Impact of repeated loading on mechanical response of a reinforced sand

Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 804e814

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Impact of repeated loading on mechanical response of a reinforced sand Aida Mehrpazhouh a, Seyed Naser Moghadas Tafreshi a, Mehdi Mirzababaei b, * a b

Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran School of Engineering and Technology, Central Queensland University, 120 Spencer Street, Melbourne, Victoria, 3000, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2018 Received in revised form 15 November 2018 Accepted 14 December 2018 Available online 13 April 2019

Pavements constructed over loosely compacted subgrades may not possess adequate California bearing ratio (CBR) to meet the requirements of pavement design codes, which may lead to a thicker pavement design for addressing the required strength. Geosynthetics have been proven to be effective for mitigating the adverse mechanical behaviors of weak soils as integrated constituents of base and sub-base layers in road construction. This study investigated the behaviors of unreinforced and reinforced sand with nonwoven geotextile using repeated CBR loading test (followed by unloading and reloading). The depth and number of geotextile reinforcement layers, as well as the compaction ratio of the soil above and below the reinforcement layer(s) and the compaction ratio of the sand bed, were set as variables in this context. Geotextile layers were placed at upper thickness ratios of 0.3, 0.6 and 0.9 and the lower thickness ratio of 0.3. The compaction ratios of the upper layer and the sand bed varied between 85% and 97% to simulate a dense layer on a medium dense sand bed for all unreinforced and reinforced testing scenarios. Repeated CBR loading tests were conducted to the target loads of 100 kgf, 150 kgf, 200 kgf and 400 kgf, respectively (1 kgf ¼ 9.8 N). The results indicated that placing one layer of reinforcement with an upper thickness ratio of 0.3 and compacting the soil above the reinforcement to compaction ratio of 97% significantly reduced the penetration of the CBR piston for all target repeated load levels. However, using two layers of reinforcement sandwiched between two dense soil layers with a compaction ratio of 97% with upper and lower thickness ratios of 0.3 resulted in the lowest penetration. Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Geosynthetics Geotextile Reinforced soil California bearing ratio (CBR) Elastic behavior Repeated loading Dense sand Medium dense sand

1. Introduction Geosynthetic-reinforced soil possesses a number of benefits including economical design, ease of installation, improved performance and reliability where weak soils may pose excessive postconstruction differential settlements due to their low bearing capacity (Abu-Farsakh and Chen, 2011; Senthil Kumar and Rajkumar, 2012; Chakravarthi and Jyotshna, 2013; Liu and Won, 2014). Geosynthetics have been also used in many areas of geotechnical engineering, e.g. construction of footings over soft soils (Deb et al., 2007; Latha and Somwanshi, 2009; Badakhshan and Noorzad, 2015), road/railway embankments (Li and Rowe, 2005; Benmebarek et al., 2015; Hussaini et al., 2015), slope stabilization

* Corresponding author. E-mail addresses: [email protected] (A. Mehrpazhouh), nas_ [email protected] (S.N. Moghadas Tafreshi), [email protected] (M. Mirzababaei). Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.

(Shukla et al., 2011; Abd and Utili, 2017), road base/sub-base construction (Kamalzare and Ziaie-Moayed, 2011), and pavement systems (Abu-Farsakh et al., 2016; Correia and Zornberg, 2016; Hou et al., 2017; Pei and Yang, 2018). A majority of previous studies have focused on the investigation of mechanical behavior of geosynthetic-reinforced soil systems under monotonic loads (Zhao and Foxworthy, 1999; Santoni and Webster, 2001; Ahmed Kamel et al., 2004; Saran, 2010; Biswas et al., 2016; Rajesh et al., 2016; Badakhshan and Noorzad, 2017; Punetha et al., 2017; Ouria and Mahmoudi, 2018). Debnath and Dey (2017) proposed that the optimum thickness of the geogridreinforced sand layer set on a group of stone columns is 0.2 times the diameter of the footing. They also observed an increase in the thickness of the sand layer and a decrease in the effectiveness of the geogrid layer in bearing capacity enhancement. Suku et al. (2017) illustrated that the resilient modulus of the reinforced section was higher than that of the unreinforced one due to the combined effect of interlocking of aggregates within the geogrid apertures and its membrane effect, resulting in a reduction in the required thickness of the reinforced base layer.

https://doi.org/10.1016/j.jrmge.2018.12.013 1674-7755 Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

A. Mehrpazhouh et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 804e814

formation on the surface of the top layer. Previous studies have clarified some valuable results in the field of pavement reinforcement, but all the effective parameters have not been fully considered. Thus in this study, some practical solutions to improving a weak subgrade such as the use of compacted soil layers with different thicknesses, in combination with geotextile layers, have been considered. The repeated CBR test is used to investigate the effect of single and double geotextile layers, the placement depths and the thickness of compacted soil layers on the load-penetration response (the elastic and plastic deformation) of unreinforced and reinforced specimens. 2. Materials In this study, a well-graded sand (SW) according to the unified soil classification system (ASTM D2487-11, 2011), with a specific gravity of 2.68, was used. Fig. 1 shows the particle size distribution curve of the soil determined using ASTM D422-63(2007)e2 (2007). The maximum dry unit weight and optimum moisture content of the sand were determined to be 19.2 kN/m3 and 11.5%, respectively, using modified Proctor compaction test (ASTM D1557-12e1, 2012) (see Fig. 2). Table 1 shows the physico-mechanical properties of the soil used in this study. A nonwoven geotextile made of polypropylene yarns was used as a reinforcement material. Table 2 shows the properties of the geotextile. 3. Testing procedure In this context, a series of repeated CBR loading tests was conducted following the recommendations by ASTM D1883-16 (2016) on a compacted soil in a CBR mold of 152 mm (d)  117 mm (H). Loading and unloading stages were carried out at an axial displacement rate of 1.27 mm/min using a 50 mm diameter standard circular penetration piston. A CBR loading cycle comprised of a loading to the target load (i.e. 100 kgf, 150 kgf, 200 kgf and 400 kgf, 1 kgf ¼ 9.8 N), followed by unloading. Preliminary repeated CBR loading tests showed that the rate of increase in penetration significantly decreases with the number of loading cycles, and a stable condition may be expected beyond 20 loading cycles, regardless of the number of geotextile layers, the density of soil layers and the amplitude of the target load. The adequacy of using 20 loading cycles was also reported in previous studies (Tavakoli Mehrjardi et al., 2012). Therefore, each test was repeated for 20 loading cycles. The above testing procedure was conducted for target loads as stated previously, and the elastic

