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Shear strength characteristics of mechanically biologically treated municipal solid waste (MBT-MSW) from Bangalore
Waste Management 39 (2015) 63–70
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Shear strength characteristics of mechanically biologically treated municipal solid waste (MBT-MSW) from Bangalore G.L. Sivakumar Babu a,⇑, P. Lakshmikanthan b,1, L.G. Santhosh b,2 a b
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India Centre for Sustainable Technologies (CST), Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 2 June 2014 Accepted 6 February 2015 Available online 4 March 2015 Keywords: Municipal solid waste Shear strength Elastic modulus Strength ratio Stiffness ratio
a b s t r a c t Strength and stiffness properties of municipal solid waste (MSW) are important in landfill design. This paper presents the results of comprehensive testing of shear strength properties of mechanically biologically treated municipal solid waste (MBT-MSW) in laboratory. Changes in shear strength of MSW as a function of unit weight and particle size were investigated by performing laboratory studies on the MSW collected from Mavallipura landfill site in Bangalore. Direct shear tests, small scale and large scale consolidated undrained and drained triaxial tests were conducted on reconstituted compost reject MSW samples. The triaxial test results showed that the MSW samples exhibited a strain-hardening behaviour and the strength of MSW increased with increase in unit weight. Consolidated drained tests showed that the mobilized shear strength of the MSW increased by 40% for a unit weight increase from 7.3 kN/m3 to 10.3 kN/m3 at 20% strain levels. The mobilized cohesion and friction angle ranged from 5 to 9 kPa and 8° to 33° corresponding to a strain level of 20%. The consolidated undrained tests exhibited reduced friction angle values compared to the consolidated drained tests. The friction angle increased with increase in the unit weight from 8° to 55° in the consolidated undrained tests. Minor variations were found in the cohesion values. Relationships for strength and stiffness of MSW in terms of strength and stiffness ratios are developed and discussed. The stiffness ratio and the strength ratio of MSW were found to be 10 and 0.43. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The characterization of shear strength of mechanically biologically treated municipal solid waste (MBT-MSW) is important for the design of slopes, vertical expansion of landfills, seismic stability evaluations and predicting failure of landfills. MBT-MSW is referred as MSW in this paper. MSW is a heterogeneous material which makes it difficult to evaluate its properties. The factors that are likely to have an effect on the shear strength of waste are; unit weight, size of waste particle, decomposition/age, normal stress and moisture content, etc. A number of researchers (Landva and Clark, 1990; Singh and Murphy, 1990; Jessberger and Kockel, ⇑ Corresponding author. Tel.: +91 80 22933124 (O), +91 80 23600671 (R), +91 9448480671 (mobile). E-mail addresses: [email protected], [email protected] (G.L. Sivakumar Babu), [email protected], [email protected] (P. Lakshmikanthan), [email protected], [email protected] (L.G. Santhosh). URL: http://civil.iisc.ernet.in/~gls/ (G.L. Sivakumar Babu). 1 Mobile: +91 9036406364. 2 Mobile: +91 9611187003. http://dx.doi.org/10.1016/j.wasman.2015.02.013 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
1993; Kavazanjian et al., 1995; Gabr and Valero, 1995; Grisolia and Napoleoni, 1996; Manassero et al., 1996; Jones et al., 1997; Machado et al., 2002; Stark et al., 2009; Bray et al., 2009) conducted experiments on MSW and also performed back-analysis of field case histories over the last two decades. The selection of appropriate shear strength parameters remains a challenging engineering design issue for a site-specific landfill. Variability in the shear strength parameters is due to the variability of MSW compositions, the strain level at failure, the choice of representative samples and testing methods. Satisfactory design of an engineered municipal landfill facility requires consideration of rational values of shear strength properties of MSW. Though considerable research has been conducted till date on the estimation of strength values for MSW in different countries, only a small amount of data is available on the strength properties of MSW that is representative of Indian conditions. Therefore an effort has been made in this study to develop an extensive database of shear strength properties of MSW. Though issues related to MSW landfills are global, contribution of this nature is important as the nature of waste and the magnitude of problem of MSW in developed countries and developing countries such as India are considerably different. Problems in
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landfill engineering require the assessment of engineering properties such as shear strength of MSW that present considerable difficulties in design and analysis.
2. Literature Different methods have been adopted by researches to evaluate the shear strength properties of MSW. Both direct shear tests and triaxial tests are reported in literature. Direct shear tests have been the preferred method for measuring the shear strength of MSW likely because of its simplicity. Landva and Clark (1986) performed a series of large direct shear tests on waste specimens from different Canadian landfills. The measured cohesion and friction angle ranged between 10 and 23 kPa and 24° and 42°. Stark et al. (2009) reviewed considerable experimental data on direct shear tests from literature and concluded that the shear strength of MSW increases with increasing strain or displacement. This leads to high strength values that are in good agreement with field observations of vertical scarps from landfill slope failures remaining near vertical for significant periods of time. They presented strength envelopes as function of displacement and strain level. They also suggested different strength parameters for confining pressures less than and greater than 200 kPa. Zekkos et al. (2010) investigated the effects of waste composition, confining stress, unit weight and loading rate on the stress– displacement response and shear strength of municipal solid waste (MSW) collected from a landfill located in the San Francisco Bay area. Based on 109 large-scale direct shear tests, they observed that the shear strength of MSW at low moisture contents is best characterized by cohesion of 15 kPa and friction angle of 36° at a normal stress of 1 atmosphere and a decrease in the friction angle of 5° for every log-cycle increase in normal stress. In general, there is great variability in the reported shear strengths in the literature. Cohesion values from 0 to 80 kPa and friction angle values from 0° to 60° have been reported in the published literature. Triaxial tests have also been used for evaluation of shear strength of municipal solid waste (MSW) both in undrained and drained conditions. Failure of landfills occurred during severe rains and a few case studies exist in literature. Drained strength of MSW refers to the long term condition and implies equilibrium in terms of full dissipation of pore water pressure in the landfill body and the total stresses are considered as effective stresses. Stress–strain response of MSW in undrained loading condition gives information on undrained shear strength parameters, stresses and strains during loading and at failure state in undrained condition and is essentially required for the analysis of slope stability. Analysis of stability of bioreactor landfills need understanding of undrained strength of MSW. Landfill failures such as Rumpke in the USA (Stark et al., 2000; Zekkos, 2005), Dona Juana in Columbia (Caicedo et al., 2002), Payatas in Philippines (Merry et al., 2005), are attributed to undrained conditions in the landfill material. In the literature, Gabr and Valero (1995), Caicedo et al., (2002a), Vilar and Carvalho (2004), Reddy et al. (2009a–c) reported triaxial compression tests under undrained loading. Gabr and Valero (1995) performed small-scale Consolidated Undrained Triaxial tests (CU-TX) and small-scale Direct Shear tests on 15– 30 year-old waste. The dry unit weight of the CU-TX specimens was 7.4–8.2 kN/m3 and had a specimen diameter of about 71 mm. The authors observed that the cohesion intercept decreased with increasing water content from 100 kPa to 40 kPa at water contents of 55% and 72% respectively. The cohesion increased with increase in the effective confining pressure. Strength parameters were evaluated at 20% axial strain. Cohesion was estimated to be equal to 17 kPa and the friction angle 34°.
