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Geotextiles and Geomembranes 24 (2006) 116128
A simple method to evaluate the pullout resistance of extruded geogrids
embedded in a compacted granular soil
Nicola Moraci, Domenico Gioffre`
Universita Mediterranea di Reggio Calabria, Dip. MECMAT, via Graziella Loc. Feo di Vito, I-89060 Reggio Calabria, Italy
Received 28 February 2005; received in revised form 26 September 2005; accepted 15 November 2005
Available online 10 January 2006
Abstract
Pullout tests are necessary in order to study the interaction behaviour between soil and geosynthetics in the anchorage zone; hence, the
resulting properties have direct implications on the design of reinforced soil structures.
Several experimental studies showed the influence of different parameters (reinforcement stiffness, geometry and length, applied
vertical effective stress, and geotechnical properties of soil) on the peak and on residual pullout resistance.
On the basis of the results of the tests carried out by Moraci and Recalcati [2005. Factors affecting the pullout behaviour of extruded
geogrids embedded in a compacted granular soil. Geotextiles and Geomembranes, submitted for publication], a new theoretical method
was developed to determine the peak and the residual pullout resistance of extruded geogrids embedded in a compacted granular soil.
The method is capable of evaluating both the bearing and the frictional components of pullout resistance, taking into account the
reinforcement extensibility and geometry as well as the non-linearity of the failure envelope of backfill soil. The comparison between
theoretical and experimental results was favourable, thus confirming the suitability of the proposed approach.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Soil dilatancy; Reinforcement extensibility; Pullout resistance; Skin friction; Bearing resistance
1. Introduction
The main interaction mechanisms affecting the pullout
resistance of extruded geogrids are the skin friction,
between soil and reinforcement solid surface, and the
bearing resistance, that develops against transversal
elements (Fig. 1).
The pullout resistance of a geogrid, assuming that the
different interaction mechanisms act at the same time with
maximum value and that they are independent of each
other, may be evaluated using the following equation:
PR PRS PRB, (1)
where PRS is the skin friction component of pullout
resistance and PRB the bearing component of pullout
resistance.
The frictional component of pullout resistance, for a
geogrid of length LR and unit width WR (Fig. 2), may be
evaluated from the following expression:
PRS 2aSLRt 2aSLRs0n tan d, (2)
where s0n is the normal effective stress, d the skin friction
angle between soil and geogrid, t the shear stress acting at
soilreinforcement interface and aS the fraction of geogrid
surface area that is solid.
To evaluate the bearing component of pullout resistance,
Jewell (1990) proposed the following expression:
PRB LR
S
aBs
0bB, (3)
where S is the spacing between geogrid bearing members,
LR=S the number of geogrid bearing members, aB thefraction of total frontal area of geogrid available for
bearing, B the bearing member thickness and s0b the
effective bearing stress on the geogrid bearing members.
For granular soils, the bearing stresses s0b on geogrid
bearing members are linked to the soil shear strength angle,
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www.elsevier.com/locate/geotexmem
0266-1144/$- see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.geotexmem.2005.11.001
Corresponding author. Tel.: +39 0965875263; fax: +39 0965875201.
E-mail addresses: [email protected] (N. Moraci),
[email protected] (D. Gioffre` ).
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the initial stress state, the interface roughness and the
reinforcement depth in relation to the sizes of the bearing
members (Rowe and Davis, 1982). For punching failure
mechanism, the ratio s0b=s0n depends only on soil shear
strength angle and may be defined as following (Jewell
et al., 1985):
s0b
s0n ep=2f
0 tan f0 tanp
4
f0
2 . (4)
For general shear failure mechanism, the ratio s0b=s0n
may be defined as follows:
s0bs0n
ep tan f0
tanp
4f0
2
. (5)
According to Jewell (1996), the Eqs. (4) and (5) represent
a lower bound and an upper bound for the bearing
component of resistance in pullout conditions.
In order to evaluate the bearing component of pullout
resistance, Matsui et al. (1996) and Bergado and Chai
(1994) proposed other relationships.
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Nomenclature
d skin friction angle between soil and geogrid
(deg.)
aB fraction of total frontal area of geogrid avail-
able for bearing (dimensionless)s0b effective bearing stress on the geogrid bearing
members (kN/m2)
s0n normal effective stress (kN/m2)
aS fraction of geogrid surface area that is solid
(dimensionless)
Ab area of each rib element (mm2)
Ar node embossment area (mm2)
At bar portion between two nodes area (mm2)
B bearing member thickness (mm)
Br node thickness (mm)
Bt thickness of the bar portion between two nodes
(mm)
Beq strip of uniform thicknessCaS reduction coefficient of geogrid area where skin
friction develops (aS)
d50 average grain size (mm)
fb soilgeosynthetic pullout interaction coefficient
(dimensionless)
L reinforcement length in the anchorage zone (m)
LR specimen length (m)
nt number of geogrid bearing members
ntb number of nodes in a transversal element
PR pullout resistance (kN/m)
PRB bearing component pullout resistance (kN/m)
PRR residual pullout resistance (kN/m)
PRS skin friction component pullout resistance(kN/m)
PRRS skin friction component pullout resistance
under residual conditions (kN/m)
S spacing between geogrid bearing members
(mm)
U uniformity coefficient (dimensionless)
wopt optimum water content (%)
Wr node width (mm)
Wt width of the bar portion between two nodes
(mm)
f0 soil shear strength angle (deg.)
f0cv soil shear strength angle at constant volume
(deg.)f0p peak shear strength angle (deg.)
gdmax maximum dry unit weight (kN/m3)
mRS=GSY soilgeosynthetic residual interface apparent
coefficient of friction (dimensionless)
mS/GSY soilgeosynthetic peak interface apparent coef-
ficient of friction (dimensionless)
Fig. 2. Definition of terms for a geogrid (Jewell, 1990).
