A Laboratory Device to Test the Pull-Out

Geotechnical Testing Journal, Vol. 28, No. 5Paper ID GTJ12212

Available online at: www.astm.org

Lok-Man Chu1 and Jian-Hua Yin2

A Laboratory Device to Test the Pull-OutBehavior of Soil Nails

ABSTRACT: Soil nailing is an in situ soil reinforcing technique adopted for stabilizing existing slopes in Hong Kong and in many other countriesand regions. For the design of a soil nail system, pull-out capacity of a soil nail is an important design parameter. Field pull-out tests are then carriedout to verify the pull-out resistance assumed at the design stage. However, the pull-out capacity of a soil nail in the field is influenced by a numberof factors, such as variation in the soil properties, variation in the installation procedures, types of soil nail, and stress levels. Thus, field testing hasa number of limitations. To overcome the limitations, a new laboratory pull-out testing apparatus was developed to investigate the interface shearstrength behavior of the soil nails and surrounding soil. A numerical analysis was carried out to assess the initial stress conditions in the laboratorypull-out test box. A series of laboratory pull-out tests was performed with a cement grouted nail in a Completely Decomposed Granite (CDG) soil.It is aimed at studying the influence of overburden pressure, soil degree of saturation, and surface roughness of soil nail on the interface shearstrength. The results indicate that the curves for pull-out tests exhibit a significant peak and post-peak shear strength behavior. And the interfaceshear strength between the nail and soil is shown to be dependent on the normal stress, the soil degree of saturation, and the surface roughness ofthe nail. A correlation of the interface shear strength with major influencing factors is then derived for practice design applications. The findings inthis study may be a useful reference for geotechnical engineering in soil nailing design and constructions and for better understanding of the soilnail pull-out performance in the soil.

KEYWORDS: soil nailing, interface shear strength, degree of saturation, surface roughness, slope

Introduction

Soil nailing, defined as the inclusion of a slender unstressed re-inforcement into an in situ soil mass, has been widely used inslope stabilization in Hong Kong and other places during the pasttwo decades due to its technical and economical advantages overconventional methods such as mechanically stabilized earth walls,sheet piles, and steep slopes. One of the most important parametersgoverning the design and safety assessment of soil nailing is theultimate shear strength at the interface between soil nails and thesurrounding soil. However, the interface shear strength of a soilnail is depending on a number of influencing factors, such as, soilconditions (dry, wet), soil nail surface (roughness), and stress con-ditions, etc. In current practice, this parameter is estimated from anassumed skin friction on the interface between the soil nail and soil,and is then verified by field pull-out tests during the constructionstage (Powell and Watkins 1990). However, the field pull-out testshave a number of limitations in the following two areas:

1. Uncertain field testing conditions:a. Vertical and lateral stresses are normally calculated using

the overburden soil pressures (or stresses). This calcula-tion is questionable since the vertical (or normal) stress onthe soil nail is normally not equal to the overburden soilpressure for a sloping ground.

Received September 23, 2003; Accepted for publication January 18, 2005;published September 2005.

1 Department of Civil and Structural Engineering, The Hong Kong Polytech-nic University, Hun Hom, Kowloon, Hong Kong, China.

2 Department of Civil and Structural Engineering, The Hong KongPolytechnic University, Hung Hom, Kowloon, Hong Kong, China. Email:[email protected]

b. After drill hole is formed in the soil slope, the stresses inthe hole are zero (or released). Cement grouting a rebarin the center of the hole is normally done under gravity(no pressure grouting in Hong Kong). The contact stressbetween a cement grout and the surrounding soil is verysmall and is unknown.

c. The curing time of the cement grout is normally sevendays in Hong Kong before field pull-out testing. The ce-ment grout might not have enough strength before pull-outtesting. In other words, a failure between the cement groutand the rebar might occur, not achieving the goal of testingthe interface between the cement grout and the surroundingsoil.

2. Nonworst testing condition for verifying soil nail pull-outperformance:The field pull-out testing is normally carried out in the no-rain weather condition. The soil is normally unsaturated orin a natural wet condition. However, the worst condition forthe soil nail is a fully or near-fully water saturated condition.Thus, the results of the field pull-out tests may not be veryuseful for verifying the soil nail pull-out performance in theworst condition.

Due to the above uncertainties, data from field pull-out tests arevery scattered and very difficult to understand. Laboratory pull-out tests can overcome the above limitations. Recently, researchershave tried to investigate the interface shear strength behavior ofsoil nail in laboratory pull-out model tests. Schlosser and Guilloux(1981), Cartier and Gigan (1983), Heymann et al. (1992), Heymann(1993), Berglund et al. (1996), and Franzen (1998) performed aseries of field pull-out tests by using different types of driven nails

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2 GEOTECHNICAL TESTING JOURNAL

and grouted nails. The scatter in the testing results was reportedand was due to influence factors in the field. Recently, Chang andMilligan (1996), Milligan and Tei (1998), Franzen (1998), and Leeet al. (2001) conducted laboratory pull-out tests of steel bars andrubber tubes in yellow Leighton Buzzard Sand, Baskarp Sand, anda loose CDG soil.

