Mixtures of Clay / EPS Particulates and Undrained Shear Strength

Nicholas T. Rocco1, M.ASCE, Ph.D., P.E. and Ronaldo Luna2, F.ASCE, Ph.D., P.E.

1Formerly, Graduate Student, Missouri University of Science & Technology, Butler – Carlton Hall, 1401 N. Pine St., Rolla, MO 65401; ntrocco@gmail.com

2Professor, Missouri University of Science & Technology, Butler – Carlton Hall, 1401 N. Pine St., Rolla, MO 65401; rluna@mst.edu

ABSTRACT: Soils mixed with expanded polystyrene (EPS) particulates could be used as lightweight fills in slopes for improved stability, embankments over compressible soils, and for reduced earth pressures in soil retention structures. The addition of very low density EPS particulates into soil has a large effect on the mass and volumetric properties of the resulting soil mixtures and their influence on mechanical properties is scarce in the literature. A laboratory characterization program of clay/EPS particulate mixtures was conducted for different dosages of EPS. The undrained shear strength of the soils modified with EPS particulates for both saturated and compacted conditions is presented herein. The preparation of unit element specimens and the influence of EPS content on the density and void ratio are presented. The reduction in dry unit weight is between 8% and 12% and the increase in equivalent void ratio is between 15% and 25% for each half percent increase in EPS content. The results indicate that, for the saturated mixtures, the undrained shear strength is relatively unaffected by EPS content. Conversely, for partially saturated compacted mixtures, a threshold EPS content was noted upon which significant reductions in the undrained shear strength were realized. INTRODUCTION The addition of expanded polystyrene (EPS) particulates has the potential to produce a soil mixture with improved performance and several practical applications. EPS particulate and cohesive soil mixtures could be used as lightweight fill in slopes or embankments if they are shown not to have a significant reduction in strength. They could also be used for reduced earth pressures against structures if the EPS particulates function as compressible inclusions within the soil matrix. To date there is little published information and data for soils mixed with EPS particulates. Various researchers reported the experimental results of field and laboratory tests on mixtures of soil, EPS particulates, and cement (Satoh et al. 2001; Tsuchida et al. 2001; Yoonz et al. 2004; Liu et al. 2006). Wei et al. (2008) and Deng and Xiao (2010) mixed EPS particulates with sand to create a lightweight fill and measured

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that stress-strain characteristics of the modified soils in the laboratory using direct shear and triaxial compression tests, respectively. Results of these tests were mixed as Deng and Xiao (2010) showed a systematic decrease in drained strength with increasing EPS content, whereas Wei et al. showed generally increasing shear strength with increasing EPS content. Nataatmadja and Illuri (2009) mixed EPS particulates into cohesive, swelling soils to act as compressible inclusions and measured their response in an oedometer. Nonetheless, the influence of EPS particulates on the strength of a composite soil is poorly understood and warrants further investigation. This paper presents the material properties, specimen preparation of mixtures, weight-volume relations, and undrained shear strength of these soil mixtures. MATERIALS Bulk samples of EPS particulates were obtained from a regional supplier of EPS materials for engineering, manufacturing, and packaging industries. These closed-cell particulates are often called “polystyrene pre-puffs” (PSPP) in the manufacturing sector. Both stereoscopy and scanning electron microscopy were utilized to visually assess the particle shape, size, texture, and internal structure. The spheroidal particle surfaces were characterized by a continuous, textured and dimpled membrane without noticeable cracks or large voids, as shown in Figure 1. The internal honeycomb structure of the particles was characterized by fused foam cells with thin, translucent cell walls. The average diameter and density (unit weight) of the EPS particles used in this research were 2.9 mm and 51.0 kg/m3 (0.50 kN/m3), respectively. Commercially available powdered kaolin with a liquid limit 56%, plastic limit of 29%, and a specific gravity of 2.64 and classifies as a CH/MH according to USCS.

Figure 1. SEM Image of EPS Particulate and 4-Phase Modified Soil Element SPECIMENS PREPARATION Different soil specimens were prepared by varying the clay : EPS mass ratio. In addition, pure clay specimens were prepared to serve as a baseline in which to analyze data trends with increasing EPS content. The materials were mixed

SOLID

WATER

EPS

AIR

VTotal

VSolid

VWater

VAir

VEPS MTotal

MAir

MEPS

MWater

MSolid

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according to the ratios listed in Table 1. Mix ratios by volume were calculated based on the measured EPS particulate density and specific gravity of the clay. Notice that the “Mixture C” specimen with 1.5% EPS by mass results in a 44% change in volume, which is significant weight-volume change.

Table 1. Test Specimen Mix Ratios

Specimen ID Measured Mix

Ratio by Mass, % (Clay : EPS)

Calculated Mix Ratio by Volume, %

(Clay : EPS) Clay 100 : 0 100 : 0 Mixture A 99.5 : 0.5 79.4 : 20.6 Mixture B 99.0 : 1.0 65.7 : 34.3 Mixture C 98.5 : 1.5 55.9 : 44.1

Two different specimen preparation techniques were utilized during this research program: (1) slurry consolidation and (2) static undercompaction. For the slurry consolidated specimens, clay and EPS particulates were mixed with water at 150% of the measured liquid limit and consolidated using a 300 mm large diameter consolidometer under a vertical pressure of 150 kPa. Specimens with dimensions of 50 mm diameter and 100 mm height were trimmed from the consolidated sample for strength testing. Specimens were trimmed with the vertical axis parallel to the consolidation direction in the consolidometer. Compacted specimens were prepared using a 71 mm diameter cylindrical, split miter box mold modified with a top collar for additional height. Clay, EPS particulates, and water were mixed at the appropriate mass ratios and allowed to hydrate for at least 24 hours prior to pressing. Specimens were remolded in five lifts using the undercompaction method (Ladd 1978). Each lift was statically pressed in a triaxial load frame under a constant rate of strain of 5 mm/min to the appropriate lift height. All compacted specimens were prepared at the optimum water content and approximately 88% or 94% of the maximum dry unit weight as determined by the Standard Proctor test (ASTM D698). Some segregation of EPS and clay was noted during transfer of the prepared material to the compaction mold, and care was taken to thoroughly mix each lift after placing the material in the mold. INFLUENCE OF EPS CONTENT ON VOID RATIO AND UNIT WEIGHT The addition of very low density EPS particulates into soil had large effect on the mass and volumetric properties of the resulting soil mixtures. The effect of increasing EPS content on the dry unit weight of slurry consolidated and compacted unit element specimens is shown in Figure 2. The reduction in dry density with each 0.5% increase in EPS content was between 8% and 12% on average.

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Figure 2. Effect of EPS Content on Dry Unit Weight

The calculation of void ratio requires the determination of the voids within a specific unit volume of soil. Voids are typically considered to include both the air space and water space between soil particles. Assuming that the closed-cell EPS particulates are completely impermeable, then each particle is comprised of a solid membrane, a honeycombed internal structure, and internal air mass. Given that the measured EPS particulate density (51 kg/m3) is closer to that of air (ρair ≈ 1.204 kg/m3 @ 20°C), rather than soil particle density (ρsoil ≈ 1400 – 2000 kg/m3), the EPS particulates were considered as void space (VEPS). Therefore, an equivalent void ratio (eeq) was defined as,

Voids EPSeq

s

V Ve

V

+= (1)

Where Vvoids is the volume of the water and air, Vs is the volume of the soil particles, and VEPS is the volume of the EPS particulates. The effect of increasing EPS content on the equivalent void ratio of slurry consolidated and compacted unit element specimens is shown in Figure 3. The increase in equivalent void ratio with each half percent increase (0.5%) in EPS content was between 15% and 25% on average.

