Tire-reinforced earthfill. Part 1: Construction of a test fill, performance, and retaining wall...

Tire-reinforced earthfill. Part 1: Construction of atest fill, performance, and retaining wall design

Vinod K. Garga and Vince O’Shaughnessy

Abstract: The satisfactory disposal of scrap tires is a major environmental problem worldwide. This waste occupiesvaluable space in landfill sites, and tire stockpiles pose serious health and fire hazards. The use of scrap tires as rein-forcement for construction of retaining walls and slopes is a viable method towards reduction of this waste. This paperdescribes the construction of a 57 m high × 17 m wide instrumented test fill, comprising both retaining wall and rein-forced slope sections. Approximately 10 000 whole tires and tires with one sidewall removed, tied together with poly-propylene rope, were used in both cohesionless and cohesive backfills. The testing program also included plate loadingtests, field pull-out tests on tire mats, water-quality assessment in the field and laboratory, and other complementarylaboratory testing. This first paper, in a series of three, demonstrates the practical feasibility of constructing reinforcedearth fills using scrap tires. Results of large plate load tests and the field behaviour with particular reference to the de-sign of the retaining wall sections are presented. The paper emphasizes the role of negative wall friction in increasingthe active thrust when the retaining wall becomes more compressible than the backfill. Recommendations for the de-sign of retaining walls using scrap tires are presented.

Key words: scrap tires, earth reinforcement, retaining walls, reinforced slopes, plate load test, construction, performance.

Résumé: L’entreposage satisfaisant des pneus usés constitue un problème environnemental majeur à travers le monde.Ces déchets occupent un espace d’une certaine valeur sur les sites d’enfouissement, alors que les empilements de pneusreprésentent un risque pour la santé et pour le feu. L’utilisation de pneus usés comme armature dans la construction demurs de soutènement et de pentes de talus constitue une méthode efficace pour la diminution de ces déchets. Cet arti-cle décrit la construction d’un remblai d’essai instrumenté de 57 m × 17 m de largeur, comprenant les sections du murde soutènement et des pentes du talus armés. Environ 10 000 pneus complets et pneus amputés d’une paroi latérale,reliés entre eux avec un câble de polypropylène ont été utilisés dans des remblais de sols tant cohérents quepulvérulents. Le programme d’essais incluait également des essais de chargement sur plaque, des essais d’arrachementde matelas de pneus sur le terrain, des essais d’évaluation de la qualité de l’eau sur le terrain et en laboratoire, etd’autres essais complémentaires en laboratoire. Ce premier article d’une série de trois démontre la faisabilité pratiquede la construction de remblais de sol arméavec des pneus usés. L’on présente les résultats d’essais de chargement surde grandes plaques et le comportement sur le terrain particulièrement en rapport avec la conception des sections demurs de soutènement. Cet article souligne le rôle du frottement négatif du mur dans l’accroissement de la résultante depoussée lorsque le mur de soutènement devient plus compressible que le remblai. L’on présente des recommandationspour la conception de murs de soutènement au moyen de pneus usés.

Mots clés: pneus usés, armature de sol, murs de soutènement, pentes de talus armées, essai de chargement de plaques,construction, performance.

[Traduit par la Rédaction] 96

Garga and O’ShaughnessyIntroduction

Scrap tires are undesired urban waste which are producedat increasing rates every year, particularly in metropolitanareas. They are nondegradable and, because of their shape,quantity, and compaction resistance, require a large amountof space for storage. It is estimated that the United Statesdiscards approximately 50 million used tires annually into

landfills (Blumenthal 1998). Canada generates over 28 mil-lion equivalent passenger tires per year, of which approxi-mately 30% are disposed in landfills or tire stockpiles(CCME 1994). Scrap tires thus require large disposal areas,since whole tires are resistant to compaction. Stockpiling ofscrap tires is also undesirable because of the potential firehazard and consequent environmental damage and can pro-vide a good breeding habitat for disease-carrying insects andvermin. Sanitary landfills are now becoming expensive engi-neered facilities and therefore it is no longer economicallyfeasible to store large volumes of waste tires. Consequently,there is an urgency to develop new, energy-efficient, benefi-cial ways to recycle and reuse large volumes of scrap tires.

A tire is composed of rubber or polymer material stronglyreinforced with synthetic fibres and high-strength steelwhich produces a material having unique properties such as

Can. Geotech. J.37: 75–96 (2000) © 2000 NRC Canada

75

Received March 10, 1998. Accepted June 17, 1999.

V.K. Garga and V. O’Shaughnessy1. Department of CivilEngineering, University of Ottawa, Ottawa, ON K1N 6N5,Canada.

1Present address: Inter-Tech Engineering Inc., 251 Bank St.,Suite 608, Ottawa, ON K2P 1X3, Canada.

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very high tensile strength, flexibility, resiliency, and highfrictional resistance. Its mechanical properties remain avail-able even after its ordinary life as a car wheel element hasexpired. These unique material properties can be exploitedto construct reinforced retaining walls and earthfills. If lay-ers of tires, side by side, are tied together to make a mat,filled with soil, and then placed in successive layers, the re-sulting structure can be used as a retaining wall or a rein-forced slope and can provide a practical alternative for theuse of this waste. The concept is similar to that employed inthe use of geosynthetics for soil reinforcement which is wellaccepted in engineering practice. The durability of tiresmakes them particularly attractive for use in earth structures.In particular, the use of tires without shredding or breakingdown as tire chips is desirable because energy is not wastedin further processing this waste material. Such structures canbe built using conventional construction techniques.

An extensive study was carried out involving the construc-tion of a large instrumented prototype test fill at an automo-bile recycling yard near Ottawa. The scope of thisinvestigation included plate loading tests, field tire reinforce-ment pull-out tests, water-quality assessment in the field andlaboratory, and other complementary laboratory testing. Thetest fill comprised gravity retaining walls constructed of tiresand tire-reinforced slopes. The objective of this paper andthe companion papers (O’Shaughnessy and Garga 2000a,2000b) is to present the results of these investigations and topropose construction and design guidelines for such struc-tures. This paper emphasizes the construction of the proto-type structure with particular reference to the design andbehaviour of the retaining wall sections and the plate loadtests. Aspects related to the behaviour and design of tire-reinforced slopes are treated in detail in O’Shaughnessy andGarga (2000a).

Tire-reinforced earthfills

Tire as a construction materialTires are fabricated with vulcanized rubber that contains

reinforcing textile cords, high-strength steel or fabric belts,

and a high-strength steel wire reinforcing bead. The differentcomponents of a radial tire are presented in Fig. 1. Thebeads consist of rubber-covered metal wires or braids thatdo not easily deform. The tire fabric is usually made ofbraided rayon cord. Today’s steel-belted passenger car tiresare manufactured with about 9.8–12.5% steel by weight(Humphrey 1996).

Tires are composed of polymeric materials, mainly vulca-nized rubber, and therefore are usually not susceptible tocorrosion. However, initial mechanical properties of the tirematerial may be altered due to physical–chemical aging, UVradiation, creep, and damage occurring during construction.Physical–chemical aging occurs as a result of the chemicalnature of polyester and polyamides that can be hydrolyzed.Hydrolysis of polyester material is a function of tempera-ture, pH, and stress level. However, hydrolysis may never beobserved within the service life of the reinforced soil struc-ture (Leclercq et al. 1990). Macromolecular chains of poly-mers are susceptible to UV radiation and ozone breakdown.These phenomena can be neglected in most cases becausethe tire-reinforcing elements are sheltered from UV radiation(covered by soil) and ozone concentration in soils is negligi-ble. Strong acid (pH≈ 1) will destroy rubber. However, re-ported pH values for acidic groundwater usually rangebetween 4 and 5. At these pH levels, deterioration of therubber is minimal. However, this parameter could be impor-tant if waste material from industrial waste or acid mine tail-ings is used as backfill. Damage during construction andfrom compaction of fill has not been observed or reported.Tires are able to withstand high stress levels imposed duringcompaction by being able to deform to the same extent asthe surrounding soil. If tires are tied together to form a tiremat, the interconnecting elements such as steel bars, polyes-ter straps, and ropes could be subjected to greater construc-tion damage than the surrounding tires. The amount ofdamage would be a function of the tensile strength and flexi-bility of the connection.

