Assessing the Potential Improvement of Fine-grained Clayey Soils by Plastic Wastes

Because of progressively dumping of plastic wastes (PWs) obtained from beverage industry it is of interest to use them as reinforcement material in civil engineering projects. For assessing potential use of plastic wastes in improvement of shear strength of fine-grained soils, two clayey soils were mixed with different amount of plastic wastes (i.e. 0.5%, 1.0%, 1.5% and 3.0% by weight) and consolidated undrained triaxial tests were performed on the compacted samples. Test results indicate that variations of shear strength and pore water pressure depend on the amount and type of plastic waste. It is observed that, irrespective of clay plasticity, adding plastic waste to the fine-grained soils improves their shear strength and plastic waste content (PWC) of 3.0%, within the range of used amounts, has the best effect on the shear strength. Moreover, adding plastic waste causes to decrease shear-induced pore water pressure slowly. Furthermore, deformability of samples changes in term of plastic waste content, type of plastic and clay type. It can be concluded that there is a possible usage of clay-plastic waste mixtures as construction materials and, thereby, plastic wastes can be managed by recycling them in the field of geotechnical engineering, thus contributing to clean up the environment.


Introduction
The bottled water is the fastest growing beverage industry in the world.International Bottled Water Association (IBWA) reported that 1.5 million tons of plastic are annually used to bottle water and 1500 bottles are dumped as garbage every second.Polyethylene terephthalate (PET) is one of the most abundant plastics in solid urban waste (de Mello et al., 2009).It has been reported that annual consumption of PET bottles is approximately 10 million tons in the world and it grows about up to 15% every year.On the other hand, the number of recycled or returned bottles is very low (ECO PET, 2007).Global bottled water consumption is estimated about 61.4 billion gallons in 2011 and total consumption swelled by 8.6 percent in 2011.Per capita consumption of 8.8 gallons represented a gain of 1.2 gallons over the course of five years (Rodwan Jr., 2012).
Biodegradation process of plastics is very slow, because plastics mainly are synthesized using non-renewable fossil resources.Therefore, the plastic wastes should be recycled to decrease these effects.For the management of plastic waste, recently their use in the civil engineering projects is taken into consideration.The advantages are the reuse of these materials and the reduction of using natural material like soil in geotechnical engineering applications.Adding polyethylene fibers of waste plastics to soil-cement mixtures showed that it improves the stress-strain response of uncemented and cemented sands (Consoli et al., 2002).A field application of fiber-reinforced cemented sand pro-posed for increasing the bearing capacity of spread foundations has been reported previously (Consoli et al., 2003).Consoli et al. (2004) by performing triaxial compression tests on cemented and uncemented sand reinforced with various types of fibers indicated that the mode of failure changes from brittle to ductile due to inclusion of fibers.Consoli et al. (2009) found that both cement and fiber insertions affect dramatically the stress-dilatancy behavior of the sand.Dutta & Rao (2007) proposed some regression based models for predicting the behavior of sand-waste plastic mixture.Numerical simulation also indicates that pull-out resistance of fibers governs the stress-strain response of random-reinforced soil (Sivakumar Babu et al., 2008).Comprehensive experimental studies on compacted soilfiber samples showed improvement in strength and stiffness response, reduction in compression indices, reduction in swelling behavior of soil.It is also observed that fibers reduce the seepage velocity of plain soil considerably and thus increase the piping resistance of soil (Sivakumar Babu & Vasudevan, 2008a, b, c).Based on critical state concepts, a constitutive model was proposed to obtain stress-strain response of coir fiber-reinforced soil as a function of fiber content (Sivakumar Babu & Chouksey, 2010).Sivakumar Babu & Chouksey (2011) investigated the effects of plastic waste on the soil behavior by performing a series of triaxial compression and one dimensional compression tests.They found that there is significant improvement in the strength of plastic waste mixed soils due to increase in friction be-tween soil and plastic waste and development of tensile stress in the plastic waste.Compression behavior of plastic waste mixed soil indicates significant reduction in compression parameters.
The main objective of the present study is to obtain the geotechnical properties of fine-grained cohesive soils by partially replacing them with plastic waste.To this end, experimental tests were conducted on clayey soils and mixtures of clayey soils with different amount of plastic waste.
The tests include a series of consolidated undrained (CU) triaxial tests to determine stress-strain and pore water pressure behavior of plastic waste mixed clayey soils.The obtained results are compared with the associated behavior of plain clays and an analysis is performed in terms of plastic waste content (PWC), type of plastic waste and clay plasticity.