0.0

4.0

8.0

12.0

16.0

0.0% 0.5% 1.0% 1.5% 2.0%

Dry

Uni

t Wei

ght (

kN/m

3 )

Percent EPS by Mass

88% Relative Compaction94% Relative CompactionSlurry Specimens

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Figure 3. Effect of EPS Content on Equivalent Void Ratio

TEST PROCEDURES Triaxial unconsolidated undrained (UU) compression tests were completed on both slurry and compacted specimens of pure clay and EPS modified clay specimens following the procedures of ASTM D2850. Properties of the slurry consolidated specimens are summarized in Table 2.

Table 2. Slurry Consolidated UU Test Specimens

Mix Ratio (% Clay : % EPS,

by mass)

σcell

(kPa) ωi

(%) γd,i

(kN/m3) eeq

S (%)

100 : 0.0 172 40.0 12.7 1.036 100 345 40.0 12.5 1.066 99.5 690 39.6 12.6 1.051 99.5

99.5 : 0.5 172 39.9 11.0 1.365 77.5 345 39.7 10.7 1.434 73.5 690 40.0 11.4 1.281 82.9

99 : 1.0 172 40.4 9.4 1.795 60.0 345 40.9 9.7 1.695 64.3 620 41.0 9.2 1.834 59.6

98.5 : 1.5 172 40.4 8.3 2.161 50.1 345 41.0 8.3 2.154 51.0 690 41.1 8.5 2.085 52.8

0.5

1.0

1.5

2.0

2.5

0.0% 0.5% 1.0% 1.5% 2.0%

Equ

ival

ent V

oid

Rat

io, e

eq

Percent EPS by Mass

88% Relative Compaction94% Relative CompactionSlurry Specimens

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Properties of the compacted specimens are summarized in Table 3. As discussed previously the EPS particulates were assumed to be entirely air void space, and thus the percent saturation (S) decreased significantly with increasing EPS content.

Table 3. Compacted UU Test Specimens

Mix Ratio (% Kaolin : % EPS,

by mass)

σcell

(kPa) ωi

(%) γd,i

(kN/m3) eeq

S (%)

Relative Compaction

(%)

100 : 0.0

69 28.4 12.5 1.078 69.6 88.0 172 28.3 12.4 1.082 69.0 87.8 345 27.9 12.5 1.072 68.7 88.2 69 28.2 13.3 0.943 79.0 94.1 172 28.2 13.4 0.934 79.7 94.6 345 28.3 13.4 0.936 79.8 94.5

99.5 : 0.5

69 27.2 11.2 1.319 54.7 88.0 172 27.9 11.2 1.330 55.7 87.7 345 28.0 11.2 1.327 56.0 87.7 69 27.6 12.0 1.176 62.3 93.8 172 27.9 12.0 1.179 62.8 93.7 345 27.8 12.0 1.172 63.0 94.0

99 : 1.0

69 27.2 10.3 1.542 47.0 87.6 172 27.2 10.2 1.555 46.6 87.0 345 27.1 10.3 1.548 46.7 87.3 69 27.1 11.0 1.384 52.2 93.3 172 27.1 11.0 1.380 52.4 93.4 345 27.1 11.0 1.386 52.1 93.2

98.5 : 1.5

69 27.6 9.1 1.891 39.1 87.6 172 27.6 9.1 1.903 38.9 87.1 345 27.5 9.0 1.907 38.6 87.0 69 27.1 9.7 1.694 42.9 93.9 172 27.3 9.7 1.700 43.0 93.6 345 27.4 9.7 1.705 43.1 93.5

RESULTS AND DISCUSSION The stress-strain data from the triaxial UU tests were analyzed to determine strength parameters and specimen stiffness. The initial tangent modulus and secant modulus were determined from the stress-strain data and generally indicated a slight decrease in stiffness with increasing EPS content, as shown in Figure 3, which is attributed to replacement of stiffer soil particles with more compliant EPS particulates.

