1 INTRODUCTION

Municipal solid waste (MSW) is a heterogeneous material consisting of a variety of constituents of varying shape and size, each exhibiting different me-chanical property. The proportion of each constituent and its position within the MSW matrix plays an im-portant role in determining the overall mechanical behavior of the waste mass.

In the geotechnical design of landfills great atten-tion must be given to the mechanical properties of the deposited waste materials, as these define landfill stability and deformations which are usually as-sessed applying soil mechanics principles. Although this has been helpful to some extent, there is an in-creasing realization that behavior of waste should be considered in the context of a separate discipline of soil mechanics. Similarities and differences in ex-perimental behavior can then lead to the develop-ment of laboratory and field tests specifically for ob-taining engineering properties of MSW.

Municipal Solid Waste Landfills (MSW) are en-vironmentally sensitive facilities that need to per-form well both under static and seismic conditions. Potential failures during an earthquake may cause significant damage to a landfill or to some of its components, such as the base containment system, and can lead to environmental disasters and signif-

cant financial costs. Damage to gas collection sys-tems or the cover system needs also to be repaired. Deep-seated slope failures involving the base con-tainment system may be difficult to repair and may require excavation and removal of the solid waste material. In the United States, significant effort has been expended to provide guidance on the design of MSW landfills under seismic loading. Federal or state regulations and guidance documents have also been developed and are regularly used in engineering practice.

According to Italian Regulation for Construction (NTC, 2008), the design of landfills include the geo-logical and geotechnical characterization of the site, a detailed description of the constructive phases and monitoring systems, taking into account the nature of waste materials and the environmental vulnerabil-ity. The design include static and seismic stability analyses, based on appropriate mechanical charac-terization of the shear strength of soil and MSW, and the evaluation of the risks related to damages of the lining system.

In the present paper, some data relating to field tests and laboratory compression tests, carried on MSW materials retrieved from the “Cozzo Vuturo” landfill (Figure 1) in the Enna area (Italy), are re-ported and discussed. The analysis of the experimen-tal results permit an upgrading of the knowledge on

Proceedings of the 5th European Geosynthetics Congress, EUROGEO 5, September 16-19, 2012 - Valencia (Spain), Vol.5, pp. 105-110, ISBN 978-84-695-4690-1

Static and dynamic waste characterization

F. Castelli, V. Lentini University of Enna “Kore”, Enna, Italy

M. Maugeri University of Catania, Catania, Italy

ABSTRACT: Mechanical properties of the Municipal solid waste (MSW) materials control many aspects of landfill lining system design and performance, including stability issues and integrity of the geosynthetic components. Stability analyses require extensive knowledge on the mechanical behavior of waste which could be provided by laboratory or in situ testing. Nevertheless, the physical and mechanical characterization of waste is a very complex task, given the testing difficulties and uncertainties adapting the theory and tech-niques used in soil mechanics. Particular attention must be devoted to MSW landfills in seismic areas, where the liners can be damaged by seismic action, as documented by recent earthquakes. In the paper the results of a geotechnical investigation carried on MSW materials retrieved from the “Cozzo Vuturo” landfill in the Enna area (Italy) are reported and analyzed. Static and dynamic properties were deter-mined by in situ and laboratory tests. In particular, Seismic Marchetti Dilatomer Tests (SMDT) have been car-ried out for the seismic characterization and the tests results are used to derive shear wave velocity profiles and the dynamic properties of the deposited waste materials. A comprehensive large-scale laboratory program was also performed to develop insights about the interpretation of the MSW compressibility.

the mechanical behavior of waste materials and fac-tors influencing the deformation processes.

Figure 1. “Cozzo Vuturo” (Enna, Italy) landfill location.

2 GEOTECHNICAL WASTE PARAMETERS

The physical and mechanical characterization of waste is a very complex task given the testing diffi-culties 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 ini-tial composition, in the type of pre-treatment car-ried out and in the way in which the waste was dumped, etc.

Waste materials differ widely in type, composi-tion, consistency and state of biochemical decay. Typical examples for different waste naturals are ex-cavated soil, construction material debris, sludges or municipal waste showing large variations in type and properties of its constituents. Generally waste types are divided into two groups: soil-like waste, defined as “granular waste”, for which conventional soil me-chanics principles are applicable, and “other waste”.

For fine-grained, soil-like waste mechanical properties - compressibility, shrinkage and swelling behavior, shear strength- are determined using con-ventional 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, it is necessary to undertake in situ tests on trial areas.

