Estimation of Shear Strength Parameters of Municipal - PDF by gpc19797

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									          Estimation of Shear Strength Parameters of Municipal Solid Waste in Landfills

                     Mylene Palaypayon                                                      Hideki Ohta
        Assistant Professor, Department of Civil Engineering         Professor, Department of International Development Engineering
               University of the Philippines Los Banos                                Tokyo Institute of Technology
                e-mail: myepalaypayon@yahoo.com                                        e-mail: ohta@ide.titech.ac.jp


                      A report submitted in participation to the JSPS Core University Program Group 3
                        as short-term exchange scientist from November 5, 2006 to February 2, 2007


                                                               ABSTRACT


The information presented in this report is a result of the simplified analysis of municipal solid waste placed in landfills
with the attempt to roughly estimate the shear strength parameters of such wastes. Shear strength parameters are defined
in terms of cohesion c and angle of internal friction ϕ (phi). Since it is known that shear strength parameters of municipal
solid wastes in landfills vary as a function of several factors such as waste composition, age of the landfill, degree of
compaction, amount of moisture, etc., it will be impossible to come up with conclusive shear strength parameters for
landfilled wastes at this time. However, using the back-calculation technique and simplified stability analysis of vertical
cut slopes for a presumed planar failure surface, a set of c and phi is estimated as a function of ‘critical’ height of
landfilled waste. For this purpose, the offshore Tokyo Bay landfill practice is presented here with emphasis on its
geotechnical and stability aspect. It is important to note that the cohesion parameters estimated from the simplified
analysis suggest minimum possible values and may even be much greater in reality as failure of the vertical cut slope has
not occurred in the site. The analysis is also subject to limitations from assumed parameters, e.g. unit weight of waste. In
addition to the simplified analysis of vertical cut slopes, an investigation of shear strength parameters of typical soil cover
used at Tokyo Bay offshore landfill is presented here using multiple-stage constant volume shear tests for various amount
of soil moisture. The results give an idea of possible shear behavior of the soil cover used in landfills.




Introduction                                                         dependent on the composition of the wastes that are
                                                                     disposed in landfills, these properties are expected to
Landfills are manmade infrastructures designed to                    vary from place to place.
address the final disposal of residual wastes, i.e. wastes
that are considered to be of no value after recovery of              Shear Strength Properties of Wastes
all recyclables and after intermediate processing such as
incineration or crushing. As the final resting place of              Shear strength parameters are important engineering
wastes, landfill facilities have to be designed and                  properties in stability analysis of landfilled wastes.
constructed with minimal negative impact to the                      Technically, sanitary waste landfills refer to facilities
environment. Geo-environmental (civil) engineers take                that are prepared with liner, cover, leachate, and gas
part in the design of landfills by studying stability of             collection systems, and operated with compaction
landfill slopes and understanding the mechanical                     equipment. These are also referred to as ‘engineered’
behavior and engineering properties of landfilled wastes.            landfills. In contrast, waste dumps refer to facilities that
Unlike soils, landfilled wastes are unique in the sense              accept wastes without the components mentioned for
that the study of their engineering properties can be                sanitary waste landfills, and can also be called ‘non-
extremely difficult because of their variable and                    engineered’ landfills. In this paper, the term ‘landfill’
hazardous nature. To this date and even for 30 years or              will pertain to both engineered and non-engineered
so, all waste landfills will continue to undergo the                 waste disposal facilities, i.e. including waste dumps.
complex stages of decomposition which will result to
wide array of engineering properties as a function of                Unfortunately, the damage caused by landfill failures
time. For example, unit weight may change through                    reported in the literature extends to loss of hundreds of
time due to decomposition of the waste. Therefore,                   lives particularly in waste dumps situated in developing
engineering properties that are reported presently in the            countries where people scavenge wastes directly at the
literature may be different from those that will be                  dumping area. Landfill failures are not exclusive to
reported several years from now. Other factors that can              non-engineered landfills, but can occur even to
affect the study of engineering properties of municipal              engineered landfill facilities that are supposed to have
solid waste in landfills include the manner in which                 been carefully planned to prevent occurrence of such
wastes are sampled, the sampling location within the                 failures. The need to investigate shear strength
landfill site, and the test methods for determining these            parameters of municipal solid waste in landfills
properties. Moreover, since engineering properties are               becomes apparent from the series of reported cases of
catastrophic failures both in engineered and non-                (Blight 2006, Merry et al. 2005, Koelsch and
engineered landfills (see Blight 2006).                          Ziemann. 2004); and in
                                                             b) Leuwigajah landfill in Bandung, Indonesia, which
To date, no standardized methodology has been                    involved 148 deaths last February 2005 (Koelsch et
established yet in determining shear strength parameters         al. 2005).
of waste, although, attempts have been made to               Both of which are examples of non-engineered landfills,
characterize these parameters from laboratory tests, or      locally known in developing countries as ‘open
from field measurements, and from back-analyses of           dumpsites’, where scavengers are allowed to recover
failed landfill slopes. The variable, site-specific          valuable materials in the dumping area.
character of the waste even makes it more difficult to
come up with reliable shear strength parameters to be        A reconnaissance of the events of the dump failure in
used in landfill design engineering and stability            Payatas landfill are discussed in Merry, Kavazanjian
analyses. Moreover, laboratory tests run samples that        and Fritz (2005) and reconstituted by Blight (2006).
are disturbed, remolded, or those with some degree of        The major reasons for its failure were identified to be
disturbance in the case of boreholed waste samples.          (i) waste saturation and (ii) over-steepening of slopes.
These tests yield results that may not actually represent    The waste saturation was due to the two consecutive
the true shear strength parameters in field conditions.      typhoons that hit the Philippines 10 days prior to the
Still, there is limited information on in-situ municipal     incident. Accordingly, the side slope reached as steep as
solid waste shear behavior (Dixon and Jones 2005).           1.5H:1V. Among the other reasons identified by a
Shear behavior of waste is dependent on several factors      separate study conducted by Koelsch and Ziemann
including, but not limited to, unit weight, water content    (2004) are (iii) low waste density due to improper waste
(pore water pressure), degree of compaction, degree of       compaction, and (iv) high content of organic materials,
decomposition or age of landfill, and manner of              based from their forensic investigation.
placement and height of landfill waste fill.
                                                             The stability analysis conducted by Merry et al. (2005)
Generally, the methodology to study the engineering          is based on effective stress analysis accounting for the
properties of soils can be applied to the study of           effect of pore pressure. A saturated depth of 15.0m was
engineering properties of landfilled wastes. In the study    predicted at the base of the 33.5m total waste depth
of failures involving waste slides, the Coulumb’s failure    immediately prior to failure. Using Spencer’s method
envelope for soils will be assumed applicable, and is        for stability analysis of the representative cross-section
expressed as:                                                shown in Figure 1, a factor of safety equal to 1 was
                 τ = c + σ tan ϕ            Eq. 1            calculated with the aid of UTEXASED computer
where : τ = shear strength                                   program.
          σ = normal stress from weight of sliding block