Soil nailing has been widely used to stabilize substandard soilslopes in Hong Kong. The most frequently encountered soil isa CDG soil. So far, pull-out testing study on this CDG soil hasbeen very limited. To carry out the soil nail stabilized slope de-sign, it is necessary to understand the interface shear behaviorbetween the grouted nail and the CDG soil and to know the designparameters. The study on a cement grouted nail in CDG soil islimited. This paper is to examine a number of factors that affectthe soil nail performance and to find quantitative interface param-eters. Laboratory pull-out tests of grouted nail in CDG soil havebeen conducted to investigate the shear strength behavior of thesoil nails and surrounding soil. The following influences on theinterface shear strength are investigated in the laboratory pull-outtests:

1. Influence of different applied stress levels of 50, 100, and300 kPa;

2. Influence of soil degree of saturation on pull-out shearstrength;

3. Influence of surface roughness of the soil nails with a reg-ular or irregular nail surface. From the experimental results,empirical equations are established to correlate the interfaceshear strength with the major influencing factors.

Laboratory Pull-Out Testing Apparatus and Procedure

Cross-sectional views of the laboratory pull-out test box areshown in Figs. 1a and 1b. Photos of the laboratory pull-out testingare shown in Fig. 1c. The pull-out test box has internal dimensionsof 700 mm long by 560 mm wide by 605 mm high. Dimensionsof the box were chosen in order to compromise between the mini-mal effects of the boundaries and the difficulty of preparing a hugevolume of soil sample. The pull-out test box was built up by 5-mmrolled steel plates and 38 mm × 38 mm steel angle bars usingwelded and bolted connections. In addition, an external steel reac-tion frame was constructed outside the steel box in order to carry thesoil compacting force, constrain the lateral soil pressure, and reactthe pull-out load. A steel plate with a 140-mm-diameter openinghole is attached to the front side of the box for the cement-groutednail to be pulled out (see Fig. 1).

The pull-out box was initially assembled and restrained by screwsuniformly fixed against the external reaction steel frame. Frictionbetween the soil and the sidewalls of the box was minimized bylubricating oil. The soil was then placed in the six layers and com-pacted to achieve a degree of compaction of 95 %. With respectto CDG soil, the degree of compaction was determined accord-ing to ASTM Test Method for Moisture-Density Relations of Soilsand Soil-Aggregate Mixtures Using 2.49 kg Rammer and 305 mmDrop (D698) and ASTM Test Method for Moisture Content Pen-etration Resistance Relationships of Fine-Grained Soils (D1558).A 100-mm-diameter hole was predrilled into the compacted soilby a rotary cutting tool with diamond bits. There is no water sup-ply in the drilling. The application of the machine is according toASTM Test Method for Obtaining and Testing Drilled Cores andSawed Beams of Concrete (C42). The lengths of the soil nails inthe tests are 500 mm and 700 mm for regular surface nails and ir-

regular surface nails, respectively. The nails with irregular surfacewere created by a diamond steel plate scratching on the drillholesurface in the drilling process (see Fig. 2). A 32-mm-diameterribbed steel bar was centralized in the drillhole horizontally anda cement paste with a water-cement ratio of 0.45 was continu-ously grouted over the drillhole by gravity flow. The grouted nailwas cured inside the soil for 28 days to achieve matured cementstrength.

For pull-out test in a water submerged condition, two 5-mm-diameter rubber pipes were connected to the bottom of the pull-outtest box. Once the grouted nail was set at seven days after ce-ment grout, carbon dioxide (CO2) was pumped through the soilsample via the rubber pipes. When the de-air operation was com-pleted, de-aired water was then pumped into the box at a slow rateand under a low water pressure. The degree of saturation of thesoil inside the pull-out test box was measured after the pull-outtest.

After 28 days of curing, the desired normal pressure was ap-plied through the pressurized water rubber diagram bag positionedbetween the topsoil surface and the top reaction plate against nor-mal load reaction frame. The rubber diagram bag was filled withwater and was used to produce a uniformly distributed pressureon the top surface of the compacted CDG soil. The top flexibleboundary is preferred for the purpose of measuring dilatancy ofthe soil-cement grout interface and slightly decreasing in frictioncoefficient (Palmeira and Milligan 1989). A pull-out test was con-ducted at a displacement rate of 0.5 mm/min. The pull-out forcewas applied by a 100 kN capacity hydraulic jack through a pull-out load reaction frame mounted in front of the external reactionframe (see Fig. 1). The pull-out load was measured by a load celllocated between the pull-out load reaction frame and the hydraulicjack. Pull-out displacements were measured by two linear variabledifferential transducers (LVDTs) pointed at the nail head. Theseinstruments were linked to a data logger that took and recordeddata at desired time intervals.

