Waste Management xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Mechanical properties of Municipal Solid Waste by SDMT Francesco Castelli a,⇑, Michele Maugeri b a b
Geotechnical Engineering, Faculty of Engineering and Architecture, Kore University of Enna, 94100 Enna, Italy Geotechnical Engineering, Department of Civil and Environmental Engineering, University of Catania, 95125 Catania, Italy
a r t i c l e
i n f o
Article history: Received 25 November 2012 Accepted 16 October 2013 Available online xxxx Keywords: Municipal Solid Waste Landfill Mechanical properties Dilatometer Marchetti Test (DMT) Laboratory tests
a b s t r a c t In the paper the results of a geotechnical investigation carried on Municipal Solid Waste (MSW) materials retrieved from the ‘‘Cozzo Vuturo’’ landfill in the Enna area (Sicily, Italy) are reported and analyzed. Mechanical properties were determined both by in situ and laboratory large-scale one dimensional compression tests. While among in situ tests, Dilatomer Marchetti Tests (DMT) is used widely in measuring soil properties, the adoption of the DMT for the measurements of MSW properties has not often been documented in literature. To validate its applicability for the estimation of MSW properties, a comparison between the seismic dilatometer (SDMT) results and the waste properties evaluated by laboratory tests was carried out. Parameters for ‘‘fresh’’ and ‘‘degraded waste’’ have been evaluated. These preliminary results seems to be promising as concerns the assessment of the friction angle of waste and the evaluation of the S-wave in terms of shear wave velocity. Further studies are certainly required to obtain more representative values of the elastic parameters according to the SDMT measurements. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Municipal Solid Waste (MSW) is a heterogeneous material consisting of a variety of constituents of varying shape and size, each exhibiting different mechanical properties. The proportion of each constituent and its position within the MSW matrix plays an important role in determining the behavior of the waste mass. In the geotechnical design of landfills great attention must be given to the mechanical properties of the deposited waste materials, as these define the overall and lining landfill stability (Jones and Dixon, 2005), the effectiveness of the final cover. Physical properties of MSW that are generally of interest for geotechnical purposes include total unit weight, in situ moisture content, hydraulic conductivity, etc., while mechanical properties include stiffness, compressibility, shear strength and interface properties. Some of these properties can be measured directly (e.g., dry density, water contents, etc.), whereas other properties, due the difficulties related with sampling, are generally obtained from indirect methods combining with the existent knowledge of waste properties. Recovery and testing of representative and undisturbed samples of Municipal Solid Waste (MSW) is very difficult, thus ⇑ Corresponding author. Address: Facoltà di Ingegneria e Architettura, Università ‘‘Kore’’ di Enna, Cittadella Universitaria Enna Bassa, 94100 Enna, Italy. Tel./fax: +39 095935481. E-mail address: [email protected] (F. Castelli).
laboratory tests are often conducted on material that is recompacted into the test apparatus. For this reason, in situ testing can be considered the most reliable method for evaluation of the MSW properties (Matasovic et al., 2011), even if, in situ testing techniques are also subject to limitations that may significantly affect their applicability. Field measurement techniques may be divided into non-intrusive measurements that do not penetrate the waste mass and intrusive measurements that penetrate the waste mass. Non-intrusive techniques range from seismological and geophysical measurements of wave propagation velocities and electrical resistivity, while intrusive techniques include measurements made in borings or soundings such as Standard Penetration Test (SPT), Cross-Hole Test (CH), Down-Hole Test (DH), Pressuremeter Test and internal measurements that include Cone Penetration Test (CPT) and Dilatometer Marchetti Test (DMT). Similarly, in situ measurements can be grouped into direct measurements of the MSW properties and indirect measurements that rely on correlations to evaluate properties of interest. The Dilatometer Marchetti Test (DMT) is used widely for in situ measurements of physical and mechanical soil properties, and the latest iteration of DMT is the seismic dilatometer (SDMT), that is the combination of the mechanical flat dilatometer (DMT) introduced by Marchetti (1980), with a seismic module for measuring the shear wave velocity Vs. The seismic dilatometer test, conceptually similar to the seismic cone penetration test (SCPT), was first introduced by Hepton
0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.10.026
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
2
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
(1988) and subsequently improved at Georgia Tech, Atlanta, USA (Martin and Mayne, 1997, 1998; Mayne et al., 1999). The new SDMT system, described in Marchetti et al. (2008), has been recently developed in Italy. Validations of Vs measurements by SDMT compared to Vs measurements by other in situ techniques at various research sites are reported in Marchetti et al. (2008). Besides the S-wave distribution, the seismic dilatometer provides the usual DMT parameters by use of common correlations (Marchetti, 1980; TC16, 2001). The small strain and large strain modules back calculated from the SDMT measurements can be combined to identify the soil modulus degradation curve (Mayne et al., 1999), representing the stiffness at intermediate strains. Despite its success, the use of DMT for in situ measurements of MSW properties has not yet been reported in literature (Matasovic et al., 2011). In the paper the results of a geotechnical investigation carried on MSW materials retrieved from the ‘‘Cozzo Vuturo’’ landfill in the Enna area (Sicily, Italy) are reported and analyzed. Seismic Dilatomer Marchetti Tests (SDMT) have been carried out for the geotechnical characterization of the deposited waste materials. A comprehensive large-scale laboratory program was also performed to develop insights about the interpretation of the MSW compressibility.
2. Mechanical waste parameters The physical and mechanical characterization of waste is a very complex task given the testing difficulties and uncertainties adapting the theory and techniques used in soil mechanics (Grisolia et al., 1995; Jessberger and Kockel, 1993a). In fact, it is difficult to extrapolate the results obtained from one situation to another, because of the differences in initial composition, in the type of pre-treatment carried out and in the way in which the waste was dumped, etc. Waste materials differ widely in type, composition, consistency and state of biochemical decay. Typical examples for different waste naturals are construction material debris, excavated soil, sludges or municipal waste showing large variations in type and properties of its constituents. According to Jessberger and Kockel (1993b) waste types can be divided into two groups: ‘‘soil-like waste’’, defined as granular waste, for which conventional soil mechanics principles are applicable, and ‘‘other waste’’. For fine-grained, ‘‘soil-like waste’’ mechanical properties – compressibility, shrinkage and swelling behavior, shear strength – are determined using conventional geotechnical test methods. For mixed and coarse-grained ‘‘soil-like waste’’ some modifications of the testing equipment are necessary, i.e. compression tests on large scale samples. For ‘‘other wastes’’ is often necessary to undertake in situ tests on trial areas. An appropriate method to classify ‘‘other wastes’’ may be the conduction of tests like grain-size analysis, ignition loss and determination of the main waste constituents by sorting out. In this case, testing methods must take into account of several factors not comparable to the testing of soils, as example, the size of sample constituents, the high compressibility, the simulation of in situ structure of waste. Generally the tendency of increasing fine-grained material with increasing age of waste can be observed. This tendency probably can be explained by different states of decomposition. A typical grain size distribution of municipal waste was reported by Jessberger and Kockel (1991). Particle size diameter ranges from 0.01 mm up to about 100 mm for waste with age ranging between 15 years and 8 months.
Recent studies (e.g. Kavazanjian et al., 1999; Dixon and Jones, 2005; Zekkos et al., 2006; Dixon and Langer, 2006; Zekkos et al., 2008; Bray et al., 2009; Zekkos et al., 2010) have identified a variety of factors that can significantly affect the mechanical properties of MSW. These properties (strength and compressibility) are dependent on the individual composition of the waste material and on the mechanical properties of its constituents, as well as, the mechanical parameter are time-dependent related to the state of decomposition. Taking account of this effects, parameters for fresh and decomposed waste must be distinguished. A large database is available on the shear strength parameters of MSW, which are generally required for slope stability analyses of landfills. Large-scale laboratory testing results on in situ and reconstituted MSW are available in the literature and have been used to aid in the development of recommended MSW mechanical properties. For example, shear strength of MSW evaluated from back analysis of landfill slopes has been reported by Kavazanjian et al. (1995), to develop their recommended MSW shear strength envelopes. Kölsch (1996) conducted tension tests in a large scale to show the important role of fibrous inclusions in generation of tensile resistance of waste mass. In line with this, Towhata et al. (2004) and Zekkos et al. (2007) conducted triaxial shear tests to show that fibers develop significant shear resistance, making the stress-strain behavior nearly linear. A comprehensive large-scale laboratory testing program using direct shear, triaxial, and simple shear tests was performed on Municipal Solid Waste by Bray et al. (2009) and Zekkos et al. (2010) to develop insights about and a framework for interpretation of the shear strength of MSW. The results indicate that direct shear test is appropriate to evaluate the shear strength of MSW along its weakest orientation, and the static shear strength of MSW is best characterized by using a stressdependent Mohr-Coulomb strength criterion with a cohesion (c’) of 15 kPa and a friction angle (/’) of 36° at normal stress of 1 atm, decreasing by 5° for every log cycle increase in normal stress. To characterize the deformation or compression behavior of waste, parameters like Young’s modulus, stiffness modulus or compression index can be used (Dixon et al., 2004). Comparable to the strength parameters, for mixed waste materials, compression parameters from literature show large variations. The amount of settlement of waste materials is dependent on the properties of the individual constituents and on their composition. So the evaluation of compression parameters requires an extensive description and classification of the waste materials. Typical values of elastic parameters (Young’s modulus E’ and Poisson’s ratio m’) for various groups of constituents are shown in Table 1 (Singh et al., 2007). It should be pointed out that the values of E’ and m’ shown in Table 1 are not the ‘‘material’’ properties, but the ‘‘matrix’’ properties, representing the elastic properties of the set of constituents. In fact, the proportion of each constituent and its position within the MSW matrix plays an important role in determining the overall mechanical behavior of the waste mass. For example, the true values of E’ for the individual constituents such as metals, glass wood, etc. are likely to be significantly greater than the values of E’ shown in Table 1. However, within a matrix of MSW, the mobilized values of E’ are likely to be not too different from the values of E’ shown in Table 1 (Singh et al., 2007).