Fig. 1. The two mechanisms for bond between reinforcement and soil(Jewell et al., 1985).
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In particular, Matsui et al. (1996) proposed an equation
based on a Prandtls mechanism, as shown in Fig. 3:
s0bs0n
ep tan f tanp
4f0
2
cos
p
4f0
2
1 sin f0 sinp
4
f0
2 . 6
The comparison between the values s0b=s0n obtained
through expression (6) and the pullout test results
performed by the authors on steel grid reinforcement
(diameter 6 mm) embedded in granular soil (f0 37:31)showed a good agreement (Fig. 4).
Assuming that there was an uniform distribution of
shear stress applied along the whole surface of reinforce-
ment, Jewell (1990) obtained a general theoretical relation-
ship to evaluate the pullout resistance of a geogrid:
PR 2aSLRs0n tand
LR
S
aBBs
0b
2fbLRs0n tanf
0, 7
where fb is the interaction coefficient in pullout conditions.
This coefficient can be evaluate based on geometrical
parameters of the reinforcement and on the soil shear
strength characteristics (Jewell, 1990).
Others studies (Palmeira, 2004; Palmeira and Milligan,
1989) emphasized the influence of scale, shape and
interference effects.
The scale effects are related to the ratio between the
transverse element thickness (B) of the reinforcement and
soil grain size D50 and to the ratio between the spacing
between geogrid bearing member (S) and soil grain sizeD50. These effects are relevant if B/D50p10 (Fig. 5,
Palmeira and Milligan, 1989) and if S/D50 is less than 3
(Jewell et al., 1985).
The shape effects are related to the geometry of the
reinforcement transverse elements. Jewell (1996), in order
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Fig. 3. Assumed failure surface for the evaluation of s0b (Matsui
et al., 1996).
Fig. 4. Comparison between proposed equation and available pullout test
data of anchor and grid reinforcements (Matsui et al., 1996).
100
75
50
25
00 20 40 60 80 100 120
Pullout displacement (mm)
Bearingres
istanceperunitwidth
ofreinforcement(kN/m)
Normal pressure
= 98.1kPan
Grid geometry
225x150x6 (mm)
N
N= 1
N= 2
N= 3
Fig. 5. Scale effect on bearing capacity (Palmeira and Milligan, 1989).
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to take into consideration the shape effects, suggested using
a shape coefficient that can be assumed to be equal to 1.0 in
the case of a circular shape and equal to 1.2 in the case of
rectangular shape.
Milligan et al. (1990), by means of photo-elastic studies,
showed that the bearing action reduced the friction
between the soil and the reinforcement (interference effect).This means that, under similar conditions, the fraction of
the ultimate pullout load due to skin friction may be
considerably smaller than that due to bearing, and smaller
than the value produced between grid area available for
friction, normal stress and the skin friction coefficient
between the soil and grid.
As the ratio between the distance between grid bearing
members and bearing member thickness increases, the
interference between these members will decrease to an
extent that they will behave as a series of isolated bearing
members being pulled out of the soil mass. Palmeira (2004)
observed metal grids embedded in dense sand and saw that,
for S/B ratios of above 40, the grid bearing members
behaved in isolation, under the experimental conditions
adopted.
As mentioned above, the passive failure surfaces that
developed against bearing members cause a reduction of
the skin friction component of the pullout resistance. This
effect can be taken into account in terms of reduction of
geogrid area where skin friction develops (aS). For this
reason a reduction coefficient, CaS, may be introduced.
The limits of theoretical expression used to evaluate the
soilgeosynthetic pullout interaction coefficient, fb, have
been investigated by different researchers (Palmeira and
Milligan, 1989; Wilson-Fahmy and Koerner, 1993; Moraciand Montanelli, 2000; Ghionna et al., 2001). In particular,
previous experimental studies (Palmeira and Milligan,
1989; Moraci and Montanelli, 2000; Ghionna et al.,
2001) have shown that the values of fb are largely
influenced by the reinforcement geometry, extensibility
and soil dilatancy. Thus, it is important to develop a new
theoretical expression that is able to include the evaluation
of all the parameters that influence the mobilization of the
interaction mechanisms (frictional and passive) during
pullout, as emphasized by previous works (Moraci et al.,
2002, 2003, 2004; Moraci and Recalcati, 2005). In the
present paper, on the basis of the test results obtained by
Moraci and Recalcati (2005), a new theoretical method was
developed to evaluate the pullout resistance of extruded
geogrids embedded in a compacted granular soil. The
method is able to evaluate both the passive and the
frictional components of pullout resistance taking into
account the reinforcement extensibility and geometry, as
well as, the non-linearity of the failure envelope of the
backfill soil.