Properties of the Soil and Cement Grout Specimens

Completely Decomposed Granite Soil

The soil used in the laboratory pull-out tests was a CDG, whichwas excavated from a cut-slope in Highland Park at Kwai Chung,Hong Kong. The grain size distribution of the soil was determinedby wet sieving and hydrometer tests as recommended in ASTMTesting Method for Wet Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil Constants (D2217) andASTM Testing Method for Particle-Size Analysis of Soils (D422),respectively. The grain size distribution of the CDG soil is shownin Fig. 2. The soil has 28.22 % of gravel, 37.19 % of sand, 19.59 %of silt, and 15.00 % of clay from the sieving test. According toASTM Testing Method for Description and Identification of Soils(D2488), the soil would be classified as yellowish brown, well-graded clayey silty gravelly sand, SM. The specific gravity Gsofthe soil is 2.64, and the permeability of the soil k is 4.54 ×10−5 m/s. The maximum dry density ρd max of the soil from astandard compaction test is 1.70 Mg/m3 and the correspondingoptimum moisture content mopt is 19.00 %. The shear strength pa-rameters (c′, φ′) of the CDG soil obtained from large size directshear tests and conventional consolidated undrained (CU) triax-ial tests are summarized in Table 1. It is noted that the speci-mens in CU triaxial tests were saturated with back pressure up to300 kPa.

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CHU AND YIN ON SOIL NAILS 3

FIG. 1—Views of a pull-out test apparatus: (a) longitudinal section, (b) cross section, and (c) photos.



0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100 mm

Per

cent

age

Fin

er

CLAYFINE

SILT

MEDIUM COARSE COARSEFINE MEDIUM

SAND

MEDIUMFINE

GRAVEL

COARSE

ST

ON

E

FIG. 2—Particle size distribution of a Completely Decomposed Granite (CDG) soil.

Cement Grouted Nail

Cement grout used for the tests was prepared from a cementpaste mixed with a water-cement ratio w/c of 0.45. The densityof cement grout is found to be 2.0 Mg/m3. The average uniaxialcompressive strength σc for 28 days cement grout samples is56.81 MPa. The tangent Young’s modulus E50 and the correspond-ing Poisson’s ratio νn is 16.37 GPa and 0.28, respectively.

FLAC-Analysis of Stress on a Soil Nail

Numerical Simulation and Modeling

A two-dimensional explicit finite difference program, FLAC(1996), was used to investigate the initial stresses around a 100-mm-diameter of soil nail inside the laboratory pull-out test box(Fig. 1). The three-dimensional (3-D) problem was simplified as atwo-dimensional (2-D) axisymmetrical problem, which consideredthe pull-out test box as a cylindrical mass with the soil nail axisas the center axis of the cylindrical mass. Figure 3 shows the 2-Daxisymmetrical FLAC model. An elastic-perfectly plastic consti-tutive model with the Mohr-Coulomb failure criterion was usedfor the soil sample and an elastic model for the soil nail in theanalysis. The properties of the soil are based on results of the con-solidated undrained triaxial tests in Table 1, and the properties ofthe soil nail are based on the laboratory testing results on the cementgrout.

L

50 mm

C

500 mm 200 mm

250 mm

x

yApplied normal stress

Gravity and stresses direction

Soil nail for 500 mm

Soil nail for 700 mm

Soil material

FIG. 3—Dimensions of a numerical model in FLAC.

In the numerical model, two soil nail lengths of 500 mm and700 mm inside the pull-out box were examined. The details ofmodeling and the boundary conditions are presented in Fig. 3. Thesize of the domain was 0.7 m × 0.3 m with grid size of 0.01 min vertical and 0.01 m in horizontal direction. The vertical sidesin the model were constrained in horizontal direction and the hor-izontal size in the bottom of the model is the axisymmetric linewith only vertical displacement constrained. The top is a pres-sure boundary with an initial pressure of zero. This pressure isincreased to 300 kPa in the FLAC modeling after the gravity isapplied.

TABLE 1—Values of shear strength parameters from the direct shear tests and consolidated undrained triaxial tests.

Natural Wet Condition Submerged Condition

Testing Methods c′, kPa φ′, degree c′, kPa φ′, degree E50, Mpa νs

Direct shear box tests Peak shear strength 45.77 30.43 26.75 24.46 · · · · · ·Shear strength at displacement 38.24 30.17 23.42 25.01 · · · · · ·

of 70 mmConsolidated undrained Critical-state shear strength · · · · · · 0.00 31.20 14.00 0.49

triaxial tests



The soil mass was initially simulated for gravity stresses, and thedisplacements due to gravity were initialized to zero. A sufficientnumber of steps were used to achieve an equilibrium state condi-tion by assessing the equilibrium ratio. Drilling of a nail hole wassimulated with the FLAC model null material zone, and a modelelastic material was filled into the null zone to simulate the soilnail material inclusion. After the soil nail installation, several stepswere carried out to reach an equilibrium state condition again. Thenormal stress σn,top on the top surface of the box was simulated byapplying a pressure of 300 kPa on the top boundary. The stressesinside the soil mass and on the soil nail were calculated using theFLAC model.