100 90 Passing percentage (%)

Although the mechanical behaviors of unreinforced soils and shallow foundations subjected to dynamic loads have been comprehensively investigated over the last decades (Cunny and Sloan, 1962; Raymond and Komos, 1978; Das and Shin, 1996; Cremer et al., 2002; Gajan et al., 2005; Chowdhury and Dasgupta, 2017; Izadi et al., 2018), the performance of the reinforced soil subjected to cyclic and repeated loads has not been comprehensively documented. Ahmed Kamel et al. (2004) conducted a series of cyclic triaxial tests on different soil samples (SP, ML and CL) reinforced with two geogrid types with different stiffnesses. They reported a reduction in the permanent strain of the reinforced soil, irrespective of the types of soil and geogrid. Al-Qadi et al. (2008) carried out a set of full-scale tests to investigate the performances of geogrid reinforced pavement systems under a moving dual-tire. They found that the optimum placement depth of reinforcement in a thin aggregate layer is at the interface between unbound aggregate and subgrade, while for a thicker base layer, the geogrid may be installed at the upper third of the layer. Correia and Zornberg (2016) investigated the response of a large-scale flexible geogrid reinforced pavement subjected to cyclic loads and concluded that the geogrid layer improves the behavior of pavement system by reducing the amount of generated strain, rutting depth and permanent lateral displacement in the surface layer due to a better stress distribution role of geogrids. Wu et al. (2015) also showed that the deflection rate of gravel base reinforced with geogrid is 60% less than that of unreinforced one at the initial cycle of loading of the loaded wheel test (LWT). Abu-Farsakh et al. (2016) investigated the performance of a hot-mix asphalt concrete paved road reinforced with geosynthetic layers using cyclic plate load test. It was reported that reinforcing with two layers of geosynthetics results in a reduced pavement rutting depth, hence reducing the thickness of the road base by one third due to the increased resilient modulus of the resulting structure. Among different standard tests, California bearing ratio (CBR) test is a simple and effective way of testing for evaluating the mechanical strengths of natural ground, subgrade and base course. It is also used to design the thickness of materials needed for the proposed road construction. Furthermore, the CBR test is a simple, quick and appropriate approach, compared to the repetitive plate load test, which is used to evaluate the resilient and permanent deformation characteristics of compacted soil layers. Singh and Gill (2012) investigated the effect of geogrid reinforcement on the CBR value and elastic modulus of the reinforced clay. They reported an increase in CBR from 2.9% for unreinforced clay to 9.4% with a single geogrid layer. Sas and Gluchowski (2013) used a series of CBR tests to investigate the cyclic deformation of subgrade stabilized with lime. They found that the amount of plastic deformation decreased with time until it became purely a resilient deformation. Molenaar (2008) compared the cyclic CBR test results with a numerical modeling and a cyclic triaxial test. Their results indicated a good correlation between resilient modulus obtained from both methods. The study of Brahmachary and Rokonuzzaman (2018) on the natural fiber-reinforced soil indicated that the CBR value in soaked and un-soaked conditions improves considerably with the addition of 1.2% bamboo fiber. Mittal and Shukla (2018) investigated the combined effect of geogrid and geotextile on the strength behavior of a silty soil. They showed that the CBR values for geosyntheticreinforced samples are significantly high but the ductility and rupture resistance are approximately constant compared to the natural soil. Geo-structures on weak soils subjected to repeated loads (e.g. moving wheel load) may experience excessive settlement due to the low bearing capacity of the underlying soil, thus resulting in progressive defects in form of uneven settlement and crack

805

80 70 60 50

D10=0.23 mm D30=0.9 mm D50= 1.95 mm D60=2.8 mm Cu=12.17 Cc=1.26

40 30 20 10 0 0.001

0.01

0.1 1 Grain diameter (mm)

10

Fig. 1. Grain size distribution curve of the sand used in this study.

100

806

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Dry unit weight (kN/m3)

20.0 19.5 19.0 18.5 18.0 17.5 17.0 0

2

4

6

8 10 12 Water content (%)

14

16

18

(a)

(b)

Fig. 3. Schematic of unreinforced specimen: (a) One medium dense sand layer (Test 1), and (b) A dense layer on a medium dense layer with u1/D of 0.3 and 0.6 (Tests 2 and 3).

Fig. 2. Modified Proctor compaction curve of the sand.

deformation (Pe) and plastic deformation (Pp) of the deformed soil were analyzed subsequently. It should be noted that as the mechanism of geotextile reinforcement for stress redistribution is based on the membrane action, the shape of the reinforcement layer deformation is a parabolic form with rutting in the middle and hence the CBR mold dimension may not fully provide support for the geotextile layer.

