Mechanical behaviour of pre-and port-treated municipal solid waste

Testo completo

(1)1. TABLE OF CONTENTS 1. INTRODUCTION ............................................................................ 11. 2. WASTE CHARACTERIZATION .................................................. 15 2.1. Composition ............................................................................................................... 15. 2.2. Classification methods ............................................................................................... 19. 2.3. Physical parameters .................................................................................................... 27. 2.3.1. Moisture content ................................................................................................. 30. 2.3.2. Unit weight ......................................................................................................... 34. 2.3.3. Particle size distribution ..................................................................................... 38. 2.4. 3. Hydraulic parameters ................................................................................................. 43. 2.4.1. Saturated conditions ........................................................................................... 45. 2.4.2. Unsaturated conditions ...................................................................................... 49. MECHANICAL PARAMETERS ................................................... 53 3.1. Differences between waste and soil ............................................................................ 54. 3.1.1. Particle compressibility ...................................................................................... 54. 3.1.2. Scales, REV and mechanical tests ...................................................................... 56. 3.2. Shear strength ............................................................................................................. 59. 3.2.1. Influence of composition and fiber content ......................................................... 68. 3.2.2. Influence of density and fiber orientation ........................................................... 69. 3.2.3. Influence of pre-treatment, degradation and age................................................ 71. 3.2.4. In situ tests and back analysis ............................................................................ 76. 3.2.5. Summary of literature review ............................................................................. 80. 3.2.6. Proposed failure envelopes ................................................................................ 81. 3.2.7. Pore gas pressures ............................................................................................. 83. 3.3. Compressibility .......................................................................................................... 85. 3.3.1. Proposed models ................................................................................................ 88. 3.3.2. Laboratory tests .................................................................................................. 95. 3.3.3. In situ tests........................................................................................................ 102. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(2) 2. 4. MATERIALS, METHODS AND EQUIPMENTS ........................105 4.1. 4.1.1. Real sample from Gioia Tauro MBT plant....................................................... 105. 4.1.2. Real sample from Monsummano Terme Landfill ............................................. 108. 4.1.3. Synthetic waste: a first attempt ........................................................................ 110. 4.1.4. Synthetic waste compositions used in this research ......................................... 113. 4.2. Validation tests ........................................................................................................ 115. 4.2.1. Composition ..................................................................................................... 115. 4.2.2. Particle size analyses ....................................................................................... 116. 4.2.3. Consolidation tests........................................................................................... 119. 4.3. Used equipment and procedures .............................................................................. 122. 4.3.1. Aerobic reactor for biological pre-treatment ................................................... 122. 4.3.2. Consolidometers .............................................................................................. 123. 4.3.3. Triaxial apparatus ........................................................................................... 134. 4.4. 5. Preliminary tests: real samples and synthetic compositions ..................................... 105. Post treatment .......................................................................................................... 135. EXPERIMENTAL RESULTS .......................................................137 5.1. Water content measurements ................................................................................... 137. 5.2. Granulometric analyses ........................................................................................... 137. 5.2.1. Raw synthetic waste ......................................................................................... 137. 5.2.2. Pre-treated synthetic waste .............................................................................. 140. 5.2.3. Comparisons .................................................................................................... 143. 5.3. Consolidation tests ................................................................................................... 145. 5.3.1. Long term test on one month pre-treated waste ............................................... 145. 5.3.2. Consolidation tests on pre-treated and post-treated synthetic waste ............... 148. 5.4. Triaxial tests ............................................................................................................ 154. 5.4.1. Triaxial tests interpretation ............................................................................. 154. 5.4.2. Triaxial tests results ......................................................................................... 157. 6. CONCLUSION AND FUTURE DEVELOPMENTS ...................177. 7. BIBLIOGRAFY ..............................................................................180. Mechanical behaviour of pre- and post-treated municipal solid waste.

(3) 3. LIST OF FIGURES Figure 2.1 Generalized phases in the generation of landfill gases (I= initial adjustment, II=transition phase, III= acid phase, IV methane fermentation and V= maturation phase) (Tchobanoglous, Theisen, & Vigil, 1993) ...................................................................... 18 Figure 2.2 Evolution with time of waste component (Gourc & Staub, Bio-HydroMechanical characterization of MSW (Municipal Solid Waste), an absolute need for the landfill design, 2012) ...................................................................................................... 19 Figure 2.3 Grisolia's classification triangular chart (Grisolia et al. 1995) ...................... 21 Figure 2.4 Kölsch's classification: identification of waste sample (Kölsch, 1995) ........ 22 Figure 2.5 Example of smaller and larger than 20 mm fractions (Zeccos, 2005) ........... 23 Figure 2.6 Classification flow chart (Dixon & Langer, 2006) ........................................ 24 Figure 2.7 Minimum and maximum range and average values of mechanical properties for components in the selected material groups (Dixon & Langer, 2006) ...................... 25 Figure 2.8 Classification: time phases (Dixon N. and Langer U., 2006) ........................ 26 Figure 2.9 Example graph for classification (Dixon N. and Langer U., 2006) ............... 27 Figure 2.10 Schematic representation of components in conceptual model (Hudson et al., 2004).......................................................................................................................... 28 Figure 2.11 Drying experiment for MSW at different temperatures on 200g samples (Gourc & Staub, 2012) .................................................................................................... 31 Figure 2.12 Variation of water content with depth from Gabr and Valero (1995); (see Manassero, Van Impe, & Bouazza, 1997) ...................................................................... 32 Figure 2.13 Variation in water content with the embedment depth (Zhan, Chen, & Ling, 2008) ............................................................................................................................... 33 Figure 2.14 Recommended unit weight profile for different amount of compaction effort and soil cover (Zeccos, 2005) ......................................................................................... 37 Figure 2.15 Average MSW density vs. applied pressure for un-shredded and shredded waste. Solid line represents density under compaction, dashed line is the density after pressure has been released (Christensen, et al., 2013) .................................................... 38 Figure 2.16 Dry sieve analyses of finer than 20 mm fraction (Zeccos, 2005) ................ 39 Figure 2.17 Grain size distribution using dry and wet testing (Gabr &Valero, 1995) (see Manassero, Van Impe, & Bouazza, 1997) ...................................................................... 40 Figure 2.18 Particle size distribution of waste samples (Machado et al., 2010) ............. 40 Figure 2.19 Particle size distribution of landfilled MSW from a landfill in Beijing, China (Wu et al., 2012) ................................................................................................... 41 Figure 2.20 Particle size distribution of MSW samples with decomposition (Hossain et al., 2010) Phase I=aerobic, Phase II=anaerobic acid; Phase III=accelerated methane production; Phase IV= decelerated methane production ................................................ 41 Figure 2.21 Particle size distribution of synthetic MSW at various phase of biodegradation (Reddy et al., 2011) R1=Anaerobic acid phase, R2=Accelerated methane phase, R3=Decelerated methane phase, R4=Methane stabilization ................. 42 Figure 2.22 Identification of waste samples – range of size (Kolsch, 1995) .................. 43. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(4) 4. Figure 2.23 Moisture retention curves for coarse sand, clay and waste material (Zardava, Powrie, & White, 2009) .................................................................................. 44 Figure 2.24 Vertical hydraulic conductivity against: a) applied stress; b) dry density; c)drainable porosity; (data from Beaven 2000 and Hudson 2011); d)average stress showing the effect of gas accumulation(data from Hudson 2011) from (Powrie, Beaven, & Hudson, 2005) ............................................................................................................. 48 Figure 2.25 Comparison between Metropolitan Center Landfill and Bandeirantes Landfill values of permeability and those reported in literature (Machado et al., 2010) 49 Figure 2.26 Typical relative permeability graphs obtained for soil (Warrik,2002) (see Stoltz, Gourc, & Oxarango, 2010) .................................................................................. 51 Figure 2.27 Relative gas permeability krG as a function of gas saturation SG (Stoltz, Gourc, & Oxarango, 2010) ............................................................................................. 51 Figure 2.28 Comparison of relative hydraulic conductivity (Wu et al., 2012) ............... 52 Figure 3.1 Grisolia’s classification ................................................................................. 53 Figure 3.2 Stress path of MSW sample with different fiber contents: on the left 0% fiber content, on the right 25% (Shariatmadari et al., 2009) ................................................... 55 Figure 3.3 Sample drilling at Ihlenberg landfill (Kolsch, Shear strenght of waste, 2009) ......................................................................................................................................... 57 Figure 3.4 Small scale: on the left small triaxial device (d=76 mm) (Zeccos, 2005); on the right small scale consolidometer (d=63,5 mm) (Durmusoglu, Sanchez, & Corapcioglu, 2006).......................................................................................................... 57 Figure 3.5 Large triaxial device (d=300 mm) (Zeccos, 2005);Large consolidometer (d=711,2 mm, h=558,8 mm) (Durmusoglu, Sanchez, & Corapcioglu, 2006); Large shear box (plane area = 1.00 m × 1.80 m) (Kolsch, 2009) ;Pitsea cell, University of Southampton (d= 2 m, h=2,5m) (Hudson et al., 2004) ................................................... 58 Figure 3.6 In situ scale (Rumpke landfill slope failure from www.dr-koelsch.de) ........ 58 Figure 3.7 Bearing behaviour of waste: model of interaction of frictional and tensile forces (Kolsch, 1995) ...................................................................................................... 60 Figure 3.8 Bearing behaviour of waste: model of increasing of total shearing resistance by tensile forces- (Kolsch, 1995) .................................................................................... 61 Figure 3.9 Shear stress in a reinforced soil (Izzo, Gawlik, & Di Nubila, 2013) ............. 61 Figure 3.10 Potential percentage distribution of incompressible, reinforcing and compressible elements between initial, post placement and final state (Dixon & Langer, 2006) ............................................................................................................................... 62 Figure 3.11 Fresh MSW results (Machado, Vilar, & Carvalho, 2008) .......................... 68 Figure 3.12 Mobilization of shear strength parameters of pre-treated waste material (Borgatto et al., 2009) ..................................................................................................... 69 Figure 3.13 Effect of unit weight on the direct shear stress-displacement response of solid waste (Zekkos et al, 2010)...................................................................................... 69 Figure 3.14 Effects of the amount of material and its orientation with respect to the shear surface (Zekkos et al., 2010) ................................................................................. 70 Figure 3.15 Scheme of specimen configuration and view of the assembling (Izzo, Gawlik, & Di Nubila, 2013)............................................................................................ 71 Figure 3.16 Effect of fibers orientation to specimens submitted to normal stresses of 50, 100 e 200 ......................................................................................................................... 71. Mechanical behaviour of pre- and post-treated municipal solid waste.

