Experimental Evaluation of Geocell-Reinforced Bases Under Repeated Loading

Accepted Manuscript

Experimental Evaluation of Geocell-Reinforced Bases under Repeated Loading

Sanat K. Pokharel, Jie Han, Dov Leshchinsky, Robert L. Parsons

PII: S1996-6814(16)30194-8 DOI: http://dx.doi.org/10.1016/j.ijprt.2017.03.007 Reference: IJPRT 82

To appear in: International Journal of Pavement Research and Technology

Received Date: 9 September 2016 Revised Date: 8 March 2017 Accepted Date: 12 March 2017

Please cite this article as: S.K. Pokharel, J. Han, D. Leshchinsky, R.L. Parsons, Experimental Evaluation of Geocell- Reinforced Bases under Repeated Loading, International Journal of Pavement Research and Technology (2017), doi: http://dx.doi.org/10.1016/j.ijprt.2017.03.007

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Experimental Evaluation of Geocell-Reinforced Bases under Repeated Loading

Sanat K. Pokharel1, Jie Han2*, Dov Leshchinsky3, and Robert L. Parsons2

1 Stratum Logics Inc., St. Albert, Alberta, T8N 7L5, Canada. Tel +17808032359; fax

+17804082259; email: [email protected]

2*Civil, Environmental, and Architectural Engineering Department, the University of Kansas,

Lawrence, Kansas 66045, USA. Tel +17858643714; fax +17858645631; email: [email protected]

3 Emeritus professor, Department of Civil and Environmental Engineering, the University of

Delaware, Newark, DE 19719, USA. Tel +13028312446; email: [email protected]

*Corresponding author.

Abstract

Geocells, one type of geosynthetics manufactured in a form of three-dimensional interconnected cells, have been reported to effectively provide lateral confinement to infill material to increase the modulus and bearing capacity of base courses. Most studies so far have been focused on the behavior of geocell-reinforced bases under static loading. Geocells used for pavement applications are subjected to repeated loading. Limited studies have been conducted so far to investigate the performance of geocell-reinforced bases under repeated loading. In this study, single and multiple geocell-reinforced granular bases with three types of infill materials

(Kansas River sand, quarry waste, and AB-3 aggregate) were tested and compared with the unreinforced bases under repeated loading. This study experimentally investigated the effect of the geocell reinforcement on the permanent deformation and percentage elastic deformation of

the granular bases. The test results showed that the geocell reinforcement reduced the permanent deformation and increased the percentage elastic deformation of the granular bases. Multiple geocell-reinforced sections demonstrated even better performance as compared with single geocell-reinforced sections.

Keywords: Geosynthetic reinforcement, geocells, permanent deformation, elastic deformation, repeated loading, base course.

Introduction

Geosynthetic reinforcement, one of the established techniques of ground improvement for over 40 years, has been developed extensively to improve the performance of both paved and unpaved roads. Geosynthetic reinforcement has been used to increase bearing capacity and modulus, reduce required base thickness, extend the service life of the pavement, reduce operational cost, and minimize maintenance requirements [1, 2]. Therefore, it is considered as a sustainable option to overcome premature pavement failure. Majority of the research on geosynthetics for pavement applications so far, has focused on planar reinforcement, such as geogrid and geotextile, and has resulted in several design methods [1, 2, 3, 4, 5]. However, the three-dimensional forms of interconnected honeycomb geosynthetic cells, known as geocells, are not that commonly used as compared with geogrids and woven geotextiles. One major reason for this situation is that theories and design methods for geocells have been lagging far behind the applications in the field [6].

The idea of cellular confinement was first developed by the United States Army Corps of

Engineers in 1970s [7]. The geocells then were made of paper soaked in phenolic water resistant

resin. Later metallic geocells were chosen to meet the strength requirements but they proved unfeasible because of handling difficulty and high cost. Cellular structures resembling geocells were also made from geogrids forming the sides and diaphragms [8, 9]. Geocells have also been made using geogrid sheets jointed by bodkin bars (for example, Carter and Dixon [10]).

Commercially available geocells are now made of high-density polyethylene (HDPE) and novel polymeric alloy (NPA). NPA geocells are explained in the sections to follow. Geocells come in different shapes and sizes; however, the most common shape of geocell is nearly circular.