3.1. Unreinforced tests The schematic of the unreinforced soil specimens is shown in Fig. 3, where Rc is the compaction ratio (i.e. the ratio of the dry unit weight of the soil layer to its maximum dry unit weight), u1 is the thickness of the top dense soil layer, h is the thickness of the lower medium dense soil layer, and D is the diameter of the CBR piston. In this study, the upper thickness ratio (u1/D) is defined as the ratio of the thickness of the top dense soil layer (u1) to the diameter of the CBR piston (D). The upper thickness ratio was varied between

0, 0.3 and 0.6. Table 3 demonstrates the arrangements of the tests conducted in this study for both unreinforced and geotextilereinforced compacted sand. In order to evaluate the reliability and repeatability of the measured data, all tests described in Table 3 were repeated three times and it was observed that the maximum difference in the recorded data is within 5%. In order to evaluate the effect of compaction density on the efficiency of planar reinforcement for improving the load-penetration behavior of sand, two compaction density values of 97% (relatively dense) and 85% (relatively medium dense) were selected. The moisture content is selected as the optimum value obtained from the Proctor compaction curve (¼11.5%), but to achieve two compaction density values of 85% and 97%, different compaction energies were used by changing the number of blows for each soil layer. The first test as shown in Fig. 3a was conducted on a soil specimen compacted at a compaction ratio of 85% (see Table 3) and provided a baseline for comparing the level of improvement due to

Table 1 Physico-mechanical properties of the sand studied. Properties Specific gravity, Gs Fine content (<75 mm) Fine sand (0.075e0.425 mm) Medium sand (0.425e2 mm) Coarse sand (2e4.75 mm) Fine gravel (4.75e19 mm) USCS soil classification Coefficient of uniformity, Cu Coefficient of curvature, Cc Maximum dry unit weight, gdmax Optimum moisture content, uopt

Unit % % % % %

kN/m3 %

Value

Standard

2.68 1.21 15.95 31.95 38.11 12.78 SW 12.17 1.26 19.2 11.5

ASTM D854-14 (2014) ASTM D422-63(2007)e2 (2007)

ASTM D2487-11 (2011)

ASTM D1557-12e1 (2012)

Table 2 Properties of the geotextile. Properties Type Material Area weight Tensile strength (machine direction, MD) Tensile strength (cross machine direction, CMD) Elongation at peak stress (MD) Permittivity CBR puncture resistance

Unit

Value

Standard

g/m2 kN/m kN/m % s1 kN

Nonwoven Polypropylene 412 16 13 50 1.2 2.1

ASTM ASTM ASTM ASTM ASTM

D4595-11 (2011) D4595-11 (2011) D4632/D4632M-15a (2015) D5493-06 (2016) D6241-14 (2014)

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807

Table 3 Arrangements of repeated CBR loading tests. Specimen type

Test No.

u1/D

Unreinforced specimen

1 2 3 4 5 6 7 8 9 10 11

0 0.3 0.6 0.3 0.6 0.9 0.3 0.6 0.3 0.3 0.3

Reinforced specimen with one layer of reinforcement (N ¼ 1)

Reinforced specimen with two layers of reinforcement (N ¼ 2)

Compaction ratio of top dense soil layer (RC-u1) (%)

u2/D

Compaction ratio of second dense soil layer (RC-u2) (%)

97 85

97 0.3 0.3 0.3

85 97

the change in compaction ratio of the soil layers and the arrangement of the reinforcement layer(s). The second and third tests were carried out to investigate the effect of the thickness of the dense layer overlying the medium dense layer on the load-penetration capacity of the unreinforced specimen (see Fig. 3b).

97 85 97

h/D

Compaction ratio of lower medium dense soil layer (RC-h) (%)

Target load (kgf)

2.34 2.04 1.74 2.04 1.74 1.44 2.04 1.74 1.74 1.74 1.74

85

100, 150, 200 200, 400 200, 400 200 200 200 200 200, 400 200, 400 400 400

85

85

diameter of the CBR piston. Tests Nos. 10 and 11 examine the effect of two reinforcement layers on the load-penetration capacity of the medium dense and the dense soil layers overlying medium dense soil (see Fig. 5). 4. Results and discussion

3.2. Reinforced tests Reinforced specimens were prepared with different arrangements including the number and depth of reinforcement layers and compaction ratio of the top and bottom of the reinforcement in order to investigate the combined effects of abovementioned parameters on the load-penetration behavior of the soil (i.e. Tests Nos. 4e11). For reinforced soil, the geotextile layer was cut in a round shape and placed in the determined depth from the top of the specimen (i.e. u1 from the top for one reinforcement layer and u1 and u1 þ u2 for double reinforced layers). Figs. 4 and 5 show the specimen conditions for single and double layers of reinforcement, respectively, for Tests Nos. 4e11. Fig. 4a and b depicts the reinforced specimen with one layer of geotextile in which the soil above the reinforcement with a thickness of u1 compacted at compaction ratios of 85% and 97%, respectively. Fig. 4c demonstrates a specimen with one layer of geotextile between two layers of dense soil with compaction ratio of 97% and thicknesses of u1 and u2 at the top and bottom of the reinforcement, respectively. For the reinforced specimen, the lower thickness ratio (i.e. u2/D) is defined as the thickness of the dense soil layer under the first reinforcement layer (i.e. u2) bounded either by the lower medium dense soil layer (see Fig. 4c) or the second reinforcement layer (see Fig. 5b) to the

Geotextile layer

(a)