AB-Malek and Stevenson (1986) studied the physical con-dition of vulcanized natural rubber submerged in 24 mof seawater for a period of 42 years. Their investigation

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76 Can. Geotech. J. Vol. 37, 2000

Fig. 1. The different components of a radial tire.



revealedthat no serious deterioration of the rubber hadoccurred. After 42 years of submersion, the maximumamount of water adsorbed was 4.7% and had no adverse ef-fect on strength properties. The limited amount of water ab-sorption was attributed to the formation of a thin surfacelayer (0.05 mm) of an iron base material. There was no vi-sual breakdown of the tire by marine organisms. The thinrubber layer prevented corrosion of the mild steel tire rein-forcement, even in water which was highly oxygenated. As aresult, even worn tires provide a very high quality reinforc-ing material that is very resistant to corrosion and deterioration.

Resistance to fireTires are a combustible material, and the reported ignition

of tire chips used in some fills (not used in the present re-search) can be a matter of concern (Humphrey 1996). How-ever, recent tire shred fills constructed in accordance withnew design guidelines which minimize internal heating haveremained inert (Whetten et al. 1997). Soil structures con-structed with buried whole tires would be difficult to ignite.The West Yorkshire Metropolitan County Council (WYMCC1977) investigated the potential fire hazard of a structureconstructed with whole tires. A 1 m high circular mound oftires filled with soil was built. The filling of the tires wasachieved by placing the soil over the tire without compac-tion. Attempts to ignite the tire mound were carried out us-ing several different procedures. A strong wind prevailedduring testing. Although the tire structures did not representan actual reinforced tire wall, several important conclusionswere drawn.

To set a tire-embedded structure on fire, a continuous andintense application of heat was required (e.g., a bonfire ofdry timber). The use of gasoline or the application of a gasblow torch was insufficient. The application of a gas blowtorch produced only slight charring of the rubber and, com-pared to more conventional building materials, considerablyless damage was produced. Burning of the tires would be re-stricted to exposed tires only and would not penetrate intothe fill. The possibility of fire hazard can be eliminated bycovering the exposed tire facing with incombustible materi-als such as bricks, concrete blocks, shotcrete, metal panels,gabions, or soil. Finally, hundreds of earthfills have beensuccessfully constructed using discarded whole tires (dis-cussed in the next section), and none have reported problemswith spontaneous combustion.

Previous experience with tires in earthfillsThe use of tire as earthfill reinforcement is not new. Many

tire structures have been constructed in other countries, e.g.,United States, Brazil, Switzerland, France, and Germany. Atthe end of 1993, over 250 tire-reinforced soil structures hadbeen constructed in France alone (Long 1993). Tire–soil-reinforced structures have numerous civil engineering appli-cations, including retaining walls, reinforced slopes, light-weight fill, energy dissipators, sound barriers, slope or riverprotection, and many other potential applications.

One of the first reported practical applications using dis-carded tires was the repair of a hillside fill instability alongCalifornia Highway 236 north of Santa Cruz in the mid-1970s (Forsyth and Egan 1976). After the removal of debrisand the placement of a drainage system, the road embank-

ment was rebuilt by reinforcing the soil with tire sidewallmats that were vertically spaced at 0.6 m. The individual tiresidewalls were joined together by steel clips to form a con-tinuous mat that was then extended beyond the embankmentface by 100–150 mm to provide erosion protection. The useof tire sidewall mats permitted the construction of a sideslope of 0.5H:1V, rather than the conventional 1.5H:1V,which resulted in a saving of some 70 000 m3 of expensivefill (Hausmann 1990).

Drescher and Newcomb (1994) mention several practicalapplications of scrap tire projects in California. One projectused scrap truck tires to control shoulder erosion of an em-bankment on Route 32 in Tehama County. Here, whole trucktires were interconnected with 2.7 mm steel reinforcing barsto form a continuous mat. The reinforcing mats were se-cured to the embankment by salvage anchor posts and thencovered with approximately 0.7 m of compacted permeablefill.

The first project in England using scrap tires was the con-struction of an experimental gravity wall in West Yorkshire(WYMCC 1977). The experimental cribwall was built toprovide extra valuable space for future office expansion orcar parking. The construction of the retaining wall usedwhole tires from cars or light commercial vehicles rangingfrom R-13 to R-15 (radius of the tire rim in inches), withcorresponding tire widths varying from 125 to 200 mm. Theheight of the tire wall varied up to 3.7 m and incorporated acurve. The maximum length of the structure, with 4500 tires,was 45 m with an average tire layer thickness of 0.15 m andcreated a useable area of approximately 100 m2. The investi-gators observed that effective interlock at the face of the tirewall was difficult for slopes steeper than 1 to 1. To stabilizethe face during construction, steel link bars 12 mm in diame-ter were used for every 15 tires (not all tires were intercon-nected). The cost of the experimental tire wall was estimatedto be approximately one-quarter the cost of a similar conven-tional retaining wall.

Dalton and Hoban (1982) report on the construction of atire wall on the west-bound exit of the M62 at Junction 26,also in England, as an alternative to the traditional gabionwall solution. The reinforced soil structure was an anchoredor tied-back tire wall. The face of the wall was constructedby placing the tires tread to tread to form a single line. Al-ternate layers of face tires, at an approximate vertical dis-tance of 0.3 m, were connected to a secured, anchored tireby 1000 kg Paraweb webbing (a polymeric strap). The an-chor tires were positioned 3 m back and centred horizontallyfrom the front line of tires. The paraweb was threadedthrough all face tires and secured to the anchor tires at regu-lar intervals. A granular backfill was used. The paraweb andtire anchors were able to prevent local failure of the tire wallface by providing sufficient tensile strength. These also pro-vided enough lateral restraint against wedge-type failure andslip-circle failure within the block.

The first research in France on soil reinforcements usingold tires was commissioned in 1976 and resulted in the sub-mission of a report to the Délégation Général à la RechercheScientifique et Technique. The Laboratoire Central des Pontset Chaussés conducted studies on reinforcement in the formof whole tires, sidewalls, or treads placed on edge or cut andlaid flat (Long 1993).

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Garga and O’Shaughnessy 77



The first experimental reinforced tire wall was 5 m highand 10 m long. It was constructed in 1982 at Nancy in theLangres region. The wall consisted of several reinforcing tiretread mats tied to flexible precast concrete facing units. Eachmat was vertically spaced no more than 0.5 m apart andback filled with a granular material. The individual tiretreads were placed edgewise and tied together with polyesterstraps. Since then, French engineers have constructed morethan 250 structures with tire reinforcement in France and 12in Algeria up to 1993 (Long 1993). One quarter of theFrench projects have used scrap tires for slope remediationand erosion protection. The French literature provides verylimited quantitative information, especially on the mechani-cal behaviour and deformation characteristics of these tirestructures, and design criteria for tire-reinforced earthfillshave not been well defined. Other applications have includedthe following: construction of retaining walls; providing alight-weight ground fill that has a unit weight between 6 and8 kN/m3, decreasing active pressures on structures; energy-absorption barriers for rockslides and snow avalanches;sound barriers; reducing load distribution above buried cul-verts by arching; and improving ground serviceability formilitary vehicles.

Tires could also be used as a structural medium in a num-ber of ways: creating artificial islands and reefs, land terrac-ing, crash barriers, bridge abutments, reinforcing soilfoundations, sea defenses, stabilizing soil heaps, and inmany other potential applications.

