INTERACTION BETWEEN TIRE SHREDS, RUBBER-SAND AND GEOSYNTHETICS

Transcription

1 Technical Paper by A. Bernal, R. Salgado, R.H. Swan Jr. and C.W. Lovell INTERACTION BETWEEN TIRE SHREDS, RUBBER-SAND AND GEOSYNTHETICS ABSTRACT: Waste tires can be shredded (tire shreds) and re-used as fill material. The fill material may be comprised solely of tire shreds or mixed with sand (rubber-sand). If geosynthetics are to be used with fills containing tire shreds, it is necessary to understand the physical interaction between tire shreds, or rubber-sand, and the geosynthetic of interest. A laboratory testing program consisting of direct shear tests, interface direct shear tests, and geosynthetic pullout tests was conducted to determine the interaction properties of tire shreds and rubber-sand fills with three different flexible geogrids and a woven geotextile. Direct shear and interface direct shear tests were conducted using a large direct shear box with dimensions of 3 mm by 3 mm and a total depth of 23 mm. Pullout tests were carried out in a large pullout box with dimensions of 1.2 m in length by.9 m in width and a total depth of.5 m. The details of the testing program, the test results, and conclusions based on the test results are presented in this paper. KEYWORDS: Direct shear test, Interface friction, Pullout test, Geogrid, Geotextile, Tire shreds, Tire chips, Rubber-Sand, Coefficient of interaction. AUTHORS: A. Bernal, Project Engineer, GeoHidra, Avenida Las Ciencias Razzetti Quinta Marbella, Caracas, Venezuela, Telephone: 58/ , Telefax: 58/ , @compuserve.com; R. Salgado, Assistant Professor School of Civil Engineering, Purdue University, West Lafayette, Indiana , USA, Telephone: 1/ , Telefax: 1/ , rodrigo@ecn.purdue.edu; R.H. Swan, Laboratory Manager, Soil-Geosynthetic Interaction Testing Laboratory, GeoSyntec Consultants, 11 Lake Hearn Dr., N.E., Suite 2, Atlanta, Georgia 3342, USA, Telephone: 1/ , Telefax: 1/ , robs@geosyntec.com; and C.W. Lovell, Professor Emeritus, School of Civil Engineering, Purdue University, West Lafayette, Indiana , USA, Telephone: 1/ , Telefax: 1/ PUBLICATION: Geosynthetics International is published by the Industrial Fabrics Association International, 181 County Road B West, Roseville, Minnesota , USA, Telephone: 1/ , Telefax: 1/ Geosynthetics International is registered under ISSN DATES: Original manuscript received 3 April 1997, revised version received 21 June 1997 and accepted 7 July Discussion open until 1 July REFERENCE: Bernal, A., Salgado, R., Swan, Jr., R.H. and Lovell, C.W., 1997, Interaction Between Tire Shreds, Rubber-Sand and Geosynthetics, Geosynthetics International, Vol. 4, No. 6, pp