2. The method
Test results obtained by Moraci and Recalcati (2005)
showed the influence of different parameters (reinforce-
ment stiffness and structure, embedded length and vertical
effective stress) on the pullout behaviour of mono-oriented
extruded geogrids embedded in a compacted uniform
medium sand.
In particular, it was found that the dilatancy of the soil
at the interface is the phenomenon that most influences the
pullout resistance and the interface apparent coefficient offriction (mS/GSY). Due to the dilatancy effects, the apparent
coefficient of skin friction mobilized at low vertical
effective confining pressures is higher than that at high
confining pressures.
Experimental results (Moraci and Recalcati, 2005) also
showed that the reinforcement extensibility influences the
peak pullout resistance. In particular, extensibility effects
were more evident in long reinforcements and in high
vertical confining stresses. In residual conditions, the
extensibility effects were negligible.
Test results (Moraci and Recalcati, 2005) also showed an
increase in peak and residual pullout resistance, and
therefore in the mobilized interface apparent coefficient
of friction, while increasing the competent bearing area of
each node, upon which the bearing mechanisms are
mobilized.
The decrease of the pullout resistance after the peak is
related to both reinforcement length and confining stress.
Finally, the apparent coefficient of friction mobilized in
residual conditions depends only on the applied vertical
stress and geogrid geometry; in these conditions mRS=GSYdoes not depend on reinforcement length.
In the case of long reinforcements and high effective
vertical stresses, reinforcement extensibility induces a
progressive mobilization of the elementary interactionmechanisms (skin friction and bearing resistance).
Vice versa, in short reinforcements (independent of
vertical effective stresses) and in long reinforcements
(subjected to low vertical effective stresses) the longitudinal
strains are small. In such cases, the reinforcement behaves
in a rigid way rigid and the interaction mechanisms are
effectively activated at the same time along the whole
length of the reinforcement.
Different experimental studies (Matsui et al., 1996;
Palmeira, 2004) showed that, for the characteristic
displacement field of pullout tests, the bearing stress, after
the displacement corresponding to the maximum value,
remains almost constant with any increase in displacement
(Figs. 6 and 7).
In order to separate the two components of the pullout
resistance, it is possible to perform pullout tests on geogrid
specimens where a portion of transverse reinforcing
elements are removed (Alagiyawanna et al., 2001; Alfaro
et al., 1995; Matsui et al., 1996; Palmeira and Milligan,
1989). Due to their structure, the bearing resistance of
the geogrids used in this study, develops both at the
node embossments and at the transverse bars. With
this reinforcement form it is not possible to deter-
mine experimentally the two components of pullout
resistance.
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In order to validate the previous findings, which may
explain the different behaviour of the three geogrids used in
the research (in terms of pullout resistance and in terms of
apparent coefficient of friction), the following approach
was used:
1. The use of a simple Eq. (8) for the determination of the
pullout resistance in geogrids and soils for which the
scale effects are negligible (i.e. S=B larger than 40 andS/D50 larger than 1000):
PR 2CaSaSLRs0n tan d ntntbAbs
0b, (8)
where CaS is the reduction coefficient of geogrid area
where skin friction develops, nt LR=S the number ofgeogrid bearing members, ntb the number of nodes in a
transversal element, Ab At Ar the area of each rib
element (including the single node and the bar portion
between two nodes) where the bearing resistance can be
mobilized (Fig. 8) and s0b the bearing stress evaluated by
Eq. (6) according to Matsui et al. (1996).
2. To take into account the particular structure of the
elements on which the bearing resistance mobilizes, the
soil dilatancy effects (non-linearity of the failure
envelope of back fill soil) and the geogrid extensibility.
3. The comparison between theoretical (Moraci and
Recalcati, 2005) and experimental values of the pullout
resistance under different conditions.
Pullout tests have been performed on three different
HDPE extruded mono-oriented geogrids (described asGG1, GG2 and GG3, respectively) (Moraci and Recalcati,
2005). The three geogrids show similar geometrical
characteristics when viewed in plan. They have the same
number of tensile elements per unit width and longitudinal
rib pitch, and similar elliptical aperture shape. On the
contrary, the three geogrids have a different cross-sectional
shape with major differences in rib and bar thickness.
A more detailed analysis of the transversal bar geometry
has shown a non-uniform shape with greater thickness at
the rib intersection. The bearing interaction mechanisms
develop both at the node embossments and at the
transverse bars. Therefore, the node embossment and the
transverse bar geometry have been carefully determined to
evaluate the bearing resistance surfaces.
The results of this analysis are reported in Table 1, where
Wr and Br are the node width and thickness, respectively,
Wt and Bt are the width and thickness of the bar portion
between two nodes, respectively (Fig. 8), and Ab is the area
of each rib element (including the node embossment and
the bar portion between two nodes At Ar) where the
bearing resistance can be mobilized.
The complex geometry of the transverse bars, including
the areas Ab in the same transverse element, was assumed
to be equivalent to that of a strip of uniform thickness (Beq)
(Fig. 9).