Materials
The two fine-grained clayey soils used in this study were retrieved from two distinct borrow areas, namely Malekan and Roshdiyyeh areas in East Azerbaijan province.For abbreviation, these soils were denoted with MC and RC letters, respectively.According to Unified Soil Classification System (USCS), both of the clays were categorized as CL (ASTM, 2011).Some index properties of the clayey soils have been listed in Table 1.As well as, grading curves of these materials have been presented in Fig. 1.
Two types of plastic wastes obtained from water bottles with different flexibility are used as reinforcing material.Plastic wastes chips were named PW1 and PW2, the PW2 type being more flexible than the PW1 type.The size of pieces for both types of plastics were selected 8 mm in length and 4 mm in width, and their specific gravities are 1.452 and 1.36, respectively.

Sample preparation
Soil mixtures were prepared by mixing clayey soils with 0%, 0. 5%, 1.0%, 1.5% and 3.0% of plastic wastes by dry weight.To study the effect of plastic flexibility, both PW1 and PW2 plastics were added to Malekan clay.In order to model samples for the triaxial tests which would re-produce field conditions as closely as possible, standard Proctor compaction tests were performed on both the clayey soils and mixtures of MC clay with PW2 plastic to determine maximum dry unit weight (g dmax ) and optimum water content (w opt ) (ASTM, 2012).Compaction test results showed that plastic waste does not significantly affect compaction parameters of MC clay.Therefore, compaction tests were not performed on the other mixtures.Triaxial samples of MC and RC clays mixed with PW1 plastic were prepared according to 0.98g dmax and w opt values of MC and RC clays, respectively.Required materials for samples made of MC clay and PW2 plastic were calculated based on 0.98g dmax and w opt values of associated sample.
To obtain a homogenous mixture, required quantity of plastic wastes was distributed over the soil and mixed uniformly and, then, required water was sprayed onto the surface of the materials and after mixing it was placed in sealed plastic bags and stored overnight in a controlled humidity room.Figures 2(a) and 2(b) show typical photos of PW2 plastic waste chips and mixture of this plastic with MC clay, respectively.The entire mixture was statically compacted in the mold, with 50 mm diameter and 100 mm in height, in four layers, and samples for triaxial testing were obtained.Table 2 shows some specifications of tested samples.

Shear testing
After extruding the samples from the mold, they were set up in triaxial cell and standard consolidated undrained (CU) triaxial testing procedures were followed (ASTM, 2004).To saturate the samples, distilled water was transmitted through them and then incremental backpressure saturation with a pressure differential of 30 kPa was applied.The backpressure was raised to a maximum of 400 kPa and B value was calculated for each increment.Saturation of the samples took approximately 4-6 days to complete until reaching a B value of at least 0.96.The sam- ples were consolidated under effective consolidation stresses of 200 kPa and then shearing was applied to the samples at a rate of 0.04 mm/min until reaching up to 20-24% strain by simultaneously measuring shear-induced pore water pressure.

Results and Discussions
Figures 3, 4, and 5 illustrate stress-strain curve, changes in pore water pressure and stress paths of MC-PW1, RC-PW1 and MC-PW2 mixtures, respectively.These figures include variations of deviatoric stress vs. axial strain (e a ), excess pore water pressure (Du) vs. e a , and deviatoric stress (q' = s' 1 -s' 3 ) vs. mean normal effective stress (p' = (s' 1 + 2s' 3 )/3).It is clearly observed that the plastic waste influences the behavior of natural soils; so that by increasing the plastic waste content the samples exhibit higher shear strength (Figs. 3(a), 4(a) and 5(a)).

Undrained shear strength
The correlation between undrained shear strength and plastic waste content is shown in Fig. 6(a).The figure shows the shear strength of samples with PWC = 0.5% is approximately equal to the shear strength of plain clay and when the amount of plastic waste changes from 0.5% to 3.0% shear strength increases gradually.The maximum improvement in the shear strength of different mixtures was obtained at plastic content of 3.0%.Maximum increments in shear strength of MC clay mixed with 3.0% PW1 and 3.0% PW2 plastics are about 49.80% and 25.73%, respectively.The increment value for RC clay mixed with 3.0% PW1 plastic was about 55.20%.In addition, this figure illustrates that the effect of PW1 plastic on the improvement of shear strength is almost twice in comparison with that of PW2 plastic.
It is observed that the effect of plastic wastes on the shear strength of clayey soils depends on the clay plasticity so that plastic wastes improve the shear strength of RC clay better than MC clay, but the difference is not noticeable.