The shear strength behaviour of municipal solid waste (MSW) in drained condition provides understanding of long term performance of slope stability and operations of landfill systems. For the long term performance of landfill system, the design parameters required are evaluated from drained condition and the change in volumetric strain is measured. The transient changes caused because of these processes affect porosity, shear strength characteristics, cause deformation or settlement. Machado et al. (2002) conducted drained tests and reported the results. Grisolia and Napoleoni (1996) performed consolidated drained Triaxial tests (CD-TX) under different cell pressures. The stress–strain plots generally suggest a strain hardening behaviour for strains in excess of 40% without reaching a peak stress. It is necessary to understand the importance of undrained and drained shear strength of MSW in landfill engineering practice. This can be explained with reference to the schematic stress paths of MSW indicated in Fig. 1. The stress paths are presented in ðp0 qÞ plot, where q is deviator stress and p0 ¼ r01 þ 2r03 =3 is the mean effective stress and more details are available in geotechnical literature (Budhu, 2008). Referring to Fig. 1, it can be noted that the undrained shear strength ðqf 1 Þ mobilized during landfilling and corresponding mean effective stress p0f 1 on critical sate line or failure envelope at point A represent the failure state. Therefore, for the safe filling in undrained condition, it is essential that undrained shear strength should not exceed the limiting value of shear strength of MSW ðqf 1 Þ corresponding to the mean effective stress applied. In the undrained condition, the filling of the landfill waste should be such that the addition of normal stress reflected in p0 should not develop high pore water pressures and that additional shear stress should not exceed maximum likely shear strength ðqf 1 Þ. Similarly, in drained condition the mobilized shear strength ðqf 2 Þ follows 1:3 slope reaching critical state line at point B and indicates that for the safe filling height for long term operations, the drained shear stress should not exceed qf 2 at mean effective stress of p0f 2 . Strength and stiffness properties are related and are required in the assessment of stability and deformation characteristics of landfills. Stiffness properties or elastic modulus are arrived at from shear modulus and elastic modulus from direct shear tests and triaxial tests as well as field tests in landfill engineering practice. Dickson and Jones (2005) used pressure meter tests and measured shear modulus values which vary from 5 MPa to 30 MPa. In geotechnical engineering literature, to estimate the engineering properties such as strength and stiffness, one engineering approach is to obtain a ratio of ðs=rÞ and ðE=sÞ where ‘S’ is the shear
Fig. 1. Stress paths for undrained and drained condition.
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strength, ‘r’ is the effective vertical consolidated stress and ‘E’ is elastic modulus) and a few studies are available (Ladd and Foot, 1974; Ladd et al., 1977; Jamiolkowski et al., 1985). Previous studies indicated that the undrained shear strength ratio ðs=rÞ of different soils varying from 0.25 to 3.19 and stiffness ratio ðE=sÞ varies from 40 to more than 30,000 depending upon type of soil and test method adopted. However, such correlations are not available for making reliable estimates of the shear strength and stiffness response of the MSW for undrained and drained conditions. The paper proposes relations among these parameters for MSW. This paper describes the strength characterisation of a particular mechanically biologically treated (MBT) waste from Bangalore, India, with respect to particle size and unit weight. Direct shear tests, triaxial consolidated undrained and consolidated drained tests were conducted in the laboratory on representative samples of MSW retrieved from a landfill. Most of the mechanical testing of wastes has been carried out using geotechnical testing methods.
3. Site description and composition analysis Municipality Solid Waste Rules in India specify that biodegradable wastes should be processed by composting, vermi composting etc. and landfilling shall be restricted to non-biodegradable inert waste and compost rejects. Several pre-treatment methods have been developed in the recent times in order to recover the materials and to minimise the organic content reaching the landfills. Composting has been adopted as a potential pre-treatment method in Mavallipura landfill located in the outskirts of Bangalore. The waste used in this study is the mechanically biologically treated compost reject collected from the Mavallipura landfill site. The MSW entering the landfill undergoes a number of processes before being landfilled. Hand sorting of recoverable waste is followed by aerobic windrow composting for a period of 2 months. Screening of the compost reject is done using large screens of size 35 mm and 16 mm to further segregate the recyclable materials. Bulk of the compost reject was found to pass through 35 mm screens which indicated that the particle size of the MSW that is landfilled is <35 mm. The compost reject contained clothes-6.34%, Plastics-28%, glass-1.28%, leather-0.8%, coconut-5.56%, stones-1.96%, rubber-0.88%, wood-0.16% and organic materials-54.2%. The compost reject had particle sizes varying from 35 mm to 4 mm. The particles of size >10 mm mostly contained large plastics, rubber shoes, leather bags and other inert materials which were hand sorted or removed by other mechanical procedures. Since it was difficult to separate particles <10 mm, these were landfilled directly. The MBT process reduces the particle size. Therefore samples of particle size 4 mm and 10 mm were considered in this study.
4. Determination of moisture content and organic content Moisture content of the waste was calculated as the ratio of the weight loss to the weight that remained after heating at a temperature of 60 °C until the specimen has dried to a constant mass. The test for total volatile solids (TVS) was performed according to the APHA-1965 (American Public Health Association) standard methods. Organic content of the compost reject <10 mm particle size was calculated as the ratio of the weight loss to the initial specimen weight after heating to a temperature of 550 °C in a muffle furnace. The initial decomposable organic content of the waste was found to be 54–55% and the inerts constituted to 45%. The specific gravity of sample was measured as 1.26 by density bottle method and pcynometer method.
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5. Laboratory tests and methods Direct shear tests, small scale and large scale consolidated undrained and drained tests were conducted on reconstituted MSW samples collected from the landfill site. A total of 27 tests (3 direct shear tests, 12 consolidated undrained tests and 12 consolidated drained tests) were conducted in the laboratory to study the variation in strength properties of compost reject with particle size and unit weight. One test was performed for each condition and 2 CD tests were repeated for 100 kPa confining pressure as deviator stress value for 50 and 100 kPa were found to be almost similar. These tests were conducted for unit weights of 7.4 kN/m3 and 10.3 kN/m3 and particle sizes of 4 mm and 10 mm. Direct shear tests were performed on samples of size 60 mm in length, 60 mm in width and 30 mm in height, unit weight 10.3 kN/m3 and moisture content of 44% for confining pressures 50, 100 and 150 kPa. All the tests were performed at a strain rate of 0.25 mm/min. Tests were performed in conformity with ASTM D 3080-94 test method for direct shear tests. The MSW was filled in the direct shear mould carefully in 4–5 layers and compacting using a wooden rammer with 4–6 blows per layer. A triaxial pneumatic machine with load cell capacity of 500 kg and loading frame capacity of 5 Mg was used for consolidated drained and undrained tests. Small scale (50 mm diameter and 100 mm height, 100 mm diameter and 200 mm height) triaxial tests were conducted on MSW samples (44% moisture content) for unit weights of 7.4 kN/m3 and 10.3 kN/m3 and particle sizes of 4 mm and 10 mm. Normally the MSW particle size varies from 10 mm to 100 mm, and therefore it is difficult to maintain and compare MSW particle size. Since most of >10 mm particles like plastics and rubber are hand sorted at the landfill site, MSW particle size <10 mm were used in the study. The 50 mm diameter sample size was used in case of 4 mm MSW particle and 100 mm diameter sample size was used in case of 10 mm MSW particle size. MSW samples of 50 mm diameter were prepared by static compaction and dynamic compaction was adopted for the 100 mm samples. In case of Static compaction pre-determined amount of MSW was fed into a metal mould and the mix was statically compacted from both ends using screw jack arrangement. The cylindrical specimen was then extruded using a screw jack. In case of dynamic compaction, MSW was compacted in a split mould in 6–7 layers with 5–6 blows for each layer. The specimens in each group were saturated and consolidated to effective confining pressures of 50, 100 and 150 kPa. The consolidated drained and consolidated undrained tests were performed in accordance with the ASTM D3080 and ASTM D4767. 6. Results and discussion Shear strength parameters derived with the Mohr–Coulomb failure criterion are commonly used to quantify MSW shear strength. The MSW exhibited a strain-hardening behaviour during the direct shear tests and triaxial compression tests. Therefore, strength was defined in terms of the mobilized shear strength corresponding to a selected strain level. The shear strength mobilized at four strain levels (i.e., 5%, 10%, 15% and 20%) was investigated in each type of tests. 6.1. Direct shear tests Tests were conducted under normal loads of 50 kPa, 100 kPa and 150 kPa. Figs. 2a and 2b show the stress–strain relationships and normal stress–shear stress relationships from direct shear tests. The tests were terminated at the maximum shear displacement of about 20 mm. The shear strength of MSW increased with
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Fig. 2a. Stress–strain relationship from direct shear tests of MBT-MSW.