Numerical Analysis Results

Two soil nail lengths of 500 mm and 700 mm were consideredin the FLAC model. Figures 4a and 4b show the normal (or radial)stress contours for a 500-mm-long nail and 700-mm-long nail. Itcan be seen from Fig. 4a that the normal stress σ′

n,nail on the 500-mm-long soil nail is nearly uniform for 90 % length of the nail. Butthere is stress concentration at the nail toe. From Fig. 4b, the normalstress σ′

n,nail on the 700-mm-long soil is nearly uniform for the fulllength of the nail. Calculated normal stresses for both 500-mm and700-mm-long nails inside the box under the applied normal stress300 kPa are given in Fig. 5. It is seen from Fig. 5 that a more uniformstress is obtained for the 700-mm-long nail, but the normal stresson the 500-mm-long nail varies more. The average normal stressacting along the nail for 500 mm and 700 mm long is the same of

L

50 mm

301 kPa

306 kPa 305 kPa

304 kPa

303 kPa

302 kPa

303 kPa

304 kPa

305 kPa

C

500 mm 200 mm

Soil Nail

250 mm

x

y (a)

L

50 mm

C

700 mm

250 mm

x

y

Soil Nail

301 kPa

306kPa

303 kPa

302 kPa

304 kPa

305 kPa

(b)

FIG. 4—Contour of vertical stress inside the laboratory pull-out testbox: (a) 500-mm-long soil nail and (b) 700-mm-long soil nail.

300

302

304

306

308

310

0 100 200 300 400 500 600 700

Distance from the nail head (mm)

Cal

cula

ted

norm

al s

tres

s (k

Pa)

700 mm long soil nail

500 mm long soil nail

FIG. 5—The vertical stresses against distance from the nail head (x-direction in Fig. 3) inside the pull-out test box from FLAC modeling.

305.9 kPa. It is found that the average normal stress σ′n,nail acting

along the nail (500 mm or 700 mm long) from the FLAC model isshown equal to the applied overburden pressure σ′

v0 at the midlevelof the nail plus the applied stress σn,top on the soil top surface, thatis, σ′

n,nail = σ′v0 + σn,top = 20 × 0.3 + 300 = 306 kPa. The average

unit weight of the CDG soil is 20 kN/m3. In this paper, overburdenstress σ′

v0 at the middle nail level is considered to evaluate thenormal stress acting on the soil nail inside the laboratory pull-outtest box.

Pull-out Capacity of a Soil Nail

The average pull-out shear stress τs (kPa) of a soil nail can becalculated from a measured pull-out load F (kN) divided by theactive nail surface area A (m2):

τs = F

A= F

(πD)Ls

(1)

The active nail surface area A is calculated by multiplying theembedded length Ls (m) of the soil nail in contact with the sur-rounding soil with the perimeter of the pulled-out nail πD(m).

In common practice, people use the following equation for cal-culation of a soil nail pull-out capacity per lineal meter T (kN/m)(Wong 1995):

T = (πDc′ + 2Dσ′v tan φ′) (2)

where D is the diameter of the soil nail, c′ is the soil cohesion, andφ′ is the internal angle of the soil friction. It is noted that T = F/Ls .This equation assumes that the interface strength is the same as thatof the soil. This might not be true.

It is noted in Eq 2 that πD is the perimeter of the nail and πDc′is the adhesion resistance per nail length (kPa/m) due to interfaceadhesion. For 2Dσ′

v tan φ′ in Eq 2, the nail is considered to be aplate of zero thickness with width D. The vertical stress on twosides of the plate is σ′

v . Thus, the total frictional resistance per naillength (kPa/m) due to the interface friction is 2Dσ′

v tan φ′. Thisapproach gets rid of the uncertainty in calculation of the lateralstress (or normal stress) on the nail.

In this paper, the following equation is proposed for the eval-uation of the pull-out capacity per meter T (kN/m) of a soil



nail, considering the interface shear strength between the nail andsoil:

T = (πDc′a + 2Dσ′

v tan δ′) (3)

where c′a is the soil adhesion at the interface, and δ′′is the angle

of the interface friction. Equation 3 has the same form as Eq 2 foreasy practical applications.

From Eq 1 and Eq 3, the average shear strength is

τs = F

A= T

πD= πDc′

a + 2Dσ′v tan δ′′

πD

= c′a + σ′

v

(2

πtan δ′

)= c′

a + σ′v tan δ′ (4)

where we introduce an interface friction angle δ′, with a relation-ship of

tan δ′′ = π

2tan δ′ (5)

Equation 4 can be used to fit the shear strength data points tofind the parameters of c′

a and δ′. Use the relationship of tan δ′′ =π2 tan δ′, δ′′ can be found.

Experimental Results and Analysis

Shear Stress-Displacement Behavior of Soil Nail Pull-Out Tests

Figure 6 shows soil nail pull-out test results for a regular soil nailsurface in a natural wet condition. Figure 7 shows a comparison ofresults from soil nail pull-out tests for a regular soil nail surface andfrom soil-soil direct shear tests on the same soil used in the pull-outtests. In the natural wet condition, the moisture contents of the soilbefore and after the direct shear tests and pull-out tests are about19 % (nearly to the optimum moisture content of the soil), and thedegree of saturation of the soil in the pull-out tests is approximately70 %.