4.1. Effect of repeated load on load-penetration behavior of unreinforced soil Fig. 6a illustrates the load-penetration curves for unreinforced specimens with a compaction ratio of 85% tested at three target repeated load levels of 100 kgf, 150 kgf and 200 kgf, respectively (see Table 3, Test 1). Fig. 6b depicts the variation of the maximum penetration with the number of loading cycles for the three tested target loads. As expected, the penetration increased with the target load level. However, for each target load (particularly the lower load levels, e.g. 100 kgf and 150 kgf), the rate of penetration declined with the number of cycles, achieving a virtual plateau with the change in deformation of the last two cycles being 2% of the total deformation of the specimen. The penetration rate for the higher target loads (i.e. 200 kgf and 400 kgf) needs further loading cycle applications to achieve an insignificant incremental change (i.e. the curve in Fig. 6b becomes approximately flat). This is mostly attributed to the membrane action of geotextile which is completely mobilized in higher target loads (Leng, 2002). Fig. 6 also indicates that the major part of the deformation, regardless of the applied target repeated load level, occurred within the first cycle that was irrecoverable and can be assumed to be

Geotextile layer

( b)

Geotextile layer

(c)

Fig. 4. Reinforced specimens with one layer of geotextile (a) in the medium dense soil (Tests Nos. 4e6), (b) underneath the dense layer overlying medium dense soil (Tests Nos. 7 and 8), and (c) between two dense layers overlying medium dense soil (Test No. 9).

808

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Geotextile layer

Geotextile layer

(a)

(b)

Fig. 5. Reinforced specimens with two layers of geotextile (a) in the medium dense soil (Test No. 10) and (b) in the dense soil overlying medium dense soil (Test No. 11).

plastic deformation. Sas and Gluchowski (2013) reported a similar finding in that the first cycle of loading and unloading contained the highest permanent deformation, which was increased by increasing the target load, but after a specific number of cycles, the rate of deformation changes was decreased significantly. 4.2. Effect of compaction ratio on load-penetration behavior of unreinforced soil In weak subgrades without appropriate compaction to sustain the surcharge of the pavement and the load exerted by the moving vehicles, excessive plastic settlement may cause the soil to collapse under repeated loads. Therefore, one of the solutions to improve the bearing capacity of the weak subgrade is to replace the upper layer of the subgrade soil with a densely compacted soil to reduce the settlement of the subgrade (KazimierowiczFrankowska, 2007). Fig. 7 shows the relationship between load and total penetration for upper soil with thickness ratios of 0, 0.3 and 0.6, which was compacted to the compaction ratio of 97%, while the lower layer was compacted to the compaction ratio of 85%. The total penetration of the medium dense specimen after 20 loading cycles was determined as 21.96 mm. However, compaction of the top layer with upper thickness ratios of 0.3 and 0.6 resulted in 42.1% and 61.9% reductions in the total penetration,

250

25

Unreinforced (100kg.f) kgf) Unreiforced (100 Unreinforced (150kg.f) kgf) Unreiforced (150 Unreinforced (200kg.f) kgf) Unreiforced (200

225

20 Total penetration (mm)

200 175 CBR load (kgf)

respectively. In fact, increasing the compaction ratio and thickness of the top layer resisted a great amount of penetrating load and resulted in less penetration. The effects of the top layer compaction ratio on plastic penetration (Pp) and elastic penetration (Pe) of the specimens with different upper thickness ratios (u1/D) are illustrated in Fig. 8. The plastic penetrations were calculated as the difference between the total deformation of the soil at the end of the unloading stage and the deformation at the onset of the subsequent loading stage (see Fig. 7), and elastic penetration was determined as the interspace between the total and plastic penetrations. As can be seen in this figure, the rate of changes in the elastic and plastic penetration values decreased with the number of loading cycles and approached a near-zero value after 20 loading cycles. In addition, a large portion of the total settlement of the soil is due to the plastic deformation, as the elastic deformation of each loading cycle is bounded to a negligible range of 0.3 mm (for repeated load of 200 kgf) to 0.4 mm (for repeated load of 400 kgf) (see Fig. 8). According to the results of Moghaddas Tafreshi et al. (2014) on repeated plate load tests, less elastic deformation (less than 0.3 mm) at lower repeated load (lower than 200 kgf) was expected. Moghaddas Tafreshi et al. (2011) found an increase in the elastic deformation of the foundation bed by increasing the compaction ratio of the soil layers.

150 125 100 75

15

10

5

50

Unreinfoced (100 kgf) Unreinforced (150 kgf) Unreinforced (200 kgf)

25 0

0 0

2

4

6

8 10 12 14 16 18 20 22 24 Penetration (mm) (a)

0

2

4

6

8 10 12 14 16 18 20 Number of cycles (b)

Fig. 6. (a) Load-penetration curves, and (b) Relationship between penetration and number of cycles for unreinforced soil compacted at Rc ¼ 85% (Test No. 1).

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500 450

Rc=97%) Unreinforced (Test 2, u2, Unreinforced (Test u1/D:0.3, Rc:97%) 1/D=0.3,

450

400

Rc=97%) Unreinforced (Test 3, u3, Unreinforced (Test u1/D:0.6, Rc:97%) 1/D=0.6,

400

350

Repeated load = 200 kgf

300 250 200

250 200 150

100

100

50

50

0 5

10

Unreinforced 3, u3, Rc=97%) Unreinforced(Test (Test u1/D:0.6, Rc:97%) 1/D=0.6, Repeated load = 400 kgf

300

150

0

Unreinforced 2, u2, Rc=97%) Unreinforced(Test (Test u1/D:0.3, Rc:97%) 1/D=0.3,

350 CBR load (kgf)

CBR load (kgf)

500

Unreinforced (Test 1, Rc=85%) Unreinforced (Test1, Rc:85%)

809

15 20 25 Penetration (mm) (a)

30

0

35

0

5

10 15 20 25 Penetration (mm) (b)

30

35

Fig. 7. Load-penetration curves for unreinforced soil with upper thickness ratios of 0.3 and 0.6 at repeated CBR loads of (a) 200 kgf and (b) 400 kgf.

Fig. 9 demonstrates the load-penetration behavior and the variation of plastic deformation of the soil specimen with the number of loading cycles for unreinforced and reinforced medium dense soils tested under a repeated load of 200 kgf (i.e. Tests Nos. 1 and 4e6). Reinforcement layer was embedded at different depths equivalent to upper thickness ratios of 0.3, 0.6 and 0.9, respectively, to investigate the effect of the placement depth of reinforcement in compacted soil. Fig. 9a shows that irrespective of upper thickness ratio, the penetration resistance of the soil increased significantly with a single layer of reinforcement, and the total/plastic deformation of the soil decreased with the reduction in the upper thickness ratio of the specimen. The initial slope of the loadpenetration curve for the first loading cycle also increased with the placement of reinforcement, resulting in an improvement in the penetration resistance of the reinforced specimen. In this study, for single reinforcement layer in the medium dense sand specimen with a compaction ratio of 85%, an upper thickness ratio of 0.3 resulted in the least plastic deformation (see Fig. 9b). It is in

30 25 20

0.45 0.40 0.35 0.30 0.25

15

0.20 0.15

10

0.10

5

0.05 0.00

0 0

2

4

6

8 10 12 14 Number of cycles (a)

16

18

20

35

0.50

0.50 0.45

30 Plastic penetration, Pp (mm)

Unreinforced UnreinforcedPPp (Test1,1,RRc:85%) p (Test c=85%) Unreinfoced P Pp (Test 2, 2, uu1/D:0.3, Rc:97%) Unreinforced p (Test 1/D=0.3, R c=97%) UnreinforcedPPp (Test3,3,u1u1/D:0.6, /D=0.6, RRc:97%) Unreinforced p (Test c=97%) UnreinforcedPPe (Test1,1,RRc:85%) Unreinforced p (Test c=85%) Unreinfoced P Pee (Test (Test 2, 2, uu1/D:0.3, Rc:97%) Unreinforced 1/D=0.3, R c=97%) Unreinfoced P Pee (Test (Test3,3,uu1/D:0.6, Rc:97%) Unreinforced 1/D=0.6, R c=97%) Repeated load = 200 kgf

Elastic penetration, Pe (mm)

Plastic penetration, Pp (mm)

35

accordance with the results of Moghaddas Tafreshi and Dawson (2010), in which the optimum placement of planar reinforcement was reported as 0.35 times the footing width. Compared to the unreinforced soil, the values of the total CBR penetration in the last cycle of the test were reduced by 58%, 39% and 33% with upper thickness ratios of 0.3, 0.6 and 0.9, respectively. Ling and Liu (2001) and Zidan (2012) indicated similar results with a decrease in the total settlement under the loaded area using the reinforcement layer. Soil does not possess an inherent tensile strength and therefore, the placement of reinforcement layers/elements such as geotextile or randomly distributed fibers within the compacted soil contributes to the development of a tension resisting behavior that can effectively withstand the externally stimulated forces (Mirzababaei et al., 2013, 2018). Therefore, the embedded geotextile layer contributed effectively to receive and re-distribute the applied penetrating force on a wider area, resulting in a reduced stress concentration. The reduction in the performance of reinforcement by increasing the upper thickness ratio of the reinforced specimen could be attributed to the departure of the reinforcement layer from the stressed zone as the reinforcement layer may not be anchored effectively in a less compacted soil.

0.40

25

0.35 0.30

20

0.25 15

Unreinfoced Pp Unreinforced Pp (Test (Test 2, 2, u1/D:0.3, u1/D=0.3,Rc:97%) Rc=97%) Unreinforced P Ppp (Test (Test 3, 3, uu1/D:0.6, Rc:97%) 1/D=0.6, R c=97%) (Test 2, 2, u1/D:0.3, u1/D=0.3,Rc:97%) Rc=97%) Unreinforced Pe (Test Unreinfoced Pe Unreinforced Pe (Test (Test 3, 3, u1/D:0.6, u1/D=0.6, Rc:97%) Rc=97%) Unreinfoced Pe Repeated load = 400 kgf

10 5 0

0.20 0.15 0.10

Elastic penetration, Pe (mm)

4.3. Effect of embedment depth of reinforcement on loadpenetration behavior of soil

0.05 0.00

0

2

4

6

8 10 12 14 16 Number of cycles

18 20

(b)

Fig. 8. Relationships between plastic and elastic penetration values with the number of cycles for the repeated CBR loads of (a) 200 kgf and (b) 400 kgf.

810

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Unreinforced 1, R1, Unreinforced(Test (Test Rc:0.85) c=85%) Rc-u1Rc-u1:85%) =85%) Reinforced 4, u4,1/D=0.3, Reinforced(Test (Test u1/D:0.3, Reinforced 5, u5,1/D=0.6, Rc-u1Rc-u1:85%) =85%) Reinforced(Test (Test u1/D:0.6, Rc-u1Rc-u1:=85%) =85%) Reinforced 6, u6,1/D=0.9, Reinforced(Test (Test u1/D:0.9,

250 225

24 22 20 Plastic penetration, Pp (mm)

200

CBR load (kgf)

175 150 125 100 75

18 16 14 12 10

50

8 6

Unreinforced Pp Pp (Test 1, Rc1, =85%) Unreinforced (Test Rc:0.85) (Test 4, u14, /D=0.3, Rc-u1=85%) Reinforced PpPp Reinforced (Test u1/D:0.3, Rc-u1:85%) Reinforced PePp (Test 5, u15, /D=0.6, Rc-u1=85%) Reinforced (Test u1/D:0.6, Rc-u1:85%) Reinforced (Test u1/D:0.9, Rc-u1:=85%) (Test 6, u16, /D=0.9, Rc-u1=85%) Reinforced PePp

4

25

2 0

0 0

2

4

6

0

8 10 12 14 16 18 20 22 24 Penetration (mm) (a)

2

4

6

8 10 12 14 Number of cycles

16

18

20

(b)

Fig. 9. Relationships between (a) repeated load and penetration and (b) plastic deformation and number of cycles for unreinforced and one-layer reinforced soils with different upper thickness ratios at target load of 200 kgf.