2 1 INTRODUCTION Scrap tires are a high-profile waste material for which several beneficial uses have been proposed and put into practice (Ahmed 1993). One approach consists of shredding the tires into small pieces that are often referred to as tire shreds or tire chips, depending on their size. For simplicity, the term tire shreds will be used in this paper. Tire shreds have been used in a variety of applications because of their unique engineering properties. The use of tire shreds or mixtures of tire shreds and sand (i.e. rubber-sand) as lightweight fill (Bernal et al. 1997) could significantly minimize the waste tire disposal problem that currently exists. The growing interest in utilizing waste materials in civil engineering applications has opened the possibility of constructing reinforced soil structures with unconventional backfills, such as tire shreds. Geogrids and woven geotextiles have been used effectively to improve the performance of embankments and backfills by reducing deflections, settlement, and earth pressures within the embankments orbackfills, and byincreasing the bearing capacity of these structures. Geogrids may be placed within tire shreds or rubber-sand backfills of earth structures to increase the lateral confinement of the system, improve the shear modulus due to vertical confinement, and spread the vertical stresses due to the tensioned membrane effect. The lateral confinement should resist the tendency of the fill to walk out under repetitive surface traffic loads on embankments (Koerner 1994). Cyclic loading tests on unreinforced and geogrid-reinforced conventional soil embankment sections under both strong and weak subgrade conditions have shown that failure occurs later in reinforced sections than in unreinforced sections for both subgrade conditions (Abdel Halim et al. 1983). Permanent deformations in geogrid-reinforced sections were also found to be less than in unreinforced sections. Geogrids have also been used to reinforce unpaved roads. The mechanisms of reinforcement in this case are increased soil strength, load spreading, and membrane support via controlled rutting. The difference in the required thickness of the granular base due to the geogrid reinforcement should be compared to the cost of the geogrid and its installation. If geogrid is the more economical alternative (as is usually the case for soil subgrades with California Bearing Ratio (CBR) values less than 5), then its use is recommended (Koerner 1994). The performance of embankments on weak subgrades would also be improved by using a lightweight fill, such as tire shreds or rubber-sand. The interest in effectively designing tire shreds and rubber-sand fills creates the need for the development of testing procedures to evaluate the interaction properties of tire shreds and rubber-sand with geosynthetics through pullout testing and direct shear testing. A direct shear and pullout testing program was conducted to evaluate the interaction of a woven geotextile and three different flexible geogrids (with different aperture sizes) within two different types of fill material. The first fill material was composed solely of tire shreds having a nominal area of 5 mm 5 mm, and the second fill material was a rubber-sand consisting of a mixture of the same tire shreds and a masonry sand. Direct shear and interface direct shear tests were conducted using a large direct shear box with dimensions of 3 mm by 3 mm and a total depth of 23 mm. Pullout tests were performed in a large pullout box with dimensions of 1.2 m in length by.9 m in width and a total depth of.5 m. Based on the results of this testing program, recom- 624

3 mendations are made for the selection of interface shear resistance values when designing earth structures with tire shreds and rubber-sand fills and geogrids and geotextiles. 2 TESTING PROGRAM 2.1 Materials Tested Tire Shreds and Rubber-Sand The direct shear and pullout tests were conducted using two different fill materials: tire shreds and rubber-sand. A volume of tire shreds has a high degree of compressibility due to the high rubber content and relatively high void ratio. This high compressibility can be reduced by mixing the tire shreds with sand in order to reduce the void ratio. The first fill material used in the testing program consisted of tire shreds with a nominal maximum area of 5 mm 5 mm. The tire shreds were processed by BFI Tire Recyclers, a tire shredder operator located in Jackson, Georgia, USA. The second fill material consisted of a rubber-sand mixture prepared by combining, by weight, 4% of the tire shreds with 6% of a medium particle size masonry sand with the following properties: Unified Soil Classification System (USCS) of SP (poorly graded sand), coefficient of uniformity, C u = 2.75, coefficient of curvature, C c = 1.1, maximum particle size of 2 mm, and an internal friction angle, φ =31_ determined from direct shear tests conducted under normal stresses of 7, 35, and 56 kpa and a shear displacement rate of 1 mm per minute. The particle size distribution curves for the tire shreds and the masonry sand are presented in Figure 1. The rubber-sand mixture was prepared by mixing tire shreds and sand in a separate container. Tire shreds were poured into a barrel and then an adequate proportion of sand was added. The fill material was then thoroughly mixed. The low moisture content of Percent passing (%) Tire shreds Sand Particle size (mm) Figure 1. Particle size distribution of the tire shreds and masonry sand used in the rubber-sand backfill. 625

4 the sand (w = 4%) helped prevent segregation of the mixture since the sand particles tended to stick to the tire shreds Geogrids and Geotextile Tested Pullout testing was carried out on three types of flexible geogrids, identified as FOR- TRAC 55/3-2 (Geogrid A), FORTRAC-OM 35/35 5 (Geogrid B), and FORTRAC- OM 35/35 1S (Geogrid C, a special product developed for this testing program), all of which were manufactured by Huesker, Inc., Charlotte, North Carolina, USA. The first number in each product name is the ultimate strength in kn/m in the machine direction, the second number is the ultimate strength in kn/m in the cross-machine direction, and the third number is the square aperture size in millimeters. The manufacturer s product data states that the geogrids are made from high tenacity polyester (PET) yarns woven into a stable, grid-like pattern and then coated with polyvinyl chloride (PVC), making the final product very pliable. The multifilament PET fibers are chemically similar to the fibers used in the manufacture of high-performance automobile tires. The geogrid members are oval shaped in cross section. The long-term geosynthetic design load values in Table 1 are calculated by dividing the ultimate load by four factors of safety that take into account different degradation mechanisms (installation damage, creep, chemical durability, and biological durability). A flexible, PET, woven geotextile was also used in the pullout testing program. The geotextile, COMTRAC R 2.45, was also supplied by Huesker, Inc. Table 1 is a summary of the properties of the geosynthetics used in the testing program, as available. Table 1. Physical properties of the geogrids and geotextile used in the testing program. Property Manufacturing process and polymer type Geosynthetic Geogrid A Geogrid B Geogrid C Geotextile High tenacity, multifilament PET fibers, woven grid coated with PVC Woven PET Aperture size (mm 2 ) NA Mass per unit area (g/m 2) Percent open area (%) Member thickness/thickness (mm) Ultimate strength in warp direction (kn/m) Ultimate strength in fill direction (kn/m) Elongation at break in warp direction (%) Elongation at break in fill direction (%) Tensile strength at 5% (geogrids) and 6% (geotextile) strain in warp direction (kn/m) Tensile strength at 5% (geogrids) and 6% (geotextile) strain in fill direction (kn/m) Long-term design load for silt and clay (kn/m) Long-term design load for 38 mm crushed stone and gravel (kn/m) Notes: PVC = polyvinyl chloride; PET = polyester; NA = not applicable. 626

5 2.2 Test Equipment Direct Shear Box A large direct shear box (Figure 2) was used to evaluate the shear strength of the two fill materials. The upper box is 3 mm by 3 mm and 15 mm deep. The lower box is 36 mm in length by 3 mm in width and 75 mm deep, allowing for 6 mm of total displacement during shear. The normal stress was applied to the test specimen using a mechanical lever arm system that can be loaded using either dead weights or an air cylinder. The shear load was applied to the test specimen through the use of a screw-advance drive system powered by an electric motor and a gear reduction system that is electronically controlled to maintain a constant rate of shear displacement. The fill-geotextile interface properties were measured in a large direct shear box (Figure 3) similar to the one described above, where the upper box contained the fill material to be tested and the lower shear box was filled with a layered sand fill and covered with the geotextile. The geotextile was fixed to the lower shear box opposite to the direction of shear. The normal pressure was applied directly onto the fill material in the upper box using the mechanical lever arm system Pullout Box A large pullout box (Figure 4) was used to evaluate the pullout resistance of the geogrid and geotextile specimens in tire shreds and rubber-sand fill materials. The pullout Lever arm Load cell 3 mm Load cell Metal plate Upper box 6 mm 15 mm LVDT Displacement Lower box 75 mm Screw advance drive Displacement system and motor 36 mm Air cylinder Figure 2. Schematic diagram of the direct shear test apparatus. 627

6 Load cell 6 mm LVDT Compacted fill Sand Displacement Air cylinder Upper box Geotextile specimen Lower box Figure 3. Schematic diagram of the interface direct shear test apparatus. Protective sand layer (25 mm) Air bladder LVDT 25 mm Compacted fill Geogrid specimen LVDT.5 m Load cell 15 mm 25 mm Compacted fill 1.22 m Figure 4. Schematic diagram of the pullout test apparatus. box has dimensions of 1.22 m in length by.9 m in width and a total depth of.5 m. Sleeve plates were attached to the front wall above and below the slot through which the geosynthetic specimen was pulled to minimize the lateral load transfer to the rigid front wall of the pullout box. The sleeve plates are L -shaped steel angles having a length and height of 15 mm, and a width the same as the pullout box (.9 m). The fill thickness above and below the geosynthetic specimen was 25 mm. A vertical normal stress was applied to the top fill layer using an air bladder system. Below the air bladder was a 25 mm-thick, protective sand layer used to prevent the possible puncture of the air bladder due to the exposed steel belts in the tire shred, or rubbersand fill materials. One end of each geosynthetic specimen was cast in an epoxy resin to form a rigid end section. The rigid geosynthetic specimen end section was then bolted between two plates which extended inside the fill to ensure that the geosynthetic remained confined and aligned during the test. A pullout loading system comprising two hydraulic cylinders mounted on each side of the pullout box, with a common pressure supply provided by a constant-rate hydraulic pump, applied the geosynthetic pullout force. A constant pullout rate (1 mm/minute) that was measured at the bolted end of the geosynthetic specimen was used for each pullout test. 628

7 2.2.3 Electronic Instrumentation An electronic load cell and a linear variable differential transformer (LVDT) were mounted on the pullout loading system to measure the pullout load and the displacement of the bolted end of the geosynthetic specimen. These instruments were connected to a computer data acquisition system comprised of a Validyne Engineering UPC-68 data acquisition card and Labtech Notebook data acquisition software. The system was used to monitor the electronic instrumentation throughout the test. A SIN- CO model total pressure cell was placed on the bottom of the pullout box under the fill material to evaluate the normal pressure transmitted to the base of the bottom fill layer (Figure 4). During each direct shear and interface direct shear test, the applied normal load and shear force were measured using electronic load cells, and the shear displacement was measured with an LVDT. These instruments were also monitored by the same computer data acquisition system during each test. 2.3 Test Procedures and Results Direct Shear Tests The direct shear tests were performed in accordance with ASTM D 38 using the large direct shear box described previously (Figure 2). The rubber-sand and tire-shred specimens were prepared by placing and compacting the initial fill layer in the lower box with a small 4 kg tamper. The upper box was then positioned and filled with three 5 mm-thick layers of the fill material. Each layer was compacted with a small tamper until the final desired height was reached, after which the specimen was covered with a metal plate. The final compacted unit weights of the tire shreds and rubber-sand fills were approximately 5.9 and 11.5 kn/m 3, respectively. The final compacted unit weight of the tire shreds was attained using compaction energies as low as 5% of the Standard Proctor compaction energy required for the rubber-sand fill (Ahmed 1993). Compacted tire shreds and rubber-sand have approximately one-third and two-thirds of the typical compacted unit weight of conventional backfill materials, respectively. The normal stresses were measured using an electronic load cell mounted on the lever arm system (Figure 2). The results of the direct shear tests are shown in Figures 5 and 6 for tire shreds and rubber-sand specimens, respectively. It may be noted in Figures 5 and 6 that with increases in normal confinement the fill materials become more ductile, requiring greater displacement to achieve a constant shear stress. The rubber-sand and tire shreds specimens did not develop well-defined peak shear strengths as the specimens were sheared. The shear strength parameters for tire shreds and rubber-sand must be defined at pre-established magnitudes of deformation. The internal friction (mobilized strength) angle and cohesion values obtained from the direct shear tests on tire shreds and rubber-sand specimens are in reasonable agreement with results reported in previous studies (Ahmed 1993; Bernal et al. 1997; Foose et al. 1996; Humphrey et al. 1993). The mobilized friction angles for tire shreds and rubber-sand specimens at various displacements are shown in Figures 7 and 8, respectively. The observed strain-hardening behavior can be attributed in part to the densification of the specimens during shear, caused by the tendency of tire shreds to interlock with each oth- 629

8 8 7 Shear stress (kpa) σ n =54kPa σ n =24kPa σ n =7kPa Figure 5. Direct shear test results for the tire shreds specimens (normal stress, σ n =7, 24, and 54 kpa). 8 7 σ n =79kPa Shear stress (kpa) σ n =43kPa σ n =7kPa Figure 6. Direct shear test results for rubber-sand specimens (normal stress, σ n =7, 43, and 79 kpa). 63

9 5 Mobilized friction angle (_) Figure 7. Mobilized friction angle values obtained from a direct shear test on a tire shreds specimen. 5 Mobilized friction angle (_) Figure 8. specimen Mobilized friction angle values from a direct shear test on a rubber-sand 631

