Influence of the Fibre Component of Soft Plastic on Shear Strength Parameters of PreTreated Municipal Solid Waste

For an appropriate stability analysis of pre-treated MSW landfill, the shear strength parameters of the material should be well defined in order to have a more realistic assessment of its geomechanical behaviour. As is well known, the shear strength of the material can be determined using two distinct components: friction (between the granular particles) and apparent cohesion. Unlike soil, where tensile strength is very low, in the case of pre-treated and fresh waste, the tensile forces are high and have a relevant participation in landfill stability due to the presence of fibres and foil material. The apparent cohesion is also called the reinforcing component, and consists basically of a wide range of substances: paper/cardboard, soft plastic, hard plastic and wood. Soft plastic is the most representative reinforcement component and due to its increasingly high volumetric concentration in the overall pre-treated MSW composition, its influence on the shear strength parameters was chosen for study. Laboratory tests were carried out with a sample of pre-treated MSW material in its original composition and modified composition (without soft plastic material). The results showed the influence of this group of materials on the strength properties of the pre-treated MSW, especially on the apparent cohesion


Introduction
The stability of pre-treated MSW landfills has become one of the most challenging topics in geo-environmental engineering.The shear strength parameters of pretreated MSW should be well defined in order to provide a more realistic assessment of landfill geotechnical behaviour and stability.So, the use of appropriate laboratory tests and calculation methods is relevant for investigation and analyses of pre-treated MSW and landfills.Simple methods of geotechnical engineering are not suitable to obtain the usual shear strength parameters of pre-treated MSW and determine the landfill slope stability.It is well known that pre-treated and fresh MSW have different geomechanical behaviour from soil material.Pre-treated and fresh municipal solid wastes differ clearly in the strength characteristics from soil material due to their fibrous components.
Pre-treated and fresh waste landfill stability can be separated into two components: the friction between the granular particles and the apparent cohesion caused by the fibre effect, i.e., the tensile forces that appear in the fibrous components, such as textiles, foils, paper/cardboard, soft plastic, hard plastic and wood.However, each of these materials has different physical and mechanical properties.In other words, each one of the listed materials influences the reinforcement in different ways.
According to Kölsch (1993Kölsch ( , 1996)), separate tests should be conducted for the study of pre-treated and fresh MSW shear strength, distinguishing the friction and tensile part by performing direct shear and tensile tests, respectively.In this way, tests with the same type of waste composition were carried out using medium-scale tensile box equipment in order to separately investigate the influence of the soft plastic material on the shear strength properties of pre-treated MSW (Borgatto, 2010).The results of tensile tests will be presented in another paper.
In particular, soft plastic material is the most representative reinforcement component due to its higher volumetric concentration in the overall pre-treated MSW composition.In Brazil, for instance, the percentage by weight of soft plastics in the composition of MSW can be higher than 30%.On the other hand, a higher percentage of soft plastic in MSW (or fibrous materials) leads to an extremely high strength, even enabling the construction of vertical temporary slopes in a landfill (Kölsch et al., 2005).Laboratory tests were carried out in order to investigate the influence of soft plastic on pre-treated MSW strength in the way of reinforcement or due to reduction of its final emplacement density.
In Germany, no landfill can be constructed today with fresh waste.Mechanical-biological treated (MBT) material (Münnich et al., 2006) was chosen for this investigation; The results showed that even with visibly reduced amount of soft plastic in its composition compared to untreated waste, the remaining part still influences pre-treated MSW strength properties.
So, the results obtained in these studies concern a landfill constructed with pre-treated MSW by a mechanical biological or equivalent process.
As indicated by the name, the mechanical biological process for pre-treatment of municipal solid waste is basically divided into two steps: (i) a mechanical procedure to remove items with large dimensions and/or special characteristics (batteries and other hazardous materials, tyres, recyclable materials), including opening of trash bags, and reduction of the size of the material not removed; and (ii) biological treatment with or without revolution, involving aerobic degradation of the material.After this process, the material is considered to be inert.

