Mechanical Properties of Municipal Solid Waste

Transcription

1 Journal of Testing and Evaluation, November 2004, Vol. 32, No. 6 Paper ID JTE11945 Available online at Orencio M. Vilar 1 and Miriam de F. Carvalho 2 Mechanical Properties of Municipal Solid Waste ABSTRACT: This paper deals with the compressibility and shear strength of Municipal Solid Waste (MSW) as measured in the laboratory. The waste studied was recovered from Bandeirantes Sanitary Landfill (São Paulo, Brazil) and is about 15 years old. More than 50 % by weight of MSW is organic paste and as much as 17 % is plastic and other strip materials. Consolidation and drained and undrained triaxial compression tests were performed in reconstituted waste specimens of large dimensions and with different unit weights considering both saturated specimens and specimens tested at natural moisture content. It is shown that the MSW exhibits a pronounced secondary compression; the influence of void ratio and stress on secondary compression index is discussed and compared with available data from MSW and soft soils. Typical stress-strain curves are presented and some peculiarities are shown, as the absence of failure even at large strains, up to 30 %, implies the need of deriving shear strength envelopes based on strain. The influence of saturation and of unit weight on shear strength is also addressed as well as the shear strength mobilization that shows that the friction tends to be fully mobilized at strains of about 20 %, while the cohesion intercept starts to be mobilized at strains of 10 % and even more and a limiting value could not be observed in the strain range attained in the tests. The development of pore water pressures in the undrained tests and the variation of undrained shear strength with effective confining pressure are also addressed. KEYWORDS: municipal solid waste, landfill, compressibility, shear strength Introduction The management and disposition of all kinds of refuse, including municipal solid waste (MSW), is an important task faced by society. As far as MSW is concerned, efforts to minimize waste production as well as to recycle or reuse some of its materials are among some of today s waste management policies. However, these policies, together with other refuse reduction techniques are unable to avoid some kind of final disposition system such as a sanitary landfill. A sanitary landfill is designed considering sound sanitary and environmental principles but it must also be seen as an engineering structure. In this sense the mechanical properties of waste are of paramount importance in order to deal with questions such as stability against failure, deformation, stress imposed to foundation soil, and the behavior of accessory elements, such as liners and gas and leachate drains, among others. As could be expected, the mechanical behavior of MSW is a very complex subject. Different material types such as paper, metal, glass, wood, garden waste, food, and plastic, with variable dimensions, are usually present. So, besides inert materials and daily soil covering there are also degradable components, such as organic matter. Each one of these components has a particular mechanical behavior with different strength and compressibility properties that can vary with time. The proportion of each component is highly variable and is related to cultural and economic aspects 1 Associate Professor, University of São Paulo, São Carlos, Brazil. E mail: orencio@sc.usp.br 2 Assistant Professor, Catholic University of Salvador, Brazil. E mail: mfcarvalho@geoamb.eng.ufba.br Copyright 2004 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA

2 2 JOURNAL OF TESTING AND EVALUATION of the region where the refuse is generated. This probably imparts particular characteristics for MSW generated in different parts of the world. These aspects and other difficulties such as getting representative samples, adequate testing procedures both in the field and in the laboratory, and the lack of reliable models to represent waste behavior have led to the study of MSW considering some typical concepts of Soil Mechanics, sometimes adapted to take into account the features of such a peculiar material [1,2]. For instance, the shear strength of MSW is usually determined from large scale triaxial compression or direct shear tests, in situ tests, or back analysis results that are interpreted according to the Mohr-Coulomb shear strength criteria. Shear strength values of MSW reported in the literature vary widely, with friction angles between 10 and 53 and cohesion values varying from 0 to 67 kpa [3,4]. This variability can be credited to the heterogeneity of the sample, the strain level used to define shear strength parameters, the choice of representative samples, and the tests used. In addition, it is possible that other variables such as the age, composition, density, and moisture content of the waste can interfere with the shear strength of MSW. Based on limited data and calling attention to the wide scattering in the available data, some authors such as Singh and Murphy [5], Kavazanjian et al. [3], Van Impe [6], and Manassero et al. [7] have proposed shear strength envelopes for practical purposes. Manassero et al. [7] proposed a shear strength envelope that depends on the normal stresses (σ). For very low normal stresses (σ<20 kpa) it is considered that MSW behaves as a cohesive soil-like material, thus showing a cohesion intercept (c) of about 20 kpa. For low to moderate normal stresses, that is, 20 < σ < 60 kpa, MSW is considered to be a purely frictional material with a shear strength angle (φ) of 38 o. Finally, for larger normal stresses, it is suggested c 20 kpa and φ 30o. These values are comparable to those suggested by Kavazanjian et al.[3]. Regarding the compressibility of MSW, many mechanisms contribute to it. It is known that MSW exhibits a pronounced secondary compression. This is related to material creep and to biological and chemical decomposition which are time-dependent processes that extend for many years, depending on environmental and fill managing factors, such as climate and leachate recirculation. In addition to these important processes, the almost instantaneous compression (credited to void ratio reduction and compression and distortion of some components) due to applied loads must be considered. Propositions about the mechanisms that determine MSW compression and how they are distributed with time can be found, for instance, in Grisolia and Napoleoni [8] and Stulgis et al. [9]. As far as the modelling of MSW mechanical behavior is concerned, compression is the topic that has received the most attention from the researchers. For instance, Sowers [10] extended the classical soil consolidation model, plus a secondary compression component, to study settlement of landfills. To deal with the accentuated secondary compression of waste, some authors have used the Gibson and Lo [11] rheological model or a power creep law [12]. The biological degradation and its influence on waste settlement have been dealt with by some authors such as Edgers et al. [13], Sanchez-Alciturri et al. [14], and Park and Lee [15]. Marques et al. [16] presented a composite model that takes into account initial mechanical compression in response to applied load and secondary compression due to mechanical creep and biodegradation. The model was implemented in a computer program to perform one-dimensional analyses of MSW landfill and was able to reproduce the settlements measured in a landfill. A more comprehensive model that encompasses stress-strain behavior and time-dependent deformation was proposed by Machado et al. [17]. The proposal considers that two different effects determine MSW mechanical behavior: the reinforcement of MSW by synthetic fibers and the MSW paste, that is, the nonfibrous-materials component of the waste. The time-dependent compression was modelled through a hyperbolic relationship and the model was capable of reproducing stress strain curves from triaxial compression tests as well as settlements recorded in a sanitary landfill. In this paper, results of triaxial compression and consolidation tests carried out on large remolded samples of MSW are presented and discussed. The influence of some variables on shear strength, such as density, moisture content and saturation, size of specimen, type of test, and the