3. Test site The investigation was conducted at ‘‘Cozzo Vuturo’’ landfill (Fig. 1) located at about 3.8 km from the city of Enna (Sicily, Italy). The landfill covers an area of about 120,000 m2 and it receives wastes from five main waste districts (Enna, Calascibetta,
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
3
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx Table 1 Elastic parameters of MSW (Singh et al., 2007). Property
Constituents
Range (% by weight)
Rigid and incompressible Soil-like material Degradable and compressible Reinforcing and tensile elements
Metals, glass, wood, ceramic Demolition waste, cover soil, ash Food, yard & animal waste Paper, cardboard, flexible & rigid plastics, tires
5–17 6–25 16–43 16–60
Elastic parameters E’ (MPa)
m’
75–110 10–20 0.5–0.7 1.5–3.0
0.26–0.49 0.25–0.33 0.05–0.15 0.28–0.32
Fig. 1. Landfill location.
Leonforte, Villarosa, Valguarnera), including more than 20 towns, for a total of about 180,000 inhabitants. The landfill is located in a hilly area, geologically made up of Numidian Flysch of Holigocene-lower age, marly and sandy brown clay of medium Miocene age, and river alluvium of Holocene age. There is a mixture of humus and clay 1–2 m deep within the area of the landfill and a clay layer of about 30–40 m below it. The average permeability of the clay layer varies in the range 2 109–7 109 cm/s. The humus layer is an aquifer but the recharge area is very small and the groundwater stays for only a short time in the aquifer, 30–40 m below the ground surface. The average daily temperature in the Enna area is about 14 °C. The temperature fluctuations caused by the day and night cycles
are in the range between 15 °C per day. Usually, the heaviest precipitations in the Enna area occur in November, December and January. The precipitation percentage in the rainy season (October–March) accounts for 75% of the total annual precipitation. The rainfall is often in the form of a quick and hard pouring rain, which stops suddenly. Normally it does not rain longer than 3 hours. The total landfill area is dived into two disposal site named B1 and B2 respectively. The B1 landfill activity took place from 1999 to 2006. The B2 landfill is located in the smaller upper part of the catchments basin, with a final volume of 330,000 m3. The landfill was designed in order to fill a naturally occurring valley (Fig. 2). According to the original project, the residual useful life of the B2 waste disposal plant is expected to be about 3 years and half (the end of the disposal activity planned in 2012, has been postponed to the end of 2013) but an extension up to about 650,000 m3 is currently considered. The landfill is about 18 m high. The upper soil layers have been removed to create space for the waste and to guarantee geotechnical
Fig. 2. Landfill construction.
Fig. 3. Actual filling status.