Therefore, in this case, the shear strength parameters
expressed as a function of cohesion c and angle of
internal friction ϕ, must be overcome before failure
against sliding can occur along the surface of shearing
waste blocks.

To be able to estimate the shear strength parameters of             Figure 1. Cross-section of Payatas slope evaluated
municipal solid waste placed in landfills, without                                by Merry et al. (2005)
having to resort to laboratory testing of remolded
wastes or disturbed samples, a simplified stability
analysis of vertical cut slopes is presented in this paper   The unit weight of fluid indicated in the figure as
based on the established principles of soil mechanics.       γfluid,equivalent equal to 20.9 kN/m3 models the build-up of
Existing height of vertical cut slope at offshore Tokyo      landfill gas within the saturated waste as excess pore
Bay landfill is used to conservatively back-calculate the    pressure in addition to the pore water pressure. Hence,
cohesion parameter by first assuming probable                this unit weight of the fluid that is proportionately
estimates for phi (ϕ) parameter. The estimated shear         greater than that of water yielded a factor of safety of
strength parameters are then used to forward-calculate       1.0, in contrast with their analysis specifying a unit
‘critical’ heights which suggest maximum allowable           weight of waste equal to that of water (9.81 kN/m3),
heights of landfilled wastes that can be placed on           which resulted to a factor of safety equal to 1.2 (Merry
vertical slopes or 90o side slope angle without failure.     et al. 2005). From the effective stress analysis, it can be
                                                             clearly seen that by increasing the pore pressure, either
Studies of Landfill Failures in the Past                     from precipitation or from the accumulation of gas
                                                             during waste anaerobic degradation, the factor of safety
Two most catastrophic failures that occurred in the past     is significantly reduced. However, no measurements on
are the waste slide in:                                      landfill gas build-up were done in their study and the
a) Payatas landfill in Quezon City, Philippines last         modified precipitation records of Tallahassee Florida
     July 2000, which involved recorded deaths of 278,       were used in the Hydrologic Evaluation of Landfill
     and from 80 to 350 missing, presumed to be dead
Performance (HELP) model to evaluate                    the    the shear strength τF was 1-1.5 kPa. Pore pressure was
accumulation of water in the waste pile.                       estimated to be 15 meters of water. At unit weight equal
                                                               to 7.5 kN/m3, while τf was estimated at 17 kPa, and τF is
Blight’s (2006) rudimentary analyses of failures, on the       equal to 26 kPa.
other hand, estimated interfacial shear strength of waste
at failure expressed as τf and at cessation of flow τF.        In summary, literature values of cohesion c and angle of
The analysis takes a rigid block which slides at an            internal friction ϕ used in the stability analysis for the
inclined angle through the interface with the block’s          two landfill failure cases mentioned above are presented
own weight as the driving force for failure. This              in Table 1.
interface can either be between the waste and the
underlying subsoil, or the waste and the liner system.          Table 1. Shear strength parameters and unit weight of
For Payatas landfill, the estimated interfacial shear           municipal solid waste in developing countries used in stability
                                                                analysis by previous studies.
strength τf at failure is equal to 11-12 kPa for an                                               Angle of           Unit
assumed unit weight of 5 kN/m3. Since the waste did               Landfill site
                                                                                  Cohesion
                                                                                              internal friction     weight
                                                                                   c (kPa)
not flow, no value for τF at cessation of flow was                                               ϕ (degrees)      γ (kN/m3)
calculated. Pore pressure was estimated to be only 2-2.4         Payatas,
                                                                                      19             28              10.2
                                                                 Philippines1
meters of water. When the unit weight was increased to           Leuwigajah,
7.5 kN/m3, calculated τf was 17 kPa.                             Indonesia2
                                                                                      10             20               11
                                                                 1
                                                                  Merry et al., Reconnaissance of the July 10, 2000 Payatas landfill failure, 2005
                                                                 2
                                                                  Koelsch et al., Stability of landfills – the Bandung dumpsite disaster, 2005
The most recent failure in Leuwigajah landfill, Bandung,
Indonesia is also believed to have failed due to               A summary table of parameters used for the two landfill
oversaturation of waste from heavy rainfall. The failure       failures are presented in Annex 2 where the extent of
in Leuwigajah occurred after 3 days of heavy rain. Both        failure, magnitude of waste failed volume, etc. are also
the Payatas and the Leuwigajah failures occurred in the        indicated.
early morning at about 4:30 am and 2:00 am,
respectively. These waste failures were marked with            Tokyo Bay Offshore Landfill Site Visit
loud cracking noises sounding like explosions from the
waste avalanche. In addition, waste smolders and burns         An ocular site visit was conducted last December 26,
were observed in both dumpsites. The failure that              2006 at the Tokyo Bay offshore landfill with the
occurred for both landfills was evidently affected by the      assistance of the executive director of the Tokyo Port
existing climatic conditions during that time.                 Terminal Corporation, Mr. Akira Tanaka and the
                                                               manager of the Plan Coordination section of the
