pullout resistance of bearing reinforcement …cec.sut.ac.th/download/published/bearing-sf.pdf ·...

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i) Associate Professor and Chair, School of Civil Engineering, Head, Construction Technology Research Unit, Suranaree University of Technol-ogy, Thailand (suksun＠g.sut.ac.th and suksun＠yahoo.com).

ii) Research Engineer, Construction Technology Research Unit, Suranaree University of Technology, Thailand.The manuscript for this paper was received for review on May 1, 2009; approved on December 2, 2009.Written discussions on this paper should be submitted before November 1, 2010 to the Japanese Geotechnical Society, 4-38-2, Sengoku,Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.

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SOILS AND FOUNDATIONS Vol. 50, No. 2, 215–226, Apr. 2010Japanese Geotechnical Society

PULLOUT RESISTANCE OF BEARING REINFORCEMENTEMBEDDED IN SAND

SUKSUN HORPIBULSUKi) and ANEK NIRAMITKORNBUREEii)

ABSTRACT

Mechanically stabilized earth (MSE) structure has been widely accepted as a retaining structure. Its construction costis mainly controlled by backˆll materials, which are generally coarse-grained soils, and reinforcement type (steelvolume). The present paper introduces a new cost-eŠective reinforcement, designated as ``Bearing Reinforcement''. Itis composed of a longitudinal member and transverse (bearing) members. The longitudinal member is made of adeformed bar, which exhibits a high pullout friction resistance. The transverse members are a set of equal angles,which provide high pullout bearing resistance. The maximum pullout bearing resistance of a single isolated transversemember, sbmax, can be determined by using the plasticity solution based on the modiˆed punching shear failuremechanism. In‰uential factors governing the mobilization of pullout bearing resistance are spacing, S, leg length, B,and numbers, n of transverse members. The larger the S/B, the lower the transverse member interference. The S/B ra-tios of º3.75 and À25 are referred to as full and free interference, respectively. The relationship between normalizedaverage pullout bearing stress, sbn/nsn and pullout displacement, d, where sbn/n is average pullout bearing stress ofthe bearing reinforcement with n transverse members and sn is applied normal stress, is practically identical for thesame level of transverse member interference. This relationship can be modelled by hyperbolic function. From thisˆnding, a suggested procedure for estimating pullout characteristics (maximum pullout resistance and pullout forceversus displacement relationship) of the bearing reinforcement for any level of transverse member interference (any S,B, and n) based on a one point test on the bearing reinforcement with a single isolated transverse member is proposed.Good agreement has been obtained between the predicted and the measured pullout characteristics. This suggestedmethod is useful for the internal stability analysis of MSE wall in terms of engineering and economic viewpoints.

Key words: bearing member interference, bearing reinforcement, mechanically stabilized earth walls, pulloutresistance (IGC: H2/K11)

INTRODUCTION

The use of inextensible reinforcements to stabilizeearth structures has grown rapidly in the past two de-cades. When used for retaining walls or steep slopes, theycan be laid continuously along the width of the reinforcedsoil system (grid type) or laid at intervals (strip type).Both grid and strip reinforcements are widely usedaround the world including Thailand. The constructioncost of the mechanically stabilized earth (MSE) wall ismainly dependent upon the transportation of backˆllfrom a suitable borrow pit and the reinforcement type.The backˆll is generally granular materials, according toa speciˆcation of the Department of Highways,Thailand. The transportation of the backˆll is thus a ˆx-ed cost for a particular construction site. As such, thereinforcement becomes the key factor. The lower the steelvolume used and the faster the installation, the lower theconstruction cost.

In Thailand, a widely used strip reinforcement is theribbed steel reinforcing strip. It is 50 mm in width and 4.2mm in thickness with yield strength of 520 MPa. Thisreinforcement is conveniently transported to a factory forgalvanization and to a construction site as well as simpleand fast to install due to its strip shape. Since it is not pro-duced in Thailand and is imported from Africa, the con-struction cost is relatively high due to the high importcharges. The steel grid reinforcement can be locallymanufactured. This reinforcement has been extensivelystudied at the Asian Institute of Technology by Prof. D.T. Bergado and his co-workers (Bergado et al., 1988,1996; Shivashankar, 1991; Chai, 1992). The advantage ofthe grid reinforcement is that the pullout bearingresistance in the resistant zone is high. However, the totalvolume (weight) of steel grid required is still high due towasted transverse (bearing) bars in the active (unstable)zone. The transportation and installation of the grid rein-forcement are less convenient than those of the strip rein-

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Fig. 1. Bearing reinforcement