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
4
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
safe conditions by the execution of a terracing profile. The waste is piled up in compacted layers of about 1.8 m thick (Fig. 3). The waste density near the surface of the landfill ranges between 7 and 8 kN/m3. The daily covering layers consist of clayey soil. 4. Field testing program This section presents the results obtained by the seismic dilatometer tests (SDMT) executed at the ‘‘Cozzo Vuturo’’ landfill (Fig. 4), as part of the geotechnical investigations planned for the mechanical characterization of the waste materials. Two dilatometer tests are located in the old catchments basin named B1, two dilatometer tests are located in the upper part of the catchments basin named B2 and, finally, one dilatometer test is located between the two disposal basin B1 and B2 (Fig. 5). A cross section of the landfill with the location of the seismic dilatometer tests is reported in Fig. 6. It can be observed that the results deductible from tests named B1a and B1b, as well as those named B2a and B2b, regard the deposited waste materials, while the results deductible from test named Arg are referred to the soil foundation. Figs. 7 and 10 show the typical profiles obtained by the Marchetti dilatometer tests. In particular, Fig. 7 reports the material index (Id) profiles, Fig. 8 reports the undrained shear strength (cu) profiles, Fig. 9 reports the friction angle (/’) profiles and Fig. 10 reports the working strain constrained modulus (M) profiles. The material index Id is used (in ‘‘normal’’ soils) to indicate the soil type and it is defined as follows:
Id ¼
p1 p0 p0 u0
ð1Þ
where p0 is the pressure required to just begin to move the membrane (‘‘lift-off’’), p1 is the pressure required to move the center of the membrane 1.1 mm against the soil (Fig. 11) and u0 is the preinsertion in situ pore pressure. According to Marchetti (1980) the ‘‘soil’’ can be identified as: – clay Id < 0.6 – silt 0.6 < Id < 1.8 – sand 1.8 < Id The data p0 are generally interpreted to estimate the undrained shear strength cu and the friction angle /’ using the following correlations (Marchetti, 1999):
cu ¼ 0:22r0v o ð0:5K d Þ1:25
for Id < 1:2
Fig. 4. Seismic Dilatometer Marchetti Tests (SDMT).
ð2Þ
Fig. 5. Location of Seismic Dilatometer Marchetti Tests. 2
/0 ¼ 28 þ 14:6 log K d 2:1log K d
for Id > 1:8
ð3Þ
where r0v 0 is the pre-insertion overburden stress and Kd is the horizontal stress index expressed as:
K d ¼ ðp0 u0 Þ=r0v o
ð4Þ
Thus, according to Marchetti (1999), when the value of Id is less than 1.2, a undrained strength can be evaluated, while the friction angle is generally estimated for values of Id greater than 1.8. Analyzing the results obtained it can be observed that according to material index (Id) derived by the SDM Tests, the degraded deposit waste materials located in the catchments basin named B1 (years from 1999 to 2006) can be classified as silt-clayey, while deposit waste materials located in the new catchments basin named B2 (years from 2006 to 2011) in most part can be classified as silt and in the case of the Test B2b as silt-sandy from a depth of around 4.0 m (Fig. 7). The undrained shear strength (cu), estimated for strata with prevalent silty and/or clayey fraction, seems to increase with depth (Fig. 8). Finally, values of the constrained modulus M ranging between 2 MPa up to about 100 MPa were determined. For the Tests B1a and B1b an increase in the M values with depth can be observed (Fig. 10). The particularity of the seismic dilatometer is the possibility to provide, in addition to the usual DMT parameters, the S-wave distribution in terms of shear wave velocity (VS) profiles. A schematic layout of the seismic dilatometer test (SDMT) is shown in Fig. 11 (Marchetti et al., 2008). The seismic module (Fig. 11a) is a cylindrical element placed above the DMT blade, provided with two receivers spaced 0.50 m. The shear wave source at the surface is a pendulum hammer ( 10 kg) which hits horizontally a steel rectangular plate, pressed vertically against the soil (by the weight of the truck) and oriented with its long axis parallel to the axis of the receivers, so that they can offer the highest sensitivity to the generated shear wave. The signal is amplified and digitized at depth. VS is obtained (Fig. 11b) as the ratio between the difference in distance between the source and the two receivers (S2–S1) and the delay of the arrival of the impulse from the first to the second receiver (interval time Dt). VS measurements are typically obtained every 0.50 m of depth (while the mechanical DMT readings are taken every 0.20 m). As shown in Fig. 12, measured values of VS generally increase with depth (expect for Test B2a), with an average value ranging between 100 and 150 m/s, in good agreement with the MSW shear wave velocity profiles reported in literature (Zekkos et al., 2011).
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
5
Fig. 6. Cross section of the landfill with the location of SDM Tests.
Fig. 7. Material index (Id) profiles.
Fig. 8. Undrained shear strength (cu) profiles.
Fig. 9. Friction angle (/’) profiles.
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
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F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
Fig. 10. Constrained modulus (M) profiles.
Fig. 11. Marchetti seismic dilatometer test. (a) DMT blade and seismic module; (b) schematic test layout and (c) po and p1 measurement (Marchetti et al., 2008).
Fig. 12. Shear wave velocity (VS) profiles.