Koelsch et al. (2005) conducted a stability analysis of        Engineering division, Mr. Jirou Ebata. An overview of
Leuwigajah landfill for two scenarios where a portion          the current landfill practice at Tokyo Bay offshore
of waste is either burned or unburned. The calculation         landfill was presented by the head of the Bureau of
method followed the technical recommendation of the            Environment Tokyo Metropolitan Government. The
German Geotechnical Society in which the                       head of the Bureau of Environment presented the
reinforcement effect due to plastic fibres and foils are       historical background of the Central Breakwater
considered. A parameter called the internal angle of           Landfill sites and the New Sea Surface Disposal site,
tensile stress ζ, which mathematically describes the           which is the last offshore landfill to be developed in
linear relation between tensile stress and normal stress       Tokyo Bay. He explained the general operation
is incorporated in the calculation. The component of the       procedures of waste transport and receipt, waste
shear strength that is created by the tensile forces is        placement and compaction, and maintenance being
called fibrous cohesion. The fibrous cohesion is               undertaken at the site, with the aid of video
calculated from the internal angle of tensile stress, the      documentation. A tour of the site was facilitated by the
normal stress σ, a transmission factor and a function          team from the Bureau of Environment after the short
considering the anisotropy, i.e. direction between the         presentation.
fibers and the shear plane (Koelsch et al. 2005). The
stability analysis of the first scenario without landfill      The Tokyo Bay landfill is a special case of landfill since
fire resulted to a factor of safety equal to 1.13. When        the wastes are actually not filled on-land but on the
the landfill fire is included in the stability analysis, the   reclaimed portion offshore. The reclaimed land is
cohesion of the burned waste is reduced from 10 to 0           constructed using dredge soil. The existing underlying
kPa resulting to a factor of safety equal to 1.0. The          soil is made of clay with the landfill perimeter secured
mode of failure assumed for both cases is translational        using the caisson method, a concrete retaining structure
passing through the subsoil. The analyses suggest that         used to keep water from entering the landfill working
waste burning reduces the reinforcement effect and             area (Wikipedia, 2006), as well as to prevent leachate
fibrous cohesion of the waste pile that led to the failure     from escaping out to the seawater. Waste collected from
of the slope.                                                  Tokyo districts do not directly go to this landfill facility,
                                                               instead the waste is brought first to intermediate
In the rudimentary failure analysis conducted by Blight        processing centers like the incineration plants, crushing
(2006) for the Leuwigajah landfill failure, the back-          plants, and iron and aluminum recovery centers. The
calculated shear strength at failure τf is 15-20 kPa for an    landfill site only accepts residuals from intermediate
assumed unit weight of 5 kN/m3. At cessation of flow,
processing, like shredded plastics, crushed bulky waste,      written in Japanese provided to the author during the
incinerator ashes, or slag.                                   landfill site visit. It is presented here to serve as basis
                                                              for geotechnical investigation of other municipal solid
Photographs [refer to Annex 1] of the actual landfill         waste landfills and for future comparison.
conditions on the site are presented in this paper to
illustrate the landfill operation they are implementing       In order to determine the geotechnical properties of
and to show existing slope conditions, the nature of the      municipal solid waste landfills, previous studies have
landfilled wastes, and the manner of waste placement at       been conducted at 15 landfill sites in Japan. Based from
the site. Most of the wastes that are landfilled compose      these studies, the cohesion value of waste ranges from
of shredded plastics and crushed bulky wastes as can be       0.4 kg/cm2 to 0.7 kg/cm2 [39.24 to 68.67 kPa] which
seen in Photo 2. Incinerator ash which is considered to       suggests relatively higher values than what was used for
be a soft weak material especially when it becomes            Payatas and Leuwigajah. The friction angle, on the
saturated with water is carefully placed in prepared          other hand, takes a value of 30o similar to that of sand.
ditches as shown in Photo 1. This method of placement         Since waste has same friction angle to that of sand,
is referred to as architrave method schematically shown       settlement of waste is not critically important, though
in Figure 2. Similarly, municipal wastewater treatment        the settlement of dredge soil may become more
sludge (color reddish brown in Photo 5) is placed in          important. Dredge soil is placed from the natural clay
designated areas. In some instances bulky wastes (see         seabed up to a height of 2 meters above sea level where
Photo 6) are encountered during landfilling operation         waste is consequently placed on top. Berms made of
and they are placed in specified storage areas within the     soil embankment are placed on the sides of the waste
landfill site. Such bulky wastes are crushed or               slopes to maintain stability. Table 2 shows the
pulverized first before final dumping.                        geotechnical properties of each material in the landfill
                                                              profile, i.e. seabed, dredge soil, embankment and waste,
                                                              which were used for the geotechnical investigation.
                          Incinerator ash                     Some units, e.g. kPa for cohesion and kN/m3 for unit
                                Or                            weight, were converted from the given units in the
                              Sludge
                                                              original document.
                              Dug hole
                                                               Table 2. Geotechnical properties of landfill components used
       Figure 2. Architrave method of waste landfilling.       in the analyses.