Excess pore water pressures (Du)
Change of pore water pressure during shearing (Du) is presented in Figs.3(b), 4(b) and 5(b).It is obvious that as the strain of samples increases to a specific value Du rises; thereafter its value reduces with straining.The rate of decline is steep in the samples with high plastic content.Also variation of maximum pore water pressure due to shearing (Du max ) (Fig. 6(b)) shows that when PC increases within the samples Du max decreases gradually.The maximum reduction takes place for MC-PW1, MC-PW2 and RC-PW1 mixtures including 3.0% PW and their values are 26.34%,15.96% and 18.24%, respectively.

Stress paths
Stress paths of the tests (Figs.3(c), 4(c), and 5(c)) explain that, at low level of strain, behavior of all the samples is contractive, but with developing shearing the samples exhibit dilative behavior.Moreover, as plastic waste increases the paths tend to move rightward; i.e. they exhibit more dilative behavior.For example, the behavior of MC clay including 3.0% plastic is completely dilative.Therefore, it can be concluded that adding plastic waste to the clay changes the tendency of samples during shearing.

Deformability
Secant deformation modulus (E 50 ) is an index of soil deformability.Therefore, the values of E 50 for all the samples obtained from the associated stress-strain curves and their variations vs. PWC have been plotted in Fig. 6c.This figure shows that the effect of plastic waste on the values of E 50 completely depends on the type of clayey soil and type of plastic.In MC clay by adding PW1 the values of secant deformation modulus increases; in other words, the deformability of samples reduces as PWC increases.The rate of increase in E 50 is considerable and it is about 186%.For the RC clay the trend is quite opposite to that for the MC clay, so that the values of E 50 decrease by increasing PWC in the mixtures; in other words, the deformability of samples increases as PWC increases.The rate of decrease is about 67% for the sample with 3.0% of PW1 plastic.It can be concluded that the plastic waste increases deformability of relatively stiff clay and reduces deformability of soft clay.A comparison between the curves of MC-PW1 and MC-PW2 in Fig. 6c indicates that plastic type strongly influences the trend of E 50 variations: while stiff plastic causes E 50 values of MC clay to increase, flexible plastic does not have any meaningful effect on clay deformability.Moreover, Figs. 7 and 8 show photographic views of plain and plastic mixed samples after failure, respectively, for mixture of MC and RC clays with PW1 plastic.It can be noted that angle of sliding surface is higher in mixed sample in comparison with those of plain soil.This is a sign of change in behavior from cohesive soil to frictional one.

Conclusions
Experiments were conducted to investigate the mechanical behavior of clayey soils mixed with plastic wastes obtained from water bottles.The compaction tests showed that the dry unit weight and optimum water content of mixed samples are not much different from that of associated clay.
The findings from this research show that the maximum shear strength for plain MC clay is 101 kPa, whereas for 3.0% PW1 plastic waste mixed clay it is about 151 kPa.The results indicate that there is 49.8% increase in the shear  strength of 3.0% plastic waste mixed MC clay as compared with plain clay.The maximum increase in the shear strength of RC clay is 55.20%.
It can be concluded that the plastic wastes influences the behavior of clay but this effect varies depending on the clay plasticity and flexibility of plastic wastes.The increase in the shear strength of soil is mainly due to development of tensile stress in the plastic waste.Pore water pressure due to shearing decreases slowly with an increase in plastic waste content.
The effect of plastic waste on the deformability of natural soils completely depends on the clay plasticity and plastic types.So that for MC clay with intermediate plasticity adding relatively stiff plastic causes a decrease in deformability, while for RC clay with low plasticity adding the same plastic causes an increase in deformability.In addition, it is observed that the type of plastic strongly influences the of deformation modulus variations.
Finally, it can be concluded that it is possible to use clay-plastic mixtures as construction materials, because of some increase in shear strength of clayey soils, and thus help to clean up the environment from the waste plastic materials.
Figure 2 -(a) PW2 plastic chips used in the research, and (b) mixture of MC clay with PW2 plastic before compaction.

Figure 6 -
Figure 6 -Effect of plastic wastes on the: (a) shear strength, (b) maximum pore water pressure, and (c) secant deformation modulus.

Table 1 -
Some index properties of clayey soils.

Table 2 -
List of samples with some specifications.