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: firstname.lastname@example.org e-mail: email@example.com 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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