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Figure 3. Stress – Strain Relationship

(88% Relative Compaction Specimens ; σCell = 690 kPa) Failure during shear was defined as the maximum deviator stress obtained prior to 15% axial strain. For each specimen the undrained shear strength (Su) was evaluated as half of the maximum deviator stress. The results of the triaxial UU tests on slurry consolidated specimens are presented in Table 4. Both the pure clay and EPS modified specimens exhibited nearly horizontal failure envelopes, and thus it was assumed that the soil matrix was saturated. Saturation of the soil matrix in EPS modified soils cannot be determined by measurement of the percent saturation (Table 2), since the particulates will always contain air space. Surprisingly the slurry consolidated specimens did not exhibit any consistent decrease in Su with increasing EPS content. The 99:1 mixture exhibited a significant decrease in strength, while the other mixtures exhibited only a slight reduction as compared to the pure clay specimens. Variations in the measured Su with increasing EPS content were attributed to variations in the load imposed on bulk specimens during the slurry consolidation process. The data appears to show a slight increase in the failure strain with increasing EPS content which is an indication of increasing ductility and the influence of the foam EPS particles. Partially saturated soils behave differently than saturated soils in triaxial UU tests, since the pore air in soil matrix is compressible. Increases in confining pressure during a triaxial UU test lead to compression of air voids thus decreasing the specimen volume and subsequently increasing the specimen density. In addition, as the volume of air is decreased, the saturation level increases.

0

100

200

300

400

0.0 2.5 5.0 7.5 10.0 12.5 15.0

Dev

iato

r St

ress

(kP

a)

Axial Strain (%)

0.0% EPS

0.5% EPS

1.0% EPS

1.5% EPS

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Generally strength increases in partially saturated specimens with increasing confining pressure. A suite of partially saturated soils tested at various confining pressures typically exhibit a curve failure envelop until a threshold confining pressure is reached. Above this threshold pressure all the air voids are compressed in the specimen until saturation is attained and a typical horizontal failure envelope for saturated specimens develops. For each specimen Su was evaluated at φ = 0, and thus it is simply half of the recorded deviator stress. The measured undrained shear strength of the triaxial UU compression tests on compacted specimens are shown in Figure 4 and Figure 5 for relative compactions of 88% and 93%, respectively. As expected, the compacted specimens exhibited a curved failure envelop. The undrained shear strength increased with increasing confining pressure due to the compression of air voids. It is not believed that complete saturation of the soil matrix was obtained at the highest confining pressure (345 kPa) as some of the isotropic pressure was likely shed by compliance of the EPS particulates, and thus undrained shear strengths would be expected to increase further at higher confining pressures. The Su appeared to be influenced by EPS contents greater than 1% by mass, as shown in Figures 4 and 5. The reduction in Su for the EPS modified specimens appeared to increase with increasing confining pressure. This is attributed to collapse and shearing of individual EPS particulates in the soil matrix as the porous nature of the EPS material is not as stiff as the surrounding soil matrix. Finally, axial strain measurements during testing could not determine if the EPS particulates compressed and/or collapsed within the soil matrix with increasing cell pressure. Compliance of EPS particulates within the soil matrix during testing could be realized with detailed measurements of radial and axial strain and future testing should focus on specimen deformations during testing.

Table 4. Undrained Shear Strength of Slurry Consolidated Specimens

Mix Ratio (% Kaolin:% EPS, by mass)

σcell

(kPa) εf

(%) σdev,f (kPa)

Su (kPa)

100 : 0 172 7.7 44.1 22.1 345 6.3 59.0 29.5 690 6.6 50.7 25.4

99.5 : 0.5 172 5.5 42.0 21.0 345 9.9 35.2 17.6 690 9.6 41.4 20.7

99 : 1 172 6.3 25.7 12.9 345 8.6 28.0 14.0 690 9.5 27.8 13.9

98.5 : 1.5 172 9.1 48.5 24.2 345 10.0 45.0 22.5 690 8.7 38.8 19.4

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Figure 4. Undrained Shear Strength for Compacted Specimens

(Relative Compaction ≈ 88%)

Figure 5. Undrained Shear Strength for Compacted Specimens

(Relative Compaction ≈ 93%)

0

50

100

150

200

0 50 100 150 200 250 300 350 400

Und

rain

ed S

hear

Str

engt

h, S

u(k

Pa)

Confining Pressure (kPa)

98.5% Clay : 1.5% EPS99% Clay : 1% EPS99.5% Clay : 0.5% EPS100% Clay : 0% EPS

0

50

100

150

200

0 50 100 150 200 250 300 350 400

Und

rain

ed S

hear

Str

engt

h, S

u(k

Pa)

Confining Pressure (kPa)

98.5% Clay : 1.5% EPS99% Clay : 1% EPS99.5% Clay : 0.5% EPS100% Clay : 0% EPS

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CONCLUSION Soil specimens were prepared by mixing EPS particulates with clay. The effect of EPS content on the unit weight and void ratio was discussed. Based on experimental measurements, a general guideline would be to estimate a 10% reduction in dry unit weight for each half percent increase in EPS content by mass. The undrained strength of the soil mixtures was investigated using triaxial UU compression tests. Tests were performed on both slurry consolidated and compacted specimens. Experimental results showed that the EPS content had little effect on the undrained shear strength of saturated specimens but played a more important role for partially saturated specimens. Compacted specimens with EPS contents less than or equal to 1% by mass exhibited undrained shear strengths similar to the pure clay specimens, but EPS contents greater than 1% showed a distinct reduction in strength. Results indicated that if the dosage of EPS particulates is carefully chosen, a lightweight fill could be produced with minimal loss in undrained shear strength. ACKNOWLEDGMENTS The authors acknowledge Mr. Pat Rosener of VersaTech Inc. for the generous donation of EPS materials. The financial support from the US Department of Education through the GAANN program is acknowledged. REFERENCES Deng, A., and Xiao, Y. (2010). “Measuring and modeling proportion-dependent stress-strain

behavior in EPS-Sand Mixture.” Intl. J. of Geomechanics, ASCE, Vol. 10 (6): 214 – 222.

Ladd, R.S. (1978). “Preparing test specimens by undercompaction.” Geotechnical Testing Journal, ASTM, Vol. 1 (1): 16 – 23.

Liu, H., Deng, A., and Chu, J. (2006). “Effect of different mixing ratios of polystyrene pre-puff beads and cement on the mechanical behavior of lightweight fill.” Geotextiles and Geomembranes, Elsevier Science Limited, Vol. 24: 331 – 338.

Nataatmadja, A., and Illuri, H.K. (2009). “Sustainable backfill materials made of clay and recycled EPS.” Proc. of the 3rd CIB Intl. Conf. on Smart and Sustainable Build Environments (SASBE 2009), Delft, Netherlands, 15 – 19 June 2009.

Satoh, T., Tsuchida, T., Mitsukuri, K., and Hong, Z. (2001). “Field placing test of lightweight treated soil under seawater in Kumamoto port.” Soils and Foundations, Japanese Geotechnical Society, Vol. 41 (5): 145 – 154.

Tsuchida, T., Porbaha, A., and Yamane, N. (2001). “Development of a geomaterial from dredged bay mud.” J. of Materials in Civil Engineering, ASCE, Vol. 13 (2): 152 – 160.

Wei, Z., Mingdong, L., Chunlei, Z., and Gan, Z. (2008). “Density and strength properties of sand – expanded polystyrene beads mixture.” GeoCongress 2008: Characterization, Monitoring, and Modeling of GeoSystems, ASCE Geotechnical Special Publication 179, pp. 36 – 42.

Yoonz, G., Jeon, S., and Kim, B. (2004). “Mechanical characteristics of light-weighted soils using dredged materials.” Marine Georesources and Geotechnology, Taylor & Francis Inc., Vol. 22: 215 – 229.

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