10 er due to the presence of exposed steel belting and their irregular shape. During shear, it is believed that the tire shreds in the lower box were pulled by tire shreds in the upper box in the opposite direction to displacement causing densification of the fill material in that region Interface Direct Shear Tests The interface direct shear tests were performed in accordance with ASTM D 5321 to measure the interface resistance between the geotextile and the two fill materials used in this study (i.e. rubber-sand and tire shreds). The tests were performed in a large direct shear box (Figure 3), which is similar to the shear box used for the direct shear tests on the fill materials alone. Specimens were prepared by first filling the lower box with compacted concrete sand which was used as a bedding layer. The geotextile was then placed on top of the bedding layer and fixed to the side of the lower box that was opposite to the shear direction. The upper box was then positioned and filled with the test fill (tire shreds or rubber-sand) in three 5 mm thick layers. Each layer was compacted with a small 4 kg tamper until the desired final height was reached. The specimen was then covered with a metal plate. The unit weights obtained during the placement and compaction of each specimen were similar to those obtained for the direct shear test specimens discussed in Section A normal pressure was applied using an air cylinder and was measured using an electronic load cell. The upper box was connected to two hydraulic cylinders and a loading harness which was used for pullout testing. The upper box was then pulled by the hydraulic cylinders, moving the fill material over the geotextile attached to the lower box (Figure 3). The results of the interface direct shear tests are shown in Figures 9 and 1 for tire shreds and rubber-sand fills, respectively. The failure envelopes obtained from the tire shreds-geotextile and rubber-sand-geotextile interface direct shear tests are shown in Figure 11. The interface friction angles are approximately 3_ and 32_ for the tire shreds-geotextile and rubber-sand-geotextile specimens, respectively. For the tire shreds-geotextile interface direct shear test with σ n = 63 kpa (Figure 9), the specimen almost reached a constant shear stress. The last interface direct shear stress value measured for each normal pressure applied (Figure 9) was used as the peak shear stress value in Figure 11 which does not generate any appreciable error Pullout Tests The pullout tests were performed in accordance with draft ASTM Z2467Z. First, three 8 mm thick layers of fill material were placed. The geosynthetic specimen was then placed on top of the third layer of fill and three more layers of fill were placed on top of the geosynthetic. Each layer of fill was compacted by hand. The amount of fill used was carefully controlled and the depth of each layer was measured at various locations in the pullout box to calculate an accurate value of the compacted unit weight. The unit weight values of the compacted material were similar to those obtained for the direct shear test specimens (Section 2.3.1). The geosynthetic was connected to the pullout loading harness. For each test, Telltale wire cables were attached to nodes near the clamp end, the midsection, and the free end of the geosynthetic. The wire cables were 632

11 Shear stress (kpa) σ n =63kPa σ n =35kPa σ n =7kPa Figure 9. Tire shreds-geotextile interface direct shear test results (normal stress, σ n = 7, 35, and 63 kpa). Shear stress (kpa) σ n =63kPa σ n =35kPa 1 5 σ n =7kPa Figure 1. Rubber-sand-geotextile interface direct shear test results (normal stress, σ n = 7, 35, and 63 kpa). 633

12 4 Peak shear stress (kpa) Rubber-Sand φ =32_ c =1.8kPa Tire shreds φ =3_ c =1.2kPa Normal pressure (kpa) 6 7 Figure 11. Failure envelopes obtained from interface direct shear tests on tire shreds-geotextile and rubber-sand-geotextile specimens. Note: φ = internal friction angle and c = soil cohesion. used to determine the displacement of the geosynthetic at the monitored locations during each test. Figure 4 illustrates the setup for the geosynthetic specimen pullout tests. Pullout tests were performed using tire shreds and rubber-sand fill materials under various confining pressures ranging from 2 to 68 kpa to simulate conditions at different depths within the backfill of a reinforced earth structure. A constant displacement rate of 1 mm/minute was used for all pullout tests. Figures 12, 13, and 14 show the pullout test results for Geogrids C, B, and A in tire shreds under various confining pressures, respectively. Figures 15, 16, and 17 show the corresponding test results for a rubbersand fill, respectively. Geogrid tension failures, which resulted from rupture of the geogrid nodes during the test, were observed in some tests. Pullout tests that ended in geogrid tension failure can be identified as those tests that ended after a smaller amount of displacement. 3 ANALYSIS OF PULLOUT TEST RESULTS The factors that influence pullout test results are generally related to the test apparatus and procedure, boundary effects, rate of loading, geosynthetic characteristics, fill properties (e.g. dry unit weight, moisture content, relative density, and particle shape and size), specimen preparation procedure, and confining pressure (Ingold 1983; Palmeira and Milligan 1989; Palmeira 1987; Alfaro et al. 1995; Abramento and Whittle 1995). The specimen size used was sufficiently large to minimize top and lateral boundary effects. To analyze the pullout test results, the shear strength parameter values of the tire 634

13 Pullout force (kn/m) σ n =6kPa σ n =52kPa σ n =36kPa 5 σ n =2kPa Figure 12. Pullout test results for Geogrid C in a tire shreds fill (normal stress, σ n =2, 36, 52, and 6 kpa) σ n =52kPa Pullout force (kn/m) σ n =47kPa 5 σ n =2kPa Figure 13. Pullout test results for Geogrid B in a tire shreds fill (normal stress, σ n =2, 47, and 52 kpa). 635

14 3 Pullout force (kn/m) σ n =59kPa σ n =47kPa 5 σ n =2kPa Figure 14. Pullout test results for Geogrid A in a tire shreds fill (normal stress, σ n =2, 47, and 59 kpa) σ n =68kPa Pullout force (kn/m) σ n =43kPa Figure 15. Pullout test results for Geogrid C in a rubber-sand fill (normal stress, σ n = 43 and 68 kpa). 636

15 3 25 Pullout force (kn/m) σ n =3kPa Figure 16. Pullout test results for Geogrid B in a rubber-sand fill (normal stress, σ n = 3kPa). 3 Pullout force (kn/m) σ n =55kPa σ n =34kPa σ n =46kPa 5 σ n =3kPa Figure 17. Pullout test results for Geogrid A in a rubber-sand fill (normal stress, σ n = 3, 34, 46, and 55 kpa). 637

16 shreds and rubber-sand fill materials are required (under the same confining pressures that were used in the pullout tests). These values were obtained from direct shear tests performed on the two fill materials under the same confining pressures. The large geogrid apertures allow the fill material (i.e. tire shreds) to pass through the plane of the geogrid and generate pullout resistance through two separate mechanisms. The first mechanism is the shear resistance generated between the fill material and the top and bottom of the longitudinal and transverse ribs of the geogrid. The second mechanism is passive resistance of the fill material against the front of the transverse ribs of the geogrid. The fill material goes into a state of passive resistance and opposes geogrid pullout (Koerner 1994). As the geogrid is pulled out of the fill, the fill material directly above and below the geogrid is sheared forming two shearing surfaces (Figure 18). The coefficient of interaction, C i, is the ratio between the average shear stress on the geosynthetic specimen in the pullout test and the mobilized shear stress in the fill material in direct shear tests under the same confining stress and can be calculated as follows: C i = F p 2 LW(σ n tan φ + c) (1) where: C i = coefficient of interaction; F p = measured pullout force; L = initial length of the geosynthetic specimen; W = initial width of the geosynthetic specimen; σ n =applied normal stress; φ = friction angle of the fill material; c = cohesive intercept of the fill material. The magnitude of C i is specific to the type of geosynthetic and fill material tested. The application of the C i concept for geogrids is not without question. It is reasonable to expect that the bearing mechanism will contribute to the pullout resistance of geogrids. The bearing mechanism is not present in the direct shear test, which is the reference test for the definition of C i values; thus, to some extent, different mechanisms are being compared when the C i concept is used. Nonetheless, the concept is useful for understanding pullout test results and providing design guidelines. Table 2 presents a summary of the pullout test results. Since the pullout force of the geogrid specimens in the tire shred and rubber-sand fills does not exhibit a defined peak value, it is necessary to define an amount of displacement at which both the pullout force and shear strength of the fill materials can be compared. The criterion used to establish this displacement value is the stiffness of each fill material upon loading. Tire shreds are a more compliant material, undergoing larger deformations than rubbersand, which is indicative of the amount of displacement required for the back of the geo- Load cell LVDT Compacted fill Geosynthetic Compacted fill Figure 18. Development of two shear zones within the fill material in a pullout test. 638