Fig. 2b. Shear stress–normal stress relationship from direct shear tests of MBTMSW.
increasing strain or displacement. The increase in shear stress is less under a normal stress of 100 kPa when compared to the normal stress above 100 kPa. The cohesion and friction angle increased with the strain levels and the measured cohesion (c) and friction angle (U) values at 20% strain levels were 12 kPa and 40°. The shape of the stress strain curve suggests that the long axes of the fibres are oriented perpendicular to the horizontal shear surface (Bray et al., 2009). The MSW exhibits initially a weak response followed by an upward curvature of the stress–displacement curve. The measured shear stress for a normal stress of 100 kPa was found to be 45 kPa at 20% strain. In Fig. 2b, for 5% strain and 10% strain the R2 value is less than 0.9 which indicate a scatter in the shear stress values. At the initial stages of shearing, the effect of normal stress is not pronounced, whereas at strain levels (>10%) the shearing resistance also increases. The scattering may be due to the non-uniform size, shape and orientation of the MSW particles at lower strain levels. 6.2. Consolidated undrained tests A total 12 small scale (50 mm 100 mm) and large scale (100 mm 200 mm) consolidated undrained triaxial compression
Fig. 3a. Stress–strain relationship from consolidated undrained triaxial tests of MBT-MSW for particle size 10 mm.
Fig. 3b. Stress–strain relationship from consolidated undrained triaxial tests of MBT-MSW for particle size 4 mm.
tests were carried out to study the shear strength characteristics of the MBT-MSW samples. Figs. 3a and 3b show the stress–strain behaviour of 10 mm and 4 mm particle size respectively. All the tests were conducted till 20% strain levels. The stress–strain curves exhibited a typical strain-hardening behaviour which increased continuously with axial strain without reaching an asymptotic value.
6.2.1. Effect of unit weight The stress–strain curves for unit weights 7.3 kN/m3 and 10.3 kN/m3 did not show any peak even at 20% strain levels. As shown in Figs. 3a and 3b, the deviator stress increased with the increase in the unit weight. This is in agreement with the results of Zekkos (2005) which report that specimens with lower initial unit weight have a softer initial response and lower mobilized shear strengths at a specified strain level. For a given strain level, it was found that the mobilized friction angle increased with an increase in the unit weight and the cohesion decreased marginally with increase in unit weight. The maximum mobilized cohesion and mobilized angle of internal friction were 6 kPa and 55° corresponding to a strain level of 20% were exhibited by the samples of
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greater unit weight. The mobilized strength of the waste increased by 30–35% for an increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3 at 20% strain levels. The implication from this observation is that if the unit weight of MSW in the landfill is higher, the strength response is better and good compaction procedures leading to improved unit weights in the placement of MSW need to be followed. 6.2.2. Effect of particle size The shear strength of 4 mm particle size samples with increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3 (Fig. 4). In case of 10 mm particle size samples, the shear strength was found to decrease marginally with the increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3. The maximum and minimum shear strength (320 kPa and 30 kPa) values were observed in the 4 mm particle size samples. The shear strength increased with increase in strain levels. The waste composition, high organic content, water content and the fibrous content present in MSW could be responsible for the lower shear strength values obtained in this study.
Fig. 5a. Stress–strain relationship from consolidated drained triaxial tests of MBTMSW for particle size 10 mm.
6.3. Consolidated drained tests Typical deviator stress versus axial strain relationships for MBT-MSW obtained from consolidated drained tests for samples of particle size 10 mm and 4 mm are shown in Fig. 4a and b. It can be noted that the deviator stress consistently increased with the axial strain without reaching a well defined peak. 6.3.1. Effect of unit weight The rate of increase in the stress is comparatively higher in dense samples than in the lesser denser samples (Figs. 5a and 5b). The increase in deviator stress is pronounced clearly at confining pressures greater than 100 kPa for both 4 mm and 10 mm particle size samples and unit weight of 7.3 kN/m3. However for 50 kPa and 100 kPa, there was a slight increase in the deviator stress values. Even the repeated tests showed similar trends. The heterogeneity of MSW, compressible nature of MSW, high organic content and water content may be the probable reasons for this typical behaviour. Certain inconsistency was observed when the friction angle increased sharply (from 8° to 33°) from 10% to 20% strain levels in the 4 mm particle size sample of unit weight 10.3 kN/m3. However the friction angle increased with the increase in unit weight and the cohesion varied between 5 kPa and 9 kPa. The mobilized strength of the waste increased by 40% for unit weight increase from 7.3 kN/m3 to 10.3 kN/m3 for strain levels of
Fig. 4. Variation of friction angle with strain for consolidated undrained triaxial tests of MBT-MSW.
Fig. 5b. Stress–strain relationship from consolidated drained triaxial tests of MBTMSW for particle size 4 mm.
20%. The mobilized cohesion and friction angle ranged from 5 to 9 kPa and 8° to 33° corresponding to a strain level of 20%. 6.3.2. Effect of particle size The 10 mm particle size samples showed a convex stress–strain curve as compared to a concave type of stress–strain curve exhibited by the 4 mm particle size samples. The rate of increase in stress is similar to that in the consolidated undrained tests for 4 mm (Fig. 6). The 4 mm particle size samples exhibited 70% greater shear strength compared to 10 mm particle size samples for a given unit weight of 10.3 kN/m3 and at strain levels 20%. Similar trends were observed in the case of unit weight of 10.3 kN/m3. The increase in the percent shear strength of 4 mm particle size samples over 10 mm particle size samples also increased with the increase in the strain levels from 5% to 20%. Particle size and its impact on strength have been studied by many researchers including Jessberger (1995), Foose et al. (1996) and Casagrande et al. (2006). Jessberger (1995) conducted large (540 mm diameter) uniaxial compression tests on two different waste types as received MSW and the MSW fraction passing a 120 mm sieve (85% MSW is smaller than 120 mm). The compression testing results showed that the sample with <120 mm material failed with a peak at 20% strain. In the present study both 4 mm
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Fig. 6. Variation of friction angle with strain for consolidated drained triaxial tests of MBT-MSW.
Table 1 Variation of deviator stress for consolidated undrained and consolidated drained tests at 10% and 20% strain levels. Parameters
Confining pressure (kPa)
Fig. 7. Variation of cohesion with friction angle for consolidated undrained tests, consolidated drained tests and direct shear tests.
Table 2 Variation of friction angle for different unit weights for CU and CD tests. Parameters
CD test
CU test
Deviator stress (kPa)
Deviator stress (kPa)
10% strain
20% strain
10% strain
20% strain
Unit weight = 10.3 kN/m3 particle size = 4 mm
50 100 150
140 157 177
329 438 466
79 81 165
225 274 310
Unit weight = 7.3 kN/m3 particle size = 4 mm
50 100 150
112 184 192
208 224 357
97 116 125
120 128 228
Unit weight = 10.3 kN/m3 particle size = 10 mm
50 100 150
64 60 103
150 155 172
71 102 139
132 158 233
Unit weight = 7.3 kN/m3 particle size = 10 mm
50 100 150
116 146 167
212 256 289
92 115 180
148 197 248
and 10 mm particle size samples did not fail at 20% strain levels but there is a reduction in the deviator stress with the increase in the particle size of MSW. Table 1 shows the variation of deviator stress for consolidated undrained and consolidated drained tests at 10% and 20% strain levels. Fig. 7 shows the variation of cohesion and friction angle. The friction angle values tend to decrease with increase in cohesion. There is certain variation in the friction angle values with unit weight and particle size as shown in Table 2. There is an increase in the friction angle values with the increase in unit weight (from 7.3 kN/m3 to 10.3 kN/m3) for 10 mm particle size in the consolidated drained tests. In the case of 4 mm particle size the friction angle decreased with increase in unit weight. The 10 mm particle size exhibited higher friction angle values than the 4 mm particle size samples. 6.5. Relationship between strength and stiffness The shear strength and stiffness correlations are widely used for both preliminary design studies in geotechnical engineering. These properties are also very useful in landfill engineering. Stiffness in terms of elastic modulus is an important mechanical property of waste which governs the deformation behaviour of MSW. The
Unit Unit Unit Unit
weight = 10.3 kN/m3 particle size = 4 mm weight = 7.3 kN/m3 particle size = 4 mm weight = 10.3 kN/m3 particle size = 10 mm weight = 7.3 kN/m3 particle size = 10 mm
Friction angle (degrees) CD test
CU test
21 24 33 12
10 25 32 55
Fig. 8a. Variation of elastic modulus with shear stress for consolidated undrained and consolidated drained tests.