3 VILAR AND CARVALHO ON SOLID WASTE 3 mobilization of shear strength parameters with deformation are addressed as well as the general characteristics of compression of waste, including some aspects of its time-dependent behavior. The Tested Waste Large consolidation and triaxial tests were performed on samples from Bandeirantes Sanitary Landfill, located in the city of São Paulo (Brazil), which is one of the greatest areas of urban solid waste disposition in the city. It extends over an area of about 100 hectares and depths of refuse vary between 20 and 100 m. The MSW tested was recovered through two mechanical auger borings (40 cm in diameter) from an experimental site of about 450 m 2 where the waste was about 15 years old and reached about 30 m in depth. Samples were collected at each 2 m and characterized through physical and chemical tests. Figure 1 presents typical grain size distribution curves of waste samples from the two boreholes (T1 and T2) obtained by sieving the waste that was dried at 70º C. A and B represent the horizons located from 0 to 18 m in depth and deeper than 18 m, respectively. The number that follows refers to the sample depth, in metres. Components larger than 50 mm were manually measured and plastics and other almost linear or planar elements of large dimensions (above 150 mm) were not included. The waste has its major portion finer than 10 mm and the largest particle dimension is about 100 mm. As can be seen, the T2B (18 30 m) sample shows about 60 % of particles finer than 10 mm, while the corresponding value for the other samples is about 45 %. Figure 2 shows the average physical composition of waste. The largest portion is of organic paste, which includes organic matter and daily covering soil. The volatile content and chemical oxygen demand were used to measure the organic content of the waste studied. It can be seen that the average organic paste content is about 55 % and it was found that this paste is composed of 12 % of organic matter and 43 % of inert materials. Whereas the average proportion of organic paste for all the samples was about 55 %, it reached 61 % in sample T2B, and was about 50 % for the other samples. It is also worthy to point out the large proportion of plastic (about 17 %) represented in the waste. The experimental site was investigated performing Standard Penetration Tests at intervals of 1 m and Cone Penetration Tests of the mechanical type. The bore holes used for the SPT measurements reached about 30 m in depth. Typical results for the Standard Penetration Test (SPT or N) are outlined in Fig. 3a as well as the boring profiles, where one can see thicknesses of waste layers varying between 2.8 and 15 m, and soil cover layers between 0.25 and 2 m thick. The number of blows increases with depth and excluding the values higher than 20, which correspond to the effort to surpass more resistant materials such as stone, rubber, and wood, the average SPT is about 7 blows for the two superficial layers. The SPT reaches about 12 blows for the third and fourth layers of refuse (between 10 and 30 m in depth) and these values are in agreement with those found in the literature [5,14,18]. Cone penetration tests (CPT) were conducted in two locations with depths ranging from 19 to 26 m. Problems of angular deflection of the probe and excessive penetration resistance resulted in the test ending before reaching foundation soil. The CPT results indicated that the cone frequently encountered stiff objects, which produced sharp peaks in the tip resistance measurements. These results are highly variable as can be seen in Fig. 3b, where lateral resistance varies between 56 and 1056 kpa and tip resistance from 1200 to kpa. In both cases, an envelope for the minimum values shows that they increase with depth. Typical tip resistance varied between 2500 and 7500 kpa and the friction ratio (shaft friction/tip resistance ) between 2.5 and 5.0 % (values larger than kpa for tip resistance and 400 kpa for lateral resistance were considered to be not representative of waste). These values plotted in identification charts used for soils fall in the region of clayey sand and silts and sandy and silty clays. Representative samples of MSW used in the laboratory tests were obtained through a mixture of similar materials collected in the two boreholes. In this sense two horizons were identified. The first