(5) 5. Figure 3.17 Relationship of mobilized strength parameters at a range of strains to the fill age of MSW (the data points connected by the dashes lines were obtained from samples with different original waste composition) (Zhan, Chen, & Ling, 2008) ....................... 72 Figure 3.18 Decrease in friction angle with decomposition (Hossain & Haque, 2009) . 73 Figure 3.19 Sample failures after triaxial compression: barreling, barreling and shear plane; clarified shear plane with special photography method (Bauer, Munnich, & Fricke, 2009) ................................................................................................................... 74 Figure 3.20 c’ and θ’ against axial strain for raw and separate collection residues (Grisolia, 2012) ............................................................................................................... 74 Figure 3.21 c’ and θ’ against axial strain for stabilized (end process), stabilized (half process), dried non sieved, dried after sieving waste (Grisolia, 2012) ........................... 75 Figure 3.22 Stress ratio and average volumetric strain against axial strain (Bhandari & Powrie, 2013) .................................................................................................................. 76 Figure 3.23 SPT test performed in (a) MCL and (b) BL (Machado ei al., 2010) ........... 77 Figure 3.24 Average CPT values in BL and MCL (Machado et al., 2010) .................... 77 Figure 3.25 DMT friction angles vs. vertical stress (Castelli & Maugeri, 2014) ........... 78 Figure 3.26 Suggested MSW shear strength envelopes for design (after Jones et al., 1997) (Dixon & Jones, 2005) .......................................................................................... 81 Figure 3.27 Large direct shear test results on MSW and recommended strength envelope (Zekkos et al., 2010)........................................................................................................ 82 Figure 3.28 Relationship of the secant value of friction angle with confining stress far all test data in the database (Zekkos et al., 2010)............................................................ 83 Figure 3.29 Effect of pore gas pressures on the waste stability (Gourc & Staub, 2012) 83 Figure 3.30 Compressibility curve proposed (Grisolia, 2012) ........................................ 86 Figure 3.31 Rheological model by Edil et al. 1990 from (Simoes & Catapreta, 2013) .. 90 Figure 3.32 The different fractions of MSW and their evolution with biodegradation (Gourc J. P. et al., 2009).................................................................................................. 92 Figure 3.33 Data and modelled results of LGCIE (on the left) and ELIA (on the right) 93 Figure 3.34 Pitsea compression cell (Hudson et al., 2004) ............................................. 95 Figure 3.35 Small and large scale devices (Durmusoglu, Sanchez, & Corapcioglu, 2006) ......................................................................................................................................... 96 Figure 3.36Strain versus log time plots in small and large scale test at: a)original moisture content; b) field capacity (Durmusoglu, Sanchez, & Corapcioglu, 2006) ....... 96 Figure 3.37 Compression cell (Olivier & Gourc, 2007) ................................................. 97 Figure 3.38 Secondary compression of waste during Test 2 on the left; evolution of Cαε with time on the right (Olivier & Gourc, 2007) .............................................................. 98 Figure 3.39 Primary (on the left) and secondary compression ratio vs. degree of decomposition (Reddy et al., 2011) ................................................................................ 98 Figure 3.40 Secondary settlement at 50 kPa (a) UK MBT, b) German MBT (Siddiqui et al., 2012).......................................................................................................................... 99 Figure 3.41 Secondary settlement at 150 kPa (a) UK MBT, b) German MBT (Siddiqui et al., 2012) ..................................................................................................................... 99 Figure 3.42 Small scale oedometer apparatus for one-dimensional compression tests, used for the original waste (left) and for the pre-treated waste (right) (Conte et al., 2013) ....................................................................................................................................... 100. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(6) 6. Figure 3.43 Strain for original wastes (Conte et al., 2013) ........................................... 101 Figure 3.44 Strains for pre-treated wastes (Conte et al., 2013) .................................... 101 Figure 3.45 Large scale one dimensional compresion cell (Castelli & Maugeri, 2014) ....................................................................................................................................... 102 Figure 3.46 Plate load tests at the Appels landfill, Dendermonde, Belgium (W.F. Van Impe et al. 1994) (see Manassero, Van Impe, & Bouazza, 1997) ................................ 103 Figure 3.47 Monitoring of the cap cover settlement at different times in months using a profile meter (probe in a flexible horizontal pipe) (Gourc & Staub, 2012) .................. 103 Figure 3.48 Experimental landfill (Simoes & Catapreta, 2013) ................................... 104 Figure 3.49 Strain measured over time (Simoes & Catapreta, 2013) ........................... 104 Figure 4.1 Vertical deformation vs time: Gioia Tauro sample ..................................... 107 Figure 4.2 Gioia Tauro sample after consolidation test ................................................ 107 Figure 4.3 Granulometric curve: MT real waste ........................................................... 110 Figure 4.4 First synthetic sample: a) components; b) mixture; c) unit weight evaluation ....................................................................................................................................... 111 Figure 4.5 Vertical deformation vs time: first synthetic composition .......................... 112 Figure 4.6 Vertical deformation vs time: comparison first synthetic composition and real MT waste................................................................................................................ 112 Figure 4.7 Material component for the synthetic waste of this study ........................... 114 Figure 4.8 Dynamic respirometer and synthetic waste ................................................. 114 Figure 4.9 Results of respirometric analises for synthetic wastes ................................ 115 Figure 4.10 Granulometric curve: synthetic vs real waste ............................................ 119 Figure 4.11 Vertical settlement vs time: synthetic vs real waste .................................. 120 Figure 4.12 Shifted curve vertical settlement vs time :synthetic vs real waste ............ 121 Figure 4.13 Vertical deformation vs log time :synthetic vs real waste ......................... 121 Figure 4.14 Aerobic reactors plant ................................................................................ 122 Figure 4.15 Realized aerobic reactors ........................................................................... 123 Figure 4.16 Gastec Detector Tubes ............................................................................... 123 Figure 4.17 Consolidometer: functioning scheme ........................................................ 125 Figure 4.18 Consolidometer plant ................................................................................. 125 Figure 4.19 Consolidometer: side elevation ................................................................. 126 Figure 4.20 Front and back view .................................................................................. 126 Figure 4.21 Particular of the cylindrical mold .............................................................. 127 Figure 4.22 Realized consolidometers .......................................................................... 127 Figure 4.23 Taking and mixing of the pretreated material ........................................... 128 Figure 4.24 Quartering operation .................................................................................. 129 Figure 4.25 Mold filling ................................................................................................ 131 Figure 4.26 Loading of consolidometer device ............................................................ 132 Figure 4.27 Loaded consolidometer.............................................................................. 133 Figure 4.28 LVDT sensors and acquisition system ...................................................... 133 Figure 4.29 Large scale triaxial apparatus .................................................................... 134 Figure 4.30 Post treatment ............................................................................................ 135 Figure 5.1 Granulometric curve raw material (wet way) .............................................. 138 Figure 5.2 Granulometric curve of raw waste: wet and dry method............................. 139 Figure 5.3 Granulometric curve of pre-treated waste (wet method) ............................. 140. Mechanical behaviour of pre- and post-treated municipal solid waste.

