Waste Management 39 (2015) 63–70
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
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
64
G.L. Sivakumar Babu et al. / Waste Management 39 (2015) 63–70
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.
G.L. Sivakumar Babu et al. / Waste Management 39 (2015) 63–70
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 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
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
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
64
G.L. Sivakumar Babu et al. / Waste Management 39 (2015) 63–70
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.
G.L. Sivakumar Babu et al. / Waste Management 39 (2015) 63–70
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 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