4 4 JOURNAL OF TESTING AND EVALUATION one, from 0 to 18 m in depth comprised visually less-degraded material, while the lower horizon, from 18 to 30 m in depth was composed of more degraded materials, with dark coloration and strong odor. So, it was possible to compose four representative samples: T1A, T1B, T2A, and T2B (T1 and T2 bore numbers 1 and 2; A more superficial horizon and B deeper horizon). After performing the chemical tests, the visual degradation was not confirmed, since Chemical Oxygen Demand and Total Volatile Solids were similar for all the representative samples. The major difference between the samples rests on the larger proportion of fines and organic paste of sample T2B. Among all the samples, T1B, T2A, and T2B were used for quantification of shear strength and compressibility properties of the waste. The unit weight of solid particles was determined using the prescriptions of applicable standards, but substituting the pyncnometer for a 2 L glass container calibrated for temperatures between 15 and 40 C. Table 1 shows the unit weight of solid particles obtained for three of the samples studied. Before molding the specimens for the consolidation and triaxial compression tests, the representative samples were mixed thoroughly to avoid segregation and reduced samples were obtained after quartering. After that, the sample was scalped and the largest particles were substituted by an equal amount of finer particles in order to keep the relationship between the largest particle size present and specimen diameter lower than 1/6. Test Equipment and Procedures Consolidation tests were carried out in a large consolidation cell (365 mm in diameter and 385 mm in height). Loads were applied through compressed air acting on the pressure chamber located in the top of the cell. Porous stones located at the top and at the base of the cell allowed drainage of the effluent produced during the consolidation process. The specimens were molded to reach unit weights between 8 and 14 kn/m3. The waste at its natural moisture content was spread in layers and each one was carefully pressed to reach the desired unit weight, until completely filling the whole height of the consolidometer. Six specimens were tested with their natural moisture content and one was soaked to simulate saturation. In general, each load step acted about 7 days but some loads remained over the specimen for 15 or more days in order to have a more complete picture of the creep features of the MSW. A large triaxial cell was constructed to test samples up to 400 mm in height and 200 mm in diameter. The triaxial chamber is made of rigid PVC, 300 mm in diameter and capable of supporting 1 MPa of cell pressure. The base, top, and cap are constituted of hard aluminum and the loading ram is stainless steel. Triaxial compression tests were carried out using statically compacted specimens with nominal unit weights of 10, 12, and 14 kn/m3, diameters of mm, and heights of mm. Saturated specimens and specimens molded at the natural moisture content were tested. In the saturated tests, back pressure was applied in steps and the pore water pressure parameter, B, reached about 0.9 for 200 kpa of pressure and did not increase any more as back pressure was increased, even for 450 kpa, which was the largest back pressure the triaxial cell could withstand. From the consolidation characteristics of waste it was found that a strain rate of 0.7 mm/min was a conservative value for the drained tests (CD). This strain rate was also adopted in the undrained tests (CU). Effective confining pressures (σ' 3c ) of 100, 200, and 400 kpa were used.

5 VILAR AND CARVALHO ON SOLID WASTE 5 Results and Discussion Consolidation Tests Table 2 gives the initial and final characteristics of all the specimens used in the consolidation tests. The number that follows the identification sample is the molding unit weight in kn/m3. Except for the sample molded with a unit weight of 8 kn/m3, all the others were compressed up to 640 kpa. After compression under 640 kpa, it can be observed that all of the samples reached unit weights larger than 15 kn/m3 and the denser specimens reached values larger than 16 kn/m3. If dry unit weight is considered, it can be seen that it practically doubled at the end of the test in the samples molded with dry unit weights approximately lower than 7 kn/m3. Not considering the additional secondary compression that still would occur, final void ratios under 640 kpa were almost independent of initial void ratio and were of the order of 1.0. Figure 4 shows typical results of dial readings against time on a logarithmic scale from the consolidation test of sample T1B10 molded at both natural moisture content and saturated conditions. As the load was applied there was an almost instantaneous compression, followed by gradual compression with time characterizing a secondary compression or creep deformation. The load stages were interrupted after some reasonable time that could allow the determination of secondary compression characteristics; however, some stages lasted about fifteen days, without any sign of deformation stabilization. The data of these tests, together with the results from the other samples, allowed the calculation of secondary compression indexes C α (C α = e/ log t) and C α (C α = C α /(1 + e o )) for each stage of loading, as shown in Fig. 5. Discarding the values related to the first stages of loading, more affected by accommodation of the waste and of the consolidometer, it can be observed that C α ranged from to 0.044, with an average value of It was not possible to relate C α to void ratio, although there was a tendency for the lower density specimens to yield larger values of C α. The saturation did not appear to influence C α since the values for the saturated and natural moisture content specimens tended to superpose. A similar tendency was observed for C' α [C' α = C α /(1 + e o )], which varied between and 0.016, with an average value of The values of C α found are lower than the ones presented by Sowers [10] and by Gabr and Valero [19], as shown in Fig. 6, but they are of the same order of magnitude of the waste tested by Landva and Clark [20]. In the range between 100 and 640 kpa, the influence of overburden stress on C α and C' α was almost negligible, although a slight decrease in C α was observed as the stress increased. Although some load steps lasted 15 days or more, the creep deformation must be almost entirely credited to material creep without any appreciable contribution of degradation creep determined by the many biological and chemical processes that act in a sanitary landfill. As an equilibrium void ratio was not attained in the time spans used in the tests, some curves for different times of loading were plotted. Figure 7 illustrates some compression curves of samples T2A and T1B molded with an initial unit weight of 10 kn/m3. The curves were almost parallel but not straight. They were rather slightly concave upwards; however, straight curves were fitted to points between 60 and 640 kpa allowing one to obtain the primary compression index [C c = e/ log(σ)]. C c values from these tests, together with the data from the other specimens, are summarized in Table 3, where it can be seen that C c varied between 0.52 and The data from Table 3 indicate that C c is dependent on initial void ratio since C c decreases with void ratio. If C c is normalized through the specific volume (1 + e o ), it can be seen that the compression coefficients C' c [C' c = C c /(1 + e o )] obtained are very close, averaging 0.21, as indicated in Table 3. C c values were among the lowest values reported by Sowers [10] and were similar to the data from Gabr and Valero [19], as shown in Fig. 8. Otherwise, C' c were near to the lowest value reported by Landva and Clark [20], when testing MSW from Canada in a large compression cell.