4.4. Combined effect of one layer of reinforcement and compaction ratio on load-penetration behavior Fig. 10 shows the combined effect of compaction ratio of the top layer on the load-penetration behavior of the reinforced soil for upper thickness ratios of 0.3 and 0.6. It is apparent that the total penetration of the specimen decreased with the compaction ratio of the upper layer, irrespective of unreinforced and reinforced specimens. However, for both upper thickness ratios of 0.3 and 0.6, unreinforced soil showed an enhanced rate of improvement upon compaction compared to the reinforced soil. On the other hand, for reinforced soil, increase in upper thickness ratio had an adverse effect on the total experienced deformation of the soil and it was

Rc=85%) Unreinforced (Test 1, Rc:85%) u1/D=0.3, Rc-u1:97%) Rc-u1=97%) Unreinforced (Test 2, u1/D:0.3, u1/D=0.3, Rc-u1:85%) Rc-u1=85%) Reinforced (Test 4, u1/D:0.3, Rc-u1=97%) u1/D=0.3, Rc-u1:97%) Reinforced (Test 7, u1/D:0.3,

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150 125 100 75

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c=85%) Unreinforced (Test 1, R Rc:85%) u1/D=0.6, Rc-u1:97%) Rc-u1=97%) Unreinforced (Test 3, u1/D:0.6, u1/D=0.6, Rc-u1:85%) Rc-u1=85%) Reinforced (Test 5, u1/D:0.6, Rc-u1=97%) u1/D=0.6, Rc-u1:97%) Reinforced (Test 8, u1/D:0.6,

250

CBR load (kgf)

CBR load (kgf)

also found that the penetration of the unreinforced soil with a compaction ratio of 97% was less than that of the reinforced soil with a compaction ratio of 85% and upper thickness ratio of 0.6. The observed behavior of the reinforced soil with a higher upper thickness ratio can be attributed to the less anchorage of the reinforcement in the medium dense soil that may result in slippage of the reinforcement under the repeated loading. Therefore, reinforcement of the medium dense soil is efficient provided that either the upper layer of the soil is compacted well close to the maximum dry unit weight of the soil or the reinforcement layer is placed relatively close to the soil surface. Since the optimum value of the upper thickness ratio of the reinforced medium dense specimen was determined to be 0.3, a

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8 10 12 14 16 18 20 22 24 Penetration (mm) (a)

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Fig. 10. Effects of compaction ratio on loadepenetration relationships for unreinforced and reinforced soils with one layer of reinforcement at CBR load of 200 kgf for (a) u1/D ¼ 0.3 and (b) u1/D ¼ 0.6.

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500

500 Unreinforced u1/D:0.6, Rc-u1:97%) Unreinforced (Test(Test 3, u13,/D=0.6, Rc-u1 =97%) Reinforced u1/D:0.6, Rc-u1:97%) Rc-u1 =85%) Reinforced (Test(Test 8, u18,/D=0.6, Reinforced u1/D=u2/D:0.3, u2/D=0.3, RRc-u1=Rc-u2:97%) Reinforced (Test(Test 9, u19,/D= c-u1= Rc-u2=97%) Repeated load = 200 kgf

450 400

Unreinforced u1/D:0.6, Rc-u1:97%) Unreinforced (Test(Test 3, u3, Rc-u1 =97%) 1/D=0.6, Reinforced u1/D:0.6, Rc-u1:97%) Reinforced (Test(Test 8, u18,/D=0.6, Rc-u1 =85%) Reinforced (Test 9, u1/D=u2/D:0.3, Reinforced (Test 9, u1/D= u2/D=0.3, RRc-u1=Rc-u2:97%) c-u1= Rc-u2=97%) Repeated load = 400 kgf

450 400 350 CBR load (kgf)

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811

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Fig. 11. Effects of compaction ratio on loadepenetration relationships for unreinforced and reinforced soils with one-layer reinforcement at target loads of (a) 200 kgf and (b) 400 kgf.

series of CBR tests was conducted by placing the reinforcement layer between two dense layers with upper and lower thickness ratios of 0.3 and compaction ratio of 97%, under target repeated loads of 200 kgf and 400 kgf. Fig. 11 shows that the use of one layer of geotextile, irrespective of its location, reduced the total settlement. This figure also shows that placing the reinforcement layer between two compacted soil layers with a thickness ratio of 0.3 resulted in a further improvement in the load-penetration behavior of the subgrade compared to unreinforced and reinforced systems with an upper compacted soil layer thickness of 0.6 times the loading plate, irrespective of the target load level. Sandwiching a geotextile layer between two compacted layers (u1/D ¼ u2/D ¼ 0.3) reduced the settlement by 10% compared to placing the geotextile layer under a thicker compacted layer (u1/D ¼ 0.6). It conforms that the optimal location of geotextile as a reinforcement layer is at the upper third of the base layer (Al-Qadi et al., 2008). Thus, if the compacted layer is supposed to be the base layer, the geotextile layer should be in the middle of the base layer. Fig. 12 compares the variation of plastic penetration for unreinforced and reinforced soils with a single layer of reinforcement sandwiched between two dense layers for upper thickness ratios of 0.3 and 0.6 and target loads of 200 kgf and 400 kgf. The impact of placing reinforcement layer between two dense soil layers with upper and lower thickness ratios of 0.3 on reduction of the plastic deformation was more noticeable at the target load of 400 kgf in comparison with the load level of 200 kgf. Increase in surface stress results in larger stress bulbs and hence the total deformation is controlled by the compaction ratio to a larger depth from the soil surface. Moreover, with the reinforcement layer between two dense layers, the interfacial frictional resistance between adjacent soil and the geotextile layer increases with normal stress and this leads to a better contribution to the tensile strength of the reinforcement layer and hence more resistance to the deformation of reinforced soil. 4.5. Effect of number of reinforcement layers on load-penetration behavior of soil The load-penetration behavior of reinforced specimens with single and double layers of geotextile (Tests Nos. 9e11) is shown in Fig. 13. For a constant compaction ratio of 85% throughout the