(7) 7. Figure 5.4 Koelsh’s classification applied on the pre-treated samples of this research 142 Figure 5.5 Granulometric curves: raw vs pre-treated waste (wet method) ................... 143 Figure 5.6 Granulometric curves for different duration of pre-treatment ..................... 144 Figure 5.7 Granulometric curves synthetic material vs real material ........................... 144 Figure 5.8 Vertical deformation vs time: pre-treated waste .......................................... 145 Figure 5.9 Vertical deformation vs log time: pre-treated waste.................................... 147 Figure 5.10 Consolidation tests on pre-treated and post- treated synthetic waste ........ 148 Figure 5.11 Vertical deformation vs time for pre-treated waste at 25, 50 e 100 kPa ... 149 Figure 5.12 Vertical deformation vs time for post treated waste at 25, 50 e 100 kPa . 150 Figure 5.13 Vertical deformation vs time at 25 kPa: pre-treated vs post treated waste 150 Figure 5.14 Vertical deformation vs time at 50 kPa: pre-treated vs post treated waste 151 Figure 5.15 Vertical deformation vs time at 100 kPa: pre-treated vs post treated waste ....................................................................................................................................... 151 Figure 5.16 Shifted curve: vertical deformation vs time at 25 kPa ............................... 152 Figure 5.17 Shifted curve: vertical deformation vs time at 50 kPa ............................... 152 Figure 5.18 Shifted curve: vertical deformation vs time at 100 kPa ............................ 153 Figure 5.19 Peat and sand specimen ............................................................................. 156 Figure 5.20 Reconstitution of synthetic pre-treated specimen in triaxial cell............... 156 Figure 5.21 Application of the two latex membranes ................................................... 157 Figure 5.22 Triaxial specimen preparation: removing of the base plate ....................... 157 Figure 5.23 Triaxial specimen preparation: positioning of the mold on the base of triaxial cell..................................................................................................................... 158 Figure 5.24 Triaxial specimen preparation: extracted specimen and height adaption .. 158 Figure 5.25 Triaxial specimen preparation: application of membranes and positioning of the specimen in triaxial cell........................................................................................... 159 Figure 5.26 Deviatoric stress vs axial deformation q- εa for pre-treated waste at 50 and 100 kPa .......................................................................................................................... 160 Figure 5.27 Pre-treated specimens after triaxial tests: on the left P5 at 50 kPa and on the right P7 at 100 kPa ........................................................................................................ 161 Figure 5.28 Deviatoric stress vs axial deformation q- εa for post-treated waste at 25, 50 and 100 kPa ................................................................................................................... 161 Figure 5.29 Post treated specimens after triaxial tests on the left (P6 at 50 kPa) and a particular of inclusions in the specimen on the right .................................................... 162 Figure 5.30 Deviatoric stress vs axial deformation q- εa for pre-treated and post-treated waste at 50 kPa .............................................................................................................. 162 Figure 5.31 Deviatoric stress vs axial deformation q- εa for pre-treated and post-treated waste at 100 kPa ............................................................................................................ 163 Figure 5.32 CU test results at confining pressure of 50 and 300 kPa a) deviatoric stress; b) pore water generation (Shariatmadari et al., 2009) ................................................... 163 Figure 5.33 Stress paths of pretreated waste at 50 kPa: total and effective .................. 164 Figure 5.34 Drained test: deviatoric stress vs axial deformation q- εa for pre-treated waste at 25, 50 and 100 kPa .......................................................................................... 165 Figure 5.35 Drained test: mobilized angle vs axial deformation ϕ’- εa for pre-treated waste at 25, 50 and 100 kPa .......................................................................................... 166. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(8) 8. Figure 5.36 Drained test: volumetric strain vs axial deformation εv - εa for pre-treated waste at 25, 50 and 100 kPa .......................................................................................... 166 Figure 5.37 Drained test: deviatoric stress vs axial deformation q- εa for post-treated waste at 25, 50 and 100 kPa .......................................................................................... 168 Figure 5.38 Drained test: mobilized angle vs axial deformation ϕ’- εa for post-treated waste at 25, 50 and 100 kPa .......................................................................................... 168 Figure 5.39 Drained test: volumetric strain vs axial deformation εv - εa for post-treated waste at 25, 50 and 100 kPa .......................................................................................... 169 Figure 5.40 Deviatoric stress vs axial deformation q- εa for pre-treated and post-treated waste at 25 kPa .............................................................................................................. 170 Figure 5.41 Deviatoric stress vs axial deformation q- εa for pre-treated and post-treated waste at 50 kPa .............................................................................................................. 170 Figure 5.42 Deviatoric stress vs axial deformation q- εa for pre-treated and post-treated waste at 100 kPa ........................................................................................................... 171 Figure 5.43 Mobilized angle vs axial deformation ϕ’- εa for pre-treated and post treated waste at 25 kPa .............................................................................................................. 171 Figure 5.44 Mobilized angle vs axial deformation ϕ’- εa for pre-treated and post treated waste at 50 kPa .............................................................................................................. 172 Figure 5.45 Mobilized angle vs axial deformation ϕ’- εa for pre-treated and post treated waste at 100 kPa ............................................................................................................ 172 Figure 5.46 Volumetric strain vs axial deformation εv - εa for pre-treated and post treated waste at 25 kPa .............................................................................................................. 173 Figure 5.47 Volumetric strain vs axial deformation εv - εa for pre-treated and post treated waste at 50 kPa ............................................................................................................. 173 Figure 5.48 Volumetric strain vs axial deformation εv - εa for pre-treated and post treated waste at 100 kPa ............................................................................................................ 174 Figure 5.49 Secant shear modulus G vs deviatoric strain εd for pre-treated and posttreated waste at 100 kPa ................................................................................................ 174 Figure 5.50 Shear modulus G vs deviatoric strain εd for pre-treated and post-treated waste at 100 kPa ............................................................................................................ 175 Figure 5.51 Shear modulus G vs deviatoric strain εd for pre-treated and post-treated waste at 25 kPa .............................................................................................................. 175 Figure 5.52 Initial shear modulus G0 vs initial mean stress p for pre-treated waste ..... 176. Mechanical behaviour of pre- and post-treated municipal solid waste.