Laboratory and field investigations

To investigate the behaviour of tire-reinforced structures, aprototype test embankment was constructed on a private au-tomobile parts recycling property near Ottawa. Large num-bers of tires were already stored on the site for several years.This site had been operational as an automobile-recyclingfacility for considerable period of time, and was thereforelicensed by the Ontario Ministry of the Environment as anapproved site for scrap tire storage. The design of the test fillincorporated three reinforced slope configurations, and threetire-reinforced gravity wall sections along its two sides. Inthe reinforced slope, the tires were used either as wholetires, or with one side wall removed. The latter is referred toin subsequent text as a cut tire. In the reinforced slope sec-tion, reinforcing tire mats in which the individual tires weretied together by polypropylene rope were placed at a verticalspacing of 0.5 m. Each tire reinforcing mat was in-filledwith a backfill layer of soil, approximately 0.3 m thick, andcompacted. Hence, the tire layers were separated by a layerof compacted soil. In the construction of the retaining wallsection, the tire mat reinforcements were stacked on top ofeach other in a staggered manner. The voids within the tire-reinforcing mat were filled with backfill soil and compactedbefore the next tire-reinforcing layer was placed. In contrastto the reinforced slope section, only a thin layer of soil ex-isted at the tire mat interfaces in the retaining wall section. Itshould be noted that neither in the reinforced fill section norin the retaining wall section were the tire mats connected inthe vertical direction. The thickness of a reinforcing tire matcomposed of whole or cut passenger tires varied between150 and 200 mm.

A large number of pull-out tests on various configurationsof tire mat assemblies were separately carried out to betterunderstand the design of the tire-reinforced earthfills. Theresults of these investigations and the design implicationsare presented in a companion paper (O’Shaughnessy andGarga 2000a). To evaluate any toxic effects of buried tireson the surrounding groundwater, a drainage system was in-stalled below the embankment and the effluent collected inthree wells. The results of the analysis of water-quality dataare also presented in another companion paper(O’Shaughnessy and Garga 2000b).

Material properties

The soil material placed within the tire mat reinforcementis largely confined, and therefore the resistance developedbetween soil and tire reinforcement is primarily frictional.Consequently, it is important to assess the material proper-ties of the backfill soil, the attachment, and interface behav-iour between the backfill and tire rubber.

Backfill soilsThe test embankment consisted of three sections. Two

sections consisted of a good quality backfill comprised of acohesionless, clean sand imported to the construction sitefrom a Ministry of Transportation of Ontario approvedquarry. The third section was composed of various silty claycuttings and discarded fill collected over time from severalconstruction sites in the Ottawa area. This waste materialwas used by the test site owner as a soil barrier around hisproperty for several years. All soil tests were performed onsamples compacted to their respective densities determinedfrom Standard Proctor tests.

Imported sandA summary of index and strength properties of imported

sand with the corresponding testing methodology is given inTable 1. Standard Proctor tests indicated a maximum drydensity of 1845 kg/m3 at a optimum water content of 10.5%and a unit weight of 20 kN/m3. The sand was uniform andfree draining, with less than 5% fines and a trace of gravel.The failure envelope indicates an effective internal frictionangle of 42°, for an average dry density of 1790 kg/m3.

On-site cohesive soilThe index and strength properties of the on-site cohesive

backfill soil are also shown in Table 1 together with the cor-responding testing method used for their determination.Standard Proctor compaction tests indicated a unit weight of19 kN/m3 and a maximum dry density of 1508 kg/m3 at anoptimum water content of 29%. Particle-size analyses wereperformed on three soil samples from different locationsaround the site. The soil varied from a sandy silt with somegravel to a clayey silt with trace gravel (Unified Soil Classi-fication System). Consolidated undrained (CU) direct sheartests were performed on soil samples compacted at their nat-ural water content. The cohesive soil samples were shearedat a high displacement rate of 0.6 mm/min to minimize pore-water dissipation. The consolidated undrained strength pa-rameters are an apparent angle of internal friction of 19° andan apparent cohesion of 68.4 kPa. The latter parameters are

© 2000 NRC Canada




in terms of total stresses, since pore-water pressure cannotbe measured in direct shear tests. Consolidated drained (CD)

tests using a displacement rate of 0.00064 mm/min werealso performed on the compacted cohesive soil. The com-pacted backfill material has a peak effective angle of inter-nal friction of 32°. The cohesive backfill has a compressionindex of 0.108 and a recompression index of 0.031. The ver-tical coefficient of consolidation (cv) varies from 2.4 × 10–7

to 1.05 × 10–6 m2/s depending on the applied vertical effec-tive stress.

Attachment properties of the polypropylene ropeA 9.4 mm (3/8 in.) diameter polypropylene rope was used

to tie the tire elements together to form a reinforcing mat.The use of other types of tire attachments such as steelclamps was not permitted under the Ontario Ministry of theEnvironment and Energy (OMEE 1991) objectives for thisresearch. Polypropylene rope is very resistant to chemicaland biological attack and is readily available. The rope wastested in a Tinius Olsen testing machine to determine its ten-sile strength and modulus of elasticity. The use of severaldifferent knots was examined to determine which type ofknot was most effective, while remaining simple to tie in thefield. A square knot was selected. The appropriate length ofrope was placed between two eye bolts in the testing ma-chine; the two loose ends were tied together by means of theselected knot, and pulled apart; displacements were alsomeasured. Testing proceeded until failure occurred. Initialtest results revealed that several wraps of the rope around thetires would be required to provide adequate attachmentstrength. Therefore, the rope was looped two and three timesaround the eye bolts, tied, and tested to failure. This allowedthe evaluation of the overall strength characteristics of therope in terms of the number of wraps used in the test em-bankment and the field pull-out tests.

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Property ASTM testing procedure SandCohesivebackfill

Relative density,Dr D854-92 2.67 2.66Unit weight, γ (kN/m3) D698-91 (Standard Proctor) 20 19Maximum dry density,δd (kg/m3) D698-91 1845 1508Optimum water content,w (%) D698-91 10.50 29Particle-size distribution (average)

Gravel (%) D422-92 5 12Sand (%) D422-92 93 24Silt (%) D422-92 2 55Clay (%) D422-92 2.8 9

Uniformity coefficient,Cu D422-92 25Average plastic limit,wP D4318-87 24.4Average liquid limit,wL D4318-87 52.8Average plasticity index,IP D4318-87 28.4Unified Soil Classification System D2487-92 SP CH

MHStrength parameters

cu (kN/m2) D3080-90 (direct shear) 68.4φ u (°) D3080-90 19φ ′ (°) D3080-90 42 32

Compression parametersCcr (consolidation) D2435-90 0.031Cc (consolidation) D2435-90 0.108

Table 1. Index and strength properties of the backfill materials.

Fig. 2. Stress–strain behaviour of polypropylene rope in terms ofthe number of wraps used.



The stress–strain relationship for polypropylene rope interms of the number of wraps around the two eye bolts isshown in Fig. 2. The linear portion of each average stress–strain curve indicates that the elastic modulus ranged from240 MPa for a single loop to 320 MPa for two to threeloops. The initial slack in the attachment scheme combinedwith the tightening of the knot at the beginning of testingwas responsible for the apparent nonlinearity observed at theinitial stages. This nonlinearity was more predominant in thecase of a single wrap.