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Fig. 7. Load displacement curve for an isolated bearing member
(Palmeira, 2004).
Fig. 8. Schematic cross-section AA of the geogrid bar.
Table 1
Structural characteristics of the different geogrids (for symbol see Fig. 8)
Geogrid Wr (mm) Wt (mm) Br (mm) Bt (mm) Ab (mm2)
GG1 11.26 6.6 3.80 3.57 66.35
GG2 11.86 6.0 4.65 4.48 82.03
GG3 12.36 5.5 5.16 4.85 90.45
2.5
2.0
1.5
1.0
0. 5
0. 0 5 10 15 20 25
B/ D50
square section
round section
B
('
/'
)s/('/
'
)
b
n
b
n
Fig. 6. Relationship between bearing resistance and pullout displacement
(Matsui et al., 1996).
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A granular soil was used in the tests. The soil was
classified as uniform medium sand with a uniformity
coefficient U d60=d10 1:5 and an average grain sized50 0:22 mm. Standard Proctor compaction tests gave amaximum dry unit weight gdmax 16:24kN=m
3at an
optimum water content wopt 13:5%.Direct shear tests, performed at an initial unit weight
equal to 95% ofgdmax (obtained at a water content of 9%),
yielded high single values of the peak shear strength angle
f0p, in the range 481 (for s0v 10 kPa) to 421 (for
s0v 100 kPa). The shear strength angle at constant
volume, f0cv, was 341.
The soil shear strength angle used to determine of the
skin friction component of the pullout resistance based on
previous experimental researches on smooth HDPE
geomembranes was assumed to have a value of d equal
to 1/3 f0 (Fannin and Raju, 1993; Raju, 1995). In order to
take into account the reinforcement extensibility, the
following assumptions were made:
1. In long reinforcements (LR 0:921:15 m) and higheffective vertical stresses, the reinforcement extensibility
induces a progressive mobilization of the two elemen-
tary interaction mechanisms. Under these conditions,
the skin friction was evaluated using an average value of
the shear strength angle between the peak and the
constant volume values, assuming a non-linear failure
envelope for the backfill soil.
2. In short reinforcements (LR 0:4 m), independent ofthe applied vertical effective stresses, and in long
reinforcements (subjected to low vertical effective
stresses), the longitudinal strain is small. In such cases,
the reinforcement behaves as a rigid material and the
interaction mechanisms are activated simultaneously
along the whole reinforcement. Under these conditions,
the peak shear strength angle can be used to evaluate
both components of the pullout resistance, assuming a
non-linear failure envelope for the backfill soil and a
suitable stress level.
On the basis of the experimental results obtained by
Matsui et al. (1996) and Palmeira (2004), the bearing
resistance component of pullout resistance was evaluated
using the peak shear strength angles corresponding to the
different vertical effective stresses, in order to take into
account the non-linearity of the failure envelope (due to
dilatancy effects) of the backfill soil.
Eq. (8) permits the evaluation of the residual pullout
resistance PRR. In this case in order to evaluate the skin
friction component of pullout strength, the soil shear
strength angle at constant volume f0cv was used.
In order to evaluate the reduction of the skin friction
component induced by the passive failure surfaces devel-
oped on bearing members, a reduction coefficient, CaS, of
the geogrid area, where skin friction develops (aS), was
used. This value, derived from the assumption that the
maximum extensions of passive failure surfaces are equal
to 40 times the thickness of the equivalent bearing members
(Fig. 9), is given by
CaS Seff
S
S 40nBeq
S. (9)
This reduction is only applied under residual conditions.
3. Experimental validation of proposed method
In order to validate the proposed method, the theoretical
values of the peak and residual pullout resistances obtained
using Eq. (8) were compared with the experimental
results obtained by Moraci and Recalcati (2005) reported
in Table 2.
Tables 35 show the peak (PexpR ) and residual (P
expRR)
experimental pullout resistances, calculated by the present
method (PtheorR and PtheorRR ), the theoretical values of the
peak and residual skin friction resistance (PtheorRS
and PtheorRRS
)
and the ratio between the skin friction resistance and
pullout resistance both at the peak and in residual
conditions (PtheorRS =PexpR and P
theorRRS =PR
expR ).
Figs. 1012 show (in the different reinforcement lengths)
the comparison between experimental and theoretical
values of the peak pullout resistances, evaluated for the
different applied vertical effective confining stresses. The
skin friction component of the peak pullout resistance is
small in comparison to the bearing component. On the
basis of the theoretical analysis (Eq. (8)), the skin friction
component PRS 2aSLRs0n tan d represents less than
20% of the peak pullout resistance. In particular, the
values vary between 9% and 19%, for GG1, and between
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Fig. 9. Assumed equivalent geometry.
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8% and 19% for GG2 and GG3 (Tables 35). The lower
values are related to the lower vertical effective stress and
to short reinforcements; the higher values are related to
the higher vertical effective stresses and to long reinforce-
ments. Figs. 1315 show the same comparison in terms of
residual pullout resistance.