results obtained in the present study have been used to estimate elastic modulus. Figs. 8a and 8b show the relationship between the values of elastic modulus (E) measured and shear strength ðS ¼ r1 r3 Þ and vertical effective stress ðr0 Þ and shear strength at 20% strain levels for both undrained and drained tests for two different unit weights of MSW. It is evident that there is a general increase in elastic modulus with increase in vertical stress as well as shear stress. A linear relationship is noted in the plot of elastic modulus versus shear strength and vertical stress. The maximum elastic modulus value (2400 kPa) was obtained in a consolidated drained test in the denser MBT-MSW samples. It can be noted that
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response as a function of overburden pressure can be captured by the above equations. The Eqs. (1) and (2) are useful in estimating the strength and stiffness as a function of overburden pressure for the Bangalore MSW. The shear strength and elastic modulus at a depth of 10 m can be found out by knowing the overburden pressure at that point. Estimation of the modulus for a MSW sample collected at depth of 10 m (assuming cb = 10.3 kN/m3).
s ¼ 0:43 10:3 10 ¼ 44:29 kN=m2 E ¼ 10 44:29 ¼ 442:9 kN=m2
Fig. 8b. Variation of shear stress with vertical effective stress for consolidated undrained and consolidated drained tests.
the test results corresponding to undrained and drained conditions as well as two different unit weights fall on the same line indicating that the strength and stiffness of MSW are uniquely related and the stiffness ratio ðE=SÞ was found to be 10 for MSW at 20% strain. This is called stiffness ratio and the typical values for a loose soil is in the range of 40–500. The fact that the value is 10 clearly indicates that the MSW material is more compressible. The following relationships are evident from Fig. 6a–c,
E ¼ 10 S
S
¼ 0:43
7. Summary and conclusions Comprehensive small-scale and large-scale laboratory testing was performed on MBT-MSW using direct shear tests, triaxial consolidated drained and undrained tests to develop a framework for interpreting the shear strength. The results of this testing program analysed the variation of the strength properties with unit weight and particle size. The conclusions that can be drawn from this study are as follows:
ð1Þ
Similarly the ratio of strength as a function of effective overburden pressure (height unit weight) or p0 ðr1 r3 =ðr1 þ 2r3 3 Þ can be obtained with reference to the results presented in Figs. 8a–8c.
r
Therefore for a MSW sample of unit weight 10.3 kN/m3 and at a depth of 10 m the shear stress and elastic modulus are calculated as 44.29 kN/m2 and 442.9 kN/m2 using the above relationships. These relationships can be used to understand the role of strength and stiffness of MSW in stability and settlement problems in landfill engineering.
ð2Þ
q ¼ 1:7 p0 The values considered correspond to a strain level of 20%. As indicated earlier both undrained and drained shear strength
1. The shear strength of MSW is axial strain dependent and increased with increasing deformation in all the tests. The friction angle at 20% strain levels were 40°, 55° and 33° from direct shear test, consolidated undrained test and consolidated drained test respectively. There was a minor variation in cohesion values (0–10 kPa). 2. The strength of MSW increases with increase in unit weight. In the present study the denser samples exhibited higher strength than the less dense samples. 3. The stiffness value increases linearly with increase in the vertical effective stress up to 450 kPa. The maximum value obtained from the tests is 2400 kPa. 4. The stiffness ratio and the strength ratio of MSW were found to be 10 and 0.43 for both consolidated undrained and consolidated drained triaxial tests.
Acknowledgements The authors thank the reviewers for their valued comments and suggestions. The work presented in this paper is a part of the research in the project CIST/MCV/GLS/0038 ‘‘Evaluation of Municipal Solid Waste (MSW) characteristics of a typical landfill in Bangalore’’ funded by Center for infrastructure, Sustainable Transportation and Urban Planning (CiSTUP), which is greatly acknowledged. The authors thank Anthony A for helping in conducting the experiments. References
Fig. 8c. p’ q plots for triaxial consolidated undrained and consolidated drained tests.
American Public Health Association, Water Pollution Control Federation, and Water Environment Federation, 1965. Standard methods for the examination of water and wastewater, vol. 11. American Public Health Association. Bray, J.D., Zekkos, D., Kavazanjian, E., Athanasopoulos, G.A., Riemer, M.F., 2009. Shear strength of municipal solid waste. J. Geotech. Geoenviron. Eng. 1356, 709–722.
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Caicedo, B., Yamin, L., Giraldo, E., Coronado, O., 2002. Geomechanical properties of municipal solid waste in Dona Juana sanitary landfill. Proc. 4th Int. Cong. Environ. Geotech., Rio de Janeiro 1, 177–182. Casagrande, M.D.T., Coop, M.R., Consoli, N.C., 2006. Behavior of a fiber reinforced bentonite at large shear displacements. J. Geotech. Geoenviron. Eng. 132 (11), 1505–1508. Dixon, N., Jones, D.R.V., 2005. Engineering properties of municipal solid waste. Geotext. Geomembr. 23 (3), 205–233, ISBN 0266-1144. Foose, G.J., Benson, C.H., Bosscher, P.J., 1996. Sand reinforced with shredded waste types. J. Geotech. Eng. 122 (9), 760–767. Gabr, M.A., Valero, S.N., 1995. Geotechnical properties of municipal solid waste. Geotech. Test. J. ASTM 18 (2), 241–251. Grisolia, M., Napoleoni, X., 1996. Geotechnical characterization of municipal solid waste: choice of design parameters. Proc. 2nd Int. Cong. Environ. Geotech., Osaka, Japan 2, 641–646. Jamiolkowski, M., Ladd, C.C., Germaine, J.T., Lancellotta, R., 1985. New developments in field and laboratory testing of soils. In: Proc. 11th Int. Conf. Soil Mech. Found. Eng. A.A. Balkema, Rotterdam, vol. 1, pp. 57–153. Jessberger, H.L., Kockel, R., 1993. Determination and assessment of the mechanical properties of waste materials. In: Proceedings Sardinia 93, 4th International Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 1993, pp. 1383–1392. Jessberger, H.L., Syllwasschy, O., Kockel, R., 1995. Investigation of waste body behaviour and waste structure interaction. In: Proceedings Sardinia 1995, Fifth International Waste Management and Landfill Symposium S. Margherita di Pula,Cagliari, Italy. Jones, D.R.V., Taylor, D.P., Dixon, N., 1997. Shear strength of waste and its use in landfill stability. In: Yong, R.N., Thomas, H.R. (Eds.), Proceedings Geoenvironmental Engineering Conference. Thomas Telford, pp. 343–350. Kavazanjian, N., Matascovic, R., Bonaparte, G.R., Schmertmazin, E., 1995. Evaluation of evaluation of MSW properties for seismic analysis. Geoenviron. 2000, Geotech. Special Publ. ASCE. 46, 1126–1141. Ladd, C.C., Foott, R., 1974. New design procedure for stability of soft clays. J. Geotech. Eng. Div. 100 (GT7), 763–786. Ladd, C.C., Foote, R., Ishihara, K., Schlosser, F., Poulos, H.G., 1977. Stress-deformation and strength characteristics. Proc. 9th Int. Conf. Soil Mech. Found. Eng., Tokyo 2, 421–494.