6 6 JOURNAL OF TESTING AND EVALUATION The relationship between C α and C c or C' α and C' c is about 0.06 which is in the range usually reported for amorphous and fibrous peats [21]. Drained Triaxial Compression Tests Stress-strain and volumetric-axial strain curves obtained from triaxial compression tests are presented in Figs. 9 and 10 for the 150 mm-diameter specimens of sample T2A compacted at natural moisture content and for saturated ones, respectively. The results shown are from specimens compacted to a unit weight of 12 kn/m3, but can be qualitatively assumed as typical of the stress strain behavior of all the samples tested. It can be observed that MSW does not show a peak or an ultimate value as is common in soils, since the deviator stress (σ 1 -σ 3 ) increases, almost continuously, with axial strain (ε a ). For larger strains, the curves are upwardly concave thus suggesting that the material is stiffening. As far as the strains are concerned, volumetric strain (ε v ) increases continuously with axial strain and the largest volume compression is related to the specimen tested with the lower confining pressure. The stress-strain curves showed are qualitatively similar to MSW experimental curves presented by Grisolia et al. [22] for specimens of reconstructed waste and by Jessberger and Kockel [23], who tested milled waste. The stress path followed in these tests plotted in the s',t plane are shown in Fig. 11, where: (1) (2) As failure cannot usually be clearly defined when testing MSW and considering the need for an operational shear strength envelope to deal with typical problems such as slope stability, a reliable procedure is to use the Mohr-Coulomb criterion related to some value of strain. This is done in Fig. 11, where shear strength envelopes for axial strains of 10, 20, and 30 % are included. Table 4 summarizes the shear strength envelopes of sample T2A, including the envelopes of Fig. 11 and the envelopes for the other unit weight used. As can be seen in Fig. 11 and Table 4, the mobilized shear strength tends to increase with strain both for the natural moisture content and saturated samples. For instance, for the natural moisture specimens with a unit weight of 12 kn/m3, cohesion departed from 20 kpa for 10 % strain and increased to 71 kpa for 30 % strain. For the same strains the friction angle increased from 22 o to 33 o. The large cohesion intercepts calculated have been credited to the fiber components (especially plastics) that provide a reinforcement mechanism [24] responsible, for instance, for the existence of vertical cuts as high as 4 m in the tested waste in the field. Shear strength parameters of different magnitudes were measured in the saturated and nonsaturated samples. For example, at 20 % strain, the samples tested at natural water content showed a cohesion intercept of 39.2 kpa and a friction angle of 29. The corresponding values for the saturated samples were 60.7 kpa and 23, respectively. Saturated samples apparently show a decrease in friction angle and an increase of cohesion when compared to the samples tested at natural water content. This is different from what happens in many soils where saturation produces a reduction on suction and on cohesion. This variation could be credited to the heterogeneity of the samples, but it is possible to speculate that the

7 VILAR AND CARVALHO ON SOLID WASTE 7 saturation could reduce the friction strength between many of the waste components (plastic, paper, metal, etc), thus reducing the friction angle. On the other hand, the cohesion intercept seems to be determined by the fibrous components that produce reinforcement and probably is not linked to the MSW water content. However, it must be recalled that the unsaturated specimens had a relatively high initial degree of saturation (above 70 %). The large volume compression during consolidation raised the degree of saturation, approaching the saturated condition, and in this respect both conditions should yield close strengths. Although shear strength parameters were different, it must be noted that the friction angle of the saturated specimen is lower than that of the unsaturated one, while the inverse occurs with cohesion; however, the available total resistance is almost the same in each case. In fact, if the results of both samples are plotted together as in Fig. 12, only a small dispersion around an average envelope is seen, in spite of the specimen condition (saturated or not). So, in this case, for each strain level, a single envelope could represent the shear strength of saturated and nonsaturated samples. Table 5 summarizes the average shear strength parameters considering the saturated and unsaturated specimens of sample T2A tested at a unit weight of 12 kn/m3. Specimens with Different Unit Weight As shown in Table 4, unit weights of 10, 12, and 14 kn/m 3 were used in this study and its influence on the stress-strain behavior can be observed through the tests performed with an effective confining stress of 100 kpa, as shown in Fig. 13. There is good agreement between the three curves, which tend to superpose. This behavior was similar for the other effective stresses used in the tests and it was possible to adjust the shear strength envelopes for different strains that were independent of unit weights, as shown in Fig. 14 and Table 6. So the unit weights showed little influence on the measured shear strength, at least for the range used in this study, from 10 to 14 kn/m 3. However, this should be considered a peculiarity of the tests presented here, since the molding conditions of the specimens provided only small variation in the physical indexes at the end of consolidation, as can be checked in Table 7. The unit dry weight varied little among all the specimens, which could account for the close results for shear strength of all of the specimens. So this finding should not be extended to any other unit weights. For instance, fresh MSW after usual compaction reaches unit weights between 3 and 6 kn/m 3 with few contact points, and until more data is available it is difficult to believe that these low densities will yield the same strength as the denser and older MSW. Table 8 presents the shear strength envelopes of sample T1B tested with 12 kn/m3 of unit weight in both the saturated and nonsaturated conditions. As can be seen, the results approach those of sample T2A, with only small differences. The results of sample T2B are shown in Table 9. Although the stress-strain curves of this sample were qualitatively similar to the curves of sample T2A, shown in Figs. 9 and 10, there were differences in the values measured. A notable deviation in friction angle occurred in sample T2B, which showed values lower than the other samples. The major presence of fines and organic paste was the most plausible explanation for the reduction of the friction angle, since the cohesion that is determined by fiber reinforcement was almost the same for samples T2A and T1B. The Influence of Specimen Dimensions Consolidated drained tests performed with 200 mmdiameter specimens reached lower deviator stress than for 150 mm-diameter specimens. This is illustrated by the stress-strain curves of specimens tested under various confining pressures shown in Fig. 15. The variation observed in all the specimens is summarized in Fig. 16, where the relationship between the deviator stresses measured in specimens with cm [(σ 1 σ 3 ) 15 ] and cm [(σ 1 σ 3 ) 20 ] is shown. For confining stresses of 100 and 200 kpa and axial strain between 10 and 25 %, the relationship reaches a maximum of about 1.2, while for 400 kpa the relationship increases continuously with strain. One possible reason for the increase of shear strength in the smaller samples is the reinforcement effect of the fibrous materials, since the same amount of fibrous material was used in both specimens. The fibers can be more uniformly