Recent studies (e.g. Kavazanjian et al., 1999; Zekkos et al., 2006; Zekkos et al., 2008; Bray et al., 2009) have identified a variety of factors that can significantly affect the mechanical properties of MSW. These properties (strength and compressibil-ity) are dependent on the individual composition of the waste material and on the mechanical properties of its constituents. In addition the mechanical pa-

rameter are time-dependent related to the state of de-composition. Taking account of this effect, parame-ters for fresh and for decomposed waste must be distinguished. In some cases to provide applicable parameters for stability or deformation analysis, it might be useful to conduct appropriate tests.

To characterize the deformation or compression behavior of waste, parameters like 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 also 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 compres-sion parameters requires an extensive description and classification of the waste materials.

An appropriate method to classify “other wastes” may be the conduction of tests like grain-size analy-sis, determination of the main waste constituents, moisture content, consistency limits, organic con-tent, etc,. 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.

3 DYNAMIC MUNICIPAL SOLID WASTE PROPERTIES

It is important to stress that the dynamic characteris-tics of solid waste materials play an important role on the seismic response of landfill (Maugeri and Sêco e Pinto, 2005).

Some properties are measured directly, such as dry density and water contents, whereas other prop-erties, due the difficulties related with sampling, are obtained from indirect methods combining with the existent knowledge of waste properties.

Both in situ (Matasovic et al., 2011) and labora-tory tests (Athanasopoulos, 2011) can be performed to evaluate these properties. MSW properties that are used as input in the seismic analyses of landfills in-clude:

− the unit weight of MSW; − the (dynamic) shear strength of MSW; − the shear wave velocity VS and small-strain

shear modulus G0 profile; − the shear strain-dependent shear modulus reduc-

tion G/G0 and material damping (λ) curves; and − the dynamic Poisson’s ratio ν.

The unit weight of MSW can vary significantly at a landfill site and its variation with depth affects sig-nificantly the seismic response of landfills. Zekkos et al. (2006), based on large-scale laboratory and in situ field data developed a hyperbolic model for the unit weight of MSW. The unit weight was found to be affected by compaction effort and composition as

well as confining stress and is described by the fol-lowing equation:

where γi is the near-surface in-place unit weight (kN/m3), z is the depth (m) at which the MSW unit weight γ is to be estimated, and α and β are modeling parameters with units of m4/kN and m3/kN, respec-tively.

Calibration of the model using laboratory and field test data, yielded values for γi, α and β that are a function of compaction effort and amount of soil and are shown in Table 1. Table 1. Unit weight parameters for different compaction effort and amount of soil cover (Zekkos et al., 2006).

Compaction effort and soil amount

iγ

kN/m3

β

m3/kN

α

m4/kN

Low 5 0.1 2

Typical 10 0.2 3

High 15.5 0.9 6

The shear strength properties of waste landfills are not easily determined since the physical compo-sition of the mixture makes it unsuitable for the con-ventional laboratory strength testing. To overcome this situation, site-specific MSW shear strength is not typically used, but data reported in the literature is used.

Limited data is available on the dynamic shear strength of MSW. Bray et al. (2011) recommended a nonlinear static shear strength envelope. On the basis of laboratory tests performed at various strain rates with no pore pressure increase, the dynamic shear strength of MSW was estimated to be about 25% greater than its static shear strength.

However, because of the scarcity of the data, a conservative estimate that the dynamic shear strength is 1.2 times its static shear strength was rec-ommended for use in practice.

This conclusion is also consistent with the back-calculations made by Augello et al. (1995). An ex-tensive review of the state of the art on the VS, the strain-dependent nonlinear dynamic properties of MSW and Poisson’s ratio has been performed by Zekkos et al. (2011). The VS will vary for different landfill sites. A summary of the available in situ data is presented in Figure 2.

Nonlinear dynamic properties of MSW have been recommended in the literature primarily based on the numerically back-calculated response of the OII landfill, with the exception of the Zekkos et al. (2008) curves that were developed based on large-scale laboratory testing and were found to be largely a function of waste composition and confining stress. The characterization of waste material proper-ties for seismic design is difficult due the heteroge-

neity of the material, requiring the procurement of large samples.

Figure 2. MSW Shear wave velocity profiles reported in the literature (Zekkos et al., 2011).

4 TEST SITE

The study was conducted at “Cozzo Vuturo” landfill (Figure 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, Leonforte, Villarosa and Valguarnera), including more than twenty towns, for a total of about 180.000 inhabi-tants.