Landva, A.O., Clark, J.I., 1986. Geotechnical testing of wastefill. In: Proceedings, 39th Canadian Geotechnical Conference Ottawa, Ontario, pp. 371–385. Landva, A.O., Clark, J.I., 1990. Geotechnics of waste fill. Geotechnics of waste fills— theory and practice, ASTM STP 1070. In: Landva, A., Knowles, D. (Eds.), American Society for Testing and Materials. Philadelphia, Pennsylvania, pp 86–103. Machado, S.L., Carvalho, F.M., Vilar, O.M., 2002. Constitutive model for municipal solid waste. J. Geotech. Geoenviron. Eng., ASCE 128 (11), 940–951. Manassero, M., Van Impe, W.F., Bouazza, A., 1996. Waste disposal and containment. Proc. 2nd Int. Cong. Environ. Geotech. Osaka, Japan 3, 1425–1474. Merry, S.M., Kavazanjian, E., Fritz, W.U., 2005. Reconnaissance of the July 10, 2000, Payatas Landfill failure. J. Perform. Constr. Facil., ASCE 19 (2), 100–107. Budhu, M., 2008. Soil Mechanics and Foundations. John Wiley & Sons. Reddy, K.R., Hettiarachchi, H., Parakalla, N., Gangathulasi, J., Bogner, J.E., Lagier, T., 2009a. Geotechnical properties of landfilled municipal solid waste under short-term leachate recirculation operations. Waste Manage. Res. 27 (6), 578– 587. Reddy, K.R., Hettiarachchi, H., Parakalla, N., Gangathulasi, J., Bogner, J.E., 2009b. Geotechnical properties of fresh municipal solid waste at Orchard Hills Landfill, USA. Waste Manage. 29 (2), 952–959. Reddy, K.R., Hettiarachchi, H., Parakalla, N., Gangathulasi, P.J., Bogner, J.E., Lagier, 2009c. Hydraulic conductivity of municipal solid waste in landfills. ASCE J. Environ. Eng. 135 (8), 677–683. Singh, S., Murphy, B., 1990. Evaluation of the stability of sanitary landfills. Geotech. Waste Fills – Theory Pract. ASTM STP 1070, 240–258. Stark, T.D., Eid, H.T., Evans, W.D., Sherry, P.E., 2000. Municipal solid waste slope failure. II: stability analyses. J. Geotech. Geoenviron. Eng., ASCE 126 (5), 408– 419. Stark, T.D., Sarihan, N.H., Li, G., 2009. Shear strength of municipal solid waste for stability analyses. Environ. Geol. 57, 1911–1923. Vilar, O.M., Carvalho, M.F., 2004. Mechanical properties of municipal solid waste. ASTM Geotech. Test. J. 32 (6), 1–12. Zekkos, D.P., 2005. Evaluation of Static and Dynamic Properties of Municipal Solid Waste. Dissertation, Doctor of Philosophy, Department of Civil and Environmental Engineering, University of California, Berkeley, California. Zekkos, D., Athanasopoulos, G.A., Bray, J.D., Grizi, A., Theodoratos, A., 2010. Large-scale direct shear testing of municipal solid waste. Waste Manage. 30 (8), 1544–1555.
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Shear strength characteristics of mechanically biologically treated municipal solid waste (MBT-MSW) from Bangalore G.L. Sivakumar Babu a,⇑, P. Lakshmikanthan b,1, L.G. Santhosh b,2 a b
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India Centre for Sustainable Technologies (CST), Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 2 June 2014 Accepted 6 February 2015 Available online 4 March 2015 Keywords: Municipal solid waste Shear strength Elastic modulus Strength ratio Stiffness ratio
a b s t r a c t Strength and stiffness properties of municipal solid waste (MSW) are important in landfill design. This paper presents the results of comprehensive testing of shear strength properties of mechanically biologically treated municipal solid waste (MBT-MSW) in laboratory. Changes in shear strength of MSW as a function of unit weight and particle size were investigated by performing laboratory studies on the MSW collected from Mavallipura landfill site in Bangalore. Direct shear tests, small scale and large scale consolidated undrained and drained triaxial tests were conducted on reconstituted compost reject MSW samples. The triaxial test results showed that the MSW samples exhibited a strain-hardening behaviour and the strength of MSW increased with increase in unit weight. Consolidated drained tests showed that the mobilized shear strength of the MSW increased by 40% for a unit weight increase from 7.3 kN/m3 to 10.3 kN/m3 at 20% strain levels. The mobilized cohesion and friction angle ranged from 5 to 9 kPa and 8° to 33° corresponding to a strain level of 20%. The consolidated undrained tests exhibited reduced friction angle values compared to the consolidated drained tests. The friction angle increased with increase in the unit weight from 8° to 55° in the consolidated undrained tests. Minor variations were found in the cohesion values. Relationships for strength and stiffness of MSW in terms of strength and stiffness ratios are developed and discussed. The stiffness ratio and the strength ratio of MSW were found to be 10 and 0.43. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The characterization of shear strength of mechanically biologically treated municipal solid waste (MBT-MSW) is important for the design of slopes, vertical expansion of landfills, seismic stability evaluations and predicting failure of landfills. MBT-MSW is referred as MSW in this paper. MSW is a heterogeneous material which makes it difficult to evaluate its properties. The factors that are likely to have an effect on the shear strength of waste are; unit weight, size of waste particle, decomposition/age, normal stress and moisture content, etc. A number of researchers (Landva and Clark, 1990; Singh and Murphy, 1990; Jessberger and Kockel, ⇑ Corresponding author. Tel.: +91 80 22933124 (O), +91 80 23600671 (R), +91 9448480671 (mobile). E-mail addresses: [email protected], [email protected] (G.L. Sivakumar Babu), [email protected], [email protected] (P. Lakshmikanthan), [email protected], [email protected] (L.G. Santhosh). URL: http://civil.iisc.ernet.in/~gls/ (G.L. Sivakumar Babu). 1 Mobile: +91 9036406364. 2 Mobile: +91 9611187003. http://dx.doi.org/10.1016/j.wasman.2015.02.013 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
1993; Kavazanjian et al., 1995; Gabr and Valero, 1995; Grisolia and Napoleoni, 1996; Manassero et al., 1996; Jones et al., 1997; Machado et al., 2002; Stark et al., 2009; Bray et al., 2009) conducted experiments on MSW and also performed back-analysis of field case histories over the last two decades. The selection of appropriate shear strength parameters remains a challenging engineering design issue for a site-specific landfill. Variability in the shear strength parameters is due to the variability of MSW compositions, the strain level at failure, the choice of representative samples and testing methods. Satisfactory design of an engineered municipal landfill facility requires consideration of rational values of shear strength properties of MSW. Though considerable research has been conducted till date on the estimation of strength values for MSW in different countries, only a small amount of data is available on the strength properties of MSW that is representative of Indian conditions. Therefore an effort has been made in this study to develop an extensive database of shear strength properties of MSW. Though issues related to MSW landfills are global, contribution of this nature is important as the nature of waste and the magnitude of problem of MSW in developed countries and developing countries such as India are considerably different. Problems in
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landfill engineering require the assessment of engineering properties such as shear strength of MSW that present considerable difficulties in design and analysis.