8 8 JOURNAL OF TESTING AND EVALUATION distributed throughout the smaller dimension specimen thus providing a more efficient reinforcement action and a larger strength. Shear Strength Mobilization Figure 17 summarizes the average shear strength parameters as a function of the axial strain for the representative MSW samples. There one can observe the similarity in shear strength parameters of samples T2A and T1B, the influence of specimen size, and the discrepant behavior of sample T2B. As already noted, there is a tendency of cohesion and friction angle to increase with strain. For instance, the friction angle, for 20 % axial strain, varies between 17 and 27 and the cohesion between 39 and 53 kpa. Indeed, the three samples yielded close values for cohesion intercept and samples T2A and T1B similar angles of shear strength. When considering the mobilization of the parameters separately, it can be observed that they do not follow the same pattern. The cohesion has relatively low initial values; however it tends to increase continuously at a rate that is much more pronounced than that of friction angle. The friction angle on the other hand has a relatively high initial value and its rate of increase suggests that it will tend to reach an ultimate value as strain increases. As already discussed, the lower values of friction angles are related to sample T2B, which showed larger portions of fines and organic paste. Undrained Tests Figure 18 shows some typical results of stress-strain and pore water pressure-strain curves from undrained triaxial tests carried out in saturated specimens of T1B sample molded with a unit weight of 12 kn/m3. The stress-strain curves obtained are similar to those obtained for the CD test, i.e., the deviator stress increases with axial strain, without reaching any peak or ultimate value. Pore water pressures increase with strain and tend to stabilize for strains larger than 30 % and the larger the effective confining pressure, the larger the pore water pressure developed. The pore water pressure parameter A that relates deviator stress with developed pore water pressures reached a maximum of about 0.85 for the three effective confining pressures used. Effective and total stress paths followed in the undrained tests are shown in Fig. 19. Effective stress paths are S shaped, reflecting the positive pore water pressures that develop at the beginning of the tests, while strains are low. As strain increases, the deviator stress increases and pore pressures remain almost constant, thus causing an increase in both t and s'. Figure 19 also shows effective shear strength envelopes for different strains. Considering the strains of 10, 20, and 30 %, it can be seen that effective strength envelopes pass through the origin of coordinate system and angles of shearing strength are larger than those measured in CD tests. The difference increases with strain. For instance, at 10 % strain, φ' is 29 o, while in the CD test the corresponding value is 20 o. At 30 % strain, φ' reaches 57 o, compared to 26 o from the drained tests. Taking these results into account it must be emphasized that care must be exerted when trying to derive effective stress parameters from CU tests until more data are available. These tests could be useful in relating undrained shear strength and effective confining pressure, as shown in Fig. 20. Considering samples T2A and T1B, which showed close values of undrained shear strength, a unique envelope can fit the test results, which for 20 % strain yields s u = 0.80 σ' 3c. Sample T2B showed s u = 0.56σ' 3c, which is in accordance with the lower strength shown by this sample in the drained tests. It must be noted that the reported values are much higher than those usually associated with clays. The beneficial influence of fiber reinforcement is responsible for the existence of cohesion in MSW samples, as one can infer from the observation of urban waste dumps where vertical slopes can reach relatively great heights without any support. This can be illustrated by the results of the unconfined compression test shown in Fig. 21. The specimen is able to maintain its shape and

9 VILAR AND CARVALHO ON SOLID WASTE 9 integrity without any confinement and its unconfined compression strength is about 60 kpa for 20 % of strain. This yields a cohesion of 30 kpa, which is comparable to the values suggested by Kavanzajian et al. [3] and Manassero et al. [7]. Conclusions Results of consolidation and triaxial compression tests on large remolded specimens of a 15 year old sample of Municipal Solid Waste (MSW) were presented and discussed. Consolidation tests showed the large compressibility of MSW where secondary compression plays a fundamental role. The secondary compression index C α (C α = e/ log t) reached an average value of and was not influenced by saturation but showed a tendency to be influenced by e o, as C α slightly increased with e o. The secondary compression index, normalized through e o, C α (C α = C α /(1 + e o )) showed a small range of variation and yielded an average value of The primary compression index (C c ) was also dependent on e o and normalized C c [C' c = C c /(1 + e o )] reached about Drained triaxial compression tests showed stress-strain curves that were concave upwards and failure or an ultimate value of deviator stress could not be reached even for large values of strain, that is, above 30 %. The shear strength parameters are strain dependent and tend to increase with the deformation. However, the friction angle and the cohesion intercept are mobilized following different patterns. Although a limiting value of friction angle was not reached in the tests, it tended to be fully mobilized at large strain values, while the cohesion that is related to the fiber components starts to be mobilized at strains of 10 % and more, and a limiting value was not observed. For the same unit weight, the shear strength was little affected by the degree of saturation of the tested specimens. The relatively high initial degree of saturation of the unsaturated specimens (above 70 %) and the large volume compression during consolidation probably raised the degree of saturation, approaching the saturated condition, thus yielding close values of shear strength. The difference between shear strength parameters of saturated and nonsaturated samples is probably related to the adjustment of shear strength envelopes and a single envelope could encompass both the saturated and nonsaturated samples. The shear strength parameters obtained from specimens with different unit weights presented small variation and an average shear strength envelope could fit the results of the samples in the range of unit weights studied (10, 12, 14 kn/m3). Regarding the specimen dimensions, the smaller samples (15 cm diameter) showed an increase in shear strength when compared to the larger specimen (20 cm diameter). Undrained tests showed an increase in pore water pressures with strain until a maximum that remained as strain increased. Maximum water pressures developed were of about 85 % of deviator stress. Pore water pressures were also proportional to the effective confining pressures. Effective stress shear strength parameters from CU tests were misleading and did not agree with the parameters from CD tests. Undrained shear strength was proportional to the effective confining pressure and the relationship between these variables was larger than that usually observed in soils. Acknowledgments The writers are grateful to FAPESP (São Paulo State Research Funding Agency) and CAPES (Brazilian Research Agency) for financing part of this research.