specimen, use of two reinforcement layers reduced the total and plastic settlements of the soil significantly compared to one-layer reinforcement. It was also observed that the slope of the loading curve for the first cycle decreased significantly with the use of two reinforcement layers. Thus, it can be concluded that use of two reinforcement layers counteracted the excessive deformation at the first loading cycle when a single reinforcement layer is used. Moghaddas Tafreshi and Dawson (2010) showed that increasing the number of planar reinforcement layers led to an increase in bearing capacity of the footing and decreased the total settlement, and this effect was gradually degraded by increasing the number of reinforcement layers to more than two layers. In general, reinforcement with two layers of geotextile in the dense sand with a compaction ratio of 97% led to a further reduction in the successive deformation with the number of cycles and to a steady state within a less number of cycles. Single layer reinforced soil with a compaction ratio of 97% within a limited thickness above and below the reinforcement performed better in limiting the deformation compared to the double-layer reinforced soil with a compaction ratio of 85% throughout the specimen. The combined effect of double-layer reinforced soil with two dense soil layers above and below the reinforcement resulted in the lowest penetration within the cases studied. Soil reinforcement with two layers leads to an increment in the shear strength of the soil with increased friction at the interface of the geotextile and the soil (Marto et al., 2013; Bazne et al., 2015) and hence improves the reinforcement performance of the geotextile layer in dense soils. 5. Limitations and scale effect The efficiency of geotextile reinforcement and compaction ratio of the soil layer above and below the reinforcement layer(s) on foundations subjected to repeated loads is demonstrated by the testing results. Although the findings could be used as recommendations for geotechnical engineers, the following points should be considered: (1) The experimental results are obtained for only one type of geosynthetic, one type of sand and one size of the loading piston size. Thus, specific applications using quantitative results should only be adopted after considering these

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35

Unreinforced (Test 2, u2, Rc-u1=97%) Unreinforced (Test u1/D:0.3, Rc:97%) 1/D=0.3, Rc-u1=97%) Unreinforced (Test 3, u3, Unreinforced (Test u1/D:0.6, Rc:97%) 1/D=0.6, Reinforced 7, u7,1/D=0.3, Rc-u1Rc:97%) =97%) Reinforced(Test (Test u1/D:0.3, Rc-u1Rc:97%) =97%) Reinforced 8, u8,1/D=0.6, Reinforced(Test (Test u1/D:0.6, u2/D=0.3, Rc-u1=Rc:97%) Rc-u2=97%) Reinforced 9, u9, 1/D= Reinforced(Test (Test u1/D=u2/D:0.3, Repeated load = 200 kgf

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25 20 15 10 Unreinforced (Test 2, u12, /D=0.3, Rc-u1=97%) Unreinforced (Test u1/D:0.3, Rc:97%) /D=0.6, Rc-u1=97%) Unreinforced (Test 3, u13, Unreinforced (Test u1/D:0.6, Rc:97%) Rc-u1=97%) Reinforced (Test 8, u8, 1/D=0.6, Reinforced (Test u1/D:0.6, Rc:97%) u2/D=0.3, Rc-u1=Rc:97%) Rc-u2=97%) Reinforced (Test 9, u9, 1/D= Reinforced (Test u1/D=u2/D:0.3,

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Fig. 12. Relationships between plastic penetration and number of cycles for one-layer reinforcement at target loads of (a) 200 kgf and (b) 400 kgf.

limitations as well as studies on other materials’ properties and model geometry. (2) Qualitatively, this study has provided insight into a basic mechanism responsible for the behavior of compacted soil under repeated loads supported on a reinforced sand bed. In studies of the behavior of granular layers with geogrid reinforcement using on large- and small-scale tests, Milligan et al. (1986) and Adams and Collin (1997) showed that the general mechanisms and behavior observed in the small model tests could be reproduced at large scale. For these reasons, the general trends obtained here are expected to be similar at full size. Nevertheless, further tests with largescale model and different material properties (e.g. stiffnesses of soil and reinforcement) must be conducted to validate the present findings and to determine the existence of scale effects.

500

Reinforced Rc-u1= Rc-u2=97%, N=1) Reinforced (Test 9, uu1/D=u2/D:0.3, N:1) 1/D= u2/D=0.3, Rc-u1=Rc-u2:97%, Reinforced (Test 10, uu1/D=u2/D:0.3, N:2) Reinforced Rc-u1= Rc-u2=85%, N=2) 1/D= u2/D=0.3, Rc-u1=Rc-u2:85%, Reinforced (Test 10, 11, uu1/D=u2/D:0.3, N:2) Rc-u1= Rc-u2=97%, N=2) Reinforced 1/D= u2/D=0.3, Rc-u1=Rc-u2:97%,

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(3) When conducting a model test at a reduced scale, the scale effects may prevent a direct comparison to a full-scale prototype. Thus, it is necessary to consider the scale effects to properly simulate material properties (e.g. soil and reinforcement) and to scale the geometrical dimensions of each effective factor (e.g. Love, 1984; Fakher and Jones, 1996; ElEmam and Bathurst, 2007; Sireesh et al., 2009). Dimensional analysis provides scaling laws that can convert design parameters from a small model into design parameters for a large prototype, in consideration of the scale effect by a factor of l (herein, the ratio of the diameter of prototype circular loading plate to the diameter of model circular loading plate). By using the scaling law proposed by Langhaar (1951) and dimensional analysis of Buckingham (1914), it was deduced that the reinforcement used at full-scale requires a stiffness l2 times that of reinforcement used in the model tests. In addition, the geometric parameters and the soil shear modulus should be increased by l.