                                                                                                             Friction                  Unit
It was raining at the time of site visit and it was evident           Landfill           Cohesion,                           Cu/P1
                                                                                                            Angle, ϕ                 Weight
                                                                       Layer              c (kPa)
in Photo 6 that low infiltration and relatively good                                                        (degrees)                (kN/m3)
degree of compaction of waste caused water to pond
over the surface of the landfill.                                  Waste                    19.62               30o            -       9.81
                                                                   Dredge soil               9.81                0           0.35     15.70
Excavated wastes as shown in the ditch (Photo 1) seem              Seabed                    4.91                0           0.35     15.70
to be able to stand almost vertically stable. Wastes are           Embankment                 0                 30o            -      17.66
                                                               1
                                                                   Ratio of Undrained Shear Strength to Confining Pressure
placed at 3 to 5 meter thickness then covered with
50 cm of soil (see Photo 7), and then waste is placed         The four (4) different case scenarios of waste placement
again like in a sandwich method, as in Figure 3, until        which were investigated against stability include:
desired height of 20 to 30 meters is achieved (see            a) Waste that is filled in heights of 3 meters and 5
Photo 4). Final cover and vegetation are placed at the            meters, respectively;
completed phase, while keeping the side slopes at             b) Berm embankment placed at heights of 3.5 meters
workable angle of 4 horizontal to 1 vertical as shown in          and 5 meters, respectively;
Photo 3.                                                      c) Two 3.5-m berm embankments on the sides of the
                Until desired height of ~20-30m
                                                                  slopes placed adjacently on top of each other.
                      Soil cover: ~50 cm
                                                              d) Landfill side slopes kept at 3.5H:1V and 4.5H:1V,
              Waste spread evenly by crawler-type
                                                                  respectively.
                         dozer: ~3m

                      Soil cover: ~50 cm
                                                              The calculated factors of safety using method of slices
              Waste spread evenly by crawler-type
                                                              and assuming circular failure surface for all of the four
                         dozer: ~3m                           case scenarios are summarized in Table 3. The critical
                                                              factors of safety against failure are those cases issuing a
                Dredge soil on top of natural clay
                                                              value less than 1.0.
        Figure 3. Sandwich method of waste landfilling.
                                                              The factor of safety of landfill slopes is determined by
Geotechnical Aspect and Stability Analysis of Tokyo           assuming various positions and shapes of the failure
Bay Offshore Landfill                                         surfaces, then calculating the factor of safety using
                                                              method of slices. The smallest factor of safety for the
The following information is a summary and English            assumed failure surface is noted. The forces acting on
translation of the document “Geotechnical Investigation       the waste landfill include: (i) self- weight, (ii) dynamic
of Landfill Construction-Stability of Landfill Slopes”        forces, (iii) pore pressures (e.g. effect of rainfall)
                                                                      d) The acceptable side slope of the landfill is 4.5H:1V
  Table 3. Factors of safety from the stability analyses of the          resulting to a factor of safety of 1.141.
 four case scenarios.
              Case Scenarios                    Factor of Safety      In the study of the geotechnical aspect of landfills, an
                3m waste height on top                                understanding of the mechanical properties of landfill
                                                      1.377           components is essential. Landfill mechanical properties
                of dredge soil
   CASE 1
                5m waste height on top                                are a function of the type of waste, soil cover used,
                                                      1.052
                of dredge soil                                        degree of compaction (or number of dozer passes),
                3.5m embankment on top
                                                      1.101
                                                                      height of landfill, water content, and age of landfill.
                of waste                                              Since waste mechanical properties change through time,
   CASE 2
                5 m embankment on top                                 where waste composition depends on lifestyle and
                                                      0.865
                of waste
                                                                      many other factors, a study of these mechanical
   CASE 3       Two 3.5m berms                        0.911
                                                                      properties as a function of time is also important.
                Side slope 3.5H:1V                    0.959
   CASE 4                                                             Examples of weak materials in landfills that can induce
                Side slope 4.5H:1V                    1.141
                                                                      failure are ash and sludge. They have the tendency to
                                                                      absorb water and become weak soft materials. Aside
                                                                      from the mechanical properties of waste itself, daily soil
Generally, failure surface is taken to be circular, or                cover is also placed on top of the waste at
sometimes a combination of straight and circular                      approximately 50 cm in thickness, and thus mechanical
surfaces. The failure surface passing through the weak                properties of the soil cover have to be likewise
material is more likely. Calculation is done for various              determined. Investigation of the mechanical properties
radii of circular failure surface covering a wider area of            of soil cover is not yet conducted; hence, shear testing
the landfill slopes. The soil materials used for berm                 of the typical soil cover used at Tokyo Bay Offshore
embankment should have the same performance as that                   landfill is presented here. Since the materials used for
of the waste itself. In the stability analysis, it is                 the soil cover varies depending on the availability,
important to also consider that the failure passes                    characterization of the soil cover properties may
through the concrete retaining structure or the caisson.              therefore vary depending on the practice of the landfill
                                                                      operator. Nevertheless, a compilation of the said soil
Allowable factors of safety recommended by three                      cover properties may be necessary to incorporate the
Japanese agencies are summarized in Table 4. All                      effect of soil cover material in landfill stability analysis.
agencies suggest a circular surface of slope failure and
the use of method of slices for the analysis although the             Berm materials, on the other hand, are crucial in the
allowable factors of safety differ for each.                          analysis of stability of landfill side slopes. Thus, a table
                                                                      was prepared for characterizing the potential materials
 Table 4. Stability analysis of slopes.
                                                                      for berm embankment. Table 5 indicates the potential
                                      Method of
       Agency
                        Failure
                                    Slope Stability
                                                         Factor of    materials for berm embankment and the guidelines for
                        Surface                           Safety      its allowable height and allowable side slope.
                                       Analysis
  Japan Highway
  Public                                  Method of                    Table 5. Potential soil materials for berm embankment.
                        Circular                              1.25
  Corporation                              Slices                            Soil              Allowable height     Allowable side slope
  Design Manual                                                          Classification              (m)                   H:V
  Japan Road                              Method of
                        Circular                         1.2 to 1.3
  Association                              Slices                       SW, GM,                          5               1.5:1 to 1.8:1
  Ministry of                                                           GC, GP                        5 to 15            1.8:1 to 2.0:1
  Transport                               Method of
                        Circular                              1.3       SP                              10               1.5:1 to 1.8:1
  Bureau of Ports                          Slices
  and Harbors                                                           Excavated                       10               1.8:1 to 2.0:1
                                                                        soil                         10 to 20            1.8:1 to 2.0:1
From the geotechnical investigation and stability                       SM, SC
                                                                                                         5               1.5:1 to 1.8:1
analyses conducted which is indicated by the computed                                                 5 to 10            1.8:1 to 2.0:1
factors of safety for each scenario, the following                      VH2                              5               1.8:1 to 2.0:1
                                                                       Legend:
recommendations were made:                                                       SW – well-graded sand            SP – poorly graded sand
a) Placing the waste up to a height of 5 meters results                          GM – silty gravel                SM – silty sands
                                                                                 GC – clayey gravel               SC – clayey sands
     to a factor of safety of 1.052, which is considerably                       GP – poorly-graded gravel        VH2 – volcanic ash
     safe. However, care must be observed when using
     bulldozers to compact the waste fill especially
     along the sides.                                                 Estimation of Shear Strength Parameters for
b) Placing a 5-meter high berm embankment on top of                   Vertical Cut Slopes
     the waste is not possible as indicated by the
     calculated factor of safety equal to 0.856. The use              Referring to Photo 1 taken at the Tokyo Bay Offshore
     of 3.5-meter embankment height is recommended                    Landfill, the excavated ditches show that this type of
     instead (factor of safety = 1.101).                              waste can practically stand vertically at a height of
c) For the case where berm is placed on top of another                about 3 meters. A simplified analysis of stability of this
     waste-soil berm, soil improvement is recommended                 vertical cut slopes is conducted to derive an expression
     to speed up consolidation.                                       for the critical height using the force equilibrium
method and Coulomb’s equation for shear strength               Table 6. Back-calculated shear strength parameters for the
                                                               type of waste in Tokyo Bay Offshore Landfill.
(Eq. 1).
                                                                     Hcr              ϕm               cm         Remarks
                                                                                 (Assumed Most
                                                                  (Observed)                    (Back-calculated)  for cm
From the cross-section of one side of the ditch drawn in                             Likely)
                                                                                                                 30                       4.25
Figure 4, the unsupported vertical cut slope is assumed
                                                                                               3                 35                       3.83
to fail along the plane surface at an angle α with respect                                                                                                         Min = 3.43
                                                                                                                 40                       3.43
to the horizontal. The forces acting on the isolated
                                                                                                                 30                       5.66
failure wedge are (i) weight of the failure wedge, (ii)                                                                                                            Ave = 5.12
                                                                                               4                 35                       5.11
the normal force acting perpendicular to the failure
                                                                                                                 40                       4.57
plane surface, and (iii) the shear force mobilized along
                                                                                                                 30                       7.08                    Max = 7.08
the failure plane surface. Assuming that the failure
                                                                                               5                 35                       6.38
wedge to be rigid, and that the waste profile consist of
                                                                                                                 40                       5.72
homogenous material, the expression for critical height
is given in Eq. 2.
                                                               Using the minimum, average, and maximum back-
                            Weight of the
                                                               calculated mobilized cohesion parameters, the
                            Failure Wedge                      following critical heights were “forward-calculated”.

                                α                              Table 7. Calculated values for critical height of vertical cut
             Assumed
         Failure Plane                                         slopes from estimated cohesion parameters.
                                                                                   ϕm                Hcr
                                              H
                                                                                (Assumed         (Forward         Remarks for
            Waste Strata                                            cm
            i.e. assume homogeneous,                                          Most Likely)      calculation)           Hcr
            take γave
                  Failure Angle, α      toe                                                                   30                         2.42
                                                                               Min
                                                                                                              35                         2.69                     Min = 2.42
                                                                               3.43
         Figure 4. Cross-section of the vertical cut slope.                                                   40                         3.00
                                                                                                              30                         3.61
The derivation of the equation below is presented in                           Ave                                                                                Ave = 4.10
                                                                                                              35                         4.01
                                                                               5.12
Annex 3.                                                                                                      40                         4.47

                         4c m  ⎛      ϕ ⎞                                    Max
                                                                                                              30                         5.00                     Max = 6.19
              H cr =        tan⎜ 45o + m ⎟             Eq. 2                                                  35                         5.55
                          γ    ⎝       2 ⎠                                   7.08
                                                                                                              40                         6.19

where:                                                         Plots of the forward calculation using cm (min-ave-max)
cm = mobilized cohesion, kPa                                   calculated from 3-4-5-meter critical heights are shown
                                                               in the following figures. The first plot is set at y-
ϕm=   mobilized angle of internal friction, degrees            intercept of zero, where at zero friction angle, critical
γ = unit weight of the material, kN/m3                         height will also be zero. The second plot linearly
                                                               extrapolates the line passing thru the three plotted
The shear strength parameters, c and ϕ were back-              points of friction angle-critical height combination.
calculated from the known heights of vertical cut slopes
existing at the landfill site in Tokyo Bay. Rearranging                                      7.000
                                                                                                                                           R2 = 0.8848
Eq. 2, the possible range of values for mobilized                                            6.000
                                                                                                                                                                    c = 3.43
                                                                  critical height (m)




cohesion from assumed most probable values of friction                                       5.000
                                                                                                                                            R2 = 0.8848             c = 5.12
                                                                                             4.000                                                                  c = 7.08
angle are determined using Eq. 3.                                                            3.000
                                                                                                                                            R2 = 0.9977
                                                                                                                                                                    Linear (c = 3.43)

                                γH cr                                                        2.000                                                                  Linear (c = 5.12)

                cm =                                  Eq. 3                                  1.000
                                                                                                                                                                    Linear (c = 7.08)

                              ⎛      ϕ ⎞
                         4 tan⎜ 45o + m ⎟                                                    0.000
                                                                                                     0   10        20         30          40             50
                              ⎝       2 ⎠                                                                     friction angle (degrees)