10 10 JOURNAL OF TESTING AND EVALUATION References [1] Grisolia, M., Napoleoni, Q., and Tancredi, G., The Use of Triaxial Tests for the Mechanical Characterization of MSW, Proceedings Sardinia 95, 5 th International Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 1995, pp [2] Jessberger, H. L., Syllwasschy, O., and Kockel, R., Investigation of Waste Body- Behaviour and Waste-Structure-Interaction, Proceedings Sardinia 95, 5 th International Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 1995, pp [3] Kavazanjian, E., Matasovic, N., Bonaparte, R., and Schmertmam, G. R., 1995, Evaluation of MSW Properties for Seismic Analysis, Geoenviromental 2000, Geotechnical Special Publication, Yalcin B. Acar and David E. Daniel, Eds., Vol. 2, No. 46, ASCE, New Orleans, LA, 1995, pp [4] Knochenmus, G., Wojnarowicz, M., and Van Impe, W. F., Stability of Municipal Solid Wastes, Proceedings of the Third International Congress on Environmental Geotechnics, Lisboa, Portugal, Sêco e Pinto, Ed., Balkema, Rotterdam, ISBN x, 1998, pp [5] Singh, S., and Murphy, B. J., Evaluation of the Stability of Sanitary Landfills, Geotechnics of Waste Fills - Theory and Practice, ASTM STP 1070, Arvid Landva and G. David Knowles, Eds., ASTM International, West Conshohocken, PA, 1990, pp [6] Van Impe, W. F., Environmental Geotechnics: ITC5 activities State of the art, Proceedings of the Third International Congress on Environmental Geotechnics, Lisboa, Portugal, Sêco e Pinto, Ed., Balkema, Rotterdam, ISBN x, 1998, pp [7] Manassero, M., Van Impe, W. F., and Bouazza, A., Waste Disposal and Containment, Proceedings of the Second International Congress on Environmental Geotechnics, Preprint of Special Lectures, Osaka, Japan, A. A. Balkema, Vol. 3, 1996, pp [8] Grisolia, M. and Napoleoni, Q., Geotechnical Characterization of Municipal Solid Waste: Choice of Design Parameters, Proceedings of the Second International Congress on Environmental Geotechnics, Osaka, Japan, A. A. Balkema, Vol. 2, 1996, pp [9] Stulgis, R. P., Soydemir, C., and Telgener, R. J., Predicting Landfill Settlement, Geonviromental 2000, Geotechnical Special Publication, Yalcin B. Acar and David E. Daniel, Eds., ASCE, Vol. 2, No. 46, New Orleans, LA, 1995, pp [10] Sowers, G. F., Settlement of Waste Disposal Fills, 8 th International Conference on Soil Mechanics and Foundation Engineerings, Vol. 2, No. 2, Moscow, 1973, pp [11] Gibson, R. E. and Lo, K. Y., A Theory of Soils Exhibiting Secondary Compression. Acta, Polytechnica Scandinavica, C10, 296, 1961, pp (apud Edil et al, 1990). [12] Edil, T. B., Ranguette, V. J., and Wuellner, W. W., Settlement of Municipal Refuse, Geotechnics of Waste Fills - Theory and Practice, ASTM STP 1070, Arvid Landva and G. David Knowles, Eds., ASTM International, West Conshohocken, PA, 1990, pp [13] Edgers, L., Noble, J. J., and Williams, E., A Biologic Model for Long Term Settlement in Landfills, Proceedings of the Mediterranean Conference on Environmental Geotechnology, Usmen and Acar, Eds., Balkema, 1992, pp [14] Sánchez-Alciturri, J. M., Palma, J., Sagaseta, C., and Canizal, J., 1993, Mechanical Properties of Wastes in a Sanitary Landfill, Proccedings of the Symposium Green 93 - Waste Disposal by Landfill, Sarsby, Ed., 1995 Balkema, Rotterdam, ISBN [15] Park, H. I. and Lee, S. R., Long-Term Settlement Behavior of Landfills with Refuse Decomposition, Journal of Solid Waste Technical Management, Vol. 24, No. 4, 1997, pp [16] Marques, A. C. M, Filz, G. M., and Vilar, O. M., Composite Compressibility Model for Municipal Solid Waste, Journal of Geotechnical and Geonvironmental Engineering, ASCE, Vol. 129, No. 4, 2003, pp [17] Machado, S. L., Carvalho, M. F., and Vilar, O. M., Constitutive Model for Municipal Solid Waste, Journal of Geotechnical and Geonvironmental Engineering, ASCE, Vol. 128, No. 11, 2002, pp [18] Sowers, G. F., Foundation Problems in Sanitary Landfills, Journal of the Sanitary Engineering Division, ASCE, Vol. 4, 1968, pp [19] Gabr, M. A. and Valero, S. N., Geotechnical Properties of Municipal Solid Waste, Geotechnical Testing Journal, Vol. 18, No. 2, June 1995, pp

11 VILAR AND CARVALHO ON SOLID WASTE 11 [20] Landva, A. O. and Clark, J. I., Geotechnics of Waste Fill, Geotechnics of Waste Fills - Theory and Practice, ASTM STP 1070, Arvid Landva and G. David Knowles, Eds., ASTM International, West Conshohocken, PA, 1990, pp [21] Mesri, G. and Godlewski, P.M., Time and Stress-Compressibility Interrelationship, Journal of Geotechnical Engineering Division, ASCE, Vol. 103, GT5, 1977, pp [22] Grisolia, M., Gasparini, A., and Saetti, G. F., Survey on Waste Compressibility, Proceedings Sardinia 93, 4 th International Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 1993, pp [23] Jessberger, H. L. and Kockel, R., Determination and Assessment of the Mechanical Properties of Waste Materials, Proceedings Sardinia 93, 4 th International Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 1993, pp [24] König, D. and Jessberger, H. L., Waste Mechanics, ISSMFE Technical Committee TC5 on Environmental Geotechnics, 1997, pp TABLE 1 Unit weight of solid particles of MSW. Sample γ s, kn/m 3 T1B 23.3 T2A 22.4 T2B 25.1 TABLE 2 Initial and final characteristics of the samples subjected to consolidation tests. Sample T2A T2B T1B Parameters T2A8 T2A10 T2A14 T2B12 T1B10 T1B10* before after before after before after before after before after before after w, % γ, kn/m γ d, kn/m γ s, kn/m e Sr, % w - water content; γ - unit weight; γ d - dry unit weight; γ s particle unit weight; e void ratio; Sr degree of saturation. * - saturated after molding.

12 12 JOURNAL OF TESTING AND EVALUATION TABLE 3 C c and C c average values of MSW. Sample Test C c C c T2A T2A T2A T2A T1B T1B T2B T2B TABLE 4 Shear strength parameters of sample T2A tested with different unit weights. Unit weight kn/m 3 Axial strain, % Shear strength envelope (s -t diagram), kpa R 2 Shear strength envelope (σ -τ diagram), kpa t =0.328 s τ=19.5+σ tg t =0.380 s τ=60.2+σ tg t =0.41 s τ=115.4+σ tg t =0.380 s τ=20.0+σ tg t =0.487 s τ=39.2+σ tg t =0.550 s τ=71.5+σ tg (*) 10 t =0.294 s τ=31.4+σ tg17 12(*) 20 t =0.395 s τ=60.7+σ tg23 12(*) 30 t =0.510 s τ=85.6+σ tg t =0.343 s τ=33.3+σ tg t =0.412 s τ=74.9+σ tg t =0.485 s τ=121.7+σ tg29 (*) saturated samples; all the others tested at natural moisture content TABLE 5 Average shear strength parameters of saturated and unsaturated sample T2A tested with unit weight of 12 kn/m 3. Axial strain, % Shear strength envelope (s -t diagram), kpa R 2 C kpa φ ( ) 10 t= s t= s t= s TABLE 6 Average shear strength parameters of sample T2A considering the specimens tested with unit weight of 10, 12, and 14 kn/m 3. Axial strain, % Shear strength envelope (s -t diagram), kpa R 2 C kpa φ ( ) 10 t= s t= s t= s

13 VILAR AND CARVALHO ON SOLID WASTE 13 TABLE 7 Physical indexes of specimens of sample T2A subjected to triaxial compression tests, after molding and after consolidation. e S e S After molding After consolidation Sample σ 3 w γ γ d γ dc % kn/m 3 kn/m 3 o r % kn/m 3 c rc % T2A T2A T2A TABLE 8 Shear strength parameters of saturated and nonsaturated sample T1B molded with unit weight of 12 kn/m 3. Unit weight, kn/m 3 Axial strain % Shear strength envelope (s -t diagram), kpa R 2 Shear strength envelope (σ -τ diagram), kpa t =0.390 s τ=9.9+σ tg23 o t =0.476 s τ=37.6+σ tg28.4 o t =0.557 s τ=68.1+σ tg33.8 o 12 (*) 10 t =0.343 s τ=26+σ tg20 o 12 (*) 20 t =0.417 s τ=71.2+σ tg24.6 o 12 (*) 30 t =0.440 s τ=150.2+σ tg26 o (*) saturated samples TABLE 9 Shear strength parameters of saturated and nonsaturated sample T2B molded with unit weight of 12 kn/m 3. Unit weight, kn/m 3 Axial strain, % Shear strength envelope (s -t diagram), kpa R 2 Shear strength envelope (σ -τ diagram), kpa t =0.173 s τ=35.7+σ tg10 o t =0.256 s τ=47.1+σ tg14.8 o t =0.202 s τ=116.4+σ tg11.6 o 12 (*) 10 t =0.286 s τ=12.9+σ tg16.6 o 12 (*) 20 t =0.325 s τ=33.3+σ tg19 o 12 (*) 30 t =0.319 s τ=71.9+σ tg18.6 o (*) saturated samples