400

6. Conclusions

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A series of repeated load CBR tests was carried out to assess the effect of geotextile reinforcement on the load-penetration behavior of sand with different compaction ratios. Compaction ratio of the soil layers, position and number of reinforcement layers, and the level of target repeated loads were varied to evaluate the soil surface settlement of the reinforced soil subjected to a repeated load. Based on the results of the testing program, the following conclusions can be drawn:

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Fig. 13. Load-penetration curves for reinforced soil with one and two layers of geotextile.

(1) Soil reinforcement with geotextile resulted in a significant reduction of the penetration during the CBR test. For a constant compaction ratio of subgrade, the optimum value of embedment depth of one geotextile layer to reduce the soil penetration was approximately 0.3 times the CBR piston diameter. Although, for a given target repeated load, the amount of penetration increased with the number of cycles, its rate reduced, achieving a plateau at a higher number of cycles (i.e. 20 cycles in this study).

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(2) Since the major amount of CBR penetration occurred within the first loading cycle, soil reinforcement resulted in a significant reduction in the penetration of the first cycle and consequently a reduction in the accumulated plastic settlement of the specimen within the consecutive loading cycles. Besides, the elastic deformation of reinforced soil has a larger share of the total deformation in comparison with the unreinforced soil. (3) A thin dense layer on a medium dense sand bed improved the repeated load response of the specimen for both unreinforced and reinforced soils. Thus, for an upper thickness ratio of 0.3, the load-penetration behavior of the specimen was significantly influenced with the reinforcement. However, with increase in the thickness of the dense layer, the effect of reinforcement was no longer pronounced. (4) It was observed that using two geotextile layers with dense sand above the reinforcement layers resulted in the least total penetration compared to the other cases investigated in this study. This implies that the combined effect of using two reinforcement layers and compacting the top layers can significantly improve the load-penetration behavior of a medium dense sand bed under repeated loading and can potentially prevent the accumulated high plastic settlement at the soil surface. Comparing a single reinforcement layer with a compacted soil layer in different thicknesses indicates that use of a geotextile layer can compensate for the insufficient compaction ratio of the first surface layer of pavement.

Conflicts of interest We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. List of notations D10 D30 D50 D60 Cu Cc Gs D d H h N u1 u2 u1/D u2/D Rc Pp Pe

Effective particle diameter (mm) particle diameter corresponding to 30% finer (mm) particle diameter corresponding to 50% finer (mm) particle diameter corresponding to 60% finer (mm) Coefficient of uniformity Coefficient of curvature Specific gravity Diameter of the CBR piston (mm) CBR mold diameter CBR mold height Thickness of the lower medium dense soil layer (mm) Number of reinforcement layers Thickness of the top dense soil layer (mm) Thickness of the second dense soil layer (mm) Upper thickness ratio Lower thickness ratio Soil compaction ratio (%) Plastic penetration value (mm) Elastic penetration value (mm)

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Aida Mehrpajouh is currently a PhD candidate at K.N. Toosi University of Technology in Geotechnical Engineering since 2015. She received her BSc degree in 2012 in Railway Engineering at Iran University of Science and Technology and her MSc degree in Geotechnical Engineering from K.N. Toosi University of Technology in 2014, and in both cases as one of the top students, she was involved in many research projects which enhanced the ability to manage and perform research experiments. Her area of interests and expertise includes experimental geotechnics, innovative soil improvement methods, thermal behavior of fine-grained soil and dust control methods. Her PhD research subject mainly concentrates on investigating the behavior of liquid polymers as soil stabilizers and the effect of various types of acrylics on improving the engineering properties of fine-grained soils.

Seyed Naser Moghaddas Tafreshi graduated from Amirkabir University of Technology with a PhD in Geotechnical Engineering in 2000. In September 2000, he began his academic career as an Assistant Professor in the Department of Civil Engineering at K.N. Toosi University of Technology. He has also done one year (2008e2009) research work at Nottingham Transportation Engineering Centre (NTEC) in University of Nottingham, UK. He was promoted to Associate Professor and Full Professor in 2008 and 2013, respectively. His main focus of research has been in the area of geosynthetic reinforced beds under monotonic, repeated and dynamic loadings using physical modeling, laboratory testing and partly by numerical modeling and analytical methods. He has also been working on the protection of buried pipes, pavement roads, footings, retaining wall, etc. using geosynthetics reinforcement, recycled materials and expanded polystyrene (EPS) geofoam material.

Mehdi Mirzababaei obtained his PhD degree in Geotechnical Engineering from the University of Bolton (UK) in 2012. He carried out a postdoctoral research project from 2012 to 2016 and afterwards he was affiliated as a Lecturer at the Central Queensland University (Melbourne campus). His main research focus is on soil stabilization, soil reinforcement and sustainable use of waste as geotechnical materials. He has also completed a number of projects for the design and development of automated (softwarecontrolled) laboratory equipment for geotechnical engineering testing and research (e.g. a recent temperature controlled consolidation test apparatus).