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 Mio-cene 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 2.10-9 ÷ 7.10-9 cm/sec. 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. There is another low flow aqui-fer, 30 ÷ 40 m below the ground surface, situated be-low the clay layer.

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

z

zi ⋅++=

βαγγ

January. The precipitation percentage in the rainy season (October-March) accounts for 75% of the to-tal annual precipitation. The rainfall is often in the form of a quick and hard torrential 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 (Figure 3); 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 natu-rally occurring valley. According to the original pro-ject, the residual useful life of the B2 waste disposal plant is expected to be about 3 years and half (end of the disposal activity in 2011) but an extension up to about 650.000 m³ is currently considered. The land-fill is about 18 m high. The upper soil layers have been removed to create space for the waste and to guarantee geotechnical safe conditions by the execu-tion of a terracing profile. The waste is piled up in compacted layers of about 1.8 m thick. The waste density in the landfill ranges between 7 and 8 kN/m³. The daily covering layers consist of clayey soil.

Figure 3: Dilatometer Marchetti Tests (SDMT) location. The refuse dumping plan considers the landfill

divided in 6 cells. As a consequence drainage of storm-waters was designed on a four level basis, ac-cording to the landfill filling sequence. One-fourth of the landfill is used as an area for leachate collec-tion. The leachate extraction is carried out only from the two lower cells. The collected leachate is treated in a external water plant. A system for biogas collec-tion is expected to be installed in the landfill and will be operated after the landfill closure.

The landfill sealing at the bottom consists of nu-merous different layers. Specifically landfill bottom is lined with recompacted clay (about 1 m depth), a geotextile protection layer and an HDPE geomem-brane to prevent any seepage of leachate into the un-derlying ground. Landfill walls were lined with a geotextile.

5 FIELD TESTING PROGRAM

Field measurement techniques may be divided into non-intrusive measurements that do not penetrate the waste mass and intrusive measurements that pene-trate the waste mass. Non-intrusive techniques range from seismological and geophysical measurements of wave propagation velocities and electrical resis-tivity, while intrusive techniques include measure-ments made in borings or soundings such as Stan-dard Penetration Test (SPT), Cross-Hole Test (CH), Down-Hole Test (DH) and 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 in-direct measurements that rely on correlations to evaluate properties of interest.

The Dilatometer Marchetti Test (DMT) is widely used for in situ measurements of physical and me-chanical 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 seis-mic module for measuring the shear wave velocity VS. The seismic dilatometer test, conceptually simi-lar to the seismic cone penetration test (SCPT), was first introduced by Hepton (1988) and subsequently improved at Georgia Tech, Atlanta, USA (Martin and Mayne, 1997; 1998; Mayne et al., 1999). A new SDMT system, described in Marchetti et al. (2008), has been recently developed in Italy. Validations of VS measurements by SDMT compared to VS meas-urements by other in situ techniques at various re-search sites are reported in Marchetti et al. (2008). Besides the shear wave velocity VS, the seismic dila-tometer provides the usual DMT parameters by use of common correlations (Marchetti, 1980; TC16, 2001). The small strain and large strain moduli back calculated from the SDMT measurements can be combined to identify the modulus degradation curve for a specific soil (Mayne et al., 1999). Despite its success in measuring soil properties, the use of DMT for in situ measurements of MSW properties has not yet been reported in literature (Matasovic et al., 2008).

This section presents the results obtained by seismic dilatometer tests executed at the “Cozzo Vu-turo” landfill (Enna, Italy), as part of the geotechni-cal investigations planned for the mechanical charac-terization of 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, one dilatometer test is located between the two disposal site B1 and B2 (Figure 3). A cross section of the landfill with the lo-cation of the seismic dilatometer tests is reported in Figure 4. It can be observed that the results deducti-ble from tests named B1a and B1b, as well as from

those named B2a and B2b, regard the deposited waste materials, while the results deductible from test named Arg are referred to the soil foundation (Figure 4).

Figure 4: Cross section of the landfill with the location of SDM Tests.

Figures 5 and 6 show the typical profiles obtained

by the Marchetti dilatometer tests and in particular: Figure 5 reports the material index (Id) profile, while Figure 6 reports the working strain constrained modulus (M) profile, obtained from the usual DMT interpretation. Figure 7 and 8 show the results ob-tainable by the seismic dilatometer (SDMT): Figure 7 reports the shear wave velocity (VS) profile and Figure 8 reports the small strain shear modulus (G0) profile (obtained from VS as G0 = ρ .VS

2). Analyzing the results obtained it can be observed

that according to material index (Id) derived by the SDM Tests, the old deposit waste materials located in the catchments basin named B1 (1999-2006) can be classified as silt-clayey material, while deposit waste materials located in the new catchments basin (2006-2011) named B2, can be classified as silt-sandy material (Figure 5).