(9) 9. LIST OF TABLES Table 2.1 Sources of solid wastes within a community (Tchobanoglous, Theisen, & Vigil, 1993) ..................................................................................................................... 15 Table 2.2 Landva’s and Clark's classification ................................................................. 20 Table 2.3 Grisolia’s classification ................................................................................... 21 Table 2.4 Kölsch’s classification .................................................................................... 22 Table 2.5 Literature review of porosity values (Staub et al., 2009) ................................ 29 Table 2.6 Moisture content: literature values and procedures ........................................ 31 Table 2.7 Methods for measuring unit weight (Dixon & Jones, 2005)........................... 34 Table 2.8 Unit weights from literature ............................................................................ 35 Table 2.9 Statistical summaries of bulk unit weight data for fresh waste Fasset et. al. 1994 (see Dixon & Jones, 2005) ..................................................................................... 36 Table 2.10 Hyperbolic parameter for different compaction effort and amount of soil cover (Zeccos, 2005) ....................................................................................................... 37 Table 2.11 Hydraulic conductivity: literature review ..................................................... 46 Table 2.12 kh/kv from large scale tests (Manassero et al., 2011)..................................... 48 Table 3.1 Particle and overall compressibility and A parameter Powrie et al. (1999) (see Shariatmadari et al., 2009) .............................................................................................. 55 Table 3.2 Summary of MSW shear strength data adapted from (Zeccos, 2005) and (Stark et al., 2009) ........................................................................................................... 63 Table 3.3 Results of direct shear tests - Adapted from (Kolsch, 2009) .......................... 72 Table 3.4 Summary of MSW landfill case histories used to back calculate MSW shear strength (Stark, et al., 2009) ............................................................................................ 79 Table 3.5 Back analysis causes used in the development of the Kavazanjian et al. (1995) shear strength recommendation (Zeccos, 2005) (back analysis assuming c= 5 kPa) ..... 79 Table 3.6 Basic mechanism associated with the three phases of settlements in waste ... 87 Table 3.7 Compressibility parameters: literature review ................................................ 87 Table 3.8 Sower’s model parameters. (see Conte & Carrubba, 2012)............................ 89 Table 4.1 Classification of Gioia Tauro sample............................................................ 106 Table 4.2 Consolidation test data for Gioia Tauro sample............................................ 106 Table 4.3 MT real sample composition......................................................................... 108 Table 4.4 Granulometric curve parameters: MT waste ................................................. 108 Table 4.5 Details of granulometric test: MT real waste ................................................ 109 Table 4.6 First synthetic sample: composition .............................................................. 110 Table 4.7 Unit weight evaluation test data: first synthetic sample ............................... 111 Table 4.8 Consolidation test data: first synthetic sample .............................................. 111 Table 4.9 Synthetic composition ................................................................................... 113 Table 4.10 Average percentages by weight of each component for real and synthetic wastes used in this study ............................................................................................... 116 Table 4.11 Results of granulometric test for real MT waste ......................................... 116 Table 4.12 Results of granulometric test for synthetic waste ....................................... 117 Table 4.13 Granulometric curve parameters: synthetic waste ...................................... 117. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(10) 10. Table 4.14 Details of granulometric test for synthetic waste ........................................ 118 Table 4.15 Laboratory consolidation test: real and synthetic waste data ...................... 119 Table 4.16 Laboratory consolidation test: used equipment .......................................... 119 Table 4.17 Laboratory compression test results: synthetic vs real waste ..................... 120 Table 5.1 Moisture content for raw and pre-treated samples ........................................ 137 Table 5.2 Details of granulometric test for synthetic raw waste (sample 5) ................. 138 Table 5.3 Details of granulometric test of synthetic pretreated waste (sample 4) ........ 141 Table 5.4 Long term consolidation data: pre-treated waste .......................................... 145 Table 5.5 Literature review of consolidation tests ........................................................ 147 Table 5.6 Consolidation tests on pre-treated and post- treated synthetic waste ........... 148 Table 5.7 Consolidation tests on pre-treated and post- treated synthetic waste: results ....................................................................................................................................... 149 Table 5.8 Primary and secondary compression coefficient for pre and post treated waste ....................................................................................................................................... 153 Table 5.9 Undrained triaxial tests: specimens data ....................................................... 160 Table 5.10 Mobilized angle of pre-treated waste ad different axial strain under undrained conditions ..................................................................................................... 164 Table 5.11 Mobilized angle of post-treated waste ad different axial strain under undrained conditions ..................................................................................................... 164 Table 5.12 Drained triaxial tests: pre-treated specimens data....................................... 165 Table 5.13 Drained triaxial tests: mobilized angles ...................................................... 167 Table 5.14 Drained triaxial tests: post treated specimens data ..................................... 167. Mechanical behaviour of pre- and post-treated municipal solid waste.