A list of the strength–deformation parameters of the9.4 mm diameter polypropylene rope as a function of thenumber of wraps around the eye bolts is given in Table 2.The ultimate tensile strength varied from 11.9 kN for a sin-gle wrap to 29.4 kN for three wraps. It was observed that thefailure of the rope would first occur in the rupture of individ-ual strands which would then lead to a progressive failure ofthe rope. Failure of the knot or the rope near the eye boltwas not observed. The estimated tensile strength per ropelength (the number of wraps times two) decreased with thenumber of wraps, from 6 to 5 kN per rope length. This be-haviour is the result of an uneven stress distribution (eachwrap does not carry the same load), and therefore the ulti-mate tensile strength is not directly proportional to the num-ber of wraps, based upon a single rope element. Thisbehaviour should be considered in design when establishingthe allowable tensile force of the reinforcement. In many in-stances, the weakest part in the tire mat construction is theattachment itself (dependent on attachment type and use),since the ultimate tensile strength of most radial tires willeasily exceed 75 kN (Long 1993). A sudden loss of strengthdue to failure of the attachment could have serious conse-quences, since the improvement in shear strength is directlyproportional to the maximum force generated within the re-inforcing tire mat. A safety factor must be employed to pre-vent this failure mode, discussed in one of the companionpapers (O’Shaughnessy and Garga 2000a). Another failuremode would be the loss of adherence between the soil andthe tire mat reinforcement, in which case a redistribution ofshear stress is possible without a sudden failure of the struc-ture.

Interface frictionThe shear stressτ along the soil–reinforcement interface is

given by

[1] τ = µσn

whereσn is the normal stress exerted on the reinforcement,andµ is the coefficient of friction between the soil and rein-forcing material. The friction coefficient,µ, is defined by theangle of interface friction (δ), which is usually determinedfrom modified direct shear tests.

The ratio of interface friction to soil friction (δ/φ) for soilsranging from sands to silts acting on various constructionsurface materials typically ranges between 0.5 and 0.8(Potyondy 1961; Yoshimi and Kishida 1981; Uesugi andKishida 1986), that is

[2] µ = tan δ ≈ (0.5 to 0.8) tanφ

whereδ is the interface friction angle between a soil and asurface, andφ is the internal friction angle of the soil. Re-sults from direct shear tests between various geotextiles anddifferent cohesionless soils have demonstrated that manygeotextiles could mobilize a high percentage of the availablesoil friction, between 80 and 90% (Koerner 1994). Coeffi-cients of interface friction between tire rubber and soil havenot been reported.

Jewell et al. (1984) provided the following expression toestimate the efficiency coefficient of resistance to direct slid-ing in a composite material:

[3] α δds = − −

1 1f

tantanφ

whereα ds is the efficiency coefficient of resistance to directsliding, φ is the angle of friction for soil in direct shear,δ isthe interface friction for soil and the reinforcement surface,and f is the fraction of the surface area of the reinforcementto the total area of the sliding plane. In a unit area of tire re-inforcement, passenger tire sidewall contributes approxi-mately 25% of the area (the area of contact between tirerubber and soil); consequently thef factor in eq. [3] for tiresis approximately equal to 0.25.

Laboratory assessment of interface behaviourThe interface friction or adherence between the tire rubber

and the soil was determined from a simple modified shearbox, approximately 100 mm × 100 mm. Interface tests wereperformed on both types of backfill materials used in theprototype test fill. Three layers of soil of interest wereplaced in the top half and compacted to the required densityby dropping a small square rod from a constant height. Asolid piece of tire sidewall rubber was secured to the bottomhalf of the box. The shear load and horizontal and verticaldeformations were monitored during the test. The test wasrepeated for several different normal loads. Both undrained(fast) and drained (slow) tests were performed.

The relationship betweenµ andδ can be stated byµ = tanδ.The dry sand gave the highest interface friction coefficient,0.58 (δ = 30°), and the cohesive backfill under undrainedconditions had the lowest value of 0.40 (δ = 22°). The inter-face friction coefficient (µ ) for the sand and tire rubber fallswithin the range reported in the literature for other materialssuch as concrete, wood, steel, and geotextiles (Potyondy

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No. of wrapsaround the eye bolt

Tu

(yield, kN)σu

(yield, MPa)ε yield

(%)Elastic modulus(MPa)

1 11.9 83.2 36.4 2422 21.0 73.7 21.4 3183 29.4 68.7 20.9 319

Note: Tu ultimate tensile strength;σu ultimate stress;ε yield strain at failure.

Table 2. Material properties of the polypropylene rope.



1961; Yoshimi and Kishida 1981; Uesugi and Kishida 1986;Koerner 1994). Sands and gravels provide better resistanceto a direct sliding failure than finer grain soils. Under pres-sure, coarser soil particles may penetrate the rubber and de-form or “roughen” the surface which increases the slidingresistance.

A summary of interface friction results is provided in Ta-ble 3 and Fig. 3. The interface strength was lower than theshear strength of the respective soils in all cases studied. Itmay be more desirable to express the interface friction interms of the Mohr–Coulomb failure criterion, since the inter-face shearing interaction is dependent on several factors, in-cluding soil type and testing conditions. The interfacefriction τinterface can be expressed as (Potyondy 1961)

[4] τ σinterface c n= +f c ftan( )φφ

wherefc = ca/c, whereca is the interface adhesion;fφ = δ/φ;andc andφ are soil shear strength parameters. The parame-ters fc and fφ for the two soils and drainage conditions deter-mined in the current study are also given in Table 3.

The efficiency coefficient of resistance to direct sliding(αds) (eq. [3]) ranged from 0.82 to 0.95 (Table 3). Thesehigh values indicate that sliding resistance of a tire mat rein-forcedstructure is predominantly governed by the shear strengthcharacteristics of the soil used in its construction. The tiremat reinforcement geometry is able to fully capitalize on theshear strength provided by the soil. This characteristic of tiremat reinforcement can allow the use of lower quality back-fill, thus further decreasing the cost of the structure.

Embankment design and construction

The reinforced embankment was designed to use conven-tional construction techniques and to reuse old tires with theminimum of processing to maximize economic benefits. Thetest embankment was constructed over a sand drainage blan-ket and has a height of 4 m, with an additional 2 m high per-manent surcharge placed at the end of construction. The plandimensions of the test embankment are 17.4 m (width) and57 m (length). The embankment geometry, tire mat layout,and soil type are presented in Fig. 4. A typical cross sectionis given in Fig. 5. The layout of the different instrumentsused to monitor the performance of the test embankment isalso shown in Figs. 4 and 5.

The reinforced earth structure for the study of both the re-inforced slope and the retaining wall is composed of threeindependent 10 m long sections (A, B, and C). Each sectionwas reinforced with tire mats for use as a retaining wallstructure on one side and as a reinforced slope along theother. The three different configurations of retaining walland reinforced slope are as follows: section A comprised cuttires (tires with one sidewall removed) filled with sand, sec-

tion B whole tires filled with sand, and section C cut tiresfilled with the cohesive soil.

Construction of the test embankmentThe field construction commenced with the levelling and

compaction of a 300 mm thick clean sand drainage blanketwhich included the installation of a 76 mm perforated pipewrapped in a geotextile, separately under each section, foreffluent water collection.

A great number of tires stockpiled at the construction siterequired the removal of the inner steel rim using a derim-ming machine. The cut tires were sliced on site to removeone sidewall by using a specially designed, lightweight, low-cost, transportable machine manufactured in Ontario. Thismachine performs the tire cutting by supporting the tire hori-zontally while a hydraulic jack fitted with a slicing carbon-steel blade cuts a sidewall as the tire is rotated. A two-personcrew was able to slice approximately 200 tires per hour, ifthe tires were clean. The sliced sidewall was placed insidethe cut tire to avoid generation of unnecessary waste.

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Soil type

Shear strengthparameter,c(kPa)

Interfaceadhesion,ca (kPa)

Frictionangle,φ(°)

Interfacefrictioncoefficient,µ

Skinfriction,δ (°)

ca/cor fc

δ /φor fφ

tan δ /tan φ

Efficiencycoefficient ofresistance,αds

Dry sand 42 0.58 30 0.71 0.64 0.91Cohesive backfill (CU) 68.4 7.6 19 0.40 22 0.11 1.16 1.17 0.82Cohesive backfill (CD) 32 0.49 26 0.81 0.78 0.95

Table 3. Summary of interface friction parameters including the direct sliding efficiency coefficient.