According to the elastic theory and assuming for the deposited waste materials a density equal to 8 kN/m3, the corresponding values of the Young’s modulus E0 range between 20.5 and 46 MPa with a good correspondence with those (Table 1) suggested by Singh et al. (2007). 5. Laboratory testing program By laboratory tests, mechanical parameters of ‘‘soil-like waste’’ (
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Mechanical properties of Municipal Solid Waste by SDMT Francesco Castelli a,⇑, Michele Maugeri b a b
Geotechnical Engineering, Faculty of Engineering and Architecture, Kore University of Enna, 94100 Enna, Italy Geotechnical Engineering, Department of Civil and Environmental Engineering, University of Catania, 95125 Catania, Italy
a r t i c l e
i n f o
Article history: Received 25 November 2012 Accepted 16 October 2013 Available online xxxx Keywords: Municipal Solid Waste Landfill Mechanical properties Dilatometer Marchetti Test (DMT) Laboratory tests
a b s t r a c t In the paper the results of a geotechnical investigation carried on Municipal Solid Waste (MSW) materials retrieved from the ‘‘Cozzo Vuturo’’ landfill in the Enna area (Sicily, Italy) are reported and analyzed. Mechanical properties were determined both by in situ and laboratory large-scale one dimensional compression tests. While among in situ tests, Dilatomer Marchetti Tests (DMT) is used widely in measuring soil properties, the adoption of the DMT for the measurements of MSW properties has not often been documented in literature. To validate its applicability for the estimation of MSW properties, a comparison between the seismic dilatometer (SDMT) results and the waste properties evaluated by laboratory tests was carried out. Parameters for ‘‘fresh’’ and ‘‘degraded waste’’ have been evaluated. These preliminary results seems to be promising as concerns the assessment of the friction angle of waste and the evaluation of the S-wave in terms of shear wave velocity. Further studies are certainly required to obtain more representative values of the elastic parameters according to the SDMT measurements. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Municipal Solid Waste (MSW) is a heterogeneous material consisting of a variety of constituents of varying shape and size, each exhibiting different mechanical properties. The proportion of each constituent and its position within the MSW matrix plays an important role in determining the behavior of the waste mass. In the geotechnical design of landfills great attention must be given to the mechanical properties of the deposited waste materials, as these define the overall and lining landfill stability (Jones and Dixon, 2005), the effectiveness of the final cover. Physical properties of MSW that are generally of interest for geotechnical purposes include total unit weight, in situ moisture content, hydraulic conductivity, etc., while mechanical properties include stiffness, compressibility, shear strength and interface properties. Some of these properties can be measured directly (e.g., dry density, water contents, etc.), whereas other properties, due the difficulties related with sampling, are generally obtained from indirect methods combining with the existent knowledge of waste properties. Recovery and testing of representative and undisturbed samples of Municipal Solid Waste (MSW) is very difficult, thus ⇑ Corresponding author. Address: Facoltà di Ingegneria e Architettura, Università ‘‘Kore’’ di Enna, Cittadella Universitaria Enna Bassa, 94100 Enna, Italy. Tel./fax: +39 095935481. E-mail address: [email protected] (F. Castelli).
laboratory tests are often conducted on material that is recompacted into the test apparatus. For this reason, in situ testing can be considered the most reliable method for evaluation of the MSW properties (Matasovic et al., 2011), even if, in situ testing techniques are also subject to limitations that may significantly affect their applicability. Field measurement techniques may be divided into non-intrusive measurements that do not penetrate the waste mass and intrusive measurements that penetrate the waste mass. Non-intrusive techniques range from seismological and geophysical measurements of wave propagation velocities and electrical resistivity, while intrusive techniques include measurements made in borings or soundings such as Standard Penetration Test (SPT), Cross-Hole Test (CH), Down-Hole Test (DH), Pressuremeter Test and internal measurements that include Cone Penetration Test (CPT) and Dilatometer Marchetti Test (DMT). Similarly, in situ measurements can be grouped into direct measurements of the MSW properties and indirect measurements that rely on correlations to evaluate properties of interest. The Dilatometer Marchetti Test (DMT) is used widely for in situ measurements of physical and mechanical soil properties, and the latest iteration of DMT is the seismic dilatometer (SDMT), that is the combination of the mechanical flat dilatometer (DMT) introduced by Marchetti (1980), with a seismic module for measuring the shear wave velocity Vs. The seismic dilatometer test, conceptually similar to the seismic cone penetration test (SCPT), was first introduced by Hepton
0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.10.026
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
2
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
(1988) and subsequently improved at Georgia Tech, Atlanta, USA (Martin and Mayne, 1997, 1998; Mayne et al., 1999). The new SDMT system, described in Marchetti et al. (2008), has been recently developed in Italy. Validations of Vs measurements by SDMT compared to Vs measurements by other in situ techniques at various research sites are reported in Marchetti et al. (2008). Besides the S-wave distribution, the seismic dilatometer provides the usual DMT parameters by use of common correlations (Marchetti, 1980; TC16, 2001). The small strain and large strain modules back calculated from the SDMT measurements can be combined to identify the soil modulus degradation curve (Mayne et al., 1999), representing the stiffness at intermediate strains. Despite its success, the use of DMT for in situ measurements of MSW properties has not yet been reported in literature (Matasovic et al., 2011). In the paper the results of a geotechnical investigation carried on MSW materials retrieved from the ‘‘Cozzo Vuturo’’ landfill in the Enna area (Sicily, Italy) are reported and analyzed. Seismic Dilatomer Marchetti Tests (SDMT) have been carried out for the geotechnical characterization of the deposited waste materials. A comprehensive large-scale laboratory program was also performed to develop insights about the interpretation of the MSW compressibility.
2. Mechanical waste parameters The physical and mechanical characterization of waste is a very complex task given the testing difficulties and uncertainties adapting the theory and techniques used in soil mechanics (Grisolia et al., 1995; Jessberger and Kockel, 1993a). In fact, it is difficult to extrapolate the results obtained from one situation to another, because of the differences in initial composition, in the type of pre-treatment carried out and in the way in which the waste was dumped, etc. Waste materials differ widely in type, composition, consistency and state of biochemical decay. Typical examples for different waste naturals are construction material debris, excavated soil, sludges or municipal waste showing large variations in type and properties of its constituents. According to Jessberger and Kockel (1993b) waste types can be divided into two groups: ‘‘soil-like waste’’, defined as granular waste, for which conventional soil mechanics principles are applicable, and ‘‘other waste’’. For fine-grained, ‘‘soil-like waste’’ mechanical properties – compressibility, shrinkage and swelling behavior, shear strength – are determined using conventional geotechnical test methods. For mixed and coarse-grained ‘‘soil-like waste’’ some modifications of the testing equipment are necessary, i.e. compression tests on large scale samples. For ‘‘other wastes’’ is often necessary to undertake in situ tests on trial areas. An appropriate method to classify ‘‘other wastes’’ may be the conduction of tests like grain-size analysis, ignition loss and determination of the main waste constituents by sorting out. In this case, testing methods must take into account of several factors not comparable to the testing of soils, as example, the size of sample constituents, the high compressibility, the simulation of in situ structure of waste. Generally the tendency of increasing fine-grained material with increasing age of waste can be observed. This tendency probably can be explained by different states of decomposition. A typical grain size distribution of municipal waste was reported by Jessberger and Kockel (1991). Particle size diameter ranges from 0.01 mm up to about 100 mm for waste with age ranging between 15 years and 8 months.
Recent studies (e.g. Kavazanjian et al., 1999; Dixon and Jones, 2005; Zekkos et al., 2006; Dixon and Langer, 2006; Zekkos et al., 2008; Bray et al., 2009; Zekkos et al., 2010) have identified a variety of factors that can significantly affect the mechanical properties of MSW. These properties (strength and compressibility) are dependent on the individual composition of the waste material and on the mechanical properties of its constituents, as well as, the mechanical parameter are time-dependent related to the state of decomposition. Taking account of this effects, parameters for fresh and decomposed waste must be distinguished. A large database is available on the shear strength parameters of MSW, which are generally required for slope stability analyses of landfills. Large-scale laboratory testing results on in situ and reconstituted MSW are available in the literature and have been used to aid in the development of recommended MSW mechanical properties. For example, shear strength of MSW evaluated from back analysis of landfill slopes has been reported by Kavazanjian et al. (1995), to develop their recommended MSW shear strength envelopes. Kölsch (1996) conducted tension tests in a large scale to show the important role of fibrous inclusions in generation of tensile resistance of waste mass. In line with this, Towhata et al. (2004) and Zekkos et al. (2007) conducted triaxial shear tests to show that fibers develop significant shear resistance, making the stress-strain behavior nearly linear. A comprehensive large-scale laboratory testing program using direct shear, triaxial, and simple shear tests was performed on Municipal Solid Waste by Bray et al. (2009) and Zekkos et al. (2010) to develop insights about and a framework for interpretation of the shear strength of MSW. The results indicate that direct shear test is appropriate to evaluate the shear strength of MSW along its weakest orientation, and the static shear strength of MSW is best characterized by using a stressdependent Mohr-Coulomb strength criterion with a cohesion (c’) of 15 kPa and a friction angle (/’) of 36° at normal stress of 1 atm, decreasing by 5° for every log cycle increase in normal stress. To characterize the deformation or compression behavior of waste, parameters like Young’s modulus, stiffness modulus or compression index can be used (Dixon et al., 2004). Comparable to the strength parameters, for mixed waste materials, compression parameters from literature show large variations. The amount of settlement of waste materials is dependent on the properties of the individual constituents and on their composition. So the evaluation of compression parameters requires an extensive description and classification of the waste materials. Typical values of elastic parameters (Young’s modulus E’ and Poisson’s ratio m’) for various groups of constituents are shown in Table 1 (Singh et al., 2007). It should be pointed out that the values of E’ and m’ shown in Table 1 are not the ‘‘material’’ properties, but the ‘‘matrix’’ properties, representing the elastic properties of the set of constituents. In fact, the proportion of each constituent and its position within the MSW matrix plays an important role in determining the overall mechanical behavior of the waste mass. For example, the true values of E’ for the individual constituents such as metals, glass wood, etc. are likely to be significantly greater than the values of E’ shown in Table 1. However, within a matrix of MSW, the mobilized values of E’ are likely to be not too different from the values of E’ shown in Table 1 (Singh et al., 2007).
3. Test site The investigation was conducted at ‘‘Cozzo Vuturo’’ landfill (Fig. 1) located at about 3.8 km from the city of Enna (Sicily, Italy). The landfill covers an area of about 120,000 m2 and it receives wastes from five main waste districts (Enna, Calascibetta,
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
3
F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx Table 1 Elastic parameters of MSW (Singh et al., 2007). Property
Constituents
Range (% by weight)
Rigid and incompressible Soil-like material Degradable and compressible Reinforcing and tensile elements
Metals, glass, wood, ceramic Demolition waste, cover soil, ash Food, yard & animal waste Paper, cardboard, flexible & rigid plastics, tires
5–17 6–25 16–43 16–60
Elastic parameters E’ (MPa)
m’
75–110 10–20 0.5–0.7 1.5–3.0
0.26–0.49 0.25–0.33 0.05–0.15 0.28–0.32
Fig. 1. Landfill location.
Leonforte, Villarosa, Valguarnera), including more than 20 towns, for a total of about 180,000 inhabitants. The landfill is located in a hilly area, geologically made up of Numidian Flysch of Holigocene-lower age, marly and sandy brown clay of medium Miocene age, and river alluvium of Holocene age. There is a mixture of humus and clay 1–2 m deep within the area of the landfill and a clay layer of about 30–40 m below it. The average permeability of the clay layer varies in the range 2 109–7 109 cm/s. The humus layer is an aquifer but the recharge area is very small and the groundwater stays for only a short time in the aquifer, 30–40 m below the ground surface. The average daily temperature in the Enna area is about 14 °C. The temperature fluctuations caused by the day and night cycles
are in the range between 15 °C per day. Usually, the heaviest precipitations in the Enna area occur in November, December and January. The precipitation percentage in the rainy season (October–March) accounts for 75% of the total annual precipitation. The rainfall is often in the form of a quick and hard pouring rain, which stops suddenly. Normally it does not rain longer than 3 hours. The total landfill area is dived into two disposal site named B1 and B2 respectively. The B1 landfill activity took place from 1999 to 2006. The B2 landfill is located in the smaller upper part of the catchments basin, with a final volume of 330,000 m3. The landfill was designed in order to fill a naturally occurring valley (Fig. 2). According to the original project, the residual useful life of the B2 waste disposal plant is expected to be about 3 years and half (the end of the disposal activity planned in 2012, has been postponed to the end of 2013) but an extension up to about 650,000 m3 is currently considered. The landfill is about 18 m high. The upper soil layers have been removed to create space for the waste and to guarantee geotechnical
Fig. 2. Landfill construction.
Fig. 3. Actual filling status.
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
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safe conditions by the execution of a terracing profile. The waste is piled up in compacted layers of about 1.8 m thick (Fig. 3). The waste density near the surface of the landfill ranges between 7 and 8 kN/m3. The daily covering layers consist of clayey soil. 4. Field testing program This section presents the results obtained by the seismic dilatometer tests (SDMT) executed at the ‘‘Cozzo Vuturo’’ landfill (Fig. 4), as part of the geotechnical investigations planned for the mechanical characterization of the waste materials. Two dilatometer tests are located in the old catchments basin named B1, two dilatometer tests are located in the upper part of the catchments basin named B2 and, finally, one dilatometer test is located between the two disposal basin B1 and B2 (Fig. 5). A cross section of the landfill with the location of the seismic dilatometer tests is reported in Fig. 6. It can be observed that the results deductible from tests named B1a and B1b, as well as those named B2a and B2b, regard the deposited waste materials, while the results deductible from test named Arg are referred to the soil foundation. Figs. 7 and 10 show the typical profiles obtained by the Marchetti dilatometer tests. In particular, Fig. 7 reports the material index (Id) profiles, Fig. 8 reports the undrained shear strength (cu) profiles, Fig. 9 reports the friction angle (/’) profiles and Fig. 10 reports the working strain constrained modulus (M) profiles. The material index Id is used (in ‘‘normal’’ soils) to indicate the soil type and it is defined as follows:
Id ¼
p1 p0 p0 u0
ð1Þ
where p0 is the pressure required to just begin to move the membrane (‘‘lift-off’’), p1 is the pressure required to move the center of the membrane 1.1 mm against the soil (Fig. 11) and u0 is the preinsertion in situ pore pressure. According to Marchetti (1980) the ‘‘soil’’ can be identified as: – clay Id < 0.6 – silt 0.6 < Id < 1.8 – sand 1.8 < Id The data p0 are generally interpreted to estimate the undrained shear strength cu and the friction angle /’ using the following correlations (Marchetti, 1999):
cu ¼ 0:22r0v o ð0:5K d Þ1:25
for Id < 1:2
Fig. 4. Seismic Dilatometer Marchetti Tests (SDMT).