For the pull-out shear stress-displacement curves, the initial pull-out shear stress increases linearly with pull-out displacement, andthen the pull-out shear stress continues to increase with the dis-placement, but at a decreasing rate until the maximum shear strengthreaches at the peak. After the peak shear strength, the pull-out shearstress is observed to decrease gradually. A smaller pull-out displace-ment is required to mobilize the peak pull-out shear strength. This isconsistent with observations from Schlosser (1990), Bergado et al.(1992), Milligan and Tei (1998), and Lee et al. (2001). The curvesfor pull-out tests exhibit an obvious peak and post-peak shear stressbehavior, whereas the curves for soil-soil direct shear box testsexhibit a relatively small peak and high post-peak shear strength.This might be due to different confinements in the soil-soil direct

0

50

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200

250

300

0 50 100 150 200

Displacement (mm)

Pul

l-out

she

ar s

tres

s (k

Pa)

σn = 55 kPa σn = 105 kPa σn = 305 kPa (a)

-0.10

0.00

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0.50

0 50 100 150 200

Horizontal displacement (mm)

Ver

tical

dis

plac

emen

t (m

m)

σn = 55 kPa σn = 105 kPa σn = 305 kPa

"-" for dilation"+" for compression

(b)

FIG. 6—Pull-out test results of regular nails in natural wet condition: (a) pull-out shear strength versus displacement and (b) vertical displacementversus horizontal displacement.



0

50

100

150

200

250

300

0 50 100 150 200

Displacement (mm)

Pul

l-out

she

ar s

tres

s (k

Pa)

σn = 50 kPa σn = 100 kPa σn = 300 kPaσn = 55 kPa σn = 105 kPa σn = 305 kPa

Soil-soil direct shear tests

Regular nail pull-out tests

FIG. 7—Comparison of shear stress versus horizontal displacement curves between the direct shear tests and regular nail surface pull-out tests.

Regular nail pull-out tests:y = 0.5652x + 63.458

Soil-soil direct shear tests:

y = 0.5874x + 45.766

0

50

100

150

200

250

300

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Peak strength - Regular nail pull-out testsPeak strength - Soil-soil direct shear testsLinear (Peak strength - Soil-soil direct shear tests)Linear (Peak strength - Regular nail pull-out tests)

Peak strength - Soil-soil direct shear tests:

φ'= 30.4o, c'= 45.8kPaPeak strength - Regular nail pull-out tests:

δ'= 29.5o, ca'= 63.5kPa

(a)

Soil-soil direct shear tests:y = 0.5813x + 38.241

Regular nail pull-out tests:y = 0.492x + 32.358

0

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100

150

200

250

300

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Strength at disp. of 70mm - Regular nail pull-out testsStrength at disp. of 70mm - Soil-soil direct shear testsLinear (Strength at disp. of 70mm - Soil-soil direct shear tests)Linear (Strength at disp. of 70mm - Regular nail pull-out tests)

Strength at disp. of 70mm - Soil-soil direct shear tests:

φ'= 30.2o, c'= 38.2kPaStrength at disp. of 70mm- Regular nail pull-out tests:

δ'= 26.2o, ca'= 32.4kPa

(b)

FIG. 8—Comparison of shear strength parameters between the direct shear box tests and pull-out tests: (a) peak shear strength and (b) shear strengthat shear displacement of 70 mm.



testing and in the pull-out testing. Direct shear box wouldhave a higher restriction on soil particles compared to the pull-outbox. In addition, the cavity arching effect of the drillhole in thepull-out box might contribute to the lower post-peak shear strengthin the pull-out test results. An empty space in the section of thedrillhole and the nail is produced in the pull-out process. Therestriction of soil particle rearrangement is smaller at the shearinginterface in the pull-out tests as the stresses near the empty areaare reduced.

Effects of Normal Stress on Pull-Out Test Results

The effects of the normal stress on the soil nail pull-out shearstrength can be studied in the laboratory pull-out test results. Theshear stress-normal stress envelopes for the pull-out tests and thesoil-soil direct shear tests are shown in Fig. 8. The results for bothtests are shown to follow the Mohr-Coulomb failure criterion: shearstress is increased with increasing the applied normal stress. Thenonlinearity of the failure envelope in the pull-out test results wasexplained by Bishop (1966) and Bolton (1986), and is attributedto soil dilatancy and considered to be due to the crushing of soil

particles during shearing. The effect of an increase in normal stressδσ′

n around a soil nail can be indicated in an apparent coefficient offriction µ∗. The apparent friction coefficient µ∗ is defined by

µ∗ = τs

σ′v

(6)

The apparent coefficient of friction µ∗ is a direct indication of theshear strength τs over the vertical effective stress σ′

v . µ∗ may indi-cate the restrained dilatancy. The restriction of dilatancy increaseswith the confining stress locally around the nail, resulting in ahigher pull-out shear strength. Figure 9 presents the relationship ofthe apparent coefficient of friction with the applied normal stress forthe laboratory pull-out tests. The apparent coefficient of friction ishigher at lower normal stress and smaller for higher normal stress.The behavior of the laboratory results for the “peak pull-out shearstrength” in Fig. 9a is generally above the curve of test results ofribbed strips in Schlosser and Guilloux (1981).