Fig. 2. Connection of the bearing reinforcement to wall facing

216 HORPIBULSUK AND NIRAMITKORNBUREE

forcement.To develop a new cost-eŠective inextensible reinforce-

ment type, four factors that need to be considered are,available raw material, simple and fast installation, con-venient transportation, and high pullout and ruptureresistances with less steel volume. At Geoform Co., Ltd.and the School of Civil Engineering, Suranaree Univer-sity of Technology, the ˆrst author have developed a newtype of reinforcement, designated as ``Bearing Reinforce-ment''. It combines the advantage of the strip and thegrid reinforcements together, which meets these four es-sential factors. Figure 1 shows the typical feature of the

bearing reinforcement, which is composed of a longitudi-nal member and transverse (bearing) members. The lon-gitudinal member is a deformed steel bar and the trans-verse members are a set of equal steel angles. The lon-gitudinal and transverse members are very strongly weld-ed. The welding strength is designed to sustain the loadnot less than the tensile strength of the longitudinal mem-ber, according to the American Institute of Steel Con-struction (AISC). Since the transverse members producehigh pullout bearing resistance, only few transverse mem-bers are needed in the resistant zone (none in the activezone) and hence the cost eŠectiveness. The reinforcement

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Fig. 3. Modes of failure

217PULLOUT RESISTANCE OF BEARING REINFORCEMENT

is connected to the wall facing (1.5×1.5 m) at the tiepoint (2 U shape steel) by a locking bar (a deformed bar)(vide Fig. 2). The vertical spacing between the tie points isusually 0.75 m and the horizontal spacing is 0.75 and0.375 m, depending upon the loading level. The laborato-ry and full-scale investigations on the pullout resistanceof this reinforcement have been commenced since 2007 inSuranaree University of Technology. This reinforcementhas been introduced into practice in Thailand since 2008by the ˆrst author and Geoform Co., Ltd. Several bear-ing reinforcement stabilized earth walls have been con-structed by Geoform Co., Ltd. in diŠerent areas; namelynorth, northeast, and south of Thailand.

For a MSE wall design, an examination of external andinternal stability is a routine design procedure. The ex-amination of external stability is generally performed us-ing the conventional method (limit equilibrium analysis)assuming that the composite backˆll-reinforcement massbehaves as a rigid body (McGown et al., 1998). The inter-nal stability deals with rupture and pullout resistances ofthe reinforcement. The potential failure plane for the in-extensible reinforcement stabilized earth structure ispractically assumed by the bilinear failure mechanism(coherent gravity structure hypothesis) (Anderson et al.,1987). The internal stability against rupture failure isgoverned by the area and the strength of the reinforce-ment. The pullout resistance of any type of reinforcementgenerally consists of two parts: friction and bearingresistances. The friction pullout resistance results fromsoil shearing on the reinforcement surface. The bearingpullout resistance results from soil bearing on bearingareas, which are normal to the pullout direction. For theribbed steel reinforcing strip, the main contribution onthe pullout resistance is the friction resistance. The soil-reinforcement interaction for the strip reinforcement is acombined 2-D and 3-D interaction mechanism (Hayashiet al., 1999). As the strip reinforcement is pulled out andshear displacement occurs along the interface, the zone ofsoil surrounding the reinforcement tends to dilate.However, the volume change is restrained by the sur-rounding nondilating soil, resulting in an increase in nor-mal stress on the soil-reinforcement interface. The resultof restrained dilatancy has been designated as the 3-D in-teraction mechanism. The methods to predict and deter-mine the pullout resistance of the ribbed steel reinforcingstrip (Alforo et al., 1995; Hayashi et al., 1999; Alforoand Pathak, 2005; AASHTO, 2002) are available.

For grid reinforcement, the pullout resistance is mainlyattributed from the bearing resistance. Due to the widewidth of the reinforcement, the soil-reinforcement inter-action mechanism is plane strain condition. The existingpullout bearing failure mechanisms for the plane straincondition are general shear failure (Peterson and Ander-son, 1980), punching shear failure (Jewell et al., 1984),and modiêd punching shear failure (Chai, 1992; Berga-do et al., 1996). The general shear failure mode assumes acharacteristic êld as shown in Fig. 3(a). The bearingcapacity equation given is the Prandtl's solution(Prandtl, 1921) of the failure stress for shallow smooth

strip foundation. The horizontal eŠective stress is consi-dered to be equal to applied vertical stress, sn. However,during pullout test, the bearing member is more or lesslike deep strip foundation so the characteristic êld maynot be the same as that for shallow foundation.

Punching shear failure mode is based on the stresscharacteristic êld as shown in Fig. 3(b). It is assumedthat the stress acting on the inclined rupture line is thevertical stress, sn. Thus, the normal stress on the ruptureline AC is sn cos q. The angle of rotational failure zonewas assumed as u2＝(45＋q/2) degrees. The general shearfailure and punching shear failure mechanisms provideapparent upper and lower bounds, respectively and can-not predict the actual pullout bearing resistance very well(Jewell et al., 1984; Palmerira and Milligan, 1989). Modi-êd punching shear failure mechanism has been proved assuitable for approximating the pullout bearing resistanceof steel grid reinforcement.