2. Literature Different methods have been adopted by researches to evaluate the shear strength properties of MSW. Both direct shear tests and triaxial tests are reported in literature. Direct shear tests have been the preferred method for measuring the shear strength of MSW likely because of its simplicity. Landva and Clark (1986) performed a series of large direct shear tests on waste specimens from different Canadian landfills. The measured cohesion and friction angle ranged between 10 and 23 kPa and 24° and 42°. Stark et al. (2009) reviewed considerable experimental data on direct shear tests from literature and concluded that the shear strength of MSW increases with increasing strain or displacement. This leads to high strength values that are in good agreement with field observations of vertical scarps from landfill slope failures remaining near vertical for significant periods of time. They presented strength envelopes as function of displacement and strain level. They also suggested different strength parameters for confining pressures less than and greater than 200 kPa. Zekkos et al. (2010) investigated the effects of waste composition, confining stress, unit weight and loading rate on the stress– displacement response and shear strength of municipal solid waste (MSW) collected from a landfill located in the San Francisco Bay area. Based on 109 large-scale direct shear tests, they observed that the shear strength of MSW at low moisture contents is best characterized by cohesion of 15 kPa and friction angle of 36° at a normal stress of 1 atmosphere and a decrease in the friction angle of 5° for every log-cycle increase in normal stress. In general, there is great variability in the reported shear strengths in the literature. Cohesion values from 0 to 80 kPa and friction angle values from 0° to 60° have been reported in the published literature. Triaxial tests have also been used for evaluation of shear strength of municipal solid waste (MSW) both in undrained and drained conditions. Failure of landfills occurred during severe rains and a few case studies exist in literature. Drained strength of MSW refers to the long term condition and implies equilibrium in terms of full dissipation of pore water pressure in the landfill body and the total stresses are considered as effective stresses. Stress–strain response of MSW in undrained loading condition gives information on undrained shear strength parameters, stresses and strains during loading and at failure state in undrained condition and is essentially required for the analysis of slope stability. Analysis of stability of bioreactor landfills need understanding of undrained strength of MSW. Landfill failures such as Rumpke in the USA (Stark et al., 2000; Zekkos, 2005), Dona Juana in Columbia (Caicedo et al., 2002), Payatas in Philippines (Merry et al., 2005), are attributed to undrained conditions in the landfill material. In the literature, Gabr and Valero (1995), Caicedo et al., (2002a), Vilar and Carvalho (2004), Reddy et al. (2009a–c) reported triaxial compression tests under undrained loading. Gabr and Valero (1995) performed small-scale Consolidated Undrained Triaxial tests (CU-TX) and small-scale Direct Shear tests on 15– 30 year-old waste. The dry unit weight of the CU-TX specimens was 7.4–8.2 kN/m3 and had a specimen diameter of about 71 mm. The authors observed that the cohesion intercept decreased with increasing water content from 100 kPa to 40 kPa at water contents of 55% and 72% respectively. The cohesion increased with increase in the effective confining pressure. Strength parameters were evaluated at 20% axial strain. Cohesion was estimated to be equal to 17 kPa and the friction angle 34°.
The shear strength behaviour of municipal solid waste (MSW) in drained condition provides understanding of long term performance of slope stability and operations of landfill systems. For the long term performance of landfill system, the design parameters required are evaluated from drained condition and the change in volumetric strain is measured. The transient changes caused because of these processes affect porosity, shear strength characteristics, cause deformation or settlement. Machado et al. (2002) conducted drained tests and reported the results. Grisolia and Napoleoni (1996) performed consolidated drained Triaxial tests (CD-TX) under different cell pressures. The stress–strain plots generally suggest a strain hardening behaviour for strains in excess of 40% without reaching a peak stress. It is necessary to understand the importance of undrained and drained shear strength of MSW in landfill engineering practice. This can be explained with reference to the schematic stress paths of MSW indicated in Fig. 1. The stress paths are presented in ðp0 qÞ plot, where q is deviator stress and p0 ¼ r01 þ 2r03 =3 is the mean effective stress and more details are available in geotechnical literature (Budhu, 2008). Referring to Fig. 1, it can be noted that the undrained shear strength ðqf 1 Þ mobilized during landfilling and corresponding mean effective stress p0f 1 on critical sate line or failure envelope at point A represent the failure state. Therefore, for the safe filling in undrained condition, it is essential that undrained shear strength should not exceed the limiting value of shear strength of MSW ðqf 1 Þ corresponding to the mean effective stress applied. In the undrained condition, the filling of the landfill waste should be such that the addition of normal stress reflected in p0 should not develop high pore water pressures and that additional shear stress should not exceed maximum likely shear strength ðqf 1 Þ. Similarly, in drained condition the mobilized shear strength ðqf 2 Þ follows 1:3 slope reaching critical state line at point B and indicates that for the safe filling height for long term operations, the drained shear stress should not exceed qf 2 at mean effective stress of p0f 2 . Strength and stiffness properties are related and are required in the assessment of stability and deformation characteristics of landfills. Stiffness properties or elastic modulus are arrived at from shear modulus and elastic modulus from direct shear tests and triaxial tests as well as field tests in landfill engineering practice. Dickson and Jones (2005) used pressure meter tests and measured shear modulus values which vary from 5 MPa to 30 MPa. In geotechnical engineering literature, to estimate the engineering properties such as strength and stiffness, one engineering approach is to obtain a ratio of ðs=rÞ and ðE=sÞ where ‘S’ is the shear
Fig. 1. Stress paths for undrained and drained condition.
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strength, ‘r’ is the effective vertical consolidated stress and ‘E’ is elastic modulus) and a few studies are available (Ladd and Foot, 1974; Ladd et al., 1977; Jamiolkowski et al., 1985). Previous studies indicated that the undrained shear strength ratio ðs=rÞ of different soils varying from 0.25 to 3.19 and stiffness ratio ðE=sÞ varies from 40 to more than 30,000 depending upon type of soil and test method adopted. However, such correlations are not available for making reliable estimates of the shear strength and stiffness response of the MSW for undrained and drained conditions. The paper proposes relations among these parameters for MSW. This paper describes the strength characterisation of a particular mechanically biologically treated (MBT) waste from Bangalore, India, with respect to particle size and unit weight. Direct shear tests, triaxial consolidated undrained and consolidated drained tests were conducted in the laboratory on representative samples of MSW retrieved from a landfill. Most of the mechanical testing of wastes has been carried out using geotechnical testing methods.
3. Site description and composition analysis Municipality Solid Waste Rules in India specify that biodegradable wastes should be processed by composting, vermi composting etc. and landfilling shall be restricted to non-biodegradable inert waste and compost rejects. Several pre-treatment methods have been developed in the recent times in order to recover the materials and to minimise the organic content reaching the landfills. Composting has been adopted as a potential pre-treatment method in Mavallipura landfill located in the outskirts of Bangalore. The waste used in this study is the mechanically biologically treated compost reject collected from the Mavallipura landfill site. The MSW entering the landfill undergoes a number of processes before being landfilled. Hand sorting of recoverable waste is followed by aerobic windrow composting for a period of 2 months. Screening of the compost reject is done using large screens of size 35 mm and 16 mm to further segregate the recyclable materials. Bulk of the compost reject was found to pass through 35 mm screens which indicated that the particle size of the MSW that is landfilled is <35 mm. The compost reject contained clothes-6.34%, Plastics-28%, glass-1.28%, leather-0.8%, coconut-5.56%, stones-1.96%, rubber-0.88%, wood-0.16% and organic materials-54.2%. The compost reject had particle sizes varying from 35 mm to 4 mm. The particles of size >10 mm mostly contained large plastics, rubber shoes, leather bags and other inert materials which were hand sorted or removed by other mechanical procedures. Since it was difficult to separate particles <10 mm, these were landfilled directly. The MBT process reduces the particle size. Therefore samples of particle size 4 mm and 10 mm were considered in this study.
4. Determination of moisture content and organic content Moisture content of the waste was calculated as the ratio of the weight loss to the weight that remained after heating at a temperature of 60 °C until the specimen has dried to a constant mass. The test for total volatile solids (TVS) was performed according to the APHA-1965 (American Public Health Association) standard methods. Organic content of the compost reject <10 mm particle size was calculated as the ratio of the weight loss to the initial specimen weight after heating to a temperature of 550 °C in a muffle furnace. The initial decomposable organic content of the waste was found to be 54–55% and the inerts constituted to 45%. The specific gravity of sample was measured as 1.26 by density bottle method and pcynometer method.
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5. Laboratory tests and methods Direct shear tests, small scale and large scale consolidated undrained and drained tests were conducted on reconstituted MSW samples collected from the landfill site. A total of 27 tests (3 direct shear tests, 12 consolidated undrained tests and 12 consolidated drained tests) were conducted in the laboratory to study the variation in strength properties of compost reject with particle size and unit weight. One test was performed for each condition and 2 CD tests were repeated for 100 kPa confining pressure as deviator stress value for 50 and 100 kPa were found to be almost similar. These tests were conducted for unit weights of 7.4 kN/m3 and 10.3 kN/m3 and particle sizes of 4 mm and 10 mm. Direct shear tests were performed on samples of size 60 mm in length, 60 mm in width and 30 mm in height, unit weight 10.3 kN/m3 and moisture content of 44% for confining pressures 50, 100 and 150 kPa. All the tests were performed at a strain rate of 0.25 mm/min. Tests were performed in conformity with ASTM D 3080-94 test method for direct shear tests. The MSW was filled in the direct shear mould carefully in 4–5 layers and compacting using a wooden rammer with 4–6 blows per layer. A triaxial pneumatic machine with load cell capacity of 500 kg and loading frame capacity of 5 Mg was used for consolidated drained and undrained tests. Small scale (50 mm diameter and 100 mm height, 100 mm diameter and 200 mm height) triaxial tests were conducted on MSW samples (44% moisture content) for unit weights of 7.4 kN/m3 and 10.3 kN/m3 and particle sizes of 4 mm and 10 mm. Normally the MSW particle size varies from 10 mm to 100 mm, and therefore it is difficult to maintain and compare MSW particle size. Since most of >10 mm particles like plastics and rubber are hand sorted at the landfill site, MSW particle size <10 mm were used in the study. The 50 mm diameter sample size was used in case of 4 mm MSW particle and 100 mm diameter sample size was used in case of 10 mm MSW particle size. MSW samples of 50 mm diameter were prepared by static compaction and dynamic compaction was adopted for the 100 mm samples. In case of Static compaction pre-determined amount of MSW was fed into a metal mould and the mix was statically compacted from both ends using screw jack arrangement. The cylindrical specimen was then extruded using a screw jack. In case of dynamic compaction, MSW was compacted in a split mould in 6–7 layers with 5–6 blows for each layer. The specimens in each group were saturated and consolidated to effective confining pressures of 50, 100 and 150 kPa. The consolidated drained and consolidated undrained tests were performed in accordance with the ASTM D3080 and ASTM D4767. 6. Results and discussion Shear strength parameters derived with the Mohr–Coulomb failure criterion are commonly used to quantify MSW shear strength. The MSW exhibited a strain-hardening behaviour during the direct shear tests and triaxial compression tests. Therefore, strength was defined in terms of the mobilized shear strength corresponding to a selected strain level. The shear strength mobilized at four strain levels (i.e., 5%, 10%, 15% and 20%) was investigated in each type of tests. 6.1. Direct shear tests Tests were conducted under normal loads of 50 kPa, 100 kPa and 150 kPa. Figs. 2a and 2b show the stress–strain relationships and normal stress–shear stress relationships from direct shear tests. The tests were terminated at the maximum shear displacement of about 20 mm. The shear strength of MSW increased with
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Fig. 2a. Stress–strain relationship from direct shear tests of MBT-MSW.
Fig. 2b. Shear stress–normal stress relationship from direct shear tests of MBTMSW.
increasing strain or displacement. The increase in shear stress is less under a normal stress of 100 kPa when compared to the normal stress above 100 kPa. The cohesion and friction angle increased with the strain levels and the measured cohesion (c) and friction angle (U) values at 20% strain levels were 12 kPa and 40°. The shape of the stress strain curve suggests that the long axes of the fibres are oriented perpendicular to the horizontal shear surface (Bray et al., 2009). The MSW exhibits initially a weak response followed by an upward curvature of the stress–displacement curve. The measured shear stress for a normal stress of 100 kPa was found to be 45 kPa at 20% strain. In Fig. 2b, for 5% strain and 10% strain the R2 value is less than 0.9 which indicate a scatter in the shear stress values. At the initial stages of shearing, the effect of normal stress is not pronounced, whereas at strain levels (>10%) the shearing resistance also increases. The scattering may be due to the non-uniform size, shape and orientation of the MSW particles at lower strain levels. 6.2. Consolidated undrained tests A total 12 small scale (50 mm 100 mm) and large scale (100 mm 200 mm) consolidated undrained triaxial compression
Fig. 3a. Stress–strain relationship from consolidated undrained triaxial tests of MBT-MSW for particle size 10 mm.
Fig. 3b. Stress–strain relationship from consolidated undrained triaxial tests of MBT-MSW for particle size 4 mm.
tests were carried out to study the shear strength characteristics of the MBT-MSW samples. Figs. 3a and 3b show the stress–strain behaviour of 10 mm and 4 mm particle size respectively. All the tests were conducted till 20% strain levels. The stress–strain curves exhibited a typical strain-hardening behaviour which increased continuously with axial strain without reaching an asymptotic value.
6.2.1. Effect of unit weight The stress–strain curves for unit weights 7.3 kN/m3 and 10.3 kN/m3 did not show any peak even at 20% strain levels. As shown in Figs. 3a and 3b, the deviator stress increased with the increase in the unit weight. This is in agreement with the results of Zekkos (2005) which report that specimens with lower initial unit weight have a softer initial response and lower mobilized shear strengths at a specified strain level. For a given strain level, it was found that the mobilized friction angle increased with an increase in the unit weight and the cohesion decreased marginally with increase in unit weight. The maximum mobilized cohesion and mobilized angle of internal friction were 6 kPa and 55° corresponding to a strain level of 20% were exhibited by the samples of
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greater unit weight. The mobilized strength of the waste increased by 30–35% for an increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3 at 20% strain levels. The implication from this observation is that if the unit weight of MSW in the landfill is higher, the strength response is better and good compaction procedures leading to improved unit weights in the placement of MSW need to be followed. 6.2.2. Effect of particle size The shear strength of 4 mm particle size samples with increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3 (Fig. 4). In case of 10 mm particle size samples, the shear strength was found to decrease marginally with the increase in unit weight from 7.3 kN/m3 to 10.3 kN/m3. The maximum and minimum shear strength (320 kPa and 30 kPa) values were observed in the 4 mm particle size samples. The shear strength increased with increase in strain levels. The waste composition, high organic content, water content and the fibrous content present in MSW could be responsible for the lower shear strength values obtained in this study.
Fig. 5a. Stress–strain relationship from consolidated drained triaxial tests of MBTMSW for particle size 10 mm.
6.3. Consolidated drained tests Typical deviator stress versus axial strain relationships for MBT-MSW obtained from consolidated drained tests for samples of particle size 10 mm and 4 mm are shown in Fig. 4a and b. It can be noted that the deviator stress consistently increased with the axial strain without reaching a well defined peak. 6.3.1. Effect of unit weight The rate of increase in the stress is comparatively higher in dense samples than in the lesser denser samples (Figs. 5a and 5b). The increase in deviator stress is pronounced clearly at confining pressures greater than 100 kPa for both 4 mm and 10 mm particle size samples and unit weight of 7.3 kN/m3. However for 50 kPa and 100 kPa, there was a slight increase in the deviator stress values. Even the repeated tests showed similar trends. The heterogeneity of MSW, compressible nature of MSW, high organic content and water content may be the probable reasons for this typical behaviour. Certain inconsistency was observed when the friction angle increased sharply (from 8° to 33°) from 10% to 20% strain levels in the 4 mm particle size sample of unit weight 10.3 kN/m3. However the friction angle increased with the increase in unit weight and the cohesion varied between 5 kPa and 9 kPa. The mobilized strength of the waste increased by 40% for unit weight increase from 7.3 kN/m3 to 10.3 kN/m3 for strain levels of
Fig. 4. Variation of friction angle with strain for consolidated undrained triaxial tests of MBT-MSW.