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Table 3
Theoretical and experimental peak (PR) and residual (PRR) pullout resistance (kN/m) for geogrid GG1
LR(m)
s0v (kPa) PexpR (kN/m) P
theorR (kN/m) P
theorRS (kN/m) P
theorRS =P
expR
(%)
PexpRR (kN/m) P
theorRR (kN/m) P
theorRRS (kN/m) P
theorRRS =P
expRR
(%)
0.40 10 9.62 7.33 0.87 9.06 5.63 6.61 0.15 2.70
0.40 25 20.26 14.29 2.08 10.28 13.29 12.59 0.38 2.87
0.40 50 30.95 22.78 3.98 12.85 18.93 19.57 0.76 4.02
0.40 100 39.79 37.03 7.58 19.04 26.43 30.98 1.52 5.76
0.90 10 16.62 14.88 1.96 11.79 12.14 13.27 0.34 2.82
0.90 25 34.55 29.10 4.69 13.56 29.79 25.28 0.86 2.88
0.90 50 52.53 45.51 7.89 15.02 50.34 39.33 1.71 3.40
0.90 100 a 74.28 15.36 a a 62.34 3.42 a
1.15 10 20 18.66 2.50 12.52 14.76 16.59 0.44 2.97
1.15 25 37.13 36.51 5.99 16.14 34.32 31.62 1.09 3.19
1.15 50 62.79 57.10 10.08 16.06 62.79 49.21 2.19 3.491.15 100 a 93.28 19.63 a a 78.02 4.38 a
aSpecimen failure.
Table 4
Theoretical and experimental peak (PR) and residual (PRR) pullout resistance (kN/m) for geogrid GG2
LR(m)
s0v (kPa) PexpR (kN/m) P
theorR (kN/m) P
theorRS (kN/m) P
theorRS =P
expR
(%)
PexpRR (kN/m) P
theorRR (kN/m) P
theorRRS (kN/m) P
theorRRS =P
expRR
(%)
0.40 10 13.42 9.11 1.12 8.37 8.44 8.19 0.20 2.33
0.40 25 24.76 17.78 2.69 10.86 15.43 15.59 0.49 3.18
0.40 50 41.17 28.38 5.13 12.45 24.04 24.23 0.98 4.08
0.40 100 56.59 46.19 9.77 17.26 37.51 38.38 1.96 5.23
0.90 10 21.32 18.51 2.53 11.86 15.43 16.42 0.44 2.86
0.90 25 39.99 36.23 6.04 15.11 32.14 31.29 1.10 3.44
0.90 50 70.07 56.68 10.18 14.52 62.46 48.71 2.21 3.54
0.90 100 103.91 92.65 19.81 19.07 103.91 77.26 4.42 4.25
1.15 10 26.96 23.20 3.23 11.98 19.53 20.54 0.56 2.89
1.15 25 51.43 45.46 7.72 15.01 44.00 39.15 1.41 3.20
1.15 50 75.62 71.13 13.00 17.19 75.62 60.95 2.82 3.73
1.15 100 a 116.37 25.32 a a 96.69 5.64 a
aSpecimen failure.
Table 2
Peak (PR) and residual (PRR) pullout resistance (kN/m) measured in the tests (Moraci and Recalcati, 2005)
Geogrid Specimen
length (m)
Normal stress s0v
10 (kPa) 25 (kPa) 50 (kPa) 100 (kPa)
PR PRR PR PRR PR PRR PR PRR
GG1 0.40 9.62 5.63 20.26 13.29 30.95 18.93 39.79 26.43
GG1 0.90 16.62 12.14 34.55 29.79 52.53 50.34 78.44a
GG1 1.15 20.00 14.76 37.13 34.32 62.79 62.79 72.48a
GG2 0.40 13.42 8.44 24.76 15.43 41.18 24.04 56.59 37.51
GG2 0.90 21.32 15.43 39.99 32.14 70.07 62.46 103.91 103.91
GG2 1.15 26.96 19.53 51.43 44.00 75.62 75.62 106.91a
GG3 0.40 12.84 7.36 22.72 13.64 37.68 25.18 58.68 49.04
GG3 0.90 19.85 15.48 41.80 34.69 72.95 61.27 97.59 97.59
GG3 1.15 24.35 19.61 47.75 43.79 81.77 81.77 115.19 115.19
aSpecimen failure.
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Thus, the following conclusions can be drawn. The skin
friction component of residual pullout resistance is also
small in comparison to the bearing component. On the
basis of the theoretical analysis (Eq. (8)), the skin friction
PRS 2CaSaSLRs0n tan d component represents less than
6% of the residual pullout resistance (Tables 35). Such
small values are due to the reduction of the skin friction
component caused by the bearing failure surfaces (inter-
ference effects).
Figs. 1012 show that the proposed method is in close
agreement with the experimental data. In particular, an
underestimation of the peak pullout resistance was
observed which was more evident for short reinforcements.
This situation could be attributed to the local increment of
the vertical effective stress due to the constrained dilatancy,
which is not considered in the simple proposed model. For
short reinforcements (LR 0:40m), the percentagedifferences between experimental results and theoretical
values in terms of peak pullout resistance, ranges between
7% and 32%; for long reinforcements (LR 0:9021:15 m), such differences are quite small (0% and 19%)(Table 6).