Modiêd punching shear failure mode is based on the

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Table 1. Gradation of tested sand compared with a speciˆcation of theDepartment of Highways, Thailand

Particle size zFiner of the tested sand zFiner according to thespeciˆcation

37 mm 100.0 1004.75 mm 99.1 30–100

0.425 mm 76.4 15–1000.150 mm 10.7 5–650.075 mm 0.0 0–15


stress characteristic êld as shown in Fig. 3(c). It is as-sumed that (a) there are only two failure zones: active(ABD) and rotational zone (ABC); (b) the stress state be-yond the rupture line AC can be expressed by normalstress, sn, and horizontal stress, Ksn, which are all theprinciple stresses and k is the horizontal earth pressurecoe‹cient; and (c) the strength on AC is fully mobilized.Bergado et al. (1996) have suggested to take b＝p/2 and k＝1.0. The b＝p/2 is the same value used for deriving thebearing capacity equation for a strip footing.

The maximum bearing stress of a single isolated trans-verse member, sbmax, in coarse-grained soil is presented inthe form:

sbmax＝Nqsn (1)

where Nq is bearing capacity factor, depending upon themode of failure, and sn is normal stress. Nq for the threefailure mechanisms is presented in terms of soil frictionangle, q, as follows:

Nq＝exp [p tan q] tan2 Øp4＋q2 »

for general shear failure (2)

Nq＝exp «Øp2＋q» tan q$ tan Øp4＋q2 »

for punching shear failure (3)

Nq＝1

cos qexp [p tan q] tan Øp4＋q

2 »for modiêd punching shear failure

(b＝p/2 and k＝1.0) (4)

For the bearing reinforcement, the pullout resistance isalso the summation of pullout friction and bearingresistances. Besides the maximum pullout resistance, thepullout force and displacement mobilization processmust be investigated since most of the MSE walls do notreach the limit equilibrium state. For analyzing andpredicting the behaviour of MSE walls, therefore, an un-derstanding of the pullout resistance mobilizationmechanism is necessary.

The present paper attempts to investigate the pulloutcharacteristics (pullout force versus pullout displacementrelationship and maximum pullout resistance) of a singleisolated transverse member and the role of in‰uential fac-tors (dimension, spacing and numbers of transversemembers, and normal stress) on the transverse memberinterference and pullout characteristics of the bearingreinforcement. Finally, a rational and practical methodof predicting the pullout characteristics of the bearingreinforcement for diŠerent levels of transverse memberinterference is proposed. Comparisons of the predictedand the measured pullout test results are presented toreinforce the applicability of the proposed method.

LABORATORY INVESTIGATION

Soil SampleThe soil used in this investigation is a clean sand. It

consists of 0.3z gravel, 97z sand, and 2.7z silt. Thegradation of the sand is presented in Table 1 togetherwith a speciˆcation of the Department of Highways,Thailand. The particle size distribution is as follows:average grain size, D50＝0.31 mm; uniformity coe‹cient,Uc＝2.4; and coe‹cient of curvature, U c?＝1.2. This sandis classiêd as poorly graded sand (SP), according to theUniêd Soil Classiˆcation System (USCS). Its speciˆcgravity is 2.77. The compaction characteristics understandard Proctor energy are optimum water content(OWC)＝6.3z and maximum dry unit weight, gd, max＝16.8 kN/m3. Strength parameters of this sand at the opti-mum point obtained from a large direct shear apparatuswith the diameter of 35 cm are c?＝0, and q?＝40 degrees.This high friction angle (greater than 36 degrees) is ac-ceptable for MSE wall construction. Generally, well-graded materials are used as backˆll due to high e‹ciencyof êld compaction. The uniform sand was however usedin this investigation for the consistency of the laboratorycompaction for each test. Even though the tested sand isuniformly graded, its gradation and internal friction an-gle meet the speciˆcation of the Department of High-ways, Thailand.

Bearing ReinforcementTo understand the role of the in‰uential factors

(dimension, spacing, and numbers of transverse membersand normal stress) on the pullout characteristics, the pull-out tests on the bearing reinforcements with diŠerentdimensions, numbers, and spacing of transverse membershave been conducted under diŠerent applied normalstresses. The leg length, B, and the length, L, of the testedtransverse members (steel equal angles) are 25, 40, and 50mm and 100, 150, and 200 mm, respectively, which aregenerally used for MSE walls. As such, B/L ratio variesbetween 0.13 and 0.5, higher the B/L ratio is possibly as-sociated with a high geometry eŠect. This geometry eŠectis the same as that for the strip shallow foundation.Generally, the geometry eŠect plays a signiˆcant role forshallow foundation that the failure mechanism isgoverned by the general shear failure. The spacing be-tween transverse members, S, varies from 15 to 150 cen-timeters, depending upon the number of transverse mem-bers. In this study, the number of transverse members, n,is 1 to 4, which is generally the case in practice. The pul-lout friction resistance of a longitudinal member is inves-tigated from the pullout test on a single longitudinalmember with a diameter of 16.0 mm and length of 2.6 m.

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Table 2. Mechanical properties of longitudinal and transverse mem-bers

Member Materialname

Tensile strength(MPa)

Yield strength(MPa)

Elongation(z)

Longitudinal SD40 560 390 15Transverse Fe24 402 235 21

Table 3. Physical properties of the tested transverse members

Dimension (mm)Sectionarea(cm2)

Unitweight(kg/m)

CG.(cm)

Cx＝Cy

Momentof inertia

(cm4)Ix＝Iy

Radius ofgyration

(cm)rx＝ry

Momentof section

(cm3)Sx＝Sy

A×B t r1 r2

25×25 3 4.0 2.0 1.42 1.12 0.72 0.797 0.747 0.448

40×40 3 4.5 2.0 2.33 1.83 1.09 3.53 1.23 1.21

50×50 4 6.5 3.0 3.89 3.06 1.37 9.06 1.53 2.49

Fig. 4. Schematic diagram of pullout test apparatus


Table 2 presents mechanical properties of the longitudi-nal and transverse members. Physical properties of thetransverse members are presented in Table 3. These phys-ical and mechanical properties are determined by theThailand Industrial Standard (TIS).

MethodologyThe pullout test apparatus used in this investigation is

made of rolled steel plates, angles, channels, and H-sec-tions welded or bolted together to give the inside dimen-sion of 2.6 m in length by 0.6 m in width by 0.8 m inheight as shown in Fig. 4. The front wall contains the up-per and lower parts with a slot in between for the reinfor-cement specimen. Friction between the tested sand andthe side walls of the apparatus was minimized by the useof a lubricated rubber member as recommended by Al-faro et al. (1995). During the pullout of the reinforce-ment, due to an arching eŠect of the front wall, the nor-mal stress on the reinforcement near the front wall mayincrease (dilate) or decrease (contract). In order to reducethis eŠect, a sleeve was installed inside the slot opening,which was 150 mm in horizontal width and 100 mm inheight to isolate the bearing reinforcement near the frontwall. The compacted sand thickness of 300 mm wasmaintained above and below the reinforcement.

Normal stress was applied with a pressurized air bagpositioned between the compacted sand and the top coverof the apparatus. Before installing the air bag, a 30 mmthick layer of ˆne sand was placed on the top of the com-

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Fig. 5. Pullout test results of a longitudinal member under diŠerentnormal stresses


pacted sand and covered by a 4 mm thick steel plate. Thepurpose of this procedure was to try to produce a uni-formly distributed normal stress on the top of the backˆllsoil (Fig. 4). The pullout force was applied by 200 kNcapacity electro-hydraulic controlled jack. The pulloutdisplacement at the front of the pullout apparatus wasmonitored by a linear variation diŠerential transformer(LVDT). The maximum applied pullout displacement(end of test) is 40 mm, which is about 10z leg length (B )of the transverse member. For deep foundation, the fullmobilization of the bearing stress is about 10z of the pilediameter (Whitaker, 1976). Similarly, this end of pullouttest can be regarded as the full mobilization of pulloutresistance. The displacement along the longitudinal mem-ber could be measured using strain gauge and hence an in-direct measurement of force along the longitudinal mem-ber. For inextensible reinforcements, measuring the dis-placement along the longitudinal member did not yieldmuch useful information (Bergado et al., 1996). Thismeasurement is thus not the goal of this study. The ap-plied normal stress was 30, 50, and 90 kPa. These diŠer-ent applied normal stresses were considered to simulatetotal vertical stress (due to dead and live loads) on thebearing reinforcement at diŠerent depths. The live loadof 20 kPa is generally used for MSE wall design inThailand. The pullout tests on the bearing reinforcementswere conducted in the compacted dry sand (at optimumpoint) since the drainage is generally provided to preventa presence of permanent and temporary water table in theMSE wall. The pullout rate of 1 mm/min was adoptedthroughout the tests.

TEST RESULTS

Pullout Friction Resistance of a Longitudinal MemberFigure 5 shows a pullout test result of a longitudinal

member with a diameter of 16 mm and a length of 2.6 m.Maximum pullout friction resistance, Pf of the longitudi-nal member can be calculated from:

Pf＝pDLsn tan d (5)

where D and L are diameter and length of the longitudi-nal member, respectively, sn is normal stress, and d is theskin friction angle. It is of interest to mention that d isquite high with its value of 58.7 degrees. Consequently,d/q ratio is greater than unity and is about 1.47. Thishigh ratio is due to the contribution of the skin roughnessof the deformed bar. This shows more advantage of theuse of a deformed bar as a longitudinal member than thatof a round bar for the same diameter, length, andstrength. It is also found that the displacement at failureis insigniˆcantly aŠected by normal stress. It is about 3.0mm for all the applied normal stresses.

The diŠerence between peak and residual strengths isinsigniˆcant. It is thus suggested that practically, the pul-lout friction force and displacement relationship can bemodelled by a linear-perfect plastic model in terms of twoparameters: skin friction angle, d, and initial linear skinfriction stiŠness, ks. This ks can be determined by the fol-

lowing equation.

ks＝snpDL tan d

df(6)

where df is the displacement at maximum pullout frictionforce and taken as 3.0 mm.

Pullout Bearing Resistance of a Single Isolated Trans-verse Member (n＝1)

The pullout bearing force at any displacement is thediŠerence between the total pullout force and the pulloutfriction force. The total pullout force is directly obtainedfrom the pullout test on the bearing reinforcement with asingle transverse member (n＝1). Figure 6 shows the typi-cal total pullout force and displacement relationship ofthe bearing reinforcement with a 1.0 m longitudinalmember and a 40×150 (B×L) mm transverse member.It is notable that initially, the pullout resistance sharplyincreases with displacement and then gradually increasesuntil failure at a large displacement of about 40 mm,which is the end of test. The initial sharp increase iscaused by the pullout friction resistance, which fullymobilizes at small displacement (about 3 mm) while thesoil-bearing capacity fully mobilizes at large displace-ment.

Figure 7 shows the relationship between the normal-ized pullout bearing stress, sb/sn and the pullout dis-

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Fig. 6. Pullout force versus displacement relationship of the bearingreinforcement with a single isolated transverse member

Fig. 7. Normalized pullout bearing stress and displacement relationship of a single isolated transverse member for diŠerent dimensions


placement, d, of the single isolated transverse member (n＝1) for B×L of 25×100, 25×150, 40×150, 40×200,and 50×150 mm. The pullout bearing stress, sb, is deter-mined by assuming that the transverse member and soil inthe transverse member act as a rigid block penetratinginto the front soil during pullout. As such, the pulloutbearing stress is calculated from the ratio of pullout bear-ing force to bearing area (B×L). This assumption wasveriêd by the identical pullout test results of a transversemember and a thick plate having the same B and L underdiŠerent normal stresses. The bearing stress at the pulloutdisplacement of 40 mm (end of test) is deˆned as the max-imum bearing stress. It is of interest to mention thatsb/sn and d relationship is practically the same for allvalues of sn, B, and L. In other words, the pullout bear-ing stress, sb is dependent only upon the applied normalstress, irrespective of B/L ratio. This implies that the geo-metry eŠect is minimal for the range of applied normalstress and B/L ratio considered. The sb/sn and d

relationship can be modelled by a hyperbolic functionand presented in the form:

sb

sn＝

d1

Ei/sn＋

dsbmax/sn

(7)

where Ei is the initial slope of pullout bearingstress—displacement curve and sbmax is the maximumpullout bearing stress of a single isolated transverse mem-ber. The Ei is the same as the initial modulus used in anon-linear hyperbolic soil model (Duncan et al., 1980). Itincreases with the applied normal stress. If the Ei/sn andsbmax/sn are known, the sb versus d relationship for anysn, B, and L can be drawn.

Three pullout bearing failure mechanisms have beenproposed; namely, general shear failure (Peterson andAnderson, 1980), punching shear failure (Jewell et al.,1984), and modiêd punching shear failure (Bergado etal., 1996). Using the three proposed solutions (Eqs. (1) to(4)), the comparison between the measured and thepredicted maximum bearing resistance of the single iso-lated transverse member for various dimensions (B andL) is shown in Fig. 8. It is found that the predicted valuesby the modiêd punching shear failure mechanism (Ber-gado et al., 1996) agree well with the measured ones. Assuch, the possible failure mechanism for a single isolatedtransverse member is illustrated in Fig. 9 based on themodiêd punching shear failure mechanism.

For a particular sand, the parameter sbmax/sn in thehyperbolic solution (Eq. (7)) for any B and L can be ap-proximated using the plasticity solution. This parameteris constant and equal to Ng, which is dependent only uponsoil friction angle. With the known Nq of 39.01, the Ei/sn

is approximated from curve-ˆtting and equal to 5.56mm－1.

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Fig. 8. Comparison of maximum pullout bearing resistance of a singleisolated transverse member

Fig. 9. Possible failure mechanism of a single isolated transversemember

Fig. 10. Measured and predicted Pbn and S/B relationship for 40×150mm transverse members


Pullout Resistance of the Bearing Reinforcement (nÀ1)In practice, the bearing reinforcement consists of

several transverse members placed at regular intervals.During the pullout of the bearing reinforcement, thetransverse members interfere with each other. A dimen-sionless parameter, transverse member spacing ratio, S/Bis introduced herein to investigate the in‰uence of spac-ing, S, and dimension (B and L) of transverse memberson the pullout bearing characteristics. Generally, thelarger the S/B, the higher the pullout bearing resistanceup to a certain maximum value, due to less interferenceamong transverse members.