Geocells provide enhanced confinement effect and impart apparent cohesion [11]; increase strength [12] and resilient modulus [13, 14]; and significantly improve the load- deformation and stress distribution characteristics of poorly-graded materials [15]. The extent of bearing capacity increase is correlated with the horizontal stiffness of the cell material [16] and the hoop stresses in the geocell wall are the most significant contributing factor towards resisting loads [17]. NPA geocell-reinforcement reduces the plastic deformation and increase the percent of elastic deformation under repeated loading [18, 19]. A series of static plate load tests conducted by Pokharel et al. [20] showed that the shape of geocell layout, the stiffness and type of geocell material, and the property of infill material all played vital roles in the behavior of geocell-reinforced bases under static loading. Pokharel et al. [20] recommended a near circular shape of geocell layout as the most efficient one.

Pavement failure is often caused by insufficient stiffness and strength of the pavement structure including subgrade, base, and asphalt or concrete surface, under heavy and repeated traffic loading. Al-Qadi and Hughes [21] reported an increase of the resilient moduli of aggregate layers by about two times due to the installation of geocells within an asphalt paved road construction. Field tests with industrial by-products as the infill material in the geocell over

a period of12-month was also found to achieve all the essential performance requirements [22].

Han et al. [23] and Thakur et al. [24] studied recycled asphalt pavement (RAP) materials used as infill materials while in the Pokharel et al. [25] study three different infill materials were used including Aggregate Base Type 3 (AB-3), quarry waste (QW), and RAP. Both studies showed the benefits of geocell reinforcement in reducing ruts if unreinforced and reinforced sections are equally compacted. Under static loading, Pokharel et al. [20] found the modulus of the single geocell-reinforced bases improved by up to two times that of the unreinforced bases while the bearing capacities of the single geocell-reinforced bases were improved by up to 2.5 times those of the unreinforced bases. Thakur et al. [24] investigated the effect of geocell confinement on the creep deformation on RAP base material and they found that the geocell confinement significantly reduced the creep deformation of the RAP base material.

Although a summary of the above-mentioned past studies on geocell reinforcement confirms that the geocell can provide confinement and increase the modulus and strength of infill material, geocell-reinforced bases under repeated loading have not been well investigated. In this study, repeated load tests on single and multiple geocell-reinforced bases were carried out using three different infill materials. In addition, NPA geocells manufactured using a new manufacture technology than HDPE were used in this study.

This paper presents the results of an experimental study conducted to with NPA geocells.

Repeated plate load tests were carried out on unreinforced, single geocell-reinforced, and multiple geocell-reinforced bases in-filled with poorly-graded Kansas River sand (KR sand),

QW, and well-graded AB-3. The influences of both single and multiple geocell reinforcements with different infill materials are compared and evaluated in terms of permanent deformation, traffic benefit ratio (TBR), and percentage elastic deformation.

Material and Test Equipment

Geocell type and characteristics

Geocell made of novel polymeric alloy (NPA) was used for the tests in this study. NPA is a nano-composite alloy of polyester/polyamide nano-fibers dispersed in polyethylene matrix and characterized by flexibility at low temperatures similar to HDPE with elastic behavior similar to engineering thermoplastic. NPA geocells have a lower thermal expansion coefficient and higher tensile stiffness and strength than HDPE geocells. The coefficient of thermal expansion (CTE) of the Neoloy element used to make the geocell, measured using ASTME831 was less than 80 ppm/0C in the measurement range from -300C to +300C. The geocell used in the experiments had the tensile strength of 19.1 MPa and the elastic modulus of 355 MPa at 2% strain. 2% strain was chosen to characterize the stress-strain of geocell because the field studies have shown that the measured strains in geosynthetics are typically within 2%. Almost identical test results on wide-width tensile property of the geocell were obtained from three samples of

100 mm wide NPA geocell tested by following the ISO-527 test method. Fig. 1 shows the stress-strain curve of the NPA geocell, which was measured at a strain rate of 10%/minute at 23o

Celsius in this study. The height of the geocells used in this study was 100 mm and the wall thickness was 1.1 mm. There were two perforations of 1 cm2 each on each pallet of the geocell.