Fig. 3. The interface shear strength of the backfill soils used inthe test embankment.



The gravity retaining wall and reinforced steep slope wereconstructed simultaneously. These two structures were builtin the same embankment with the retaining wall and rein-forced steep slope on opposite sides. Layers of tires wereplaced side by side, tied together with three turns of a 2.5 mlong, 9.5 mm diameter polypropylene rope (Fig. 6), filledwith the appropriate soil, and compacted. A single personcould tie approximately 60 connections per hour. Experiencehas shown that for large-scale production, it may be more ef-ficient to preprepare the tied tire mats in manageable sec-tions on a raised production platform, and then transportthese to the embankment.

The sand and cohesive backfills were spread using bothfront-end loader and a DC-3 bulldozer (Fig. 7). It is impor-tant to mention that the cohesive soil used was lumpy andwet and would not have met the usually accepted construc-tion specifications for an earthfill. Poor quality clay fills aregenerally not acceptable for conventional embankment con-struction. A photograph depicting the retaining tire walls atthe end of construction is shown in Fig. 8. The constructionwas interrupted only occasionally for installation of pressurecells located behind the retaining wall and a few inclinome-ter casings. After completion of the embankment, the settle-ment gauges and the remaining inclinometer tubes wereinstalled in drilled holes. Most of the instrumentation wasplaced in the centre of each section to measure plane strainconditions and to eliminate any edge effects.

Relatively little construction equipment and work forcewas required. The total construction of the prototype fill wascompleted within 2 months by a three-person crew with afront-end loader and a self-propelled vibrating-drum roller.The fill was compacted by a lightweight smooth drum vi-brating roller, a Super Pac 540C model that was able to de-liver a centrifugal force of 67 kN. Figure 9 shows the degreeof infilling and compaction of cut tire reinforcements con-

structed with the cohesive backfill material (retaining wallsection). The final surcharge load consisted of approxi-mately 600 m3 of low-quality backfill, equivalent to a 2 mheight of fill, which was placed with a small excavator andlightly compacted. The entire embankment was hydroseededat the end of the project.

InstrumentationA plan view of the instrumentation layout is given in

Fig. 4, and the typical cross section also provides informa-tion on instrumentation locations (Fig. 5). Four inclinometercasings were installed in each section, two for each retainingwall structure and two for each reinforced slope to monitorlateral movements. Deep magnetic settlement gauges wereplaced in the reinforced slope and the unreinforced sectionbetween the retaining wall and steep slope to observe settle-ments. Surface topographical monuments were also installedafter the final height was reached.

Experience at landfill sites indicates that it is often verydifficult to drill through tires, since the steel belts and wiresentangle with the rotating bit. Consequently, during con-struction of the fill, care was taken to ensure that no tireswould be located along the vertical alignment of the drillholes which would subsequently be drilled for the installa-tion of instrumentation. This was ensured by using a systemof reference guide wires to locate the preplanned drill-holelocations.

Three pneumatic pressure cells were placed behind eachof the three retaining walls at different heights to measurethe lateral stresses and to determine the lateral earth pressurecoefficients. Their locations are at depths of approximately1.0, 2.0, and 3.75 m (height of the wall is 4 m). Eachpneumatic pressure cell was calibrated in an air-pressurechamber. An accurate assessment of soil stresses can only bemade by first calibrating the pressure cell in an air chamber

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Fig. 4. Plan view of the test embankment showing the geometry, tire reinforcement layout, soil type, instrumentation, and location ofthe plate load tests.



and then in the soil of interest subjected to known stresses.This calibration procedure provides an “action factor” forthe cell which will vary according to the soil type, porosity,moisture content, particle-size distribution, particle shape,compressibility, stress ratio, and stress history in addition tothe characteristics of the cell itself (Dunn and Billam 1966).Action factors for the pressure cells installed at the back ofeach tire retaining wall were regrettably not determined.Therefore, the recorded stresses should be considered an ap-proximate measurement of lateral stresses in the backfill.

Surface monuments were also installed at various loca-tions in all three sections to evaluate the overall settlementof the structure. These monuments were measured using anautomatic level.

Three large plate load tests were performed in the rein-forced slope sections after completion of the embankmentbut before the placement of the 2 m high final surcharge load.

In situ density of backfill layersThe in situ density of the compacted sand layers was veri-

fied by the rubber balloon method (ASTM D2167). The av-erage dry density of the sand sections was 1680 kg/m3. The

degree of compaction, based upon the Standard Proctor test(ASTM D698) was approximately 90%.

The in situ density of the compacted cohesive backfill wasdetermined by pushing a calibrated hollow cylinder ofknown weight and dimension into the soil. The density ofthe cohesive backfill exceeded the expected value evaluatedfrom the Standard Proctor test. The average in situ unitweight of the cohesive backfill was approximately 20 kN/m3,with a corresponding dry density of 1700 kg/m3.

Tire–soil densityThe in situ density of tire–soil material was also deter-

mined by means of a box arrangement during the construc-tion of the test embankment. The tire was placed in a largesquare rigid box, open at one end, covered with the nextlayer of soil (0.5 m), and compacted. The box was then ex-cavated from the surrounding soil, removed, and the excesssoil trimmed off. Care was taken not to disturb the soilwithin the box. The weight of soil and tire within the boxwas determined by suspending it by a cable from a tripodequipped with a calibrated load cell. To account for thevariability in tire sizes, three boxes sizes were used. A typical

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Fig. 5. A typical cross-sectional view of the test fill.



box size was 0.71 m × 0.71 m × 0.27 m and had an averagevolume of 0.14 m3.

The tire–soil densities reported below are average valuesbased on a minimum of three trials. The reinforcement layercomprised of cut tires filled with sand indicated an averagetotal unit weight of approximately 17 kN/m3. The reductionin total density was about 5% compared with the measuredin situ density of the sand layer. The small decrease in den-sity is attributed to the lower density of the tire elements(approx. 1100 kg/m3). Tests performed on whole tire rein-

forcing mats filled with sand indicated an average total unitweight of 16 kN/m3, representing a decrease in total densityof about 12% compared with the in situ density of the sand.The greater decrease in density when compared with cuttires filled with sand is associated with the formation of avoid space at the top of the tire. Compaction of whole tires,which are initially partially filled with soil, results in thecollapse of the sidewall, as shown in Fig. 10. The soiltrapped within the tires is pushed farther into the tire ele-ment, resulting in the formation of a small gap. The collapse

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Fig. 6. Photograph showing tire mat (cut tires).

Fig. 7. General overview of the test embankment during construction.



of the sidewall is beneficial because it decreases the amountof unfilled space within the tire element and produces a tirereinforcement of greater density.

A complete infilling of the cut tire reinforcement with thelumpy cohesive backfill was difficult. The measured averagetotal unit weight of 16.4 kN/m3 reflects this observation.

This represents a 20% decrease in density when comparedwith that of the compacted cohesive backfill layer. Compres-sion of the void spaces may lead to an undesirable amountof settlement. The lower unit weight of the reinforcing layershould be considered in design when using cohesive soils orwaste materials as backfill. With the benefit of this experi-

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Fig. 8. Completion of the three retaining walls to a height of 4 m (before placement of the 2 m highsurcharge).

Fig. 9. Photograph showing the degree of infilling of cut tires with the cohesive backfill after compaction (retaining wall section).



ence, it is recommended that cohesive soils be compacted insmaller lifts and a that compactor with a “sheep’s foot” vi-brating drum be used (a smooth vibrating drum was used onthis project). However, care should be exercised due to thepossibility of damage to the attachment, especially if thethickness of the cover soil is minimal.

