond the toe of the slope. While the specific location of
the slide plane could not be determined in situ, there appears to be
good agreement between the remnants of the observed failure
surface and that predicted by the analyses.
Acknowledgments
The reconnaissance efforts were completed through funding by
the National Science Foundation under Grant No. 0092700. The
writers would like to thank Dr. Victor Pulmano and Dr. Mark
Zarco of the University of the Philippines, Diliman for their help
in making contacts at the Payatas Landfill and in performing the
interview, and Mr. Per Olof Seman of SWECO International for
providing the 1992 VBB VIAK report. Finally, the writers would
like to thank the wonderful people of Payatas for their openness
in providing interviews about their terrible experience.
References
Bacalzo, S. J. 共2001兲. “Garbage still being dumped at Payatas.” Balik
Kalikasan 共Tagalog, Return to Nature, 6共2兲, 具http://www.bwf.org/bk/
2001/02/nគ03.html 共Feb. 1, 2005兲.典
Fritz, W. U. 共2003兲. “The effect of gas on pore pressures in wet landfills.”
PhD dissertation, Univ. of Arizona, Tucson.
GeoSyntec Consultants. 共1998兲. “Investigation of the causes of 27 September 1997 Zone II slope failure, Dona Juana Sanitary Landfill, Santafe de Bogota, Columbia, South America.” Technical Report prepared for ProSantana Ltda., GeoSyntec Consultants, Chicago.
Kavazanjian, E. K., Jr. 共2001兲. “Mechanical properties of municipal solid
waste.” Proc., Sardinia 2001, 8th Int. Waste Management and Landfilling Symposium, Sardinia, Italy, Vol. 3, 415–424.
Marinay, M. B., and Andrade, J. 共2000兲. “30 dead in QC dump collapse.”
The Manila Times, July 11.
Merry, S. M., Fritz, W. U., and Budhu, M. 共2005兲. “The effect of gas on
pore pressures in wet landfills.” J. Geotech. Geoenviron. Eng., in
press.
Pulley, R. V. 共2003兲. “Down in the dumps and rising from it!.” in newsletter, City Development Strategies Executive Association
具www.cdsea.org/Archives/Kcontrib/downគdumps.htm 共Feb. 1, 2005兲.典
Schroeder, P. R. et al. 共1994兲. “The hydrologic evaluation of landfill
共HELP兲 model—Engineering documentation for Version 3.” EPA/600/
9-94/168b, U.S. Environmental Protection Agency Risk Reduction
Engineering Laboratory, Cincinnati.
Sison, M. N. 共2001兲. “Firm linked to Estrada got Metro Manila garbage
contract.” in report, Philippine Center for Investigative Journalism,
January 24–25, 具www.pcij.org/stories/2001/garbage.html 共Feb. 1,
2005兲.典
Seman, P. O., and Rydergren, A. 共1992兲. “Payatas sanitary landfill, final
design report.” VBB VIAK Engineers, Stockholm, Sweden.
Spencer, E. 共1967兲. “A method of analysis of the stability of embankments assuming parallel interslice forces.” Geotechnique, 17共1兲, 11–
26.
Wright, S. G. 共1996兲. “UTEXASED–Software for Slope Stability Calculations,” Shinoak Software, Austin, Tex.

Reconnaissance of the July 10, 2000, Payatas Landfill Failure

Transcription

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