The spacing between the weld at two ends of a single NPA cell and when expanded the cell had internal dimension of 205 mm x 235 mm. Fig. 2 shows the picture of the geocell used in this research.

Tensile stress (MPa) Tensile stress 4

0 0 4 8 12 16 20 Strain (%)

Fig. 1. Tensile stress-strain curve of the NPA geocell.

Fig. 2. Picture of the NPA geocell used in the research.

Base course material

In the present study, three different base materials, Kansas River (KR) sand, quarry waste

(QW), and AB-3 aggregate, were used as infill material. KR sand is locally available in the

Lawrence area in Kansas, USA. KR sand was chosen in this research to examine the possibility of poorly graded and comparatively cheaper material in the future with Neoloy geocells. AB-3 is crushed limestone aggregate available in local quarry sites in Kansas and is the most common aggregate used in the road bases and unpaved road construction in Kansas. QW is the waste material produced during aggregate production in quarries. The alternative QW was chosen to check if the waste material can be used as an infill in the geocell thus enhancing the sustainable use of the waste at the quarry sites. The QW and AB-3 aggregate used in the tests were brought from a local limestone quarry site in Kansas. The grain size distribution curves of these infill materials are shown in Fig. 3.

The KR sand is a poorly-graded sub-rounded river sand having a mean particle size (d50)

= 2.6 mm, minimum void ratio = 0.354, maximum void ratio = 0.583, specific gravity = 2.65 at

20oC, coefficient of curvature = 0.98, and coefficient of uniformity = 2.73. The KR sand is classified as SP according to the Unified Soil Classification System (USCS). The peak angle of internal friction of the KR sand at 70% relative density was 41o, which was obtained from three triaxial tests. The details of these triaxial tests on the KR sand can be found in the paper by

Bhandari and Han [26]. The QW used for the tests had a mean particle size (d50) = 1.2 mm, liquid limit = 20, plastic limit = 12, specific gravity = 2.76 at 20oC, coefficient of curvature =

0.77, and coefficient of uniformity = 12. The QW is classified as SP-SC according to the USCS.

Standard Proctor tests and California Bearing Ratio (CBR) tests were conducted for the QW. As shown in Fig. 4, the optimum moisture content of the QW was 9% and its maximum dry density was 2.1 g/cm3. Fig. 5 shows the CBR value of 57% at 7% moisture content and 38% at the

optimum moisture content. The AB-3 aggregate used for the tests is a well-graded base material widely used in pavement applications in Kansas (USA). It had a mean particle size (d50) = 7.0 mm, liquid limit = 20, plastic limit = 13, specific gravity = 2.69 at 20oC, optimum moisture content = 10%, California bearing ratio (CBR) = 75% at 7.1% moisture content and 49% at the optimum moisture content, coefficient of curvature = 1.55, and coefficient of uniformity = 21.

The AB-3 is classified as GW-GC according to the USCS. Standard Proctor tests and CBR tests were conducted for the AB-3 also. As shown in Fig. 4, the optimum moisture content of the AB-

3 was 10% and its maximum dry density was 2.06 g/cm3. The compaction and CBR curves for the AB-3 aggregate are also shown in Figs. 4 and 5.

100

40 % passing %

Kansas River sand 20 Quarry waste AB-3 aggregate 0 0.01 0.1 1 10 100 particle size in mm

Fig. 3. Grain size distribution curves of KR sand, QW, and AB-3 aggregate.

2.10 ) 3 2.05

2.00

Dry density (Mg/m density Dry 1.95 Quarry waste AB-3 aggregate 1.90 6 7 8 9 10 11 12 Moisture content (%)

Fig. 4. Standard Proctor compaction curves of QW and AB-3 aggregate.

45 CBR (%) CBR 30

15 Quarry waste AB-3 aggregate 0 6 7 8 9 10 11 Moisture content (%)

Fig. 5. California Bearing Ratio curves of QW and AB-3 aggregate.

Model test setup

Model tests were conducted in a medium-scale loading apparatus designed and fabricated at Department of Civil, Environmental, and Architectural Engineering at the University of

Kansas. The loading system had a 15-cm diameter air cylinder with a maximum air pressure of

900 kPa. The loading plate was 15 cm in diameter. Fig. 6 shows the details of the wooden test box, which was square and had a plan area of 6400 cm2 with an adjustable height. Fig. 7 shows the pictures of the loading machine and Fig. 8 shows the single and multiple geocell reinforcement placed inside the box. For single geocell-reinforced sections, the geocell was placed at the center of the box and laid out in a near circular shape as suggested by Pokharel et al. [20]. The geocells were placed in a near circular shape as well for multiple geocell-reinforced bases. All geocells in this study were 10 cm high with a 2-cm thick fill cover (same material as the base). For all the tests, the geocells were filled and embedded in the infill material. The KR sand was placed and compacted to 70% relative density inside and outside the cell in three layers: two 5-cm thick layers and one 2-cm cover layer. The QW was compacted to a dry density approximately equal to 95% of the maximum dry density on the drier side

(approximately 7% moisture content) of the compaction curve. The AB-3 was also compacted to a dry density approximately equal to 95% of the maximum dry density on the drier side

(approximately 9% moisture content) of the compaction curve. For comparison purposes, unreinforced bases were prepared in a similar way. No subgrade existed for all the tests because the primary purpose of this research was to evaluate the influence of the geocell on the behavior of the base course.

A loading plate was placed at the center of the geocell for the reinforced case or at the center of the box for the unreinforced case. Loads were applied in increments by adjusting air pressure in the air cylinder. The repeated load was applied at 1 min/cycle in a trapezoidal form.

The load was increased from the minimum (0 kPa) to the maximum (345 kPa or 552 kPa) in 15 sec. The maximum load was applied for 20 sec and the load was released slowly in a period of

15 sec to the minimum, which was maintained for 20 sec before starting the next loading cycle.

The test was terminated when the percentage of elastic deformation reached constant at many loading cycles. To establish consistency in the comparison, all the tests therefore were terminated after 150 cycles except the unreinforced sand that could not hold the applied load

(i.e., failure occurred). As Pokharel et al. [18, 19, 20] and Han et al. [27] showed, the ultimate bearing capacities of the unreinforced and geocell-reinforced AB-3 and QW bases are higher than the tire pressure of a typical highway truck (i.e., 552 kPa). Therefore, a repeated load of

552 kPa was applied for all the tests with AB-3 and QW bases. Pokharel et al. [20] showed that the ultimate bearing capacities of multiple geocell-reinforced, single geocell-reinforced, and unreinforced KR sands were 715, 500, and 230 kPa, respectively. Since the ultimate bearing capacity of the multiple geocell-reinforced KR sand is higher than 552 kPa, a repeated load of

552 kPa was applied on the multiple geocell-reinforced KR sand. Since the ultimate bearing capacity of the single geocell-reinforced KR sand was lower than 552 kPa, the repeated load test on the single geocell-reinforced KR sand was done at an applied pressure of 345 kPa

(corresponding to approximately 70% of its ultimate bearing capacity, which is also close to the tire pressure of typical construction equipment). Due to the low ultimate bearing capacity of the unreinforced KR sand (230 kPa), a repeated load test at either 552 kPa or 345 kPa pressure was impossible. For a comparison purpose, a repeated load test was also performed on the multiple geocell-reinforced KR sand at an applied pressure of 345 kPa.

Fig. 6. Test box with geocell layout.

Fig. 7. Loading machine.

(a) Single geocell (b) Multiple geocells

Fig. 8. Geocell layout.

Test Results and Discussion

Before the repeated tests, the possible boundary effect of the test box and the repeatability of the test results were examined. As discussed by Pokharel et al. [20], the box size of 80 cm x 80 cm is large enough to eliminate the boundary effect. Pokharel et al. [20] also showed that this test device had good repeatability of the test results. Test results of unreinforced, single geocell-reinforced, and multiple geocell-reinforced bases with three different infill materials (KR sand, QW, and AB-3) under repeated loading are discussed below.

For single geocell tests, unconfined and confined tests were conducted. For an unconfined test, the geocell was filled with granular material inside the cell only without any surrounding soil outside the geocell. For a confined test, the geocell was filled and embedded in the granular material. The infill material was placed into the box including the geocell and compacted to the desired density. The layout of single and multiple geocell in the test box are shown in Figs. 8a and b. A single geocell-reinforced section after the test is shown in Fig. 9.

Fig. 9. Single geocell-reinforced section after the test.

Pressure-displacement cycles

The pressure-displacement cycles of the geocell-reinforced KR sand under repeated loading of 345 kPa were presented in Pokharel et al. [18] as a preliminary study. The pressure- displacement cycles of the unreinforced and multiple geocell-reinforced AB-3 under repeated loading of 552 kPa are presented in Fig. 10. The pressure-displacement curves with the unloading and re-loading process were obtained for all the unreinforced, single geocell- reinforced, and multiple geocell-reinforced sections. However, due to the space limit, only two curves from the tests on AB-3 aggregate sections are shown in Fig. 10(a) and (b). The permanent deformations of the KR sand, QW, and AB-3 bases under repeated loading are shown in Figs. 11, 12, and 13, respectively. The permanent deformation kept on accumulating with the increasing number of the loading cycles during the test. It is shown that in each cycle, there are elastic and plastic (also called permanent deformation) deformations.

The modulus values of the unreinforced and reinforced bases at the first loading cycle can be determined based on the slopes of the initial linear portions of the pressure-displacement curves. In a similar test with the KR sand, the modulus of the single geocell-reinforced section was approximately 1.5 times that of the unreinforced section [18]. In the present study, the modulud improvement factors of the reinforced base to the unreinforced base are provided in

Table 1. The modulus improvement factor is defined as the modulus ratio of a reinforced base to an unreinforced base with the same infill material.

Applied pressure (kPa) 0 100 200 300 400 500 600 0

3 1 cycle

4 5 cycle Displacement (mm)Displacement 10 cycle

5 25 cycle 50 cycle 6 100 cycle 150 cycle

(a) Unreinforced AB-3 aggregate

Applied pressure (kPa) 0 100 200 300 400 500 600 0

2 1 cycle

3 5 cycle 10 cycle 25 cycle 4 50 cycle Displacement (mm)Displacement 100 cycle 150 cycle 5

(b) Multiple geocell-reinforced AB-3 aggregate

Fig. 10. Pressure-displacement cycles under repeated loading.

Table 1

Modulus Improvement Factors of the Bases.

Reinforced Base Modulus Improvement

Factor

Multiple geocell-reinforced KR 2.04

Single geocell-reinforced QW 1.26

Multiple geocell-reinforced QW 1.46

Single geocell-reinforced AB-3 1.32

Multiple geocell-reinforced AB-3 1.73

Permanent deformation vs. number of cycles

Fig. 11 presents the cumulative deformation vs. number of cycles of the KR sand under three different conditions. For each test, there are two lines, which correspond to the cumulative deformations under a load and zero load. The difference between these two curves at the same number of cycles is the elastic deformation. Since the unreinforced KR sand section failed at

230 kPa [19, 27], a repeated load test under a pressure of 345 kPa was not possible. However, the single geocell-reinforced KR sand section under the repeated loading of 345 kPa survived

150 cycles of loading. The same load in the case of multiple geocell-reinforced KR sand section produced only 84% of that deformation after 150 loading cycles. This comparison demonstrates that multiple geocell reinforcement further improved the performance of reinforced bases.

Under a higher applied pressure of 552 kPa, the multiple geocell-reinforced section deformed more than that under a lower pressure of 345 kPa. It is interesting to note that the deformation

vs. number of cycle curves for the multiple geocell-reinforced sections under these two different pressures are nearly parallel after 50 cycles.

8 Single cell-reinforced - 0 kPa (unloaded from 345 kPa) Single cell-reinforced - 345 kPa Multicell-reinforced - 0 kPa (unloaded from 345 kPa) 4 Multicell-reinforced - 345 kPa Cumulative deformation (mm) Multicell-reinforced - 0 kPa (unloaded from 552 kPa) Multicell-reinforced - 552 kPa 0 0 25 50 75 100 125 150 Number of loading cycle Fig. 11. Deformations of the KR sand bases under repeated loading.