ð2Þ
Fig. 5. Location of Seismic Dilatometer Marchetti Tests. 2
/0 ¼ 28 þ 14:6 log K d 2:1log K d
for Id > 1:8
ð3Þ
where r0v 0 is the pre-insertion overburden stress and Kd is the horizontal stress index expressed as:
K d ¼ ðp0 u0 Þ=r0v o
ð4Þ
Thus, according to Marchetti (1999), when the value of Id is less than 1.2, a undrained strength can be evaluated, while the friction angle is generally estimated for values of Id greater than 1.8. Analyzing the results obtained it can be observed that according to material index (Id) derived by the SDM Tests, the degraded deposit waste materials located in the catchments basin named B1 (years from 1999 to 2006) can be classified as silt-clayey, while deposit waste materials located in the new catchments basin named B2 (years from 2006 to 2011) in most part can be classified as silt and in the case of the Test B2b as silt-sandy from a depth of around 4.0 m (Fig. 7). The undrained shear strength (cu), estimated for strata with prevalent silty and/or clayey fraction, seems to increase with depth (Fig. 8). Finally, values of the constrained modulus M ranging between 2 MPa up to about 100 MPa were determined. For the Tests B1a and B1b an increase in the M values with depth can be observed (Fig. 10). The particularity of the seismic dilatometer is the possibility to provide, in addition to the usual DMT parameters, the S-wave distribution in terms of shear wave velocity (VS) profiles. A schematic layout of the seismic dilatometer test (SDMT) is shown in Fig. 11 (Marchetti et al., 2008). The seismic module (Fig. 11a) is a cylindrical element placed above the DMT blade, provided with two receivers spaced 0.50 m. The shear wave source at the surface is a pendulum hammer ( 10 kg) which hits horizontally a steel rectangular plate, pressed vertically against the soil (by the weight of the truck) and oriented with its long axis parallel to the axis of the receivers, so that they can offer the highest sensitivity to the generated shear wave. The signal is amplified and digitized at depth. VS is obtained (Fig. 11b) as the ratio between the difference in distance between the source and the two receivers (S2–S1) and the delay of the arrival of the impulse from the first to the second receiver (interval time Dt). VS measurements are typically obtained every 0.50 m of depth (while the mechanical DMT readings are taken every 0.20 m). As shown in Fig. 12, measured values of VS generally increase with depth (expect for Test B2a), with an average value ranging between 100 and 150 m/s, in good agreement with the MSW shear wave velocity profiles reported in literature (Zekkos et al., 2011).
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
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Fig. 6. Cross section of the landfill with the location of SDM Tests.
Fig. 7. Material index (Id) profiles.
Fig. 8. Undrained shear strength (cu) profiles.
Fig. 9. Friction angle (/’) profiles.
Please cite this article in press as: Castelli, F., Maugeri, M. Mechanical properties of Municipal Solid Waste by SDMT. Waste Management (2013), http:// dx.doi.org/10.1016/j.wasman.2013.10.026
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F. Castelli, M. Maugeri / Waste Management xxx (2013) xxx–xxx
Fig. 10. Constrained modulus (M) profiles.
Fig. 11. Marchetti seismic dilatometer test. (a) DMT blade and seismic module; (b) schematic test layout and (c) po and p1 measurement (Marchetti et al., 2008).
Fig. 12. Shear wave velocity (VS) profiles.
According to the elastic theory and assuming for the deposited waste materials a density equal to 8 kN/m3, the corresponding values of the Young’s modulus E0 range between 20.5 and 46 MPa with a good correspondence with those (Table 1) suggested by Singh et al. (2007). 5. Laboratory testing program By laboratory tests, mechanical parameters of ‘‘soil-like waste’’ (