14 14 JOURNAL OF TESTING AND EVALUATION Figure Captions FIG. 1 Particle size curves of the tested MSW compared with the curves reported by Jessberger (1994). FIG. 2 MSW components expressed as weight percentages. FIG. 3 Test site at Bandeirantes Sanitary Landfill. a) Typical Bore Hole Profile, including SPT; b) Typical Cone Penetration Test. FIG. 4 Results of consolidation tests of sample T1B molded with unit weight of 10 kn/m 3. a) Specimen prepared at natural moisture content and initial void ratio of 3.35; b) Saturated specimen and with initial void ratio of FIG. 5 Secondary compression indexes (C ) and (C ) as a function of vertical stress. FIG. 6 Synthesis of various secondary compression index C as a function of void ratio of MSW. FIG. 7 Void ratio-vertical stress from consolidation test of MSW for different time of loading. a) Sample T2A with initial unit weigth of 10 kn/m 3 ; b) Sample T1B with initial unit weigth of 10 kn/m 3. FIG. 8 Synthesis of various primary compression index C c as a function of void ratio of MSW. FIG. 9 Drained tests on samples of MSW molded at natural moisture content (67 %), unit weight of 12 kn/m 3 and initial degree of saturation of 72 %. a) Deviator stress-axial strain curve; b) Volume change-axial strain curve. FIG. 10 Drained tests on saturated samples of MSW molded with unit weight of 12 kn/m 3. a) Deviator stress-axial strain curve; b) Volume change-axial strain curve. FIG. 11 Stress paths and shear strength envelopes for different strain of MSW with unit weight of 12 kn/m 3. a) Natural moisture content sample; b) Saturated sample. FIG. 12 Shear strength envelopes in a t versus s diagram considering saturated and nonsaturated samples together. Unit weight 12 kn/m 3. Filled marker - natural water content specimens; empty markers - saturated samples. FIG. 13 Results from consolidated drained (CD) triaxial compression tests for the sample T2A molded at its natural moisture content and unit weights of 10, 12, and 14 kn/m 3. Effective consolidation pressure (σ' 3c ): 100 kpa. FIG. 14 Shear strength envelopes considering together the specimens with unit weights of 10, 12, and 14 kn/m 3. FIG. 15 Comparison of stress-strain curves of 15 and 20 cm diameter specimens. T2A sample tested with unit weight of 12 kn/m 3, saturated condition, and consolidated with confining pressures of 100, 200, and 400 kpa. FIG. 16 Relation between deviator stress obtained from CD test in specimens of 15 and 20 cm of diameter, for different strains. T2A sample tested with unit weight of 12 kn/m 3, saturated condition and consolidated with confining pressures of 100, 200, and 400 kpa. FIG. 17 Mobilization of shear strength parameters with strain for the samples tested. Specimen size in parentheses. FIG. 18 Typical results from consolidated undrained triaxial compression test (CU) of saturated MSW. Sample T1B molded with 12 kn/m 3. a) Deviator stress-axial strain curve; b) Pore water pressure-axial strain curve. FIG. 19 Stress path and shear strength envelopes of MSW for different strains. Sample T1B, unit weigtht of 12 kn/m3. a) Total stress; b) Effective stress. FIG. 20 Undrained shear strength (s u ) plotted against effective confining pressure. Specimens molded with 12 kn/m 3 of unit weight. FIG. 21 Specimen of sample T1B subjected to unconfined compression test.

15 VILAR AND CARVALHO ON SOLID WASTE 15 FIG. 1 FIG. 2

16 16 JOURNAL OF TESTING AND EVALUATION a) b) FIG. 3

17 VILAR AND CARVALHO ON SOLID WASTE 17 a) b) FIG. 4

18 18 JOURNAL OF TESTING AND EVALUATION Cα Vertical Stress (kpa) T2A8 T2A10 T2A14 T1B10 T1B10sat T2B C'α Vertical Stress (kpa) T2A8 T2A10 T2A14 T1B10 T1B10sat T2B12 FIG. 5 FIG. 6

19 VILAR AND CARVALHO ON SOLID WASTE 19 a) b) FIG. 7 FIG. 8

20 20 JOURNAL OF TESTING AND EVALUATION Deviator stress (kpa) Axial strain (%) 100 kpa 200 kpa 400 kpa Volumetric strain (%) a) Axial strain (%) 100 kpa 200 kpa 400 kpa b) FIG. 9

21 VILAR AND CARVALHO ON SOLID WASTE Deviator stress (kpa) Axial strain (%) 100 kpa 200 kpa 400 kpa Volumetric strain (%) a) Axial strain (%) 100 kpa 200 kpa 400 kpa b) FIG. 10

22 22 JOURNAL OF TESTING AND EVALUATION t (kpa) s' (kpa) strain 10% strain 20% strain 30% a) t (kpa) s' (kpa) strain 10% strain 20% strain 30% b) FIG. 11 t (kpa) s' (kpa) strain 10% strain 20% strain 30% FIG. 12

23 VILAR AND CARVALHO ON SOLID WASTE Deviator stress (kpa) Axial strain (%) 10 kn/m3 12 kn/m3 14 kn/m3 a) Volumetric strain (%) Axial strain (%) 10 kn/m3 12 kn/m3 14 kn/m3 b) FIG. 13 t (kpa) kn/m3 12 kn/m3 14 kn/m3 strain 10% strain 20% s' (kpa) FIG. 14 strain 30%

24 24 JOURNAL OF TESTING AND EVALUATION Deviator stress (kpa) x40 (100kPa) 20x40 (200kPa) 20x40 (400kPa) 15x30 (100kPa) 15x30 (200kPa) Axial strain (%) 15x30 (400kPa) FIG. 15 FIG. 16

25 VILAR AND CARVALHO ON SOLID WASTE 25 Cohesion (kpa) Axial strain (%) T2A (15x30) T2B (15x30) T1B (15x30) T2A (20x40) T1B(20x40) T2B (20x40) Friction angle (degree) T2A (15x30) T2B (15x30) T1B (15x30) T2A (20x40) T1B(20x40) 10 T2B (20x40) Axial strain (%) FIG. 17

26 26 JOURNAL OF TESTING AND EVALUATION 1000 Deviator stress (kpa) Axial strain (%) 108kPa 208kPa 408kPa a) Pore water pressure (kpa) Axial strain (%) 108kPa 208kPa 408kPa b) FIG. 18

27 VILAR AND CARVALHO ON SOLID WASTE t (kpa) s (kpa) strain 10% strain 20% strain 30% a) t (kpa) s' (kpa) strain 10% strain 20% strain 30% b) FIG. 19 FIG. 20

28 28 JOURNAL OF TESTING AND EVALUATION FIG. 21