(11) 11. 1 INTRODUCTION The landfill is an important component of integrated waste management as well as the most used disposal method in developed countries. People have always disposed wastes: in the past wastes were placed into the soil, in depressions of Earth's surface or deposited in the deep ocean in a totally uncontrolled way. The social and industrial development and environmental awareness have led to more sophisticated technologies, with a steady increase in disposal costs. Nowadays, the research aims achieving the virtuous objective of minimization of environmental impacts in the perspective of an integrated waste management including not only the reduction of quantities in advance, but also recycling, energy recovery and, only at the end, controlled tipping. Landfills can be compared to reactors where materials in liquid, solid and gaseous phases, react resulting in liquid (leachate) and gaseous (biogas) emissions and a solid phase (the waste in place) which represents the source of potential residual emissions. The European Landfill Directive (Directive 1999/31/CE) and the current Italian Legislation (Legislative Decree No. 36/2003, Legislative Decree No. 152/2006) are aimed at reducing the total quantity of organic matter disposed in landfills, by setting reduction targets that States have to achieve gradually over time. Along with the reduction in the amount of waste sent to landfill, pre-treatment and post-treatment, the control of degradation of deposited waste, and their controlled leaching, are the ways for controlling and reducing long-term impacts, in the light of environmental sustainability. A sustainable landfill shall present, at the end of the financially guaranteed period (30 years after closure), a concentration of pollutants that is environmentally acceptable. Despite the improvement in the technical design and plants management, and the control of short and long term emissions, landfills still do not meet a sufficient level of acceptance among population. For this reason, and because of the strict environmental laws in matter of zonation, and the extensive urbanization, the problem of landfills location became very urgent. The possibility of placing larger quantities of waste, than that estimated in the original design, in the existing landfills expanding them both vertically and horizontally is an attractive method on which world attention is concentrated. The higher landfills are, the higher stresses and the higher strength must be mobilized in the waste body to ensure stability. This solution underlines the need of more specific engineering analysis. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(12) 12. in terms of safety and performance of plants. Those investigations require an extensive characterization of waste mechanical response even though the knowledge of waste mechanics is still poor and limited. In the past decades, catastrophic failures occurred and the investigations highlighted high pore pressure (caused both by leachate and gasses) and inadequate shear strength in the interface (waste - foundation soil, waste - geosynthetic or within the liner system) as failure causes. The design and management of landfills are complicated by the fact that wastes, that are an "unconventional" heterogeneous material, change their composition and characteristics (even the mechanical ones) during time because of degradation. The quantification of the mechanical properties is difficult for the above mentioned reasons and for the following ones: .  . Municipal Solid Waste (hereafter referred to as MSW) are heterogeneous and diversified in geographical, seasonal, time scale and dimensional terms; it is difficult to preserve in situ conditions; there are no standard procedures for sampling, classification and laboratory. tests;  choice of the correct device scale and the representative elementary volume suitable for the different purposes (mechanical or biological testing). The research in this field still needs to make important steps, and in this framework the aim of this work finds place. The theme of the thesis is part of the PON research project “Tecnologie e materiali innovativi per la difesa del territorio e la tutela dell’ambiente” carried out by researchers of DICEAM Department of University of Reggio Calabria. The literature study carried out in this thesis is mainly concentrated in mechanical issues. At the same time an experimental activity has been carried out: it consisted in some preliminary analysis of real samples and in the realization of synthetic specimens. The aim is related to the identification of synthetic samples that are suitable to represent Italian wastes. Using synthetic representative samples in laboratory tests is an advantage because:  bureaucratic and operational problems (such as extraction, transport and storage) are overcome;  reliable results are expected since there is a better control on the specimens preparation; . better level of representativeness of average conditions in waste composition. Mechanical behaviour of pre- and post-treated municipal solid waste.

(13) 13. and therefore applicability of the results to larger geographical context and not only to a single site. In this work, a standard procedure to realize representative synthetic pre-treated samples on which performing the subsequent mechanical testing was individuated. In order to fulfil this target, two prototypes were designed to simulate the aerobic biological pre-treatment and the consolidation that waste undergoes on site. A post-treatment suitable to be applied on wastes at the end of the mechanicalbiological pre-treatment was individuated with the aim of improving the mechanical behaviour and therefore the overall response of landfills in terms of stability. In particular, plastic inclusions, derived from discards of geosynthetics production, were used. Tests were performed on both the pre-treated and the post-treated synthetic materials. After pre-treatment, classification analyses and consolidation, samples were subjected to standard triaxial tests in a large scale apparatus in order to investigate the mechanical response of the two synthetic materials.. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(14) 14. Mechanical behaviour of pre- and post-treated municipal solid waste.

(15) 15. 2 WASTE CHARACTERIZATION 2.1 Composition MSW indicates a mixture of waste deriving from domestic and commercial sources and including different materials of a wide size range (Dixon & Jones, 2005). Table 2.1 is a schematic summary of the sources of MSW within a community (Tchobanoglous, Theisen, & Vigil, 1993). Table 2.1 Sources of solid wastes within a community (Tchobanoglous, Theisen, & Vigil, 1993). Typical facilities, activities, or locations where wastes are generated. Types of solid wastes. Residential. Sigle family and multifamily detached dwellings, low-, medium- and high-rise apartments, etc.. Food wastes, paper, cardboard, plastics, textiles, leather, yard wastes, wood, glass, tin cans, aluminium, other metals, ashes, street leaves, special wastes (including bulky items, consumer electronics, white goods, yard wastes collected separately, batteries, oil and tires), household hazardous wastes. Commercial. Stores, restaurant, markets, office buildings, hotels, motels, print shops, service stations, auto repair shops, etc.. Paper, cardboard, plastics, textiles, leather, yard wastes, wood, food wastes, glass, metals, special wastes (see above), hazardous wastes, etc.. Institutional. Schools, hospitals, prisons, governmental centers. As above in commercial. Construction and demolition. New constructions sites, road repair/renovation sites, razing of buildings, broken pavement. Wood, steel, concrete, dirt, etc.. Municipal services (excluding treatment facilities). Street cleaning, landscaping , catch basin cleaning, parks and beaches, other recreational areas. Special wastes, rubbish, street sweepings, landscape and tree trimmings, catch basin debris, general wastes from parks, beaches, and recreational areas. Treatment plant sites; municipal incinerators. Water, wastewater, and industrial treatment processes, etc.. Treatment plant wastes, principally composed of residual sludges. Source. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(16) 16. The term “composition” is used to describe groups of materials that compose the whole waste and their percentage by weight (Tchobanoglous, Theisen, & Vigil, 1993) The composition differs from one place to another and also within a site itself because of seasonal sequence and management activities (recycling, source sorting, pretreatment etc.). Moreover, the complexity of such heterogenic material is increased by the fact that it evolves with time because of physical-chemical-biological processes (Gourc & Staub, 2012) which waste undergoes in landfill after placement. The degradation of wastes in a landfill occurs in several stages (Figure 2.1) described below (Tchobanoglous, Theisen, & Vigil, 1993; Cossu & De Fraja Frangipane, 1995): . Phase 1 - Initial Adjustment. After the placement of waste in landfill, readily biodegradable organic compounds are decomposed aerobically with CO2 production; once the oxygen trapped in the voids is spent, the anaerobic degradation starts.. . Phase 2 - Transition phase. During this stage anaerobic conditions start to develop. Nitrates and sulfates, which act as electron acceptors in the biological conversion reactions, are reduced to nitrogen gas and hydrogen sulfide. The pH of leachate starts to decrease due to the presence of organic acids and high concentration of CO2. Phase 3 - Acid phase. The bacterial activity accelerates, leading to the. . production of high quantities of organic acids and little amount of hydrogen. The first step of the process is the hydrolysis of organic solid complexes (such as lipids, polysaccharides, proteins and nucleic acids) that are transferred in solution and transformed in a more assimilable form easier to be digested by micro-organisms. Subsequently, the fermentative bacteria digest the solubilized organic substance and produce volatile fatty acids (such as ethanol and lactic acid), CO2 and H2. The acid phase proceeds with a second step of acidogenesis, during which volatile fatty acids and alcohols are transformed, by the acidogens bacteria, in intermediate compounds with lower molecular weight, such as acetic acid (CH3COOH) and small concentrations of other organic acids. The biogas produced in this phase is mainly composed of carbon dioxide and small fraction of hydrogen. The pH is acid in the range 5÷6, due to the presence of organic acids and high. Mechanical behaviour of pre- and post-treated municipal solid waste.

(17) 17. concentrations of CO2. Other typical features are high concentrations of ammonia nitrogen (resulting from the hydrolysis and fermentation of. . nitrogenous compounds such as proteins), iron and solubilized heavy metals. The acid phase lasts from few months to two - three years. It is important to note that the process of anaerobic degradation can be stopped during the acid phase by excess of organic acids, which inhibits the methanogens bacteria. Phase 4 - Methane fermentation phase. During the initial step of this phase (called unstable) there is a slow growth of methanogens bacteria which convert the products of the previous phase in CH4 and CO2. These bacteria, that are strictly anaerobes, are very sensitive to environmental factors (pH, temperature and redox potential), and can be distinguished into two groups: the first one converts the acetic acid to methane and carbon dioxide, and the second one converts hydrogen and carbon dioxide into methane. The first group is generally responsible for approximately the 70% of methane production. The overall conversion reaction of organic matter to methane and carbon dioxide can be expressed analytically as follows (Cossu & De Fraja Frangipane, 1995): ( (. 𝑎. 𝑏. 𝑐. 𝑑. ). 𝑎. 𝑏 (. 𝑎. 𝑐 𝑏. 𝑑 𝑐. ) 𝑑. ). 𝑑 2.1. During this phase CH4 concentration in biogas increases, while the concentration of hydrogen and carbon dioxide decreases. The leachate exhibits a reduction in acid concentration, an increase in pH and alkalinity, which results in a lesser solubilization of calcium, iron, manganese and heavy metals (which precipitate largely as sulfides). During the second step, called stable, the acidogens and methanogens bacteria are in dynamic equilibrium and the methane production is almost constant, with concentrations equal of 50÷65% by volume. The remaining concentration consists mostly of carbon dioxide and small percentages of sulfur compounds and micro-pollutants. The leachate produced during this stage is. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(18) 18. hardly degradable. The pH is weakly alkaline (7÷8). This phase could last more than 30 years. . Phase 5 - Maturation phase. When the residual organic fraction is insensitive to gasification, the CH4 production reduces and an increase of nitrogen concentration in biogas is recorded due to diffusion of air. The remaining substrate in landfills is slowly biodegradable. In this phase the leachate often contains humic and fulvic acids that are hardly biodegradable.. Figure 2.1 Generalized phases in the generation of landfill gases (I= initial adjustment, II=transition phase, III= acid phase, IV methane fermentation and V= maturation phase) (Tchobanoglous, Theisen, & Vigil, 1993). This mass transfer process from the solid phase to the gas and the liquid phase induces the generation of new voids in waste body and therefore settlements that could last up to thirty years even after degradation is completed. Degradation and other alteration such as physical-chemical processes modify the. Mechanical behaviour of pre- and post-treated municipal solid waste.

(19) 19. behaviour of waste over time and influence the performance of the entire landfill during its lifespan (Machado, Vilar, & Carvalho, 2008). For these reasons all these factors have to be taken into account when studying and reckoning of waste engineering parameters (Dixon & Jones, 2005) that, as a consequence, are closely related to the time (Gourc & Staub, 2012).. Figure 2.2 Evolution with time of waste component (Gourc & Staub, Bio-Hydro-Mechanical characterization of MSW (Municipal Solid Waste), an absolute need for the landfill design, 2012). 2.2 Classification methods The starting point of a mechanical investigation is the classification of the used materials. The general approach of researchers is to employ soils classification methods with adequate changes taking into account that dimensional information are not enough to characterize such a complex material as waste. In literature several classification methods are proposed. One of the first suggested classification is that by Siegel et al. 1990 (see Dixon & Langer, 2006) during the analysis of wastes deriving from drilling operation in a Californian landfill. It consists of a simple report of percentages by weight of materials (on wet basis) considering for example material groups like metal, glass, wood, rocks and bricks, soil (including also not easily separable degraded material), rubber and plastic, paper, miscellaneous (other materials). The classification of Landva and Clark (1990) (see Dixon & Langer, 2006) stresses the importance of degradation phenomena that can alter the mechanical behaviour over. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(20) 20. time. They group materials in four classes as shown in the Table 2.2. The last three categories (ON, ID, IN) may contain voids influencing the mechanical behaviour of waste. For this reason, the authors distinguished:  hollow containers such as boxes, bottles, cans, pipe, tubing, etc.;  platy or elongated items such as sheets and plates;  bulky items like furniture, appliances, etc. Table 2.2 Landva’s and Clark's classification. Organic Putrescible (OP- monomers and low resistance polymers, readily biodegradable). -. Food waste Garden waste Animal waste Materials contaminated by such waste. Organic Non Putrescible (ON- highly resistance polymers, slowly biodegradable) -. Paper Wood Textiles Leather Plastic, rubber Paint, oil, grease, chemicals, organic sludge. Inorganic degradable (ID). Inorganic Non degradable (IN) -. - Metals (corrodible to varying degrees). -. Glass, ceramics Mineral soil, rubble Tailings, slimes Ash Concrete, masonry (construction debris). According to the Authors this material partition is not sufficient to fully characterize wastes for engineering purposes, and parameters such as the water content, the particle size distribution and the organic content have to be determined. Even the European Technical Committee provides technical recommendations about classification of waste in the section regarding the Geothechnics of Landfill Design and Remedial Works (GLR). The ETC n. 8 (1993) divides the waste into two groups: . soil-like waste that can be compared to soil for composition and. geotechnical behavior;  other wastes that include all materials whose mechanical behaviour must be characterized. The classification by Grisolia et al. (1995) is an extension of that by Landva and Clark's (Table 2.3). Such classification groups materials in terms of mechanical behaviour and degradability, emphasizing how each group can influence the response of the entire body of waste in terms of failure and resistance (Grisolia & Napoleoni, 2004).. Mechanical behaviour of pre- and post-treated municipal solid waste.

(21) 21. The Authors also stresses also the importance of time, presence of fluids and high deformable elements which differentiate waste from soils and call for a classification method that takes into account these factors (Grisolia, 2012). Table 2.3 Grisolia’s classification. Class A – Inert stable material Materials that are unlike to deform or degrade in the medium term. - Soils - Metals - Glass - Ceramics - Construction & demolition waste - Ash - Wood. Class B – Highly deformable elements Materials which undergo a large initial settlement due to modification of the initial shape. Some of these materials present creep effects. - Paper - Cardboard - Textiles - Leather - Plastics - Rubber - Nappies - Tyres. Class C – Readily biodegradable elements Readily biodegradable materials that generate biogas and leachate and a loss of volume. - Food waste - Garden waste - Animal waste - Underscreen (d< 20mm). The Figure 2.3 shows an example of the proposed diagram in which the percentages of each material group are included. It provides a useful and simple tool in understanding compositional variation between sites or during time in a single site.. Figure 2.3 Grisolia's classification triangular chart (Grisolia et al. 1995). Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(22) 22. Kölsch (1995) performed shear tests on different kinds of waste (raw, treated and degraded) to investigate the reinforcing effect of fibrous elements in the mass of waste. The Author classified samples by material and size of particles as shown in Table 2.4. Table 2.4 Kölsch’s classification. Material group - Paper and cardboard - Smooth synthetic (sheets, rubber, leather, textiles) - Hard synthetic (plastics, hard leather) - Metals - Minerals (including glass, ceramics, soil) - Wood - Organics (including organic waste, grass, leaves). Particle size class -. ≥1000 mm 500÷1000 mm 120÷500 mm 40÷120 mm 8÷40 mm ≤ 8 mm. Dimension class -. Dim 0: grain Dim 1: fibres Dim 2: foils Dim 3: box. Particles smaller than 8mm or between 8 and 40 mm are not distinguished by material type. In Dim 0 elements have dimension ≤ 8 mm; The tested material was divided into four groups according to the different origin: fresh (not pre-treated collected from dust chart), residual (like the fresh group but without organic matter), rotted 18 (urban waste after 18 month aerobic pre-treatment), site (5 years old urban waste collected from landfill.. Figure 2.4 Kölsch's classification: identification of waste sample (Kölsch, 1995). Mechanical behaviour of pre- and post-treated municipal solid waste.

(23) 23. The Author found out that the pre-treated or the decomposed material have a huge percentage (nearly 65%) with particles size ≤ 40 mm material with respect to the percentages of the fresh and residual samples (Figure 2.4). This can be explained by the fact that pre-treatments and degradation phenomena reduce particle size. Zeccos (2005) proposes a characterization scheme in three phases before the mechanical investigation:  in situ-characterization;  primary geotechnical characterization; secondary geotechnical characterization. The in situ characterization provides a first overall description of the landfilled material. It consists in drilling operation and collection of waste samples from different . landfill depth to determine (i) composition (from a visual description), (ii) temperature (immediately after the collection), (iii) age (from landfill document), (iv) in situ-unit weight tests, (v) the qualitative definition of moisture content level (dry, damp, wet, standing water) and (vi) degradation level (none, slight, moderate, high) after a first visual investigation. The primary geotechnical characterization divides material into two portions: smaller and larger than 20 mm (Figure 2.5).. Figure 2.5 Example of smaller and larger than 20 mm fractions (Zeccos, 2005). The small fraction was subjected to conventional geotechnical tests (such as moisture content, sieve analysis, organic content, material loss etc.) with traditional equipment. The large fraction are grouped by material type during the secondary geotechnical characterization phase. During the secondary geotechnical characterization phase. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(24) 24. materials like paper, soft plastics, wood, stiff plastics, gravel, metals, glass, organics and smaller than 20 mm articles trapped by larger ones are considered. The division into large and small fractions was aimed at realizing specimens with different percentages of fiber material and of different sizes in order to investigate different equipment scales. Dixon & Langer (2006) propose a classification system including the following information:  material type;  engineering properties (i.e. shear, tensile and compressive strength,  . modulus of elasticity and elongation at break); particle size; shape;. degradation potential. The classification consists in the following steps (Figure 2.6): . Figure 2.6 Classification flow chart (Dixon & Langer, 2006). 1. Description of the components. The starting point of this classification system is that of identifying the different material components (percentages by weight). 2. Definition of the mechanical properties of each material group. Mechanical parameters have to be determined for each material class and, for a given. Mechanical behaviour of pre- and post-treated municipal solid waste.

(25) 25. mechanical parameter, the influence of a component with respect to another has to be considered according to the percentages of the waste type (Figure 2.7). The aim of this step is to highlight values variability within a class and possibly identify similarities that allow to group material in terms of mechanical behaviour (Dixon & Langer, 2006).. Figure 2.7 Minimum and maximum range and average values of mechanical properties for. components in the selected material groups (Dixon & Langer, 2006). 3. Individuation of sub-groups related to the particle shape  Reinforcement components: one or two-dimensional (e.g. plastic bags . and sheets of paper) Three-dimensional components -. Compressible components: elements with high compressibility (putrescible materials, plastic packaging) and low compressibility(e.g. cans) - Incompressible components (e.g. bricks, or pieces of metal) 4. Particle size distribution of each material components. The dimensional ranges suggested are the same ranges of Kölsch’s classification: ≤ 8mm, 8÷40mm, 40÷120mm, 120÷500 mm, 500÷1000 mm, ≥ 1000 mm. Until now. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(26) 26. data described are related to materials in their initial state (i.e. before landfilling). Compaction operations in landfill, applied loads of overlying wastes and degradation could cause a change in the distribution of material components into the sub-groups described before. For example a compressible element could become a reinforcement if it undergoes flattening and subsequently traction. 5. Potential of degradation. The Authors refer to the sub-division proposed by Landva and Clark among rapidly, moderately and slowly biodegradable materials. To assess the biological activity of waste, factors such as total organic carbon (TOC) or biological oxygen demand (BOD) should be determined. After collecting size information, sub-division of material should be reviewed in order to reduce the number of material groups. After landfilling, the percentage of compressible elements decreases, while the percentage of reinforcing and incompressible elements increases. The classification by Dixon & Langer (2006) considers three phases corresponding to the delivering of waste to site, in situ placement and long-term degradation as shown in Figure 2.8. Compression mobilizes components such as paper, plastics and flexible organic materials. Moreover, in long-term, degradation occurs and materials such as hard plastics, wood or leather change nature and it is assumed they become either incompressible (<40 mm) or reinforcement (>40 mm) elements. Metals and minerals remain in their initial state (Figure 2.8).. Figure 2.8 Classification: time phases (Dixon N. and Langer U., 2006). Mechanical behaviour of pre- and post-treated municipal solid waste.

(27) 27. Dixon & Langer (2006) propose a relevant graph useful for classification purposes (Figure 2.9) in which there are gradation curves for each material group and a cumulative curve. The distinction is also made in term of degradability percentage of each group.. Figure 2.9 Example graph for classification (Dixon N. and Langer U., 2006). 2.3 Physical parameters The analyses of physical parameters (such as moisture content, particle size distribution, unit weight) starts from considering the porous structure of waste since this material is very compressible. Thus, most of relevant parameters should be determined as a function of porosity (Manassero, Van Impe, & Bouazza, 1997). A schematic representation of an elementary volume of waste shows that a typical MSW is composed by three phases (solid, liquid and gas) in which the fluid phase is not uniformly distributed in the porous matrix (Figure 2.10). Moreover, part of the fluid phase is free to flow under gravity and the remaining part is entrapped into the micro-pores within the solid particles, in particular the organic ones (Hudson et al., 2004).. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(28) 28. Figure 2.10 Schematic representation of components in conceptual model (Hudson et al., 2004). The first parameter strongly characterizing a porous medium is porosity. which is the ratio between voids volume and total volume. Considering waste materials, the porosity is called “total porosity” and is given by Staub et al. (2009):. 2.2. where Vgt is the volume of gas entrapped in solid particles and that cannot flow, Vv is the volume of voids, Vs is the volume of solids and V is the total volume of the element. The total porosity of MSW takes values in the range 30÷70% with changes of confining stress level between 0÷700 kPa (Manassero et al., 2011). According to Stoltz et al. (2010) the evolution of the total porosity, as a consequence of both mechanical and biological changes, is very relevant and can be computed at each loading step as:. Mechanical behaviour of pre- and post-treated municipal solid waste.

