shear behaviour of geosynthetics in the inclined plane test –

8/18/2019 Shear Behaviour of Geosynthetics in the Inclined Plane Test –

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GEOSYNTHETICS INTERNATIONAL • 2001, VOL. 8, NO. 4 327

Technical Paper by P.C. Lopes, M.L. Lopes, and M.P. Lopes

SHEAR BEHAVIOUR OF GEOSYNTHETICS IN THEINCLINED PLANE TEST – INFLUENCE OF SOILPARTICLE SIZE AND GEOSYNTHETIC STRUCTURE

ABSTRACT: This paper reports the investigation of the shear behaviour of six geo-

synthetics with two granular soils. The test equipment, soils, and geosynthetics prop-

erties are described and the soil-geosynthetic interaction behaviour is studied. The

influence of soil particle size, geosynthetic structure, and test method are discussed by

analysing the results of inclined plane tests. The main conclusions of the study are as

follows: geosynthetic structure has an important influence on the soil-geosynthetic

interface friction angle; higher soil-geosynthetic interface friction angles are measured

when the geosynthetic surface has significantly sized apertures (e.g., geogrids) or

allows the penetration of soil particles into the geosynthetic (e.g., nonwoven, spun- bonded geotextiles); geosynthetic surface roughness (e.g., geomembranes) is associated

with higher soil-geosynthetic interface friction angles; soil particle size has an impor-

tant influence on the soil-geosynthetic interface friction angle; broadly graded soils

with large average soil particle sizes allow an increase in the soil-geosynthetic interface

resistance; the method of test does not significantly influence the soil-geomembrane or

soil-geotextile interface friction angles (geosynthetics with continuous surfaces); and

the validity of evaluating the soil-geogrid interface resistance using a rigid support (Test

method 1) depends on the structure of the geogrid. It is suggested that site conditions is

the greatest factor to consider when selecting the most appropriate test method.

KEYWORDS: Inclined plane shear, Geogrid, Geotextile, Geomembrane, Interaction,

Particle size, Geosynthetic structure, