Arab J Sci Eng DOI 10.1007/s13369-016-2352-7

RESEARCH ARTICLE - CIVIL ENGINEERING

Static Characteristics of Footings on Tire Shred-Reinforced Granular Trench Mahmoud Ghazavi1 · Abas Mohebi1 · Milad Namdari1

Received: 27 May 2016 / Accepted: 13 October 2016 © King Fahd University of Petroleum & Minerals 2016

Abstract The use of waste tire for improvement of loadsettlement behavior of loose soil is of interest to civil engineers. In recent decades, numerous investigations have been devoted to the subject. The results show that waste tires-reinforced soil can increase shear strength of soils. The aim of this study is to discover the optimum volume of tire shreds randomly mixed in the granular trench supporting shallow footings. For this purpose, tire shreds with size of 0.2, 0.3, and 0.4 times footing width were used and mixed with gravel. The results showed that the optimum percent of tire shreds by volume in granular trench is 10–15. In addition, by increasing the size of tire shreds, the footing bearing capacity improves. Furthermore, the footing capacity improves further with increasing the height of tire shred-mixed zone in the granular trench. Keywords Strip footing · Granular trench · Tire shreds · Reinforcement

1 Introduction Granular trench and stone column are reinforcement methods used to improve load-settlement characteristics of loose soil supporting foundations. However, the problem is that in this method significant bulging occurs due to footing loading causing a significant increase in the footing settlement and reduction in the bearing capacity. In the past decades, a large amount of research have been conducted to study the effect of stone columns in load-carrying capacity of foun-

B 1

Milad Namdari [email protected] Department of Civil Engineering, K. N. Toosi University of Technology, Valiasr St., Mirdamad Cr., Tehran, Iran

dations. In terms of failure mechanism, granular piles fail in different modes comprising bulging [1], general shear failure [2], and sliding [3]. The use of tire shreds-reinforced granular material may improve load-carrying behavior of footings. Although comprehensive studies have been performed on the improvement of loose soil mixed with waste tires, to the best knowledge of the authors, no report has yet indicated any research work carried out on granular trench reinforced by tire shreds. Hamed et al. [4] performed some tests to find the optimum height of granular trench. They used soil classified as ML and SP for bed and granular trench, respectively. They discovered that the best load-settlement behavior appeared when the height of granular trench was three times the footing width. Yoon et al. [5] reinforced sand by three layers of tire cells resulting in the reduction in settlement and the increase in bearing capacity of loose soil. Bosscher et al. [6] covered soil–rubber mixture by a layer of soil, and their investigation showed an improvement in load-settlement characteristics of soil in addition to preventing the rubber from ignition in this way. Ghazavi and Amel Sakhi [7] investigated the effect of the aspect ratio of tire shred (the ratio of the length to the width) on the California bearing ratio (CBR). The research showed that a certain aspect ratio for each width of tire shreds is obtainable. The optimum aspect ratios of tire shred width of 2, 3, and 4 cm, were 5, 4, and 2, respectively. Ghazavi and Amel Sakhi [8] carried out a series of large direct shear tests on tire shred-mixed sand. They observed that tire shred aspect ratio, tire shred content, relative density, normal stress, and shred width have a significant impact on friction angle and thus shear strength of the sand–tire shred mixture. Damerchilu [9] carried out series of laboratory tests to find out the optimum values of width and length of granular trench showing that when W = 1.5B and H = 3B, the highest bearing

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capacity is reachable. In the current investigation, the results of Damerchilu’s study have been used to prepare granular trench in all the tests. As mentioned above, there is not any research work on the granular trench reinforced with waste tire shreds. The main objective of this paper is to find the variations of load-settlement characteristics of footing placed on granular trench, by the size of tire shreds, the volume percentage of tire shreds in the trench, and the height of tire shred mixture making a part of the trench.

Sand Gravel

80 60 40 20 0 0.01

0.1

1

10

100

Particle Size

Fig. 1 Grain size distribution for sand and gravel

2 Experimental Investigation Table 2 Properties of waste tire shreds

2.1 Materials In northern parts of Iran, along sea shores, where so much fine and loose sand susceptible to settlement exists and low height buildings are being constructed on this soil, trenches may not only improve load-settlement response of the ground but also minimize liquefaction risk [10]. The characteristics of sand classified as SP in unified classification system are presented in Table 1. For the trench, gravel material having the size of 4–12 mm was used (Table 1). Figure 1 shows the grain size distribution for the sand and gravel. The tire shreds were obtained by cutting the smooth waste tires of car. The width of waste tire shreds (d) was chosen 2, 3, and 4 cm, and an aspect ratio (the proportion of the length to the width of tire shreds) of 1 was selected for all tire shreds. The properties of tire shreds in use are given in Table 2. 2.2 Experimental Setup To prepare the soil bed, a rectangular tank with 1000 mm length, 210 mm width, and 700 mm height was used. The granular trench was located and fixed by two steel plates held in their position using sticky substances and also separated

Table 1 Properties of sand and gravel Specification

Material Sand

Gravel

Specific gravity

2.62

2.65

Void ratio

0.63

0.7–0.8

Effective particle size, D10 (mm)

0.1

7

Mean particle size, D50 (mm)

0.2

12

Internal friction angle (◦ )

37

45

Unit weight (kN/m3 )

16.08

15.00a

Uniformity coefficient (Cu )

2.1

1.78

Coefficient of curvature (Cc )

1.37

1.38

a

Matrix unit weight of gravel

123

Parameters

Value

Specific gravity

11.4 kN/m3

Thickness

2–6 mm

Failure tensile strength

1115 kN/m2

Elasticity modulus

23,530 kN/m

from the soil bed. The granular trench was placed 250 mm above the bottom of the tank. To make the results comparable for tests on various sand–tire mixtures for the trench, the density of granular material should be constant. To achieve this, a matrix density index for mixtures is defined based on Foose et al. [11] and Ghazavi and Amel Sakhi [7,8]. By performing some density tests on the mixtures, this value is chosen 15 kN/m3 in all tests. Achieving density more than 15 kN/m3 was not attainable through compaction of mixture in light of high elastic characteristic of tire shreds. A strip footing made of steel and having 100 mm width (B), 200 mm length, and 10 mm thickness was used as the footing model. A pneumatic jack applies the load on the footing until failure occurs, through plunger and proving ring with 20kN capacity at a constant rate of 1.2 mm/min, and a digital gauge measures displacements simultaneously. The width and the height (H ) of the granular trench were taken 150 and 300 mm, respectively, in all tests. The first test was carried out on sand bed at relative density of 60% without trench. The other tests were performed on soil improved by granular trench with 15 kN/m3 density and on soil improved by tire shred-reinforced granular trench. Figures 2 and 3 show the schematic view of experimental setup. A summary of tests is illustrated in Tables 3 and 4. To avoid the size effect on the results and considering the size of laboratory equipment such as footing and test tank, the maximum size of tire shreds was limited to 4 cm and tire shreds were not mixed in the upper 20% of the height of granular trench in most of the tests.

Arab J Sci Eng

2.4 Preparation of Granular Trench

Fig. 2 Schematic view of test setup: 1 Reaction frame; 2 Test tank; 3 Pneumatic jack; 4 Load cell; 5 LVDT; 6 Model footing; 7 Data logger

After the sand bed reached the height of 250 mm, two steel plates with 15 mm distance from one another were located to fixate and hold the granular trench. As mentioned above, the matrix density index was used in all the tests to make entire results meaningful and comparable as it defines a constant unit weight for gravels regardless of tire shreds of gravel volume in trench mass (γmat = VolumeWeight of gravel+Void volume ). To achieve this, required mass of tire shreds and gravel material in trench is determined and mixed. Since granular material is hardly compressible, the reinforced trench was compacted using extra pressure offered by pneumatic loading jack until the trench–tire shred mixture reached the specific volume. Similarly, the surrounding soil of the trench was prepared (see Sect. 2.3). Finally, the steel plates were pulled out to start the test.

3 Results and Discussion

Fig. 3 Test tank

2.3 Preparation of Sand Bed An identical procedure was performed to prepare the sand bed at 60% relative density in all tests. To gain the same unit weight for the sand bed in each test, the tank was filled in layers of 50 mm thickness, and a constant pressure was exerted on each layer using a 100 mm × 300 mm square steel plate.

The pressure-settlement variation of unreinforced and trenchreinforced sand bed is shown in Fig. 4. As seen, granular trench increases the footing bearing capacity compared with sand bed alone. As shown, the footing bearing capacity grows by 2.7 times. To determine the optimum percent of tire shred with the size of 2 × 2 mixed into granular trench, four tests were performed. These tests were applied on mixtures with tire shred contents (Vt ) of 5%, 10, 15, and 20% by volume, and results are shown in Fig. 5. As observed, the optimum percent of Vt is 10% for 10, 20, and 30% of settlement to width of footing ratio (S/B). In addition, the maximum ultimate bearing capacity of footing was obtained when Vt was 15%. Four tests were carried out on trench reinforced with tire shreds dimensions of 3 × 3 cm, and results are illustrated in Fig. 6. As seen, there is a close similarity in load-settlement behavior of reinforced granular trench by tire shreds with 2 × 2 and 3 × 3 dimensions. Three tests have been conducted to find out the optimum percent of 4 × 4 cm tire shreds mixed with gravel for the trench and results are shown in Fig. 7. As seen, the maximum bearing capacity is obtained when Vt is 10%. A summary of above results is presented in Fig. 8 where BCR is defined by:

BCR =

Ultimate bearing capacity of reinforced soil with granular trench Ultimate bearing capacity of unreinforced soil

(1)

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Arab J Sci Eng Table 3 Summary of experimental program

Table 4 Details of mass and volume of gravel and tire shred

No. of tests

Trench width (mm)

Dimension of tire shreds (cm2 )

Percent of tire shreds

Height of confinement

1

–

–

–

–

1

150

–

–

–

4

150

2×2

5%, 10%, 15%, 20%

H

4

150

3×3

5%, 10%, 15%, 20%

0.8H

3

150

4×4

5%, 10%, 15%

0.8H

4

150

2×2

10%

0.33H, 0.5H, 0.67H, H

Percent of tire shred

Mass of tire shreds (gr)

0

Volume of tire shreds (cm3 )

Volume of gravels (cm3 )

Volume of air (cm3 ) 4101

0

0

14,175

5349

5%

539

473

13,466

5081

3896

10%

1077

945

12,758

4814

3691

15%

1616

1418

12,048

4546

3486

20%

2154

1890

11,340

4279

3281

450

700

400

600

Pressure (kPa)

350

Pressure (Kpa)

Mass of gravels (gr)

300 250 200 150 Trench With (kN/m3)

100

15

500 400 No Tire Shreds

300 200 100 0

No Trench

50

(a)

0

20

40

60

80

S/B (%)

0 0

10

20

30

40

50

60

S/B (%)

700

Fig. 4 Variations of pressure settlement of unreinforced and trenchreinforced sand bed

123

Pressure (kPa)

Apparently, these three figures are similar. In fact, in all these figures, there is an initial increase in the BCR value reaching a peak it decreases. However, it is clear that there is a major difference between the results of 4 × 4 cm tire shreds on one hand with those of 2 × 2 and 3 × 3 cm on the other hand. To explain this difference, some reasons are elaborated. First, when dimensions of tire shreds are not large enough in order to contact with more than one gravel particle, on their surface, tire shreds can only improve the frictional resistance of mixture. So, by using tire shreds of 4 × 4 cm in the present study, not only the friction angle of the mixture enhances, but also the tensile

(b)

600 500 400 300 200 100 Qu 0 0

5

10

15

20

25

Vt (%) Fig. 5 a Pressure-settlement variation of footing for 2×2 cm tire shredmixed trench and b variation of bearing pressure versus Vt with 2 × 2 tire shred at various S/B

Arab J Sci Eng 800

(a)