(29) 29. (. ) (. ) 2.3. where H0 is the initial height of waste , ΔH is the settlement and n0 is the initial total porosity. From 2.3 it is evident that the total porosity does not quantify the entire voids volume because of the presence of the gas trapped within solid particles. Considering that in general waste are partially saturated material, a drainable porosity must be defined as the portion of void affected by drainage phenomenon (Hudson et al., 2004).. 2.4. It is simply inferred that the drainable porosity or effective porosity is the difference between the total porosity and the amount of volume that contains the liquid phase in the pore space that cannot be drained under the effect of gravity V’w and the gaseous phase V’g free to move in voids (Hudson et al., 2004). In other term, it is the difference between the volumetric water content at saturation and the water that waste keeps despite gravity force (i.e. field capacity). Typical effective porosity values are included in the range between 5÷30% and for confining stress levels > 200 kPa and < 50 kPa, respectively (Manassero et al., 2011). Porosity values from laboratory tests in literature are reported in the Table 2.5. Table 2.5 Literature review of porosity values (Staub et al., 2009). Authors. Porosity [%]. Type of porosity. Beaven and Powrie (1995). 28% to 33.5% 1.6% to 22.7%. Initial effective porosity Effective porosity under stress. Zeiss (1997). 47% to 57%. Initial effective porosity. Hudson et al. (2004). 45.5% to 55.5% 1.5% to 14.4%. Total porosity under stress Effective porosity under stress. Stoltz and Gourc (2007). 45% to 62%. Total porosity under stress. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(30) 30. Laboratory determination of effective porosity should be more accurate during drainage tests when the waste is completely saturated because during saturation even micro-pores are filled (Staub et al., 2009). A third definition of porosity has to be mentioned, the so called “open porosity” that quantifies all the voids in a volume of waste, even the micro-pores (Olivier & Gourc, 2007).. 2.5. 2.3.1 Moisture content Moisture content can be expressed on wet or dry basis as follows:. 2.6. 2.7. where W0 is the initial weight of sample and W1 is the weight after drying. No specific procedure are available for the determination of moisture content of MSW (Gourc & Staub, 2012). The drying standard procedure for soils requires a temperature of 105 °C until the stabilization of weight. The presence of organic compounds, sensitive to the drying process, makes the determination of water content a hard task (Gourc & Staub, 2012). High temperature can be inadequate due to the risk of fire and the presence of VOCs compounds (volatile organic carbon that tends to gasify under adequate temperature conditions) that can led to an overestimation of the water content. On the other hand, low temperature may produce underestimations if the drying process does not last for an adequate period of time (Figure 2.11).. Mechanical behaviour of pre- and post-treated municipal solid waste.

(31) 31. Figure 2.11 Drying experiment for MSW at different temperatures on 200g samples (Gourc & Staub, 2012). Moisture content depends on:  waste composition and age;  local climatic conditions;  operating procedures; effectiveness of leachate collection system;  biological decomposition rate;  daily and final cover systems. In literature values between 10÷150% (on dry basis) are reported (Machado et al., 2010). This wide range of values derives from several testing procedures and different kind of waste tested (Table 2.6). . Table 2.6 Moisture content: literature values and procedures. Author Siegel et al. (1990) (see Zeccos, 2005) Gabr & Valero (1995) (see Zeccos, 2005). Method. wd. 60°C. 1÷ 45% small samples. 60°C. 30÷130% smaller fraction (smaller values are for shallower wastes). Coumolos et al. (1995) (see Zeccos, 2005). -. Gomes et al. (2002) (see Zeccos, 2005). -. Olivier & Gourc, 2007. 105 °C per 24 h. Olivier & Gourc, 2007. 60°C per 72 h. Zeccos, 2005. 55°C. 30÷75% Greek wastes (decreasing with depth profile) 61÷96% (recent wastes near the surface) 117% (2÷3 years wastes) 150% (rainy season) 38.36% raw household waste derived from a sorting unit 16.25% only constitutive water in particle synthetic waste 12% (25.6÷26.2 m depth, 15 years) 13% (7.6÷9.6 m depth, <1 year) 23% (3.5÷4.5 m depth, 2 year). Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.

(32) 32. Staub et al., 2009. 60°C for 3 days and then at 105°C until stabilization. 30.7% (70 mm French fresh waste) 50.2% (40 mm French fresh waste) 59.5% Anaerobic acid phase 59.7% Accelerated methane production 63.3% Decelerated methane production 64.7% Stabilization phase. Hossain et al., 2010. -. Machado et al., 2010. 70°C until reaching weight stabilization. Each material component was dried separately. Fresh ≈70÷120% Aged ≈40÷110%. Sivakumar Babu et al., 2010. -. 43.7% fresh MSW 43.3% landfill MSW 134.6% synthetic MSW. Karimpour-Fard et al., 2011. 70°C until reaching weight stabilization. 85÷110% Brazilian wastes. Siddiqui et al., 2012. -. wet basis 37.9÷38.5% UK MBT waste wet basis 37.3÷37.8% German MBT. Repetti et al., 2012. -. Grisolia, 2012. -. Conte et al., 2013. 80°C for 72 hours. Bhandari & Powrie, 2013. -. 17÷ 69% Different depth (in the range 5÷34 m) 45% Raw without separate collection 55÷60% Separate collection (25%) ≈100% Separate collection (15%) ≈ 60÷70% optimum values for MBT or thermally pre-treated waste 26.3% Raw waste 43.2% Pre-treated waste 33.7% Full fraction 40.5% Finer fraction. Generally speaking weather conditions (especially rain) and high landfill depths (that means aged wastes) result in high moisture content (Figure 2.12).. Figure 2.12 Variation of water content with depth from Gabr and Valero (1995); (see Manassero, Van Impe, & Bouazza, 1997). Mechanical behaviour of pre- and post-treated municipal solid waste.

(33) 33. But in some cases opposite trends, that is a decreasing water content with depth, can be observed (Figure 2.13). Zeccos (2005) calculates the moisture content as the ratio between the weight of less than 20 mm particles material loss and the weight of the remaining material after heating at a temperature of 55°C. The results show a moisture content of about 13% for samples deriving from high and medium depth and of about 23% for the shallower ones. These quite low values are due to the fact that wastes tested are collected from a conventional landfill, not operating as bioreactor, and from a warm climate zone.. Figure 2.13 Variation in water content with the embedment depth (Zhan, Chen, & Ling, 2008). Landfill working procedures influence moisture content values: new generation plants, such as bioreactor or landfills with leachate recirculation, have higher water content than the old generation plants because water or leachate are added to reach an optimal value for biodegradation in order to speed up stabilization both in biological and mechanical terms. The material obtained from Mechanical Biological Treatment (hereafter referred to as MBT) have higher in situ densities due to smaller particle size compared to raw waste, so flow velocity and rain infiltration are reduced and this leads to the reduction of leachate volume at the base of landfill. The estimation of the correct water content in a landfill is important also for unit weight determination.. Vella Jennifer Maria Ph.D. in Geotechnical Engineering XXVII cycle.