The apparent coefficient of friction at a pull-out displacement of200 mm is plotted against the normal stress in Fig. 9b. A nearlyconstant apparent friction coefficient is observed in the results andthe values are close to the limit values for smooth strips as suggested

0.00

0.50

1.00

1.50

2.00

0 50 100 150 200 250 300 350

Normal stress (kPa)

App

aren

t coe

ffici

ent o

f fric

tion,

µ *

Peak apparent coefficient of friction(a)

Ribbed strip (Schlosser and Guilloux 1981)

Smooth strip(Schlosser and

Guilloux 1981)

µ∗ = 0.4

µ∗ = τ'/σn'=0.788

0.00

0.50

1.00

1.50

2.00

0 50 100 150 200 250 300 350Normal stress (kPa)

App

aren

t coe

ffici

ent o

f fric

tion,

µ *

'Strength at disp. of 200 mm' apparent coefficient of friction(b)

Smooth strip (Schlosser and Guilloux 1981)

Ribbed strip (Schlosser and Guilloux 1981)

µ∗ = τ'/σn' = 0.749

µ∗ = 0.4

FIG. 9—Apparent coefficient of friction for the pull-out tests in a natural wet condition: (a) peak strength and (b) strength at shear displacement of200 mm.



TABLE 2—Values of apparent coefficient of friction µ∗ from the laboratory pull-out tests.

Apparent Coefficient of Friction, µ∗

Natural Wet Soil Condition (Sr = 70%) Submerged Soil Condition (Sr = 86%)

σ′v = 55 kPa σ′

v = 105 kPa σ′v = 305 kPa σ′

v = 52 kPa σ′v = 102 kPa σ′

v = 302 kPa

Peak shear strength Pull-out tests (regular nail surface) 1.52 1.30 0.76 0.88 0.68 0.36Pull-out tests (irregular nail surface) 1.61 1.24 0.82 0.84 0.63 0.39

Shear strength Pull-out tests (regular nail surface) 0.62 0.57 0.49 0.24 0.22 0.11at displacement Pull-out tests (irregular nail surface) 0.63 0.54 0.52 0.40 0.24 0.17of 200 mm

0

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100

150

200

250

300

0 50 100 150 200

Displacement (mm)

Pul

l-out

she

ar s

tres

s (k

Pa)

Sr=70% Sr=74% Sr=78% Sr=86% (a)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 50 100 150 200


Ver

tical

dis

plac

emen

t (m

m)

Sr=70% Sr=74% Sr=78% Sr=86%


(b)

FIG. 10—Pull-out test results of regular nails in the degree of saturation, Sr of 70 %, 74 %, 78 %, and 86 % under the normal stress of 300 kPa:(a) pull-out shear strength versus displacement and (b) vertical displacement versus horizontal displacement.

by Schlosser and Guilloux (1981), indicating that the restraineddilatancy of the nail no longer exists and the nail behaves as asmooth inclusion in a larger pull-out displacement. The values ofapparent coefficient of friction obtained from the laboratory pull-out tests are given in Table 2.

Effects of Soil Degree of Saturation on Pull-Out Shear Strength

Figure 10 shows the pull-out test results for a regular surface ofsoil nails for different values of degree of saturation Sr of 70 %,

74 %, 78 %, and 86 % under the normal stress of 300 kPa.Table 3 presents the results of the laboratory pull-out tests of reg-ular surface nails in a natural wet condition (Sr = 70 %) and in asubmerged condition (Sr = 86 %). The pull-out shear strength isshown significantly dependent on the degree of saturation of thesoil. The peak angle of interface friction δ′ is reduced from 29.5◦in the natural wet condition (Sr = 70 %) to 13.6◦ in the water sub-merged condition (Sr = 86 %), while the “shear strength at sheardisplacement of 200 mm” angle of interface friction δ′ is reducedfrom 24.6◦ in the natural wet condition (Sr = 70 %) to 4.3◦ in



TABLE 3—Values of shear strength parameters and coefficients fa and fδ from the laboratory pull-out tests.

Submerged Soil ConditionNatural Wet Soil Condition (Sr = 70%) (Sr = 86%)

c′a , kPa δ′, degree fa fδ c′

a , kPa δ′, degree

Peak shear strength Pull-out tests (regular nail surface) 63.46 29.48 1.39 0.97 38.52 13.56Pull-out tests (irregular nail surface) 58.21 32.21 1.27 1.06 30.90 16.30

Shear strength at Pull-out tests (regular nail surface) 32.36 26.20 0.85 0.87 24.76 7.72displacement of 70 mm Pull-out tests (irregular nail surface) 36.82 25.27 0.96 0.84 25.49 12.29

Shear strength at Pull-out tests (regular nail surface) 10.28 24.60 11.32 4.28displacement of 200 mm Pull-out tests (irregular nail surface) 6.08 26.40 12.35 7.24

y = 3E+08x-3.3203

R2 = 0.8119

y = 1E+15x-7.0788

R2 = 0.9431

0

50

100

150

200

250

300

65 70 75 80 85 90

Degree of saturation, Sr (%)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Peak pull-out shear strength

Pull-out shear strength at disp. of 200mm

FIG. 11—Correlation between pull-out shear strength and the degree of saturation, Sr of 70 %, 74 %, 78 %, and 86 % under the normal stressof 300 kPa.

the water submerged condition (Sr = 86 %). A comparison of thepull-out test results with the different degrees of saturation, Sr,

of 70 %, 74 %, 78 %, and 86 % under the applied normal stressof 300 kPa is shown in Fig. 11. The main trend for the “peakpull-out shear strength” and the “pull-out shear strength at dis-placement of 200 mm” of pull-out tests is a nonlinear behavior andthe pull-out shear strength decreases as the degree of saturation Sr

increases.For estimating the pull-out shear strength, τs,300, for the soil

degree of saturation in the range of 70–86 % under the appliednormal stress of 300 kPa in the design of a soil nail system, thefollowing empirical equations were derived:

τs,300 = 3 × 108 × S−3.3203r (7)

for “peak pull-out shear strength”, and

τs,300 = 1 × 1015 × S−7.0788r (8)

for “pull-out shear strength at displacement of 200 mm.” These de-sign parameters are for slopes recompacted to degree of compactionto 95 %.