As concern the constrained modulus (M), the val-ues seem to be affected by a consistent variation with depth and M ranges between 2 MPa up to about 100 MPa (Figure 6).

The particularity of the seismic dilatometer is the possibility to provide, with the usual DMT parame-ters, the shear wave velocity VS profile. In this case values of VS generally increasing with depth from about 100 m/sec up to more of 400 m/sec were de-rived (Figure 7), in good agreement with the MSW shear wave velocity profiles reported in the literature (Zekkos et al., 2011). Consequently, the values of the small strain shear modulus G0 range from about 18 MPa up to more of 200 MPa (Figure 8).

Figure 5: Material index (Id) profiles.

Figure 6: Constrained modulus (M) profiles.

Figure 7: Shear wave velocity (VS) profiles.

Figure 8: Small strain shear modulus (G0) profiles.

6 LABORATORY TESTING PROGRAM

Compression parameters of soil-like waste can be determined using conventional geotechnical testing methods. For other wastes, appropriate modifica-tions of the testing equipment and/or procedures are necessary. In this last case testing methods must take account of several factors not comparable to the test-ing of soils: size of samples, high compressibility, simulation of in situ placement methods, simulation of in situ structure of waste. Thus laboratory testing of waste materials requires equipment that provides the possibility of testing large-sized samples in com-bination with large strains.

The compression behavior of municipal waste is characterized by large amounts of settlements which are time-dependent. By the help of confined com-pression tests on waste materials load induced, me-chanical settlements of a landfill can be assessed. In combination with a suitable approach for settlements

caused by biochemical degradation processes, a total settlement provision may be possible (Jessberger & Kockel, 1993b). Considering an ideal compressibil-ity test, different deformation phases can be pointed out: A) a first phase regulating the initial overall set-tlement of wastes and the big deformations of highly deformable components. B) a second longer phase connected to the release of overpressures of fluids absorbed. C) a last phase related to the viscous be-havior of solid materials.

To estimate the compressibility of MSW and to analyze the evolution of settlements, compression testing have been conducted by the use of a large-scale one-dimensional compression cell (Maugeri and Castelli, 2008).

New and 5+ year old waste materials were re-trieved and stored separately in sealed drums. As ex-ample, the time-vertical strain curves obtained for the 5+ years old wastes tested are shown in Figure 9.

Figure 9: Time-vertical strain curves of “old waste” tested.

7 REFERENCES

Athanasopoulos, G.A. 2011. Chapter 7: Laboratory Testing of Municipal Solid Waste, in Geotechnical characterization, Field Measurements, and Laboratory Testing of Municipal Solid Waste. In D. Zekkos (eds.), ASCE Geotechnical Spe-cial Publication no.209: 112-134.

Augello, A.J., Matasovic, N., Bray, J.D., Kavazanjian, Jr., E. and Seed, R.B. 1995. Evaluation of solid waste landfill per-formance during the Northridge earthquake. In M.K. Yegian & W.D.L. Finn (eds.), Earthquake design and per-formance of solid waste landfills, ASCE Geotechnical Spe-cial Publication no.54: 17-50.

Bray, J.D., Zekkos, D., Kavazanjian, E. Jr., Athanasopoulos, G.A. & Riemer, M.F. 2009. Shear Strength of Municipal Solid Waste. Journal of Geotechnical and Geoenvironmen-tal Engineering, ASCE 135(6): 709-722.

Bray, J.D., Zekkos, D., Kavazanjian, E. Jr., Athanasopoulos, G.A. & Riemer, M.F. 2011. Closure of Shear Strength of Municipal Solid Waste. J. of Geotechnical and Geoenvi-ronmental Engineering, ASCE, 136(12): 1731-1732.

Dixon, N., Ng’ambi, S. & Jones, D.R.V. 2004. Structural per-formance of a steep slope landfill lining system. Proceed-ings ICE 157: 115-125.

Grisolia, M., Napoleoni, Q. & Tancredi, G. 1995. Contribution to a technical classification of MSW. Proc. 5th Int. Landfill

Symposium Sardinia 1995, CISA, Cagliari: 703-710. Hepton, P. 1988. Shear wave velocity measurements during

penetration testing. Proceedings Penetration Testing in the UK, ICE: 275-278.