                                                                 Figure 5. Plot of critical height as a function of friction angle
Assuming that the heights 3, 4 and 5 meters for vertical                                 (y-intercept = 0)
cut slopes of landfilled waste to be critical and taking
the three probable values of angle of internal friction
equal to 30, 35 and 40 degrees, respectively, a range of                                     7.000
                                                                                                                                               R2 = 0.8848
cohesion parameters were calculated as indicated in                                          6.000
                                                                                                                                                                    c = 3.43
                                                                       critical height (m)




                                                                                             5.000
Table 6. The probable values of angle of internal                                                                                              R2 = 0.9977
                                                                                                                                                                    c = 5.12
                                                                                             4.000                                                                  c = 7.08
friction correspond particularly to the type of waste at                                     3.000
                                                                                                                                               R2 = 0.9977          Linear (c = 3.43)
Tokyo Bay landfill site and it is generally composed of                                      2.000                                                                  Linear (c = 5.12)
                                                                                                                                                                    Linear (c = 7.08)
shredded plastics and soil (see Photo 2).                                                    1.000

                                                                                             0.000
                                                                                                     0   10         20         30          40                50

                                                                                                              friction angle (degrees)


                                                                 Figure 6. Plot of critical height as function of friction angle
                                                                                        (y-intercept ≠ 0)
The relation of critical height as a function of cohesion                                                             reconsolidated at 312.8 kPa and tested for third stage
and friction angle can otherwise be plotted with                                                                      shearing up to 1.5 mm.
cohesion parameter in the x-axis as shown in Figures 7
and 8.


                           7.00

                           6.00                                           y = 0.8744x
                                                                          y = 0.7833x             phi = 30
 c ritic al he ight (m )




                           5.00                                           y = 0.7062x             phi = 35
                           4.00                                                                   phi = 40
                           3.00                                                                   Linear (phi = 30)
                           2.00                                                                   Linear (phi = 40)
                           1.00                                                                   Linear (phi = 35)

                           0.00
                               0.00   1.00   2.00   3.00   4.00    5.00     6.00    7.00   8.00
                                                      cohesion (kPa)
                                                                                                                          Figure 9. Loosely poured soil in the shear box apparatus

                            Figure 7. Plot of critical height as a function of cohesion
                                                 (y-intercept = 0)



                           7.00
                           6.00                                   y = 0.8744x - 1E-14
                                                                  y = 0.7833x + 1E-14             phi = 30
  c ritical he ight (m )




                           5.00                                            y = 0.7062x            phi = 35
                           4.00                                                                   phi = 40
                           3.00                                                                   Linear (phi = 30)
                           2.00                                                                   Linear (phi = 40)         Figure 10. Shear test apparatus used for soil testing
                           1.00                                                                   Linear (phi = 35)

                           0.00
                               0.00   1.00   2.00   3.00   4.00    5.00     6.00    7.00   8.00
                                                      cohesion (kPa)
                                                                                                                      Measurements of soil displacement in mm, shear stress
                                                                                                                      in kPa, and normal stress in kPa were recorded using
                             Figure 8. Plot of critical height as function of cohesion                                the data logger shown in Figure 11.
                                                 (y-intercept ≠ 0)

Investigation of shear strength parameters of landfill
soil cover

As discussed previously, wastes at Tokyo Bay Offshore
landfill is placed using sandwich method in which 3-m
thick wastes are covered with 50 cm soil. A study was
conducted to estimate the shear strength parameters of
the typical soil cover used at Tokyo Bay Offshore
landfill using direct shear test apparatus at Ohta
laboratory. A multiple-stage constant volume shear test
was carried out to the soil for three moisture conditions:                                                                       Figure 11. Data logger used for shear test
a) dry state with moisture content of about 13%, (b)
medium state with moisture content of about 18% and
(c) wet state with moisture content of about 23% with                                                                 The test procedure was repeated for the three soil
two test replicates for each moisture condition.                                                                      moisture contents for 2 test trials.

Procedure. The soil sample was first sieved at 2mm to                                                                 Results. Shear stress appears to increase with
remove presence of large particles such as gravel or                                                                  increasing normal stress where peak stress can not be
stone. The as-delivered state of the soil sample from its                                                             clearly observed. As the shear test apparatus can only
source was considered to be the medium moisture                                                                       allow small shear displacement, the shear test has to be
content condition of the soil. Approximately 60 grams                                                                 stopped when no sudden changes in stresses were
of soil was poured loosely to the shear box as shown in                                                               observed, which is at about 1.5 mm displacement. Plots
Figure 9. The diameter of the soil is 6cm with a                                                                      of the shear stress and normal stress for the three
thickness of 2cm, thus with a shear surface area of                                                                   moisture conditions tested are shown in Figures 12-14,
approximately 28 cm2.                                                                                                 respectively and in Figure 15 for all conditions. In all
                                                                                                                      cases, soil cover material appears to be cohesionless. It
Initally, the soil is consolidated slowly up to a normal                                                              can also be clearly seen from Figure 15 that soil cover
stress of 74.5 kPa and then tested for first stage of                                                                 at dry condition exhibited higher shear stress which
shearing as shown in Figure 10 up to 1.5 mm                                                                           suggest higher friction between soil particles. For this
displacement. The specimen was reconsolidated at                                                                      case, the value of friction angle is highest among the
153.9 kPa and again tested for second stage shearing up                                                               three conditions tested. However, the shear behavior of
to 1.5 mm displacement. Lastly, the specimen, was                                                                     soil with significant amount of moisture did not vary
                                                                                                                      too much as indicated by almost overlapping curves for
both medium and wet moisture conditions. Presence of                                                                                                              250




significant amount of water reduces friction between
soil particles that were sheared. The replicate for the                                                                                                           200




three moisture conditions yield nearly same results. The
values of angle of internal friction may range from 27.5