Similar results were obtained in terms of residual pull-
out resistance. In this case, the method agrees well with
the experimental data (Figs. 1315). In short reinforce-
ments, the differences between the experimental results
and the theoretical values vary between 1% and 26%
(Table 6).
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Table 5
Theoretical and experimental peak (PR) and residual (PRR) pullout resistance (kN/m) for geogrid GG3
LR(m)
s0v (kPa) PexpR (kN/m) P
theorR (kN/m) P
theorRS (kN/m) P
theorRS =P
expR
(%)
PexpRR (kN/m) P
theorRR (kN/m) P
theorRRS (kN/m) P
theorRRS =P
expRR
(%)
0.40 10 12.84 9.79 0.99 7.68 7.36 8.98 0.17 2.34
0.40 25 22.72 19.00 2.36 10.37 13.64 17.08 0.43 3.16
0.40 50 37.68 30.14 4.50 11.94 25.18 26.50 0.86 3.42
0.40 100 58.68 48.73 8.57 14.61 49.04 41.88 1.72 3.51
0.90 10 19.85 19.84 2.22 11.17 15.48 18.01 0.39 2.51
0.90 25 41.8 38.59 5.30 12.69 34.69 34.26 0.97 2.79
0.90 50 72.95 60.21 8.93 12.24 61.27 53.22 1.94 3.16
0.90 100 97.59 97.70 17.39 17.82 97.59 84.20 3.88 3.97
1.15 10 24.35 24.86 2.83 11.65 19.61 22.52 0.50 2.52
1.15 25 47.74 48.39 6.78 14.19 43.79 42.85 1.24 2.83
1.15 50 81.77 75.51 11.41 13.95 81.77 66.58 2.48 3.03
1.15 100 115.15 122.61 22.22 19.29 115.19 105.35 4.95 4.30
100
80
60
40
20
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=1.15m
LR
=0.90m
LR
=0.40m
GG1 - peak
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 10. Comparison between experimental and theoretical values of peak
pullout resistance for GG1.
120
80
40
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=0.40m
LR
=0.90m
LR
=1.15m
GG2 - peak
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 11. Comparison between experimental and theoretical values of peak
pullout resistance for GG2.
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For the tests in which confined tensile failure occurs the
method gives higher values of pullout resistance than in-air
tensile resistance evaluated at the same pullout test rate
(Moraci and Recalcati, 2005). Thus, the method is also
useful in the evaluation of the combination of s0v and LRthat produces confined reinforced pullout failures.
To evaluate the soilgeosynthetic interface apparent
coefficient of friction, mS/GSY, the following equation
can be used:
mS=GSY PR
2LRs0v
2aSLRs0n tan d ntntbAbs
0b
2LRs0v. (10)
The analysis was performed both in terms of the peak
and the residual soilgeosynthetic interface apparent
coefficient of friction.
ARTICLE IN PRESS
160
120
80
40
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=0.40m
LR
=0.90mLR =1.15m
GG3 - peak
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 12. Comparison between experimental and theoretical values of peak
pullout resistance for GG3.
100
80
60
40
20
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=0.40m
LR
=0.90m
LR
=1.15m
GG1 - residual
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 13. Comparison between experimental and theoretical values of
residual pullout resistance for GG1.
120
80
40
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=0.40m
LR
=0.90m
LR
=1.15m
GG2 - residual
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 14. Comparison between experimental and theoretical values of
residual pullout resistance for GG2.
160
120
80
40
0
0 20 40 60 80 100 120 140
Pullout failure
LR
=0.40m
LR
=0.90m
LR
=1.15m
GG3 - residual
experimental theoretical
PR
[kN/m]
v[kPa]
Fig. 15. Comparison between experimental and theoretical values of
residual pullout resistance for GG3.
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Figs. 1618 show the trends of the experimental and
theoretical values of the peak pullout interface apparent
coefficient of friction mS=GSY, as a function of the vertical
effective applied stress, for the three different reinforce-
ment specimen lengths used.
In all cases, it is possible to observe a reduction in the
mobilized peak pullout interface apparent friction coeffi-
cient with an increase in the applied vertical effective stress.
Moreover, it is possible to note that the lower values of
mS/GSY are given with the longer reinforcement specimens.
These results are due to two different phenomena:
The first, of greater importance, is related to soil dilatancy
that develops in conjunction with the three-dimensional
passive failure surfaces that arise at the node embossments
and at the geogrid transverse reinforcing elements. Due to
soil dilatancy, which decreases with an increase in the
confining vertical effective stress, two main effects
develop: the first is due to the different work made to
expand the dilatancy surface at different vertical effective
confining stresses; the second effect is due to the
restriction of the dilatancy connected to the nearby soil
stiffness (constrained dilatancy), which produces a local
increment of the effective confining stress.
The second effect, of less intensity, is due to theextensibility of the reinforcement which modifies the
interface tangential stress distribution and the corre-
sponding pullout strength.