Performance of the test embankment

Plate load testsThe bearing capacity and modulus of compressibility of

each reinforced fill section were determined by performingthree large plate load tests. The square loading plate

(1.2 m × 1.2 m) comprised two 12.7 mm thick steel plates.Smaller steel plates were stacked over the large square plateto reduce bending effects. The plate load tests were con-ducted following the ASTM D1194-72 procedures. The setupfor conducting the plate load test is presented in Fig. 11. Theimpressive supporting platform was loaded up to 40 t usingprecast concrete roadway dividers and was able to apply areactive stress greater than 225 kPa to the soil structure(Fig. 12). A reference H beam attached with dial gauges andsimple liquid settlement transducers (manufactured in theUniversity of Ottawa workshop) was used to measure theplate and surrounding soil movements. The precision was±0.01 mm for the dial gauges and ±0.05 mm for the liquid

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Fig. 10. Photograph showing the collapse of the sidewall during compaction of a whole tire. Rope connection was manually exposedand revealed no damage.

Fig. 11. General layout of the plate load test.



transducers. The plate was loaded in increments rangingfrom 10 to 20 kPa; this load was maintained until settlementhad essentially ceased. The test was terminated when thepeak loading capacity of the system was reached.

The load per unit area (q) versus settlement (s) relation-ships for all three plate loading tests are given in Fig. 13. Theultimate bearing capacity (qult) for both sand sections couldnot be determined, sinceqult exceeded the maximum loadingcapacity of the system which was approximately 240 kPa.The ultimate bearing capacity is defined as the lesser of theload that produces a settlement of 25 mm or that which re-sults in failure. The tire-reinforced soils did not fail in shear.Displacement in excess of 25 mm at an applied stress of240 kPa was observed only in the case of cohesive backfill(Fig. 13). It is of interest to note that even in the case ofwhole tires in sand the test indicates that a settlement of only12 mm had occurred at the maximum loading of 240 kPa (aloading approximately equivalent to a 15 story high building).This settlement is half of the usual criteria of 25 mm for ac-ceptable settlements in conventional structures. The resultsalso indicate that should a more stringent settlement criterionbecome necessary, then recourse should be made to the use ofcut tires. It should also be remembered that the plate loadingtests were carried out on top of the embankment, with the testplate located only 1.8 m from the edge of the slope.

The settlement of the plate can be estimated by the fol-lowing expression:

[5] s C q Cqq

= +

rc c m

c

log log

wheres is the settlement of the plate (m);q is the appliedload per unit area (kN/m2); qc is the compaction stress

(kN/m2); andCrc andCm are the recompressibility and com-pressibility indices (m), respectively. The compression ofthe plate and surrounding soil, plotted on a semilogarithmicgraph, for the plate load test performed on cut tires embed-ded in the cohesive backfill is given in Fig. 14, and the cor-responding compression parameters are given in Table 4.The stress imposed on the foundation was able to compressthe soil surface beyond a distance of 1.2 m from the edge ofthe plate (Fig. 14).

The higher compressibility index observed for whole tires(Cm = 0.02) compared with that for cut tires embedded insand (Cm = 0.01) was due to the compression of the voidspaces within the reinforcing layer. Consequently, the use ofwhole tires will result in higher settlement than that for cuttires (double the settlement at a stress of 240 kPa). This mayindicate that whole tire reinforced sections may be morecompressible than the unreinforced sections. The compress-ibility index for cut tires was four times greater in the cohe-sive backfill than in the sand section. Clearly, in tire-reinforcedfills, the settlement criterion is the predominant one. The set-tlement of the fill is particularly significant for tire retainingwalls which are not heavily compacted; the tire wall maythen undergo greater vertical compression when comparedwith that of the adjacent backfill. The resulting downwardmovement of the tire wall relative to the backfill will gener-ate negative wall friction. Hence, the active thrust will actupward against the back of the tire retaining wall. The direc-tion of the active thrust is critical for stability analysis.

Lateral stress distribution from earth pressure cellsThe lateral stress distribution behind each retaining wall

was directly measured by pneumatic total earth pressure cells.The cells used in this study were calibrated in a pressurized

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Fig. 12. Photograph showing the loading platform for the plate load tests, viewed from the reinforced slope section.



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Section ReinforcementCompaction stress,qc (kN/m2)

Recompressibilityindex, Crc (m)

Compressibilityindex, Cm (m)

A Cut tires embedded in sand 70 8.2×10–4 0.01B Whole tires embedded in sand 70 1.1×10–3 0.02C Cut tires embedded in cohesive backfill 80 8.5×10–4 0.04

Table 4. Compressibility characteristics determined from plate load tests.

Fig. 13. Measured plate settlement for the three plate load tests performed on each of the tire-reinforced sections.

Fig. 14. Compression of the plate and surrounding soil for the plate load test performed on cut tires embedded in cohesive backfill andthe corresponding compressibility indices.



air chamber and therefore the measurements are approxi-mate because of the lack of an in-soil calibration.

An estimate of the lateral earth pressure coefficients be-hind tire walls constructed with the sand backfill (sections Aand B) are shown in Figs. 15 and 16, respectively. Lateralearth pressure coefficients were also evaluated for the tirewall constructed with the cohesive backfill (Fig. 17). Fieldresults are compared with both Rankine and Coulomb theo-retical active earth pressure coefficients. Rankine determinedthe state of stress for a cohesionless soil mass adjacent to africtionless wall. However, the development of wall frictioncan significantly affect the earth pressures acting on the wall(Quigley and Duncan 1978). Coulomb’s theory and its sub-sequent development by others takes into account such fac-tors as wall friction, sloping backfill, and inclination of thewall. Results from the plate load tests and settlement mea-surements clearly indicate that the tire retaining walls weremore compressible relative to the unreinforced backfill, thusresulting in the development of a negative wall friction. Con-sequently, the lateral active pressure coefficients, using Cou-lomb’s theory, were determined for a level backfill, a wallinclination of 80°, and a negative wall friction equal to thebackfill friction angle (δ = –φ′). The lateral earth pressurecoefficients based on Rankine theory and ignoring the devel-opment of negative wallfriction can be underestimated,especially at large negative values of wall friction (Figs. 15–17).

During construction, high horizontal soil stresses were de-veloped behind each tire retaining wall. The high pressureswere attributed to the compaction process. Broms (1971)summarized that the effect of compaction behind unyielding

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Fig. 15. Lateral earth pressure coefficients behind the wall con-structed with cut tires and sand.

Fig. 16. Lateral earth pressure coefficients behind the wall con-structed with whole tires and sand.

Fig. 17. Lateral earth pressure coefficients behind the wall con-structed with cut tires and cohesive backfill.



walls was to increase the horizontal pressures aboveK0 val-ues within backfills behind low walls or near the surface ofbackfills behind high walls, whereK0 is the coefficient ofearth pressure at rest.K0 values for sand and cohesive back-fills, based on the theoretical relationship 1 – sinφ′, are 0.33and 0.47, respectively. The state of stress behind each wallwas well above theK0 condition.

Retaining walls constructed with tire mat reinforcementsfilled with soil and placed in successive layers produce aflexible structure. Tire retaining walls, unlike conventionalconcrete gravity retaining walls, are able to yield substan-tially as to fully induce an active state of stress within thebackfill adjacent to the wall. The reduction in lateral earthpressure registered in the lower cells during construction isattributed to the progressive yielding of the wall. After thefinal 2 m surcharge was placed, outward movement of thewalls resulted in a substantial reduction in the horizontal soilstresses, indicating that sufficient wall yielding had occurredto fully diminish compaction-induced stresses. Lateral earthpressure coefficients for all three walls were reduced to val-ues between the Rankine and Coulomb theoretical coeffi-cients. The increase in horizontal stress with time, after theplacement of surcharge, is attributed to the development ofnegative wall friction. This gradual increase in the earthpressure coincided with the vertical compression of each tirewall (Fig. 21). Regrettably, the two deeper earth pressurecells, located in sections A and B, failed after 1 year of oper-ation.