Effects of Surface Roughness of the Soil Nail on Pull-OutShear Strength

Surface roughness of shearing materials is one of the importantfactors influencing the interface shear behavior (Potyondy 1961;

Kulhawy and Peterson 1979; and Uesugi et al. 1990). An irreg-ular surface of soil nails was created in a natural wet condition(see Fig. 12) and in a water submerged condition. The asperityheight of the irregular surface is about 2 mm into the soil and theasperity spacing is about 3 mm. Figure 13 shows the pull-out testresults of irregular nails in a natural wet condition and Fig. 14shows the pull-out test results of irregular nails in submerged con-dition. A comparison of the shear strength parameters betweenthe regular and irregular surface nails in the natural wet condition(Sr = 70 %) and in the submerged condition (Sr = 86 %) is givenin Table 3. Ratio of soil adhesion over soil cohesion fa and ratio

FIG. 12—An irregular drillhole surface created in a natural wet condi-tion by a rotary cutting tool with diamond bits.



0

50

100

150

200

250

300

0 50 100 150 200

Displacement (mm)

Pul

l-out

she

ar s

tren

gth

(kP

a) σn = 55 kPa σn = 105 kPa σn = 305 kPa (a)

0.00

0.20

0.40

0.60

0.80

1.00

0 50 100 150 200


Ver

tical

dis

plac

emen

t (m

m)


"-" for dilation

"+" for compression

(b)

FIG. 13—Pull-out test results of irregular nails in a natural wet condition: (a) pull-out shear strength versus displacement and (b) vertical displacementversus horizontal displacement.

of interface friction angle over soil friction angle fδ are defined asfollows:

fa = c′a

c′ (9)

fδ = δ′

φ′ (10)

Values of fa and fδ are presented in Table 3 and are used toexamine the interfacial shear behavior in the pull-out tests.

The ratiofδ can be used to indicate the interface frictional an-gle of different soil nail surface roughness referring to the fric-tion angle of the soil. For the “peak shear strength,” the ratio fδ

is increased from 0.97 for the regular (smooth) surface nails to1.06 for the irregular (more rough) surface nails. In addition, theratiofa is reduced from 1.39 for the regular surface nails to 1.27for the irregular (more rough) surface nails. It can be seen thatthere is a great advantage of the nail surface roughness in theinterface friction. On the contrary, the ratio fδ is reduced from0.87 for the regular (smooth) surface nails to 0.84 for the irregu-lar (more rough) surface and the coefficient fa is increased from

0.85 for the regular (smooth) surface nails to 0.96 for the irregular(more rough) surface nails in the “shear strength at displacementof 70 mm.” The interface surface for the irregular (more rough)surface nails shows smooth interfacial shear behavior in the largershear displacement. The lower interface friction for the irregular(more rough) surface nails in the “shear strength at displacementof 70 mm” is mainly due to the crushing and becoming looser ofthe soil particles at the interface shear surface in the large sheardisplacement.

Figure 15 shows the comparison of the pull-out shear strengthparameters between the regular (smooth) surface nails and irregu-lar (more rough) surface nails in a natural wet condition. The peakshear strength line for irregular surface of soil nails is mostly abovethe peak shear strength line for regular surface of soil nails. Theinterface friction angle of irregular surface soil nails is approxi-mately 9.3 % higher than that of regular (smooth) surface nails.However, at the lower normal stress, the interface adhesion forthe irregular (more rough) surface nails is reduced approximately8.3 %. The shear strength line at displacement 200 mm for irregularsurface of soil nails is all above the line for regular surface of soilnails.



0

50

100

150

0 50 100 150 200

Displacement (mm)

Pul

l-out

she

ar s

tren

gth

(kP

a)


(a)

0.00

2.00

4.00

6.00

8.00

0 50 100 150 200


Ver

tical

dis

plac

emen

t (m

m)



(b)

FIG. 14—Pull-out test results of irregular nails in a submerged condition: (a) pull-out shear strength versus displacement and (b) vertical displacementversus horizontal displacement.

A comparison of the pull-out shear strength parameters betweenthe regular (smooth) surface nails and irregular (more rough) sur-face nails in a submerged condition is shown in Fig. 16. The peakshear strength line for irregular surface of soil nails is above thepeak shear strength line for regular surface of soil nails at highernormal stress, but smaller at lower normal stress. The shear strengthline at displacement 200 mm for irregular surface of soil nails is allabove the line for regular surface of soil nails. All above data indi-cate that irregular (more rough) surface nails have higher interfaceshear strength than that of regular (smooth) surface nails at largerdisplacements.