17 synthetic specimen to start moving. The approximate average displacement required for movement to be seen at the end of the specimen was taken as the reference displacement: 62.5 mm for tire shreds and 5 mm for the rubber-sand. Figures 19 and 2 show the relationship between C i and the normal stress, σ n, for tire shreds and rubber-sand, respectively. Table 2. Pullout test results. Test Geosynthetic Material Normal stress σ n (kpa) Pullout force F p (kn) Coefficient of interaction, C i Failure* 1 Geogrid C Tire shreds No 2 Geogrid C Tire shreds No 3 Geogrid C Tire shreds Rows Geogrid C Tire shreds Rows Geogrid B Tire shreds No 6 Geogrid B Tire shreds All rows 7 Geogrid B Tire shreds All rows 8 Geogrid A Tire shreds No 9 Geogrid A Tire shreds No 1 Geogrid A Tire shreds Row 1 11 Geotextile Tire shreds No 12 Geotextile Tire shreds No 13 Geotextile Tire shreds No 14 Geotextile Tire shreds No 15 Geogrid C Rubber-Sand Rows Geogrid C Rubber-Sand Rows Geogrid B Rubber-Sand No 18 Geogrid A Rubber-Sand No 19 Geogrid A Rubber-Sand No 2 Geogrid A Rubber-Sand Rows Geogrid A Rubber-Sand Rows Geotextile Rubber-Sand No 23 Geotextile Rubber-Sand No 24 Geotextile Rubber-Sand No Note: *Failure was determined in geogrids at the end of the pullout test by observing the number of rows of cross machine members in which the junctions hadbroken, with reference to the front of the specimen. Nosigns of material failure were observed in the geotextile specimens. 639

18 .5 Coefficient of interaction, C i Geogrid B Geogrid A Geogrid C Normal stress (kpa) Figure 19. Coefficient of interaction, C i, for Geogrids A, B, and C in a tire shreds fill at a shear displacement of 62.5 mm..5 Coefficient of interaction, C i Geogrid B Geogrid A Geogrid C Normal stress (kpa) Figure 2. Coefficient of interaction, C i, Geogrids A, B, and C in a rubber-sand fill at a shear displacement of 5 mm. 64

19 Table 3. Recommended coefficient of interaction, C i, values. Backfill material Coefficient of interaction, C i Geotextile Geogrid A Geogrid B Geogrid C Tire shreds Rubber-Sand Based on the limited test data in Table 2, some observations can be made. The aperture size of the geogrid appears to have a strong influence on the value of C i : as the aperture size decreases, the value of C i increases, reaching a maximum value for the geotextiles tested. (Geotextiles may be considered as geogrids that possess a zero aperture size.) The higher C i values for geotextiles may be the result of a strong bond between the rubber in the tire shreds and the geotextile. This bond probably does not develop to the same extent between the tire shreds and the geogrids because of the different geometrical characteristics of geogrids; other interpretations of this behavior are also possible. The Geogrid A and the geotextile have small C i values at low confining stresses. This is probably caused by slippage within the fill material because a small amount of shear resistance was mobilized due to the low normal stresses, and the passive resistance may be considered negligible because the 5 mm tire shred particles could not pass through the geogrid apertures. The values of C i for Geogrid A increase with increases in confining stress. It is recommended that the normal pressure on each reinforcement layer be determined and that the appropriate value of C i be selected for the fill material and geogrid used in the design of a geogrid-reinforced wall. Table 3 lists the recommended values of C i for tire shreds and rubber-sand fill materials based on the testing program described in this paper. The values in Table 3 are less than the typical C i values (of approximately.9 to 1.1) published by the manufacturer for a typical sand and the same type of geogrid used in this study. This suggests that it would be unconservative (i.e. less safe) to use typical values of C i (i.e. values used for sand fills) when designing for tire shreds or rubber-sand fills. 4 CONCLUSIONS It is preferable to re-use waste tires instead of using landfills as a means of disposal. Waste tires can be shredded and the resulting particulate material can be used as fill, either alone or mixed with sand (i.e. rubber-sand). If geosynthetics are used within fills comprised entirely or partly of tire shreds, the geosynthetic-tire shreds fill interaction properties should be determined. The results of the pullout tests presented in this paper indicate that the coefficient of interaction values for tire shreds and rubber-sand fills are lower than those observed for sand. Recommended coefficient of interaction values necessary for the design of a geosynthetic-reinforced earth structure with a tire shreds or rubber-sand backfill were proposed, but should be used with caution in view of the limited amount of data upon which these values are based. The reasonably high strength and low unit weight of tire shreds and rubber-sand make them ideal for use as fills placed on weak, compressible soils. A compacted soil cover 641