Figs. 12 and 13 present the cumulative deformations of unreinforced, single geocell- reinforced, and multiple geocell-reinforced QW and AB-3 bases, respectively under repeated loading of 552 kPa. It is clearly shown that both single and multiple geocell reinforcements reduced the cumulative deformations as compared with the unreinforced base. The reduction in the cumulative deformation started from the first loading cycle. The cumulative deformations for the QW and AB-3 bases with a single geocell reinforcement measured at 150 cycles were reduced by a factor of 1.50 and 1.33 as compared with the unreinforced QW and AB-3 bases, respectively, at the same number of cycles. The inclusion of multiple geocells reduced the cumulative deformations of the QW and AB-3 bases by factors of 1.55 and 1.40, respectively.

These comparisons demonstrate that multiple geocell reinforcement further improved the 18

performance of reinforced bases slightly. Pokharel et al. [20] showed that the geocell reinforcement did not show any benefit in the performance of QW bases under static loading because QW had apparent cohesion. This study clearly shows that geocell reinforcement improved the performance of QW and AB-3 bases under dynamic loading even though these materials have apparent cohesion.

3 Unreinforced - 0 kPa ( unloaded from 552 kPa) Unreinforced - 552 kPa 2 Single cell-reinforced - 0 kPa (unloaded from 552 kPa) Single cell-reinforced - 552 kPa 1 Multicell-reinforced - 0 kPa (unloaded from 552 kPa) Cumulative displacement (mm) Multicell-reinforced - 552 kPa pressure 0 0 25 50 75 100 125 150 Number of loading cycle Fig. 12. Cumulative deformations of the QW bases under repeated loading.

Fig. 14 presents a special case where the cumulative deformation of the confined, single geocell-reinforced QW under repeated loading is compared with an unconfined case. It is clearly shown that the confinement of the geocell by the surrounding soil reduced the cumulative deformation. It is understandable that soil confinement and geocell confinement have the same effect.

Fig. 15 shows a comparison of the permanent deformations of unreinforced and geocell- reinforced bases with three different infill materials under repeated loading of 552 kPa. Since

the unreinforced KR sand could not sustain the applied pressure of 552 kPa, no test data is shown in this figure. However, the test data of the multiple geocell-reinforced KR sand in Fig. 15 clearly show the significant benefit of geocell reinforcement in stabilizing the KR sand. Fig. 15 also shows that the AB-3 and QW bases had the similar performance under the repeated loading, which was much better than the reinforced KR sand. It is worth pointing out that since the QW is more sensitive to moisture than the AB-3, it may behave differently from the AB-3 when they are saturated. Further research is needed to evaluate their behavior under a saturated condition.

3 Unreinforced - 0 kPa (unloaded from 552 kPa) Unreinforced - 552 kPa 2 Single cell-reinforced - 0 kPa (unloaded from 552 kPa) Single cell-reinforced - 552 kPa 1 Multicell-reinforced - 0 kPa (unloaded from 552 kPa) Cumulative Cumulative displacement (mm) Multicell-reinforced - 552 kPa 0 0 25 50 75 100 125 150 Number of loading cycle Fig. 13. Cumulative deformations of the AB-3 bases under repeated loading.

2 Single cell-reinforced confined - 0 kPa Single cell-reinforced confined - 552 kPa

1 Single cell-reinforced unconfined - 0 kPa Cumulative Cumulative deformation (mm) Single cell-reinforced unconfined - 552 kPa 0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 14. Cumulative deformations of unconfined and confined, single geocell-reinforced QW.

Multicell-reinforced sand 12 Unreinforced QW Multicell-reinforced QW Unreinforced AB-3 8 Multicell-reinforced AB-3

4 Permanent displacement (mm)

0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 15. Comparison of permanent deformations under 552 kPa repeated loading.