(a)

700

700

600

600

500 400 No Tire Shreds

300 200

Pressure (KPa)

Pressure (kPa)

800

500 400 No Tire Shreds

300 200

100

100 0 0

S/B (%) 40

0

60

0

20

40

60

80

S/B (%)

800

(b)

700

800

(b)

600

700

500

600

300 200 100

Qu

0 0

5

10

15

20

25

Vt (%) Fig. 6 a Variation of pressure settlement for footing on 3 × 3 cm tire shred-mixed trench and b variation of bearing pressure versus Vt with 3 × 3 cm tire shreds for various S/B

capacity of tire shreds would mobilize in light of the sufficient area of rubber surface contacting with more gravel particles. It also could be said that the flexibility of 4 × 4 cm tire shreds is apparently more than the other two as they are bent by hand easily. Hence, in high percentage by volume of these tire shreds in mixture (Vt = 15%), this characteristic of 4 × 4 cm tire shreds causes soft behavior of mixture due to which bearing capacity of reinforced soil substantially decreases. On the other hand, in lower percentage by volume of these tire shreds (Vt = 5%) considering the large size of them, the number of tire particles is low while dispersed in the mixture so that they cannot significantly affect the bearing capacity of mixture. To conclude, when Vt is 10% the number of tire particles is neither too low to be distributed in the mixture separately nor too much to cause soft behavior. Size effect can be another reason of this difference due to increasing the size of tire shreds in comparison with the

500 400 300 200 100

Qu

0 0

5

10

15

20

Vt (%) Fig. 7 a Variation of pressure settlement of footing for 4 × 4 cm tire shred-mixed trench and b variation of bearing pressure versus Vt for with 4 × 4 cm tire shred at various S/B 6 5 4

BCR

400

Pressure (KPa)

Pressure (KPa)

20

3 2

2*2 Tire Shreds 3*3 Tire Shreds

1 4*4 Tire Shreds 0

Vt (%) Fig. 8 Variation of BCR versus Vt for reinforced granular trench with tire shred

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Arab J Sci Eng 800

600

(a)

700

500

Pressure (KPa)

qu(KPa)

600 500 400 300 200 100 0 0.2

0.3

400 300

No Tire Shreds

200 100

0.4

d/B

0 0

Fig. 9 Variation of ultimate bearing capacity versus dimension of tire shred to width of footing ratio

40

60

S/B (%) 600

700

(b) 500

Pressure (KPa)

600

Pressure (KPa)

20

500 400 300 200

400 300 200

100

100

Qu

0 0.2

0.3

0.4

0.5

d/B

0 0

Fig. 10 Variation of bearing pressure versus dimension of tire shred to width of footing ratio (Vt = 10%)

0.25

0.5

0.75

1

1.25

Normalised Reinforsed Height (h/H) Fig. 12 a Variation of pressure settlement for footing on various height of reinforcement and b variation of pressure versus height of reinforcement

700

Pressure (KPa)

600 500 400 300 200 100 0 0.2

0.3

d/B

0.4

0.5

Fig. 11 Variation of bearing capacity versus dimension of tire shred to width of footing ratio (Vt = 15%)

width of footing, making the results inaccurate. If the second reason was true, it could be said that the allowable size of tire shreds in these tests is 0.3 times the width of footing.

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As observed in Fig. 8, the optimum volume percent of tire shreds to mix is between 10 and 15% by volume. It may be said that when 10% of the entire volume of trench is occupied by tire shreds, the ultimate bearing capacity of the footing on the reinforced trench increases by using greater size for tire shreds (Fig. 9) due to the better interlocking length. Conversely, when Vt is 15%, the ultimate bearing capacity decreases by increasing the size of tire shreds as shown in Fig. 9. This is because of the soft behavior of tire shreds in mixture in high percentage of Vt . In the mixture with Vt of 10% before failure of trench happens, when 10% < S/B < 40%, tire shreds reinforced with the size of 3 × 3 cm gain the maximum bearing capacity (Fig. 10) because 3 × 3 cm tire shreds have larger area compared with 2 × 2 cm tire shreds. Therefore, contact area