The progressive development of negative wall friction hasa significant impact on the stability of a tire retaining walland must be addressed in design. Tschebotarioff (1973) re-ported the bulging of the upper portion of a 10.2 m high,double-cell crib wall. The wall was constructed with precastconcrete headers and stretchers filled with loose granularmaterial. The sloping backfill (12°) behind the wall was

compacted. It was surmised that the observed distress proba-bly resulted from an increase in active pressure on the backof the wall caused by downward settlement of the wall withrespect to the backfill and a corresponding change in wallfriction from a positive to a negative value. This peculiarrelative movement between wall and backfill was attributedto an overload of the foundation below the heel of the wall.

Lateral displacement of retaining wallsThe measured lateral displacements of the three tire walls

are shown in Figs. 18–20. In Figs. 18 and 19, the initial pro-file of the retaining wall is outlined with a broken line inwhich the horizontal scale is reduced by a factor of five(horizontal scale 1:5). The horizontal scale for the retaining-wall profile is reduced by a factor of three (horizontal scale1:3) in Fig. 20. The lateral displacements shown in these fig-ures represent the actual measured horizontal movements.The horizontal displacements relative to the base were minorbelow line A, the slope of which is equal to the internal fric-tion angle of the backfill,φ′, times the direct sliding effi-ciency coefficientα ds (αds × φ′). The results indicate thatmost of the horizontal displacements occurred above thisline. The lateral displacement in the three tire retaining wallsarose from deformation within the tire reinforcement, defor-mation of the unreinforced zone behind the wall, and move-ments due to construction.

The displacement ratios (defined as the maximum lateraldisplacement,∆, divided by the wall height,H, or the heightat which the inclinometer tube exits the wall face) for eachretaining wall are given in Table 5. The wall constructedwith cut tires in sand demonstrated the highest stiffness ratio(greatest resistance to horizontal movement), and the wallusing the cohesive backfill showed the lowest stiffness ratio.The postconstruction monitoring has shown a continuous

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Fig. 18. Lateral displacement of the retaining wall constructed with cut tires and sand with reference to inclinometer location and time.



horizontal movement with time but with a decreasing rateafter a period of approximately 1 year.

The tire retaining wall in section C, constructed with cuttires in a cohesive backfill, reported the largest lateraldisplacement (395 mm after 745 days), indicating its highlevel of flexibility. An inward movement of this tire wallinto the backfill below the A line is shown in Fig. 20. Thisobservation may indicate that the reinforced wall section is

squeezing laterally due to the vertical compression of thewall, and consequently the face of the wall moves outwardwhile the heel of the reinforced zone moves into the backfill.

The smallest lateral movement in the retaining wall wasmonitored in section A with cut tires filled with sand. Therecorded movement after placing the surcharge was approxi-mately 80 mm. The advantage of using cut tires is clearlyevident. The deformations in the sand-filled sections,

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Fig. 19. Lateral displacement of the retaining wall constructed with whole tires and sand with reference to inclinometer locationand time.

Fig. 20. Lateral displacement of the retaining wall constructed with cut tires and cohesive backfill with reference to inclinometer loca-tion and time.



especially using cut tires, are similar in magnitude to thoseexperienced with geosynthetic-reinforced walls (Christopheret al. 1990). The postconstruction lateral deformations in thecohesive soil section are large (approx. 11%). These mea-surements indicate that a more careful control of placement,water content, and degree of compaction are necessary withclayey soils.

Horizontal deformation due to constructionThe three frontal inclinometers were installed prior to

construction of the retaining walls. Horizontal deformationsoccurring during construction were primarily due to com-paction of the overlying layers. The tire retaining wall con-structed using cut tires and sand reported a normalizedhorizontal deformation (∆ /H) of about 1.2%, whereas forwhole tires it was approximately 1.9%.

The tire wall constructed using cut tires and the cohesivesoil showed the largest lateral movement of about 4.7%.This represents, on average, a threefold increase in the mea-sured horizontal deformation due to construction when com-pared with the other two walls. As previously mentioned, itwas difficult to ensure proper infilling of the cut tires withthe cohesive backfill even after compaction, since voids werestill visible. The presence of voids would induce greaterdeformability of the tire reinforcement fill. Also, it wasfound to be impractical to level off the cohesive soil surfacewith the top of the tire reinforcement. This produced an un-even interface layer of varying thickness between the differ-ent tire reinforcing layers, and the interface strength at somelocations may be reduced. Therefore, some slippage betweensuccessive tire mat reinforcements could have occurred.

Since the tire wall was constructed in successive lifts, theoutward movement developed in the lower reinforcing lay-ers, over which the subsequent tire mat reinforcement wasaligned, resulted in a change in the face angle of the wall. Itis difficult to predict the magnitude of this type of construc-tion movement. If the amount of lateral deformations is animportant criterion, it would be appropriate to provide alarger prebatter to the retaining wall face. The amount ofhorizontal movement attributed to construction accountedfor approximately 30% of the total measured displacement,after 745 days of monitoring.

Horizontal deformation within the tire wallThe inclinometer located behind the retaining wall was in-

stalled after construction but before placement of the sur-charge load. Hence, these measured displacements indicatedeformation only due to placement of the surcharge. There-

fore, the difference in the measured horizontal displacementbetween the inclinometer located within the tire wall (1.2 mbehind the tire wall face), minus construction movement,and the inclinometer located behind the tire wall(unreinforced zone) indicates postconstruction lateral defor-mation that the reinforced tire wall has undergone (Table 5).The retaining wall in section A, cut tires in sand, showedmovements within the wall after 745 days (normalized lat-eral deformation,∆ /H = 2.3%) which were less than thosemeasured within the unreinforced zone by an inclinometerlocated behind the wall (∆ /H = 2.7%). As stated earlier, cuttires were able to provide strong interlocking resistancewhen using a good quality backfill. This structure behavedas an integral mass, and therefore little or no deformationoccurred within the reinforced tire wall.

The measured normalized horizontal deformation in thereinforced section B, using whole tires and sand, after745 days (excluding displacements associated with construc-tion) was approximately 1.1% (Table 5). Since the amountof wall face movement was not measured, the inclinometerlocated within the retaining wall was assumed to representthe maximum displacement of the structure. Deformationwithin the retaining wall represented 20% of the total mea-sured displacement. The weaker interlocking resistance gen-erated at the interface and the presence of voids within thetire reinforcing mat reduced the relative stiffness of the rein-forcement and produced higher deformations.

The normalized horizontal deformation within the rein-forced zone of the tire wall constructed with cut tires and alow-quality backfill (section C) was 5.6% (excluding con-struction movement). This displacement represents 35% ofthe total amount of movement after 745 days of monitoring.

Horizontal deformation behind the tire wall (unreinforcedzone)

The measured horizontal displacements from inclinom-eters installed behind the two retaining walls constructed us-ing the sand backfill showed similar results (∆ /H = 2.7% forsection A and 2.5% for section B). Movement of the unrein-forced sand backfill behind the cut tire wall (section A) ac-counted for 77% of the total measured horizontal movementat the face of the wall. For the whole-tire retaining wall (sec-tion B), this ratio accounted for only 45%. For the tire re-taining wall in section C, deformation of the unreinforcedbackfill accounts for 35% of the measured lateral movementat the face and represents a normalized horizontal deforma-tion (∆ /H) of 5.5%. This indicates a twofold increase in themeasured horizontal deformation in the unreinforced zone

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Section Reinforcement

Inclinometer 1.2 m behind tire wallInclinometerbehind tire walla

Afterconstruction

Surcharge+ 745 days

Surcharge + 745 days –construction movement

Surcharge + 745days

A Cut tires in sand 1.2 3.5 2.3 2.7B Whole tires in sand 1.9 5.5 3.6 2.5C Cut tires in cohesive backfill 4.7 15.8 11.1 5.5

Note: Displacements occurring within the tire walls are defined by subtracting values in the last column from those in the second to lastcolumn and are as follows: section A, –0.4%; section B, 1.1%; section C, 5.6%.

a The inclinometers located behind the wall were installed after construction but before placement of the surcharge load. Hence, themeasured displacements do not include movements related to construction activities.