Traffic benefit ratio

The benefit of geocell reinforcement for extending pavement life can be evaluated using a parameter of Traffic Benefit Ratio (TBR). TBR is defined as the ratio of the number of cycles necessary to reach a given rut depth (i.e., the permanent deformation) for a geocell-reinforced test section to that for an unreinforced section at the same rut depth with the same section thickness and base and subgrade properties. The base thickness in all the test sections was 12 cm and the subgrade in the present study was a hard wood surface. Past research on other geosynthetic-reinforced bases showed that the TBR values depended on the level of permanent deformation. Since the permanent deformations of most base sections at the end of 150 loading cycles in this study were less than 4 mm, the TRB values were calculated at the permanent deformation of 3 mm. Since the unreinforced KR sand failed under static loading before the maximum pressure was applied, the number of cycles required to reach a permanent deformation of 3 mm would be less than 1. Therefore, although the benefit of geocell reinforcement in the weak and poorly-graded KR sand was very high compared to that of the stronger materials QW and AB-3, the TBR values for KR sand sections were not calculated. Table 2 presents the TBR values for single and multiple geocell-reinforced sections having different infill material. The geocell-reinforced QW and AB-3 bases had comparable TRB values. The multiple geocell reinforcement had higher TBR values than the single geocell reinforcement. It should be pointed out that these TRB values were calculated to demonstrate the benefit of geocell reinforcement in pavement life, but they should not be used for design because actual pavement sections may be much different from the base sections in this study, for example, the bases in this study were on a firm subgrade.

Table 2

Calculated Traffic Benefit Ratios (TBR).

Infill material Reinforcement type Maximum applied TBR

pressure (kPa)

QW Single geocell 552 8.0

QW Multiple Geocell 552 12.0

AB-3 Single geocell 552 8.5

AB-3 Multiple Geocell 552 12.5

Elastic deformation

The elastic deformations of unreinforced and single geocell-reinforced sections with the number of loading cycles are plotted in Fig. 16 for the QW and AB-3 sections. The elastic deformations of the multiple geocell-reinforced KR sand and unreinforced and multiple geocell- reinforced QW and AB-3 sections are plotted in Fig. 17. Fig. 16 shows that the single NPA geocell-reinforced sections had higher elastic deformation than the unreinforced sections of QW and AB-3. This comparison demonstrates that the geocell-reinforced bases had more elastic responses than the unreinforced bases due to the inclusion of the geocell. The amount of elastic deformation was slightly higher at the beginning but later stabilized to a constant value. Fig. 17 shows that the multiple geocell-reinforced AB-3 base had more elastic response than the multiple geocell-reinforced sand, followed by the quarry waste.

0.9

0.6

Unreinforced QW 0.3 Single geocell-reinforced QW

Elastic deformation (mm) Unreinforced AB-3 Single geocell-reinforced AB-3 0.0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 16. Elastic deformations of unreinforced and single geocell-reinforced QW and AB-3 bases under 552 kPa repeated loading.

0.9

0.6

Multiple geocell-reinforced sand 0.3

Elastic deformation (mm) Multiple geocell-reinforced QW

Multiple geocell-reinforced AB-3 0.0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 17. Elastic deformations of multiple geocell-reinforced bases under 552 kPa repeated loading.

Percentage of elastic deformation

Fig. 18 presents the elastic deformation as a percentage of the total deformation for geocell-reinforced KR sand with the number of loading cycles under the repeated loading of 345 kPa and 553 kPa. Fig. 19 presents the percentages of elastic deformation of multiple geocell- reinforced sections with different base materials under 552 kPa repeated loading. Similar results were obtained for all three base materials: reinforced KR sand and both unreinforced and reinforced QW and AB-3 bases. Both Figs. 18 and 19 represent the trend of those curves as well; therefore, those curves are not included here. The percentage of elastic deformation was calculated by dividing the elastic deformation induced by each load cycle to the total deformation (i.e., the sum of elastic and plastic deformations) at that cycle.

100

40 Single cell-reinforced loaded at 345 kPa Multicell-reinforced loaded at 345 kPa % elastic deformation 20 Multicell-reinforced loaded at 552 kPa

0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 18. Percentage of elastic deformation for geocell-reinforced KR sand with loading cycle.

For each loading cycle, there are elastic and plastic deformations. The percentage of elastic deformation is defined as the ratio of the elastic deformation to the total deformation in each cycle, multiplied by 100%. Figs. 18 and 19 show that the representative percentage of elastic deformation for all the test sections increased with the number of the loading cycles. At the initial loading cycles, the plastic deformation was more pronounced, however, at around 10 cycles, the percentage of elastic deformation increased rapidly with the loading cycles and became relatively stable. After 10 cycles, the percentage of elastic deformation was more than

80% for the single geocell-reinforced KR sand and more than 95% for the geocell-reinforced

QW and AB-3 bases. At 150 load cycles, the percentage of elastic deformation was 95.2% for the geocell-reinforced sand and more than 99% for the geocell-reinforced QW and AB-3 bases.