20 should be placed on top of fills containing tire shreds to prevent steel belts in the tire shreds from puncturing overlying geosynthetics. The advantages of constructing geosynthetic-reinforced fills containing tire shreds include ease of construction (because material handling is similar to that of conventional materials), reduction of earth pressures, limited deformation of the facing, and a reduction in the contact pressure with the underlying soils. Full-scale field experiments or demonstration projects are required to further validate the conclusions and recommendations presented in this paper. REFERENCES Abdel Halim, A.O., Haas, R. and Chang, W.A., 1983, Geogrid Reinforcement of Asphalt Pavements and Verification of Elastic Layer Theory, Transportation Research Record 949, pp Abramento, M. and Whittle, A.J., 1995, Experimental Evaluation of Pullout Analyses for Planar Reinforcements, Journal of Geotechnical Engineering, Vol. 121, No. 6, pp Ahmed, I., 1993, Laboratory Study on Properties of Rubber Soils, Report No. FHWA/ IN/JHRP 93/4, School of Civil Engineering, Purdue University, West Lafayette, Indiana, USA, 348 p. Alfaro, M.C., Hayashi, S., Miura, N. and Watanabe, K., 1995, Pullout Interaction Mechanism of Geogrid Strip Reinforcement, Geosynthetics International, Vol.2, No. 4, pp ASTM D 38, Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions, American Society of Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM D 5321, Standard Test Method for Determining the Coefficient of Soil and Geosynthetic or Geosynthetic and Geosynthetic Friction by the Direct Shear Method, American Society of Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM Z2467Z, Draft Standard Test Method for Measuring Geosynthetic Pullout Resistance in Soil, American Society of Testing and Materials, West Conshohocken, Pennsylvania, USA. Bernal, A., Salgado, R. and Lovell, C.W., 1997, Tire Shreds and Rubber-sand as Lightweight Backfill Material, accepted for publication as a technical paper in the Journal of Geotechnical and Geoenvironmental Engineering. Foose, G.J., Benson, C.H. and Bosscher, P.J., 1996, Sand Reinforced with Shredded Waste Tires, Journal of Geotechnical Engineering, Vol. 122, No. 9, pp Humphrey, D.N., Sandford, T.C., Cribbs, M.M., Gharegrat, H. and Manion, W.P., 1993, Shear Strength and Compressibility of Tire Chips for Use as Retaining Wall Backfill, Transportation Research Record 1422, pp Ingold, T.S., 1983, Laboratory Pull-out Testing of Grid Reinforcements in Sand, Geotechnical Testing Journal, Vol. 6, No. 3, pp

21 Koerner, R.M., 1994, Designing with Geosynthetics, 3rd Edition, Prentice Hall, Englewood Cliffs, New Jersey, USA, 783 p. Palmeira, E.M., 1987, The Study of Soil-Reinforcement Interaction by Means of Large Scale Laboratory Tests, D.Phil. Thesis, University of Oxford, United Kingdom, 237 p. Palmeira, E.M. and Milligan, G.W.E., 1989, Scale and Other Factors Affecting the Results of Pullout Tests of Grids Buried in Sand, Geotechnique, Vol. 39, No. 3, pp