ARTICLE IN PRESS
Table 6
Percentage differences between experimental results and theoretical values
LR (m) s0v (kPa) GG1 GG2 GG3
PtheorR
PexpR
P
exp
R
%Ptheor
RRP
expRR
P
exp
RR
%Ptheor
RP
expR
P
exp
R
%Ptheor
RRP
expRR
P
exp
RR
%Ptheor
RP
expR
P
exp
R
%Ptheor
RRP
expRR
P
exp
RR
%
0.40 10 24 17 32 3 24 22
0.40 25 29 5 28 1 16 25
0.40 50 26 3 31 1 20 5
0.40 100 7 17 18 2 17 15
0.90 10 10 9 13 6 0 16
0.90 25 16 15 9 3 8 1
0.90 50 13 22 19 22 17 13
0.90 100 11 26 0 14
1.15 10 7 12 14 5 2 15
1.15 25 2 8 12 11 1 2
1.15 50 9 22 6 19 8 19
1.15 100 6 9
Specimen failure.
1.4
1.2
0.8
0.6
0.4
0 20 40 60 80 100 120 140
1
S/GSY
GG1 - peak
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
Fig. 16. Comparison between experimental and theoretical values of peak
soilgeosynthetic interface apparent coefficient of friction for GG1.
2
1.6
1.2
0.8
0.4
0 20 40 60 80 100 120 140
S/GSY
GG2 - peak
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
Fig. 17. Comparison between experimental and theoretical values of peak
soilgeosynthetic interface apparent coefficient of friction for GG2.
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Moraci and Recalcati (2005) compared the experimental
results of the tests carried out on the three different
geogrids, with the same anchorage lengths and normal
stress, so that they were not influenced by the reinforce-
ment extensibility and the dilatancy effects. They observed
that the experimental results, interpreted as a function of
the different longitudinal tensile stiffnesses, do not show a
specific correlation. Vice versa, the test results confirmed
that the values of the soilgeosynthetic peak interface
apparent coefficient of friction, mS/GSY, are mainly influ-
enced by the structural characteristics (geometry and
shape) of the geogrids. In particular, the maximumpercentage differences of the values of mS/GSY are close to
the percentage differences of the competent bearing areas
(Ab) between geogrid types against which the passive
resistance is mobilized (Fig. 19). Figs. 2022 show the same
curves obtained in terms of the residual soilgeosynthetic
interface apparent coefficients of friction mRS=GSY.
The experimental results obtained by Moraci and
Recalcati (2005) showed that the residual pullout interface
apparent coefficient of friction does not depend on the
reinforcement length but only on the applied confining
stress.
Comparison of the results obtained for the three
different geogrids shows that mRS=GSY depends on geogrid
geometry. The differences between the predicted and the
experimental values range from 0% to 32% under peak
conditions and from 1% to 26% under residual ones.
The results indicate that the proposed model is suitable
to predict the interface apparent coefficient of friction,
particularly in the case of extensible reinforcements. In the
case of rigid reinforcements, the proposed method under-
estimates the interface apparent coefficient of friction.
ARTICLE IN PRESS
2
1.6
1.2
0.8
0.4
0 20 40 60 80 100 120 140
S/GSY
GG3 - peak
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
Fig. 18. Comparison between experimental and theoretical values of peak
soilgeosynthetic interface apparent coefficient of friction for GG3.
2
1.6
1.2
0.8
0.4
0
1.6
1.2
0.8
0.4
0
0 20 40 60 80 100 120
LR
=0.40 m L
R=
0.90 m
LR
=1.15 m
GG3
GG2
GG1
GG3
GG2
GG1
GG3
GG2
GG1
Normal stress v[kPa]
0 20 40 60 80 100 120
Normal stress v[kPa]
0 20 40 60 80 100 120
Normal stress v[kPa]
S/GSY
S/GSY
1.6
1.2
0.8
0.4
0
S/GSY
Fig. 19. Peak interface apparent coefficient of friction vs. s0 for different geogrids (Moraci and Recalcati, 2005).
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4. Conclusions
Comparison between the theoretical and experimental
permits the following conclusions to be drawn:
The proposed method (which takes into account theeffects of soil dilatancy, reinforcement extensibility,
geogrid structure and geometry, vertical effective
stresses and reinforcement length) predicts the experi-
mental data well, both in terms of the pullout resistance
and in terms of the interface apparent coefficient of
friction, especially for extensible reinforcements.
In the case of extruded geogrids embedded in compacteduniform medium sand, the skin friction components of
the peak pullout resistance are small in comparison to
the bearing component. The skin friction componentrepresents less than 20% of the peak pullout resistance.
The skin friction components of the pullout resistance
are small in comparison to the bearing component, in
residual conditions.
The proposed method can be used also to evaluate thecombination of s0v and LR relating to the confined
reinforcement pullout failure.
References
Alagiyawanna, A.M.N., Sugimoto, M., Sato, S., Toyota, H., 2001.Influence of longitudinal and transverse members on geogrid pullout
behaviour during deformation. Geotextiles and Geomembranes 19,
483507.
Alfaro, M.C., Miura, N., Bergado, D.T., 1995. Soilgeogrid reinforcement
interaction by pullout and direct shear tests. Geotechnical Testing
Journal 18, 157167.
Bergado, D.T., Chai, J.C., 1994. Pullout force/displacement relationship
of extensible grid reinforcements. Geotextiles and Geomembranes 13,
295316.