The higher percentage of elastic deformation is desirable for a longer service life of a pavement section. Figs. 18 and 19 show that geocell reinforcement increased the percentage of elastic deformation in the reinforced section as compared with the unreinforced section, especially for the sections with the KR sand base.

100

Sand 40 QW

% elastic deformation AB-3 20

0 0 25 50 75 100 125 150 Number of loading cycle

Fig. 19. Percentages of elastic deformation of multiple geocell-reinforced sections with different base materials under 552 kPa repeated loading.

Conclusions

This paper presents the results of experimental work conducted to investigate the behavior of novel polymer alloy (NPA) geocell-reinforced bases under repeated loading. The unreinforced and geocell-reinforced base courses with three different infill materials, Kansas

River sand (KR sand), quarry waste (QW), and AB-3 aggregates were tested under repeated loading. The experimental investigations included the effect of infill material on the performance of geocell-reinforced granular bases. The following conclusions can be drawn from this experimental study:

1. The geocell-reinforced bases had higher initial modulus than the unreinforced bases. Their

modulus improvement factors ranged from 1.26 to 2.04.

2. Geocell reinforcement significantly reduced the permanent deformation as compared with

the unreinforced bases of all three infill materials under repeated loading. Multiple

geocells further reduced the permanent deformation as compared with single geocell.

3. The traffic benefit ratio (TBR) values from this study were equal or greater than 8.0 for the

single geocell-reinforced bases and 12.0 for multiple geocell-reinforced bases, respectively.

4. The geocell-reinforced QW and AB-3 bases had a higher percentage of elastic deformation

than the unreinforced bases due to the contribution of the geocell. Except for the sand

bases, geocell-reinforced AB-3 and QW bases reached 90% elastic deformation after the

initial few cycles (mostly 10 cycles).

Acknowledgements

This research was funded jointly by the University of Kansas, Transportation Research

Institute from Grant #DT0S59-06-G-00047, provided by the US Department of Transportation –

Research and Innovative Technology Administration and PRS Mediterranean, Inc. in Israel.

This support is greatly appreciated. Mr. Milad Jowkar, a former graduate student in the

Department of Civil, Environmental, and Architectural Engineering at the University of Kansas provided assistance in the testing.

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Figure Captions

Fig. 1. Tensile stress-strain curve of the NPA geocell.

Fig. 2. Picture of the NPA geocell used in the research.

Fig. 3. Grain size distribution curves of KR sand, QW, and AB-3 aggregate.

Fig. 4. Standard Proctor compaction curves of QW and AB-3 aggregate.

Fig. 5. California Bearing Ratio curves of QW and AB-3 aggregate.

Fig. 6. Test box with geocell layout.

Fig. 7. Loading machine.

Fig. 8. Geocell layout.

Fig. 9. Single geocell-reinforced section after the test.

Fig. 10. Pressure-displacement cycles under repeated loading.

Fig. 11. Deformations of the KR sand bases under repeated loading.

Fig. 12. Cumulative deformations of the QW bases under repeated loading.

Fig. 13. Cumulative deformations of the AB-3 bases under repeated loading.

Fig. 14. Cumulative deformations of unconfined and confined, single geocell-reinforced QW.

Fig. 15. Comparison of permanent deformations under 552 kPa repeated loading.

Fig. 16. Elastic deformations of unreinforced and single geocell-reinforced QW and AB-3 bases under 552 kPa repeated loading.

Fig. 17. Elastic deformations of multiple geocell-reinforced bases under 552 kPa repeated loading.

Fig. 18. Percentage of elastic deformation for geocell-reinforced KR sand with loading cycle.

Fig. 19. Percentages of elastic deformation of multiple geocell-reinforced sections with different base materials under 552 kPa repeated loading.

Table Captions

Table 1 Modulus Improvement Factors of the Bases.

Table 2 Calculated Traffic Benefit Ratios (TBR)