Summary of Observations

On the basis of the testing results, discussions, and evalua-tions presented above, a summary of observations is presented asfollows:

1. The shear stress-displacement curves of soil nail pull-outtests exhibit an obvious peak and post-peak shear stress-displacement behavior, whereas the curves for the soil-soildirect shear box tests have a relatively small peak and highpost-peak shear strength.

2. The pull-out shear strength increases with applied normalstress. A modified strength equation based on the Mohr-Coulomb failure criterion is proposed to take into accountof the interface shear strength of cement grouted soil nails.This modified interface shear strength equation is used to an-alyze data from soil nail pull-out tests. From the analysis, theinterface shear strength parameters can be obtained. The mod-ified interface shear strength equation can be used for designof soil nail stabilized slopes.

3. Values of the apparent coefficient of friction µ∗ from pull-out tests is large at a smaller normal stress and is graduallyreduced under a higher normal stress.

4. The peak apparent coefficient of friction µ∗ from pull-outtests ina natural wet condition (Sr = 70 %) is higher than the



Irregular surface nails:y = 0.6299x + 58.207 Regular surface nails:

y = 0.5652x + 63.458

0

50

100

150

200

250

300

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Peak strength - Irregular surface nailsPeak strength - Regular surface nailsLinear (Peak strength - Irregular surface nails)Linear (Peak strength - Regular surface nails)

Peak strength - Irregular surface nails:δ'= 32.2°, ca

'= 58.2kPaPeak strength - Regular surface nails:δ'= 29.5°, ca

'= 63.5kPa

(a)

Irregular surface nails:y = 0.4963x + 6.0801

Regular surface nails: y = 0.4579x + 10.277

0

50

100

150

200

250

300

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Strength at disp. of 200mm - Irregular surface nailsStrength at disp. of 200mm- Regular surface nailsLinear (Strength at disp. of 200mm - Irregular surface nails)Linear (Strength at disp. of 200mm- Regular surface nails)

Strength at disp. of 200mm - Irregular surface nails:

δ'= 26.4°, ca'= 6.1kPaStrength at disp. of 200mm- Regular surface nails:

δ'= 24.6°, ca'= 10.3 kPa

(b)

FIG. 15—Comparison of pull-out shear strength parameters between regular surface nails and irregular surface nails in a natural wet condition:(a) peak shear strength and (b) shear strength at shear displacement of 200 mm.

suggested value for ribbed strips in Schlosser and Guilloux(1981). However, the peak apparent friction coefficient µ∗ forthe pull-out tests in the submerged condition (Sr = 86 %) isslightly lower than the suggested value for ribbed strips inSchlosser and Guilloux (1981).

5. The degree of saturation of the CDG soil has a significantinfluence on the soil nail pull-out shear strength. The rela-tionship between the pull-out shear strength and the soil de-gree of saturation for the CDG soil recompacted to degree ofcompaction to 95 % is shown nonlinear, and empirical equa-tions are derived for the relationship between the pull-outshear strength and the soil degree of saturation (Sr rang-ing from 70 % to 86 %) under the applied normal stress of300 kPa.

6. The interface shear behavior of soil nails in the pull-out testscan be assessed by using the ratio of soil adhesion over soilcohesion fa and ratio of interface friction angle over soilfriction anglefδ. The peak shear strength line for irregular(more rough) surface of soil nails is above the peak shearstrength line for regular surface of soil nails at higher normalstress, but smaller at lower normal stress for both a naturalwet condition and a water submerged condition. The shearstrength line at displacement 200 mm for irregular surfaceof soil nails is all above the line for regular surface of soilnails for both a natural wet condition and a water submergedcondition. All above data indicate that irregular (more rough)surface nails have higher interface shear strength than that ofregular (smooth) surface nails at larger displacements.





0

50

100

150

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Peak strength - Irregular surface nailsPeak strength - Regular surface nailsLinear (Peak strength - Regular surface nails)Linear (Peak strength - Irregular surface nails)

Peak strength - Irregular surface nails:

δ'= 16.3o, ca'= 30.9kPaPeak strength - Regular surface nails:

δ'= 13.6o, ca'= 38.5kPa

(a)



0

50

100

150

0 100 200 300 400 500 600 700

Normal stress (kPa)

Pul

l-out

she

ar s

tren

gth

(kP

a)

Strength at disp. of 200mm - Irregular surface nailsStrength at disp. of 200mm- Regular surface nailsLinear (Strength at disp. of 200mm - Irregular surface nails)Linear (Strength at disp. of 200mm- Regular surface nails)

Strength at disp. of 200mm - Irregular surface nails:δ'= 7.2o, ca'= 12.4 kPaStrength at disp. of 200mm - Regular surface nails:δ'= 4.3o, ca'= 11.3 kPa

(b)

FIG. 16—Comparison of pull-out shear strength parameters between regular surface nails and irregular surface nails in a submerged condition:(a) peak shear strength and (b) shear strength at shear displacement of 200 mm.

Acknowledgments

The work presented in this paper has received financial supportsfrom a RGC grant and from The Hong Kong Polytechnic University.These financial supports are gratefully acknowledged.

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A Laboratory Device to Test the Pull-Out