Table 5. Normalized lateral deformations (∆/H, in %) from inclinometer data.



behind the wall with cohesive backfill when compared withthe other two walls. The lower shear strength and greaterdeformability of the cohesive backfill resulted in an increasein horizontal deformation of the reinforced tire wall.

Settlement of the test fillThe settlement of each section measured from the mag-

netic settlement gauges under a 2 m high postconstructionsurcharge is presented in Fig. 21. The settlement of sectionsA and B, tire reinforcements in sand, was approximately25 mm. There was no substantial difference between theunreinforced and reinforced sand in each of these sections ofthe prototype embankment, suggesting that the unreinforcedsand was well confined.

The settlement measured in section C (Fig. 21), cut tiresin a cohesive backfill, was greater than 200 mm. This largesettlement arose from the difficulty in infilling of the lumpy,wet, plastic backfill in the tires. The initial settlement of150 mm, after just 76 days, originated from the compressionof the void space as a result of ongoing creep deformation ofthe structure, and consolidation of the fill is responsible forthe remaining settlement. The amount of settlement substan-tially decreased approximately 1 year after completion of theprototype embankment. Consequently, proper compactionduring construction is important to minimize long-term set-tlements in cohesive backfills. If the amount of settlement isimportant, and the use of cohesive backfill is unavoidable,then other construction techniques could be used to reducethe potential settlement problems, such as preloading thestructure or construction in stages. This is similar to mea-sures adopted with conventional cohesive backfills. How-

ever, the results strongly indicate that only cut tires shouldbe used with cohesive backfill.

The settlement of the ground surface of the prototype em-bankment after 745 days, on top of the permanent surcharge,for each section is given in Fig. 22. The minimum surfacesettlement was measured in section A, and the greatest insection C. The voids within the tire-reinforced soils, particu-larly in sections B and C, decreased with time and generatedgreater vertical settlements. The reinforced slope constructedwith whole tires in sand settled, in general, approximately30% more than the same slope constructed with cut tire rein-forcement. It is important to note in Fig. 22 that in all casesthe retaining wall settled more than the backfill, giving riseto possible negative wall friction. This observation is consis-tent with measurements from earth pressure cells.

Design recommendations for tire-reinforcedretaining walls

A retaining wall constructed with tire reinforcement inwhich the tire mats are stacked on top of each other is simi-lar to a crib retaining wall. If properly designed, it shouldbehave as a gravity wall constructed of a “homogeneous”composite material. The various potential failure mecha-nisms which should be taken into account are summarized inFig. 23. The potential failure modes are (1) sliding of thetire wall at the base; (2) overturning of the retaining wall, in-cluding overturning at some elevation above the toe;(3) bearing-capacity failure or excessive settlement of thefoundation soil; (4) deep-seated stability failure, or slipalong an internal plane of weakness; and (5) loss of service-ability due to excessive deformation.

The above are conventional modes of failure and are welldiscussed in the geotechnical literature. Results of this studyindicate that the reinforced tire wall may be more compress-ible than the backfill. The greater compressibility of the tirewall relative to the backfill can generate significant negativewall friction, and therefore must be considered in the estima-tion of lateral earth pressure. Consequently, the active thrustwill act upwards against the back of the tire retaining wall.

As an example, Table 6 shows the different factors ofsafety obtained for the three retaining walls calculated usingRankine active earth pressure theory (Ka) and a trial-wedgemethod in which the negative wall friction (–δ) was ac-counted for. The interface friction angles used in the analy-ses were determined from the direct shear tests. It should benoted that the factors of safety using negative wall frictionwere substantially less than those obtained from the conven-tional Rankine theory, which does not allow for this effect.Therefore, the use of Coulomb’s theory with considerationof appropriate wall friction is recommended.

Conclusions

(1) The ease of construction of the prototype embankmentdemonstrated the practical feasibility of using nonshreddedscrap tires as a soil reinforcement for both tire-reinforcedslopes and tire-reinforced gravity retaining walls. These struc-tures can be constructed with conventional fill-placementequipment. Virtually no damage was observed as the trucksand the lightweight compactors traversed over the tires.

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Fig. 21. The settlement of each section measured from magneticgauges after 745 days of postconstruction surcharge.



(2) Reinforced earth structures using discarded tires canbe constructed with both cohesionless and cohesive soils.However, it is recommended that only tires with one side-wall removed should be used with cohesive backfills. Thesefills do require careful compaction to ensure proper infillingof the tire reinforcement.

(3) The higher compressibility of tire retaining walls com-pared with the backfills may result in the development of

negative wall friction at the back of the wall. This negativewall friction significantly increases the active pressure act-ing on the back of the wall. A reduction in the active thrustcan be achieved by inclining the wall.

(4) Research indicates that the angle of inclination of theretaining walls should not exceed 70° when using a low-quality backfill which is compressible. Compaction behindthe retaining wall should be carefully carried out to limit the

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Fig. 22. Settlement of surface monuments with time for all sections.

Fig. 23. Potential failure modes for a retaining wall constructed with tire reinforcement.



development of high lateral stresses and to reduce the out-ward lateral deformation. Overhang of tires must not be per-mitted.

(5) The lateral deformation of the wall section constructedwith whole tires was significantly larger than that with cuttires (one sidewall removed). The walls with sand fill indi-cated that lateral displacements using whole tires were ap-proximately 60% larger than those measured in the cut tirewall section. The use of cut tires and lightly compacted co-hesive backfill resulted in a lateral movement that was 4.5times that at the wall section with cut tires and sand.

(6) Plate load tests indicate that the tire-reinforced fillscan provide satisfactory foundation for medium- to light-weight structures.

Acknowledgements

This research was made possible by a grant to the first au-thor through the Industrial Waste Diversion Program of theWaste Reduction Branch, Ontario Ministry of the Environ-ment and Energy (OMEE). This research would have beenimpossible without the generous cooperation of Mr. JimConroy, P.Eng., owner of Conroy Auto-Parts Recycling,Cumberland, Ontario, who provided the free use of hisOMEE-approved premises for test site location as well as forhis numerous contributions towards construction activities.Thanks are also expressed to Mr. Al Blank, foreman forDeschenes Construction (Ontario) Ltd., for his cooperationduring the construction and field testing activities, and toMr. Richard Moore, Technical Officer in the GeotechnicalLaboratory at the University of Ottawa, who provided nu-merous innovative ideas towards this research. The contribu-tion of Dr. Luciano Medeiros, PUCRIO, Brasil, informulating the research proposal and for numerous stimu-lating discussions is gratefully acknowledged.

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© 2000 NRC Canada


Factor of safety

Method Sliding Rotation Slope stability

Cut tires embedded in sand 1.8Rankine (Ka) 3.0 3.6Trial wedge (Ka) 1.7 2.0

Whole tires embedded in sand 1.7Rankine (Ka) 2.9 3.5Trial wedge (Ka)

a 1.6 1.9Cut tires embedded in cohesive backfillDuring construction (undrained conditions) 1.9b 1.4b 3.5Long term (drained conditions) 1.4

Rankine (Ka) 1.9 2.0Trial wedge (Ka) 1.1 1.2

Minimum recommended value 1.5 2.0 1.3aThe trial-wedge method results include the effect of negative wall friction.bLateral earth pressure distribution was based upon actual measured values obtained from pressure cells

located behind the wall, just after placement of the surcharge load.

Table 6. Estimated factors of safety for each retaining wall structure.



© 2000 NRC Canada


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