Fannin, R.J., Raju, D.M., 1993. Large-scale pull-out test results on
geosynthetics. Proceedings of Geosynthetics 93 Conference, vol. 2.
Vancouver, Canada, pp. 633643.
Ghionna, V.N., Moraci, N., Rimoldi, P., 2001. Experimental evaluation of
the factors affecting pullout test results on geogrids. In: Proceedings of
the International Symposium: Earth Reinforcement, Fukuoka, Japan,
ARTICLE IN PRESS
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0 20 40 60 80 100 120 140
RS/GSY
GG1 - residual
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
Fig. 20. Comparison between experimental and theoretical values of
residual soilgeosynthetic interface apparent coefficient of friction for
GG1.
1.2
0.8
0.6
0.4
1
0 20 40 60 80 100 120 140
GG2 - residual
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
RS/GSY
Fig. 21. Comparison between experimental and theoretical values of
residual soilgeosynthetic interface apparent coefficient of friction for
GG2.
1.2
0.8
0.6
0.4
1
0 20 40 60 80 100 120 140
RS/GSY
GG3 - residual
v[kPa]
experimental Lr = 0.40 m theoretical Lr = 0.40 m
theoretical Lr = 0.90 m
theoretical Lr = 1.15 m
experimental Lr = 0.90 m
experimental Lr = 1.15 m
Fig. 22. Comparison between experimental and theoretical values of
residual soilgeosynthetic interface apparent coefficient of friction for
GG3.
N. Moraci, D. Gioffre / Geotextiles and Geomembranes 24 (2006) 116128 127
7/30/2019 Geotextiles 1.pdf
13/13
1416 November 2001IS Kyushu 2001 Landmarks in Earth
Reinforcement, vol. 1. Balkema Publisher, pp. 3136.
Jewell, R.A., 1990. Reinforcement bond capacity. Ge otechnique 40 (3),
513518.
Jewell, R.A., 1996. Soil Reinforcement with Geotextiles. CIRIA Special
Publication 123, Thomas Telford.
Jewell, R.A., Milligan, G.W.E., Sarsby, R.W., Dubois, D.D., 1985.
Interactions between soil and geogrids. In: Proceedings from theSymposium on Polymer Grid Reinforcement in Civil Engineering.
Thomas Telford, London, pp. 1830.
Matsui, T., San, K.C., Nabesahirna, Y., Arnii, U.N., 1996. Bearing
mechanism of steel reinforcement in pull-out test. In: Proceedings of
the International Symposium: Earth Reinforcement, Fukuoka,
Kyushu, Japan. Balkema Publisher, pp. 101105.
Milligan, G.W.E, Earl, R.F., Bush, D.I., 1990. Observations of photo-
elastic pullout tests on geotextiles and geogrids. In: IV International
Conference on Geotextiles, Geomembranes and Related Products, The
Hague, The Netherlands, vol. 2. Balkema Publisher, pp. 747751.
Moraci, N., Montanelli, F., 2000. Analisi di prove di sf`lamento di
geogriglie estruse installate in terreno granulare compattato. Rivista
Italiana di Geotecnica 4/2000, 521.
Moraci, N., Recalcati, P.G., 2005. Factors affecting the pullout behaviour
of extruded geogrids embedded in a compacted granular soil.Geotextiles and Geomembranes, in review.
Moraci, N., Gioffre, D., Romano, G., Montanelli, F., Rimoldi, P., 2002.
Pullout behaviour of geogrid embedded in granular soils. In:
Proceedings of the Seventh International Conference on Geosyn-
thetics, Nice, France, vol. 4. Balkema Publisher, pp. 13451348.
Moraci, N., Montanelli, F., Romano, G., 2003. Interface pullout
behaviour of geogrids embedded in compacted granular soils. In:
Proceedings of the Third European Conference on Soil Mechanics and
Geotechnical Engineering, Praga, Rep. Ceka, vol. 1. CICE Publishers,
pp. 837841.
Moraci, N., Romano, G., Montanelli, F., 2004. Factors affecting theinterface apparent coefficient of friction mobilised in pullout condi-
tion. In: DGGT, TUM-ZG (Eds.), Third European Geosynthetics
Conference, March, Munich, pp. 313318.
Palmeira, E.M., Milligan, G.W.E., 1989. Scale and other factors affecting
the results of pull-out tests of grid buried in sand. Ge otechinique 11
(3), 511524.
Palmeira, E.M., 2004. Bearing force mobilization in pull-out tests on
geogrids. Geotextiles and Geomembranes 22, 481509.
Raju, D.M., 1995. Monotonic and cyclic pullout resistance of geosyn-
thetic. Ph.D. Thesis. The University of British Columbia, Vancouver,
Canada.
Rowe, R.K., Davis, E.H., 1982. The behaviour of anchor plates in sand.
Ge otechinique 32 (1), 2541.
Wilson-Fahmy, R.F., Koerner, R.M., 1993. Finite element modelling ofsoilgeogrid interaction with application to the behavior of geogrids in
a pullout loading condition. Geotextiles and Geomembranes 12,
479501.
ARTICLE IN PRESS
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