Test material
The pre-treated waste material used in this investigation was obtained from Blankenhagen sanitary landfill, in the district of Northeim, Germany.It is an output material of mechanical biological treatment (MBT) with maximum particle size of 60 mm after screening.Approximately 300 kg of this material was collected from a static pile after the biological stabilization period (windrow system) and taken to the laboratory.This pre-treated waste was characterized in accordance with GDA E 1-7 DGGT (1994), a technical recommendation issued by the German Society of Geotechnics.This recommendation entails waste identification and description according to waste mechanics.The waste is classified according to type, identification and its condition (Borgatto et al., 2006).
From the determination of the waste type, the indications for analysis into groups of substances (gravimetric composition) are obtained in a second stage.Afterwards, the morphological characterisation (dimensions and shape of waste particles) was performed considering size, length and volume.In this way, materials with one significant dimension (e.g., rope), two significant dimensions (e.g., foil), three significant dimensions (e.g., boxes) and small particles (e.g., grains) were identified and separated (Borgatto et al., 2006).Likewise, further analyses were carried out, such as determining water content, particle size distribution and chemical analyses.
The segregation into groups of substances consists of separating pre-treated MSW samples in such a way that each group of substances presented similar characteristics with respect to mechanical behaviour and biochemical stability.
The groups of substances were: large pieces; paper/cardboard -waste substances consisting basically of paper or paper-similar fibres, such as cardboard, paper packaging, carpets, diapers, etc.; soft plastics -waste consisting basically of soft synthetic substances or with similar characteristics, such as soft plastic, packaging, plastic foil, textiles, soft rubber, soft leather, etc.; hard plastics -waste composed basically of hard synthetic substances, such as rigid plastic packaging, PET bottles, rigid plastics, rigid leather, hard rubber, etc.; metals -ferrous and non-ferrous metal; minerals -waste consisting basically of mineral substances or with similar mechanical or biological behaviour (inert) such as glass, pottery, etc.; wood; and organic matter -waste of natural origin, organic, e.g., vegetables, grass cuttings, plants, dry leaves.
The morphological classification of pre-treated MSW in terms of shape and size according to relevant mechanical characteristics comprised the following groups associated with geometric shape: Dimension 0 -grains (no significant dimension, i.e., £ 8 mm); Dimension 1 -Fibre (one significant side compared to the others); Dimension 2 -Foil (two significant sides compared to the third side); Dimension 3 -Volume (three significant sides).
With regard to the particle size distribution curve, tests were carried out according to the German standard DIN 18123.The following sieve meshes were used: large sieving equipment (#60 mm, 40 mm, 20 mm and 8 mm), and standard sieving equipment (#4 mm, 2 mm and 1 mm).

Testing procedures
Normal Proctor tests were carried out based in the German technical standard DIN 18127.These tests were performed with pre-treated waste in its original composition and, in a second step, with samples of the same material but without soft plastic material.
Direct shear tests were used in this work to investigate the pre-treated waste strength properties.Direct shear tests were carried out using medium-scale device (300 x 300 x 150 mm).This equipment consists of two metal frames: bottom and top with a height of 60 mm and 90 mm respectively.The vertical load is applied through a hydraulic cylinder placed above the metal frame with maximum capacity of about 1000 kN/m 2 .The shear strength is measured through a load cell with maximum capacity of 500 kN.Horizontal and vertical displacements are measured by automatic reading sensors.The direct shear device is shown in Fig. 1.
First, the pre-treated waste samples were prepared and moulded into the metal frame (shear box) and compacted to the optimum water content from a Proctor test.The normal stress was applied and the pre-treated waste samples were left to consolidate for a period of 24 h.After the consolidation stage, direct shear tests were conducted as a consolidated drained test (CD) with a displacement speed of 0.05 mm/min, which is usually used for clay soils, to prevent development of excess pore water pressure.
In order to investigate the influence of soft plastic on MSW strength, direct shear tests were performed with pre-treated waste material as follows: • Sample A -pre-treated MSW with original composition, compacted to the maximum dry density according to the Proctor test; • Sample B -pre-treated MSW with modified composition (without soft plastic material), compacted to the maximum dry density according to the Proctor test.

Test material
The results of the classification into groups of substances of the pre-treated waste material under study are given in percentages of the total mass related to dry weight.The values are presented in Fig. 2.
The findings were typical for pre-treated waste material (MBT).The largest fraction corresponded to mixed materials with the particle size smaller than 20 mm, representing 83% water content dry of the total sample (Ø < 8 mm + Ø < 20 mm).Soft plastic material with 4% water content dry. of the total sample corresponded to 29% water content dry of the total reinforcement material (soft plastic + paper/cardboard + textiles + wood + hard plastic = 14%wt.). Figure 3 shows the results of morphological characterization of the pre-treated material.
Concerning the increases in shear strength, the percentage of fibrous components with dimension 1 and 2 are the most interesting due to the reinforcement generated.A value of 14%wt.was found in this study for materials with dimension 1+2, consisting mostly of soft plastic material (5%wt.).In tests performed by Kölsch (1996) with samples of MBT material, values of 16%wt.were found for dimension 1+2.
As a result of organic decay during pre-treatment, a large quantity of granular material was found (dimension 0).
The results of the particle size curve distribution of the pre-treated waste material under study are presented in Fig. 4. As in the previous tests, due to the mechanical biological pre-treatment (MBT), the largest fraction corresponded to particles smaller than 20 mm, representing 83%wt. of the total sample.