Figure 10 shows the typical relationship between maxi-mum pullout bearing force, Pbn and transverse memberspacing ratio, S/B for 40×150 mm transverse members(n＝2 to 4) under diŠerent applied normal stresses com-pared with maximum pullout bearing force of a singleisolated transverse member (n＝1), Pb1. It is found thatwhen S/B is larger than 25, there would be no more trans-verse member interference. Thus, this ratio is referred toas free interference spacing ratio. When S/B is less than3.75, the shear surface caused by each transverse member

joins together to form a rough shear surface and only theˆrst transverse member causes bearing resistance. In thiscase, all the transverse members would act like a roughblock. As such, the maximum pullout bearing resistanceis determined from the summation of the friction on theblock sides and the bearing capacity of the ˆrst transversemember. Since the bearing capacity is more dominant,the pullout bearing resistance is close to that of a singleisolated transverse member. This S/B ratio is thus deˆnedas a rough block spacing ratio. From this ˆnding, thefailure mechanism of the bearing reinforcement is classi-ˆed into three zones, depending upon S/B ratio as shownin Fig. 10. Zone 1 is referred to as block failure whenS/BÃ3.75. Zone 2 is regarded as member interferencefailure when 3.75ºS/Bº25. Zone 3 (S/BÆ25) is in-dividual failure where soil in front of each transversemember fails individually.

The level of transverse member interference can be ex-pressed by the interference factor, F. It is deˆned as theratio of the average maximum pullout bearing force ofthe bearing reinforcement with n transverse members tothat of a single isolated transverse member.

F＝Pbn

nPb1(8)

The higher the level of transverse member interference

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Table 4. Predicted and measured pullout resistance of bearing reinforcement with 25×150 mm transverse members for n＝2 to 4

n S/B sn (kPa) Predicted Pf (kN) Predicted Pb1 (kN) F Predicted Pbn (kN) Predicted Pn (kN) Measured Pn (kN)

2 5.4 30 6.4 4.4 0.60 5.2 11.6 11.25.4 50 10.7 7.3 0.60 8.7 19.4 22.25.4 90 19.3 13.1 0.60 15.6 34.9 33.7

10.7 30 6.4 4.4 0.78 6.8 13.2 13.210.7 50 10.7 7.3 0.78 11.4 22.1 24.210.7 90 19.3 13.1 0.78 20.4 39.7 36.8

21.4 30 6.4 4.4 0.96 8.4 14.8 15.321.4 50 10.7 7.3 0.96 14.0 24.7 30.221.4 90 19.3 13.1 0.96 25.2 44.5 45.5

32.1 30 6.4 4.4 1.00 8.8 15.2 15.432.1 50 10.7 7.3 1.00 14.6 25.3 31.232.1 90 19.3 13.1 1.00 26.3 45.6 45.9

42.9 30 6.4 4.4 1.00 8.8 15.2 15.442.9 50 10.7 7.3 1.00 14.6 25.3 31.442.9 90 19.3 13.1 1.00 26.3 45.6 46.8

53.6 30 6.4 4.4 1.00 8.8 15.2 15.353.6 50 10.7 7.3 1.00 14.6 25.3 31.353.6 90 19.3 13.1 1.00 26.3 45.6 46.5

3 5.4 30 6.4 4.4 0.46 6.0 12.4 11.15.4 50 10.7 7.3 0.46 10.0 20.7 21.75.4 90 19.3 13.1 0.46 18.0 37.3 37.3

10.7 30 6.4 4.4 0.70 9.2 15.6 17.210.7 50 10.7 7.3 0.70 15.3 26.1 36.910.7 90 19.3 13.1 0.70 27.6 46.9 52.3

21.4 30 6.4 4.4 0.94 12.4 18.8 19.721.4 50 10.7 7.3 0.94 20.6 31.4 40.321.4 90 19.3 13.1 0.94 37.2 56.5 60.0

32.1 30 6.4 4.4 1.00 13.1 19.6 19.532.1 50 10.7 7.3 1.00 21.9 32.6 39.432.1 90 19.3 13.1 1.00 39.4 58.7 59.5

4 5.4 30 6.4 4.4 0.39 6.8 13.3 14.75.4 50 10.7 7.3 0.39 11.4 22.1 28.05.4 90 19.3 13.1 0.39 20.5 39.8 42.8

10.7 30 6.4 4.4 0.66 11.6 18.1 16.910.7 50 10.7 7.3 0.66 19.4 30.1 35.410.7 90 19.3 13.1 0.66 34.9 54.2 56.9

21.4 30 6.4 4.4 0.94 16.4 22.8 24.621.4 50 10.7 7.3 0.94 27.4 38.1 46.621.4 90 19.3 13.1 0.94 49.2 68.5 78.0


(the lower the S/B ), the lower the Pbn, and hence the low-er the F. Based on the analysis of the test data, it is foundthat the interference factor is mainly dependent uponS/B, and n, irrespective of L and applied normal stress.Since the geometry eŠect is minimal (the soil-reinforce-ment interaction is likely plane strain condition), thelength of transverse member, L, does not play any sig-niˆcant role on the interference factor. The followingequation for interference factor is hence:

F＝a＋b ln Ø SB » (9)

where a and b are constants, depending upon n. Thesetwo constants can be obtained with the two physical con-ditions: 1) when S/B equals 3.75, the interference factor

equals 1/n since Pbn and Pb1 are the same, and 2) whenS/B equals 25, the interference factor equals unity. Thesetwo conditions establish the lower and upper values of Fat corresponding values of S/B＝3.75 and 25, respec-tively. From these two conditions, the constants a and bcan be determined by the following equations:

b＝0.527«1－1n$ (10)

a＝1－3.219b (11)

As such, a and b values are 0.152 and 0.264, －0.132and 0.351, and －0.273 and 0.395 for n＝2, 3, and 4, re-spectively. Using these a and b values for diŠerent n, themaximum pullout bearing resistance can be predicted as

224

Fig. 11. Comparison between measured and predicted maximum pull-out resistances

Fig. 12. Normalized average pullout bearing stress and pullout dis-placement relationship for 40×150 mm transverse members, n＝2and diŠerent spacing ratios


shown by the solid lines in Fig. 10. The laboratory Pb1

values (Pb1＝6.4, 12.7, and 19.7 kN for n＝2, 3, and 4,respectively) are used for this prediction. In addition,Table 4 shows an example of a prediction of maximumpullout force, Pn, of the bearing reinforcement with 25×150 mm transverse members for n＝2 to 4 placed atdiŠerent spacing ratios under the three applied normalstresses. The Pn is the summation of the maximum pull-out friction and bearing forces. In this prediction, Pb1 isapproximated from Eqs. (1) and (4). By the same way,Fig. 11 shows the comparison between the predicted andthe measured pullout force of all the studied bearing rein-forcements. It is found that the predicted and the meas-ured values are in very good agreement. This reinforcesthe applicability of the proposed equations (Eqs. (8) to(11)). These equations have been successfully used for theMSE wall design of the Department of Highways,Thailand by Geoform Co., Ltd.

Now the relationship between pullout bearing forceand displacement of the bearing reinforcement for anydimension of transverse members placed at diŠerentspacing under diŠerent applied normal stresses is investi-gated. For a particular sn, the interference factor aŠectsnot only the maximum pullout bearing resistance but alsothe mobilization of the pullout bearing resistance asdepicted in Fig. 12. It is found that normalized averagepullout bearing stress, sbn/nsn versus d relationship,where sbn/n is average pullout bearing stress of the bear-ing reinforcement with n transverse members, is identicalfor the same F. The lower the F (the lower the S/B ), thelower the strength and the initial slope of the pulloutforce versus pullout displacement relationship. From theanalysis of the test data, the relationship between the nor-malized average pullout bearing stress and displacementfor (1/n)ÃFÃ1.0 is proposed in the form:

sbn

nsn＝F

d1

Ei/sn＋

dNq

(12)

The applicability of this equation is also illustrated inFig. 12 by comparing the predicted and the measured

normalized average pullout bearing stress and displace-ment relationship. The advantage of this proposedrelationship is that the pullout bearing force versus dis-placement relationship for diŠerent normal stresses andinterference factors (S, B, n) can be rapidly assessed froma test result of the bearing reinforcement with a single iso-lated transverse member (one point test) as illustrated inthe next section.

SUGGESTED PROCEDURE OF ESTIMATINGPULLOUT CHARACTERISTICS

Horpibulsuk (2010) and Horpibulsuk et al. (2010) haveperformed a full-scale test on a bearing reinforcementstabilized earth (BRE) wall, constructed on a hard soil.They have revealed that the possible failure plane (maxi-mum tensile plane) can be approximated from the coher-ent gravity structure hypothesis, which is recommendedfor the inextensible reinforcement stabilized earth walls(Anderson et al., 1987). In practice (eŠective MSE walldesign), for a shallow depth (generally less than 3 meters)where the pullout failure criterion is governed and em-bedded length in the resistant zone is less, small diameterof a longitudinal member and short leg length, B (about25 mm), and long length, L, of transverse membersplaced at a close spacing (about 500 mm) are recommend-ed. The short B, provides a high S/B ratio and hence in-terference factor while the long L provides large bearingarea, and hence high pullout resistance in this limited em-bedded length. On the other hand, for a deeper depth,where the rupture failure criterion is governed and em-bedded length is relatively long, large diameter of a lon-gitudinal member and large dimension (large B and L) oftransverse members placed at a far spacing (about 800 to1000 mm) are recommended. It is recommended to con-sider the pullout resistance mobilized only due to axialpullout and ignore the contribution of transverse defor-mation of the reinforcement. A suggested procedure forestimating pullout characteristics of the bearing reinfor-cement for any interference factor based on a one point

225

Fig. 13. Predicted and measured pullout bearing force and displace-ment relationship for 40×150 mm transverse members, n＝4, andS＝600 mm

Fig. 14. Predicted and measured pullout force and displacementrelationship for 40×150 mm transverse members, n＝4, and S＝600 mm


test on the bearing reinforcement with a single isolatedtransverse member (n＝1) is proposed as follows:

1. Perform a large direct shear test on the backˆllmaterial to determine shear strength parameters andthen determine Nq using the plasticity solutionbased on the modiêd punching shear failuremechanism (Eq. (4)).