Kavazanjian, E. Jr., Matasović, N. & Bachus, R. C. 1999. Large diameter static and cyclic laboratory testing of mu-nicipal solid waste. Proc. 7th Int. Waste Management and Landfill Symposium, Sardinia 1999, Vol.3, Cagliari 3: 437-444.

Jessberger, H.L. & Kockel, R. 1993a. Determination and as-sessment of the mechanical properties of waste. Waste Dis-posal by Landfill - Green ’93. In R.W. Sarsby (ed): 313-322. Rotterdam: Balkema.

Jessberger, H.L. & Kockel, R. 1993b. Determination and as-sessment of the mechanical properties of waste materials. Proceedings 4th Int. Landfill Symposium, Sardinia 93, Cagliari: 1383-1392.

Marchetti S. 1980. In Situ Tests by Flat Dilatometer. Journal of Geotechnical Engineering, ASCE 106(GT3): 299-321.

Marchetti, S., Monaco, P., Totani, G., Marchetti, D. 2008. In Situ Tests by Seismic Dilatometer (SDMT), From Research to Practice in Geotechnical Engineering. In J.E. Laier, D.K. Crapps & M.H. Hussein (eds.), ASCE Geotechnical Special Publication (180):292-311.

Martin, G.K & Mayne, P.W. 1997. Seismic Flat Dilatometer Tests in Connecticut Valley Varved Clay. Geotechnical Testing Journal, ASTM 20(3): 357-361.

Martin, G.K & Mayne, P.W. 1998. Seismic flat dilatometer in Piedmont residual soils. Proc. 1st Int. Conf. on Site Charac-terization, Atlanta. In P.K. Robertson & P.W. Mayne (eds.), 2: 837-843. Rotterdam: Balkema.

Matasovic, N., El-Sherbiny, R. & Kavazanjian, E. Jr. 2008. In-situ measurements of MSW properties. Geotechnical Char-acterization, Field Measurement and Laboratory Testing of Municipal Solid Waste. In D. Zekkos (ed.), ASCE Geotech-nical Special Publication no.209: 153-193.

Matasovic, N., El-Sherbiny, R., & Kavazanjian, E. Jr. 2011. Chapter 6: In-situ measurements of MSW properties, in Geotechnical characterization, Field Measurements, and Laboratory Testing of Municipal Solid Waste. ASCE Geo-technical Special Publication no.209: 112-134.

Maugeri, M., & Castelli, F. 2008. Experimental analysis of waste compressibility. Proceedings Geocongress 2008. ASCE Geotechnical Special Publication no. 177: 208-215.

Maugeri M. & Sêco e Pinto P.S. 2005. Seismic Design of Solid Waste. Landfills and Lining Systems. Chapter 5 TC5 Re-port, September 2005.

Mayne, P.W., Schneider, J.A., Martin, G.K. 1999. Smalland large-strain soil properties from seismic flat dilatometer tests. Proceedings 2nd Int. Sym. on Pre-Failure Deforma-tion Characteristics of Geomaterials, Torino 1: 419-427.

TC16 2001. The Flat Dilatometer Test (DMT) in Soil Investi-gations - A Report by the ISSMGE Committee TC16. In R.A. Failmezger & J.B. Anderson (eds.): 7-48. Proc. 2nd Int. Conf. on Flat Dilatometer, Washington.

Zekkos, D. P., Bray, J. D., Kavazanjian, E., Matasovic, N., Rathje, E., Riemer, M., and Stokoe, K. H. 2006. Unit weight of municipal solid waste. J. Geotechnical and Geoenvironmental Engineering ASCE 132(10): 1250-1261.

Zekkos, D., Bray, J.D., Riemer M.F. 2008. Shear modulus and material damping of municipal solid waste based on large-scale cyclic triaxial testing. Canadian Geotechnical Journal 45(1): 45-58.

Zekkos, D., Matasovic, N., El-Sherbiny, R., Athanasopoulos, A., Towhata, I. & Maugeri, M. 2011. Chapter 4: Dynamic Properties of Municipal Solid Waste, in Geotechnical char-acterization, Field Measurements, and Laboratory Testing of Municipal Solid Waste. In D. Zekkos (eds.), ASCE Geo-technical Special Publication no.209: 112-134.

0%

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0,1 1 10 100 1000 10000

Time [min]

Ver

tical

Str

ain

εε εε [

%]

20 kPa 40 kPa 80 kPa 150 kPa 300 kPa 600 kPa