Testing procedures
According to the testing procedure regarding compaction tests (normal Proctor test), two types of pre-treated waste material composition were tested: original composition and modified composition (without plastic material).The results are compiled in Table 1.
The waste material sample with modified composition (without soft plastic) reached, as expected, a higher Proctor density (maximum dry density) compared to the sample with original composition.A comparison between    Figure 5 -Normal proctor curves of pre-treated waste material.
Figure 6 -Shear strength vs. horizontal displacement curves for pre-treated waste material.
pre-treated waste material under study are presented in Fig. 6.
First of all, according to the stress-strain curves presented, no material failure occurred during the test.As reported by Mahler & De Lamare (2006) and Bauer et al. (2007), that finding is common for pre-treated waste materials containing fibrous components.Consequently, deformation or displacement-dependent shear strength parameters should be used to calculate landfill stability.Based on the German soil testing standard for direct shear testing (DIN 18137-3 (2002)), deformation of 15% is stated for estimating the shear strength parameters.In the test done, this corresponds to a horizontal displacement of 45 mm.
The second point to be observed is the fact that with the increase of horizontal displacement, the mobilisation of the shear strength was always greater in the sample with original waste material composition, at all levels of normal stress.This behaviour may evidence the influence of soft plastic material on the shear strength of pre-treated MSW, mainly when the levels of horizontal displacement are high (due to fibrous components).As pointed out by Fucale et al. (2007), the strength properties of the pre-treated waste material are first mobilised by the friction components (basic matrix) for a low level of horizontal displacement.
Then, with the increase in displacement, the strength properties are mobilised as a result of tensile forces from fibrous components such as soft plastic.
The mobilisation of shear strength parameters of the pre-treated waste material samples (cohesion and angle of internal friction) vs. horizontal displacement are presented in Fig. 7.According to results, soft plastic material has more influence on the strength parameter cohesion than on the angle of friction.Observing Fig. 7 (b), the curves of cohesion mobilisation of the two samples are parallel to each other, at least qualitatively, up to a horizontal displacement of around 45 mm.After that point, both curves behave as mentioned previously, higher mobilisation in the sample with original waste composition (due to presence of fibrous components -soft plastic, in this case) compared to the sample with modified composition.
Assuming the Mohr-Coulomb failure criteria and considering the DIN 18137-3 (2002) (15% strain), the shear strength envelopes for the pre-treated waste material are presented in Fig. 8.
As a result, the soft plastic material has almost no influence on the angle of friction.Only a slight difference between the angle of friction for both tested samples (original waste composition and modified sample composition) was found.Despite the angle of friction, higher values for apparent cohesion parameters were found for the sample with original pre-treated waste composition compared to the sample with modified waste composition.The strength parameters are summarised in Table 2.
In comparison with strength parameter values provided in the literature on MBT waste, it is possible to affirm  that the findings are in the same range.Klumpfer (1998) found for samples of MBT material with Ø max.< 60 mm, a cohesion value of c = 32.7 kN/m 2 and angle of friction of F = 41.0°.Fucale et al. (2007) reported cohesion value of c' = 34.7 kN/m 2 and angle of friction of F = 45.4°determined in direct shear tests with MBT material.As mentioned before, when estimating the shear strength parameters of pre-treated MSW to be used in landfill stability calculation, deformation criteria must be considered.According to German soil testing standards, a limit deformation of 15% should be used to estimate the shear strength parameters.The results are presented in Table 3.
However, the finding for apparent cohesion is divided into two parts: one in terms of friction properties (ba-sic matrix) and the other in terms of fibre cohesion (fibrous components like soft plastic).

Conclusion
The results obtained in these studies are relevant for landfills constructed with MSW pre-treated by a mechanical biological or equivalent process.
Even though soft plastic material corresponded only to 4%wt. of the total pre-treated waste material under study, it had a relevant influence on shear strength properties.In fact, that behaviour was more evidenced in the apparent cohesion parameters after a high level of deformation.According to the results of the tests performed in a mediumscale direct shear device, the value of apparent cohesion showed a reduction of nearly 17% in the modified sample (without soft plastic material) compared to the sample with original material.As mentioned before, the parameter angle of internal friction had almost no influence, with a value in the range of 2% reduction in the sample with modified waste composition compared to the sample with original pre-treated material.
Concerning compaction testing (Proctor test), by removing the soft plastic material from the original waste composition, there was a gain in the value of maximum attainable dry density of around 7%.On one hand, this effect looks interesting from the point of view of air space gain (larger amount of waste can be landfilled in the same area) and a lower rate of water seepage leading to lower generation of leachate.As mentioned, lower values of waste emplacement density is one of the factors responsible for triggering landfill failures.On the other hand, the absence of soft plastic in the pre-treated waste composition negatively influences the shear strength properties of the pretreated MSW.Thus, the stability of the whole pre-treated landfill body will be greatly reduced.

Figure 2 -
Figure 2 -Classification into groups of substances of pre-treated waste material.

Figure 3 -
Figure 3 -Morphological classification of the pre-treated waste material.

Figure 4 -
Figure 4 -Particle size curve distribution of the pre-treated waste material.Figure5-Normal proctor curves of pre-treated waste material.
Soils and Rocks, São Paulo, 37(2): 151-157, May-August, 2014.155 Influence of the Fibre Component of Soft Plastic on Shear Strength Parameters of Pre-Treated Municipal Solid Waste

Table 1 -
Results of the proctor test on pre-treated waste material.
Figure 7 -Mobilisation of shear strength parameters of pre-treated waste material.