Fig. 5b. Stress–strain relationship from consolidated drained triaxial tests of MBTMSW for particle size 4 mm.
20%. The mobilized cohesion and friction angle ranged from 5 to 9 kPa and 8° to 33° corresponding to a strain level of 20%. 6.3.2. Effect of particle size The 10 mm particle size samples showed a convex stress–strain curve as compared to a concave type of stress–strain curve exhibited by the 4 mm particle size samples. The rate of increase in stress is similar to that in the consolidated undrained tests for 4 mm (Fig. 6). The 4 mm particle size samples exhibited 70% greater shear strength compared to 10 mm particle size samples for a given unit weight of 10.3 kN/m3 and at strain levels 20%. Similar trends were observed in the case of unit weight of 10.3 kN/m3. The increase in the percent shear strength of 4 mm particle size samples over 10 mm particle size samples also increased with the increase in the strain levels from 5% to 20%. Particle size and its impact on strength have been studied by many researchers including Jessberger (1995), Foose et al. (1996) and Casagrande et al. (2006). Jessberger (1995) conducted large (540 mm diameter) uniaxial compression tests on two different waste types as received MSW and the MSW fraction passing a 120 mm sieve (85% MSW is smaller than 120 mm). The compression testing results showed that the sample with <120 mm material failed with a peak at 20% strain. In the present study both 4 mm
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Fig. 6. Variation of friction angle with strain for consolidated drained triaxial tests of MBT-MSW.
Table 1 Variation of deviator stress for consolidated undrained and consolidated drained tests at 10% and 20% strain levels. Parameters
Confining pressure (kPa)
Fig. 7. Variation of cohesion with friction angle for consolidated undrained tests, consolidated drained tests and direct shear tests.
Table 2 Variation of friction angle for different unit weights for CU and CD tests. Parameters
CD test
CU test
Deviator stress (kPa)
Deviator stress (kPa)
10% strain
20% strain
10% strain
20% strain
Unit weight = 10.3 kN/m3 particle size = 4 mm
50 100 150
140 157 177
329 438 466
79 81 165
225 274 310
Unit weight = 7.3 kN/m3 particle size = 4 mm
50 100 150
112 184 192
208 224 357
97 116 125
120 128 228
Unit weight = 10.3 kN/m3 particle size = 10 mm
50 100 150
64 60 103
150 155 172
71 102 139
132 158 233
Unit weight = 7.3 kN/m3 particle size = 10 mm
50 100 150
116 146 167
212 256 289
92 115 180
148 197 248
and 10 mm particle size samples did not fail at 20% strain levels but there is a reduction in the deviator stress with the increase in the particle size of MSW. Table 1 shows the variation of deviator stress for consolidated undrained and consolidated drained tests at 10% and 20% strain levels. Fig. 7 shows the variation of cohesion and friction angle. The friction angle values tend to decrease with increase in cohesion. There is certain variation in the friction angle values with unit weight and particle size as shown in Table 2. There is an increase in the friction angle values with the increase in unit weight (from 7.3 kN/m3 to 10.3 kN/m3) for 10 mm particle size in the consolidated drained tests. In the case of 4 mm particle size the friction angle decreased with increase in unit weight. The 10 mm particle size exhibited higher friction angle values than the 4 mm particle size samples. 6.5. Relationship between strength and stiffness The shear strength and stiffness correlations are widely used for both preliminary design studies in geotechnical engineering. These properties are also very useful in landfill engineering. Stiffness in terms of elastic modulus is an important mechanical property of waste which governs the deformation behaviour of MSW. The
Unit Unit Unit Unit
weight = 10.3 kN/m3 particle size = 4 mm weight = 7.3 kN/m3 particle size = 4 mm weight = 10.3 kN/m3 particle size = 10 mm weight = 7.3 kN/m3 particle size = 10 mm
Friction angle (degrees) CD test
CU test
21 24 33 12
10 25 32 55
Fig. 8a. Variation of elastic modulus with shear stress for consolidated undrained and consolidated drained tests.
results obtained in the present study have been used to estimate elastic modulus. Figs. 8a and 8b show the relationship between the values of elastic modulus (E) measured and shear strength ðS ¼ r1 r3 Þ and vertical effective stress ðr0 Þ and shear strength at 20% strain levels for both undrained and drained tests for two different unit weights of MSW. It is evident that there is a general increase in elastic modulus with increase in vertical stress as well as shear stress. A linear relationship is noted in the plot of elastic modulus versus shear strength and vertical stress. The maximum elastic modulus value (2400 kPa) was obtained in a consolidated drained test in the denser MBT-MSW samples. It can be noted that
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response as a function of overburden pressure can be captured by the above equations. The Eqs. (1) and (2) are useful in estimating the strength and stiffness as a function of overburden pressure for the Bangalore MSW. The shear strength and elastic modulus at a depth of 10 m can be found out by knowing the overburden pressure at that point. Estimation of the modulus for a MSW sample collected at depth of 10 m (assuming cb = 10.3 kN/m3).
s ¼ 0:43 10:3 10 ¼ 44:29 kN=m2 E ¼ 10 44:29 ¼ 442:9 kN=m2
Fig. 8b. Variation of shear stress with vertical effective stress for consolidated undrained and consolidated drained tests.
the test results corresponding to undrained and drained conditions as well as two different unit weights fall on the same line indicating that the strength and stiffness of MSW are uniquely related and the stiffness ratio ðE=SÞ was found to be 10 for MSW at 20% strain. This is called stiffness ratio and the typical values for a loose soil is in the range of 40–500. The fact that the value is 10 clearly indicates that the MSW material is more compressible. The following relationships are evident from Fig. 6a–c,
E ¼ 10 S
S
¼ 0:43
7. Summary and conclusions Comprehensive small-scale and large-scale laboratory testing was performed on MBT-MSW using direct shear tests, triaxial consolidated drained and undrained tests to develop a framework for interpreting the shear strength. The results of this testing program analysed the variation of the strength properties with unit weight and particle size. The conclusions that can be drawn from this study are as follows:
ð1Þ
Similarly the ratio of strength as a function of effective overburden pressure (height unit weight) or p0 ðr1 r3 =ðr1 þ 2r3 3 Þ can be obtained with reference to the results presented in Figs. 8a–8c.
r
Therefore for a MSW sample of unit weight 10.3 kN/m3 and at a depth of 10 m the shear stress and elastic modulus are calculated as 44.29 kN/m2 and 442.9 kN/m2 using the above relationships. These relationships can be used to understand the role of strength and stiffness of MSW in stability and settlement problems in landfill engineering.
ð2Þ
q ¼ 1:7 p0 The values considered correspond to a strain level of 20%. As indicated earlier both undrained and drained shear strength
1. The shear strength of MSW is axial strain dependent and increased with increasing deformation in all the tests. The friction angle at 20% strain levels were 40°, 55° and 33° from direct shear test, consolidated undrained test and consolidated drained test respectively. There was a minor variation in cohesion values (0–10 kPa). 2. The strength of MSW increases with increase in unit weight. In the present study the denser samples exhibited higher strength than the less dense samples. 3. The stiffness value increases linearly with increase in the vertical effective stress up to 450 kPa. The maximum value obtained from the tests is 2400 kPa. 4. The stiffness ratio and the strength ratio of MSW were found to be 10 and 0.43 for both consolidated undrained and consolidated drained triaxial tests.
Acknowledgements The authors thank the reviewers for their valued comments and suggestions. The work presented in this paper is a part of the research in the project CIST/MCV/GLS/0038 ‘‘Evaluation of Municipal Solid Waste (MSW) characteristics of a typical landfill in Bangalore’’ funded by Center for infrastructure, Sustainable Transportation and Urban Planning (CiSTUP), which is greatly acknowledged. The authors thank Anthony A for helping in conducting the experiments. References
Fig. 8c. p’ q plots for triaxial consolidated undrained and consolidated drained tests.
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