2. Determine d, which can be directly obtained from apullout test on a longitudinal member or approxi-mated from d/q＝1.47.

3. Determine sb and d relationship and sbmax of a singleisolated transverse member, by conducting a pull-out test on the bearing reinforcement with a singleisolated transverse member under diŠerent normalstresses. It should be more or less the same as thatcalculated from the plasticity solution (Eq. (1)).

4. From sb and d relationship and sbmax (obtainedfrom step 3), determine Ei/sn which is practicallythe same for any other dimension (B and L) oftransverse member.

5. Determine the interference factor, F, of the re-quired transverse members (required n, S, B, and L)using Eqs. (9) to (11).

6. Determine Pbmax, and the pullout bearing force ver-sus displacement relationship for the required trans-verse members using Eq. (12).

7. Assuming that the maximum pullout friction forceoccurs at 3.0 mm and the linear-perfect plasticmodel is valid for pullout friction characteristics,determine ks using Eq. (6) and hence the pulloutfriction force versus displacement relationship forthe required longitudinal member.

8. From the pullout bearing force versus displacementand the pullout friction force versus displacementrelationships obtained from steps (6) and (7), re-spectively, determine the pullout force versus dis-placement relationship and the maximum pulloutforce, Pn of the required bearing reinforcement.

Figures 13 and 14 show the predicted and the measuredpullout test results of the bearing reinforcement with four(n＝4) 40×150 mm transverse members placed at spacing(S ) of 600 mm. In the prediction of pullout bearing forceversus displacement relationship (vide Fig. 13), Nq andEi/sn are taken as 39.01 and 5.56 mm－1, respectively.Taking df＝3.0 mm and d＝58.7 degrees, ks at diŠerentnormal stresses can be obtained from Eq. (6). Hence, thepullout force versus displacement relationship can bedrawn (vide Fig. 14). These two ˆgures reinforce theapplicability of the suggested method. The proposedmethod is conservative since in reality, the reinforce-ments are subjected to transverse displacement and ob-lique pull due to the deformation of the backˆll (Shew-bridge and Sitar, 1989; Leschisky and Reinschmidt, 1985;Athanasapoulos, 1993; Bergado et al., 2000; Madhav andUmashankar, 2003; Kumar and Madhav, 2009). Conse-quently, the êld pullout resistance is higher than thelaboratory one. This suggested method is on sound prin-ciples and possibly applied for the other MSE backˆlls inaccordance with the speciˆcation of the Department of

Highways, Thailand.

CONCLUSIONS

This paper deals with the introduction of a new cost-eŠective reinforcement, designated as ``Bearing Reinfor-cement'' and the development of a rational method ofpredicting its maximum pullout resistance and pulloutforce versus displacement relationship. The conclusionscan be drawn as follows:

1. From this study, the bearing reinforcement ispresented as an alternative reinforcement for MSEwall. In Thailand, it has been used for constructingseveral MSE walls under the supervision of theDepartment of Highways. The maximum pulloutbearing resistance of the bearing reinforcement witha single isolated transverse member (n＝1) can beapproximated by the plasticity solution based on themodiêd punching shear failure mechanism.

2. The transverse member interference zones are clas-siêd into three zones. Zone 1 is block failure whereall transverse members act like a rough block. Zone


2 (3.75ºS/Bº25) is member interference failure.Zone 3 is individual failure. In this zone, all trans-verse members individually mobilize their bearingcapacity (free transverse member interference).

3. The transverse member interference can be ex-pressed by the interference factor, F, in terms ofS/B and two constants, a and b, depending upon n.The higher the S/B, the lower the F, and hence thelower the pullout bearing resistance. The maximumpullout bearing force of the bearing reinforcementcan be approximated using F and the plasticity solu-tion.

4. The transverse member interference aŠects not onlythe pullout bearing resistance but also the pulloutbearing force mobilization. The lower the F (thelower the S/B ), the slower the pullout bearing forcemobilization (the lower the stiŠness of pullout bear-ing force versus displacement relationship). Theproposed normalized pullout bearing stress equa-tion in terms of F and d is useful for rapidly assess-ing the pullout bearing force and displacementrelationship for various levels of transverse memberinterference.

5. The suggested procedure for estimating pulloutcharacteristics (maximum pullout force and pulloutforce versus displacement relationship) of the bear-ing resistance for any interference factor based on aone point test is introduced. It is useful for the ex-amination of the internal stability of the MSE walls.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the ˆnancialsupport provided by the Thailand Research Fund (TRF),the O‹ce of Small and Medium Enterprises Promotion(OSMEP), and Geoform Co., Ltd. under the contractIUG508008. Science and Technology Research Grant bythe Thailand Toray Science Foundation (TTSF) is alsoappreciated. The authors are grateful to Suranaree Uni-versity of Technology for the facilities and equipmentsprovided.

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