Arab J Sci Eng Fig. 13 Deformed shape of trench made of gravel–tire shreds. a h/H = 100% and b h/H = 33%

between gravel particle and tire shreds is more and it would increase friction angle of mixture. On the other hand, before gravel particles firmly interlock with tire shreds, the possibility of bending in 4 × 4 cm tire shreds is high in low pressure; thus, soft behavior is dominant. Accordingly, it is reasonable that 3×3 cm tire shreds reach the maximum bearing capacity compared with tire shreds with 2 × 2 and 4 × 4 cm in size. Figure 11 shows the variation of bearing capacity before failure versus the size of tire shreds when Vt is 15%. This figure is similar to Fig. 10 with the same reason aforementioned for mixture with Vt = 10%. Four tests have been carried out on granular trench reinforced with 2 ×2 cm tire shreds and various gravel–tire shred mixture heights to discover the effect of height of reinforcement on bearing capacity of footing on granular trench where the height of mixture is the height of the trench from which the tire shreds are mixed toward the higher place of trench in a haphazard manner. In all these tests, Vt was 10%. The results are shown in Fig. 12. As seen, obviously by increasing the height of the mixture, the mixture acts as a relatively rigid body and transfers load to its underneath. Thus, the loadsettlement characteristic of the trench is improved because the friction angle of lower unreinforced zone of trench acting as the base of above part improves. Therefore, the bulging effect under the footing is reduced and the pressure of footing is distributed within the entire height of the trench. Figure 13 shows the bulging in reinforced trench for h/H = 100% and h/H = 33%, where h is the height of reinforcement and H is the trench height. One test has been carried out to investigate failure mechanism of sand bed in which two thin layers of surrounding sand were colored. Comparing the former and latter thickness of the layer during the test, the increase in thickness is apparent

Fig. 14 Failure mechanism: a before the test and b after the test

(Fig. 14a, b), and it might be due to sand particles moving along slip surface [12] until existing shear stress overcomes soil shear strength resulting in failure.

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4 Conclusions A series of laboratory tests on non-reinforced and reinforced granular trench have been done in order to find out the optimum percentage of tire shreds by volume in the whole mass of granular trench to obtain the highest bearing capacity, during which the effect of increasing the size of tire shreds on load-settlement behavior of footing was observed. In these tests, tire shreds with the size of 0.2, 0.3 and 0.4 times the footing width were used. Moreover, the variation of bearing capacity of footing by increasing the height of the tire shredmixed trench was studied. From the present research study, the following concluding remarks can be extracted: 1. Using tire shred-mixed granular trench in sand improves the load-carrying capacity and decreases the settlement of footings supported by the trench because of increasing the friction angle of granular material. 2. The optimum volume percentage of tire shreds with dimensions of 2 × 2, 3 × 3, and 4 × 4 cm are 15, 15, and 10%, respectively. In small settlements, the greatest footing bearing capacity is achieved when 3 × 3 cm tire shreds with Vt = 10% are used and 4 × 4 cm tire shred showed the highest ultimate bearing capacity in light of sufficient surface provided for interacting with gravel particles. By increasing the percentage of tire shred form a specific value (15% for 2×2 cm and 3×3 cm and 10% for 4×4 cm), the load-carrying capacity of mixture decreases since the ductile behavior of tire shreds is dominated. 3. The effectiveness of reinforcing trench with tire shreds at low pressures is insignificant because of the elastic behavior of mixture; however, by increasing the gravel–shred mixture stiffness, the influence of tire shred becomes more apparent. 4. The footing bearing capacity increases with increasing the height of gravel–tire shred mixtures (h/H ). For instance, by reinforcing 33% of the height of trench, the ultimate bearing capacity enhances by 6%, while the improvement around 41% was observed when the whole height of trench is reinforced by tire shreds.

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5. Dilation behavior was seen in sand bed when the footing was subjected to loading. Thickness of surrounding sand layer increases due to rolling through sand particle and failure occurs when shear stress overcomes shear strength.

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