                                                                                                                                              Shear Stress, kPa
                                                                                                                                                                  150
                                                                                                                                                                                                                                                    Dry Trial 1
                                                                                                                                                                                               tan ϕ = 0.6                                          Dry Trial 2
                                                                                                                                                                                               ϕ = 31 deg
                                                                                                                                                                                                                                                    Med Trial 1
degrees to 31 degrees for moisture content of 13% to                                                                                                                                                               tan ϕ = 0.52
                                                                                                                                                                                                                   ϕ = 27.5 deg                     Med Trial 2
                                                                                                                                                                                                                                                    Wet Trial 1

23% with the lowest moisture content experiencing                                                                                                                 100                                                                               Wet Trial 2



highest friction. These roughly estimated values of
friction angle for soil cover in Figure 15 is interestingly,                                                                                                      50




on the average, equal to the friction angle used for
wastes in the geotechnical stability analysis of Tokyo                                                                                                             0
                                                                                                                                                                        0   50   100   150   200             250         300      350   400   450
                                                                                                                                                                                             Normal Stress, kPa
Bay offshore landfill indicated in Table 2.
                                                                                                                                             Figure 15. Shear behavior of soil cover for all moisture conditions
                     250




                     200                                                                                                                     Summary and Conclusion

                                                                                                                                             The open ditches of landfilled waste mainly composing
 Shear Stress, KPa




                     150


                                                                                                                               Dry Trial 1
                                                                                                                               Dry Trial 2
                                                                                                                                             of plastic materials at Tokyo Bay Offshore landfill
                     100                                                                                                                     illustrates that waste can practically stand at vertical
                                                                                                                                             heights of up to about 3 meters. From this scenario, the
                     50                                                                                                                      minimum possible values of cohesion ranges from 3.4
                                                                                                                                             to 7 kPa with an average of 5 kPa for friction angles 30,
                       0                                                                                                                     35 and 40 degrees respectively. Therefore, the critical
                           0       50    100     150         200          250           300         350     400    450   500
                                                                   Normal Stress, kPa                                                        heights of vertically piled wastes are from 2 to 6 meters
                               Figure 12. Shear behavior of soil cover at dry condition                                                      from the simplified analysis of vertical cut slopes. In
                                                   (MC = 13%)                                                                                reality, wastes are piled for a height of 20 to 30 meters,
                                                                                                                                             with some degree of side slope, or even up to 60 meters
                     250
                                                                                                                                             for the case of Leuwigajah landfill, which however
                                                                                                                                             failed against sliding. Soil cover, on the other hand,
                     200
                                                                                                                                             showed a range of friction angle from 27.5 to 31
                                                                                                                                             degrees with zero cohesion, which is similar to sand.
                                                                                                                                             The estimated friction angle for soil cover is
 Shear Stress, kPa




                     150


                                                                                                                               Med Trial 1
                                                                                                                               Med Trial 2
                                                                                                                                             interestingly, the same as that of the friction angle used
                     100
                                                                                                                                             in the geotechnical stability analysis for Tokyo Bay
                                                                                                                                             offshore landfill, which is on the average equal to 30o.
                     50
                                                                                                                                             Therefore, it is safe to say that shear behavior of waste
                                                                                                                                             alone may be the same as that of waste plus soil cover
                       0
                           0        50     100         150         200          250           300         350     400    450                 in sandwich method of landfill operation.
                                                                   Normal Stress, kPa



                     Figure 13. Shear behavior of soil cover at medium condition
                                            (MC = 18%)
                                                                                                                                             Recommendations
                                                                                                                                             The study presented in this report shows only a
                     250
                                                                                                                                             simplified stability analysis of vertical cut slopes of
                                                                                                                                             landfilled wastes and thus gives conservative estimates
                     200
                                                                                                                                             for shear strength parameters of landfilled wastes. More
                                                                                                                                             detailed study can be conducted using effective stress
                                                                                                                                             analysis and other methods of stability analysis when
 Shear Stress, kPa




                     150


                                                                                                                               Wet Trial 1   more data are available, such as pore water pressure
                                                                                                                               Wet Trial 2


                     100
                                                                                                                                             measurements, as well as pore pressure from landfill
                                                                                                                                             gases. A detailed study of stability of open dumpsites in
                     50
                                                                                                                                             the Philippines is therefore necessary to evaluate the
                                                                                                                                             current stability conditions of such waste disposal
                       0
                                                                                                                                             facilities, which is home and at the same time threat to
                           0       50    100     150         200          250
                                                                   Normal Stress, kPa
                                                                                        300         350     400    450   500
                                                                                                                                             scavengers in the area.
                               Figure 14. Shear behavior of soil cover at wet condition
                                                   (MC = 23%)




                                                                                                                                             References
Blight, G.E. (2006), “A survey of lethal failures in       The author is indebted to the following persons who
municipal solid waste dumps and landfills”, 5th            made this research possible:
International Congress on Environmental Geotechnics,       • Prof. Hideki Ohta for his supportive and inspiring
Thomas Telford, London, 13-42.                                 nature which motivated the author to enhance her
                                                               knowledge in soil mechanics and geotechnical
Dixon, N. and Jones, D.R.V. (2005), “Engineering               engineering. His comments and suggestions were
properties of municipal solid waste”, Geotextiles and          major parts of this report.
Geomembranes, 23 (3), 205-233.                             • Prof. Thirapong Pipatpongsa for his assistance in
                                                               making the laboratory facilities available for the
Koelsch, F., Fricke, K., Mahler, C., Damanhuri, E.             conduct of this research.
(2005), “Stability of landfills – the Bandung dumpsite     • Prof. Jiro Takemura and Prof. Shigeyoshi Imaizumi
disaster, CISA (Hrsg.): Proceedings of the 10th Int.           for accommodating the author in the landfill site
Landfill Symposium, Cagliari (Italy).                          visit at Utsunomiya.
                                                           • The students of Ohta laboratory, Suga, Nipon,
Koelsch, F. and Ziehmann. (2004), “Landfill stability –        Watanabe, Kusaka, who assisted me in the Tokyo
risks and challenges”, Waste Management World, Issue:          Bay Offshore Landfill site visit and in the conduct
May-June, ISWA, Copenhagen.                                    of the direct shear testing of typical landfill soil
                                                               cover. Suga translated the documents written in
                                                               Japanese to English.
Merry, S.M., Kavazanjian E., Jr., and Fritz, W.U.
(2005), “Reconnaissance of the July 10, 2000 Payatas
                                                           The administrative assistance made by Ms. Akiko
landfill failure”, Journal of Performance of Constructed
                                                           Nozawa and Ms. Machiko Ishii during the JSPS 90-day
Facilities, 19(2), 100-107.
                                                           visit at Tokyo Institute of Technology is very well
                                                           appreciated.




Acknowledgement
Annex 1. Photographs during Tokyo Bay Offshore
Landfill Site Visit



                                                                                                     Completed height
                                                                                                  ranges from 20-30 m




            Almost Vertical Cut



                    height ≈ 3 m


                                                                 Photo 4. Another view of the completed side slope prepared
                                                                            for final cover and future vegetation.




  Photo 1. Prepared ditch at the landfill site for dumping of
                      incinerator ash.


                                                                        Municipal wastewater
                                                                        treatment sludge




          Significant Amount
                    of Plastics



                                                                   Photo 5. Disposal of municipal wastewater sludge in the
                                                                                          landfill.




      Photo 2. Waste profile mostly composed of plastics.




                                                                        Bulky wastes




                  Completed height             1
                  ranges from 20-30 m      4
                                                                Photo 6. Storage area for bulky wastes within the landfill site.




                                                                                         Soil cover




   Photo 3. Completed 4H:1V side slope at the Tokyo Harbor
                        landfill site.



                                                                                           Crawler-type dozer




                                                                Photo 7. Actual placing of soil cover over the landfilled waste.
Annex 2. Summary of previous studies conducted to analyze stability of landfill failed slopes and estimate shear strength parameters.

                                                                           Effective Stress Shear
                                                                                                       Landfill Geometry        Slide Geometry
                                                                            Strength Parameter
                     Waste                                        Waste                                                                             Shear Strength
                                                  Volume of
                    Inflow      Proposed                           Unit                                                                               Parameter
    Landfill Site                                 waste slide
                    per day   Failure Mode                       Weight                                                                             Determination
                                                    (m3)
                     (tons)                                      (kN/m3)                Friction                                                      Procedure
                                                                           Cohesion                 Area    Side     Height
                                                                                         Angle                                 Width    Length
                                                                            (kPa)                   (ha)    slope     (m)
                                                                                         (deg)




                                                                                                                                                      Based from
                              Circular failure2
      Payatas                                     10 to 12000                                                                                      GeoSyntec (1998) -
                                                                                                                                          40
     (Manila,       1500 to                       (278 deaths,                                              1.5H                                    comparing well
                                     Or                           10.2        19           28                         ∼30        -      (Blight,
    Philippines,     1800                           80-350                                                  :1V                                     with Dona Juana
                                                                                                                                         2006)
    July 2000)1                                    missing)3                                                                                        Landfill Failure,
                               Translational
                                                                                                                                                   Bogota Columbia,




    Leuwigajah
    (Bandung,                                                                                                                                      Based from German
                               Translational         2.7 M                                                                     200 to
    Indonesia,       4500                                          11         10           20       > 6.5     -     60 to 70              900         Geotechnical
                                  failure         (147 deaths)                                                                  250
     February                                                                                                                                           Society
      2005)2



1
  Merry, S.M., Kavazanjian, E., and Fritz, W.F., 2005
2
  Koelsch, F., Fricke, K., Mahler C. and Damanhuri, E., 2005
3
  Blight, G., 2006
                              COMMENT TO JSPS CORE UNIVERSITY PROGRAM

The author is grateful for the experience acquired and lessons learned from conducting research at Tokyo Institute of
Technology under her host professor HIDEKI OHTA. Research activities during the 90-day visit consisted of the
following:

    1.   Meeting and introduction to the members of the Soil Mechanics and Geotechnical Engineering Laboratory: Prof.
         Osamu Kusakabe, Prof. Hideki Ohta, Prof. Pipatpongsa Thirapong, Prof. Takashi Nakamura, Prof. Jiro
         Takemura, and all staff and students.
    2.   Attendance to course lectures by Prof. Jiro Takemura and Prof. Taro Urase (Civil and Environmental
         Engineering)
    3.   Site visit to Tokyo Bay Offshore Landfill facility, Utsunomiya Landfill facility and Incineration Plant,
         Fukushima Nuclear Power Plant.
    4.   Participation to monthly held “Terakoya” or study tour and information exchange at Ohta Laboratory.
    5.   Review of literature in geotechnical aspect of municipal solid waste landfills.
    6.   Conduct of direct shear tests for typical landfill soil cover at Tokyo Bay Offshore Landfill.
    7.   Experiencing the research culture and lifestyle at Tokyo Institute of Technology.
    8.   Research report writing and presentation.

The experience and information gathered during the research activities listed above helped the author enhance her
background in geo-environmental engineering. The 3-month research program became a venue to experience the research
culture in Japan Universities like Tokyo Institute of Technology where facilities for testing and research are always
available. An experimental research using the direct shear apparatus was also made possible by Prof. Hideki Ohta using
typical soil cover material used at Tokyo Bay offshore landfill. This gave the author hands-on experience in generating
results using actual shear testing of the soil samples. Information exchange with students of the laboratory was also
possible since most students at the Department of International Development Engineering can speak English. All in all,
the 90-day visit at Tokyo Institute of Technology as a JSPS researcher is indeed a worthwhile experience.

								
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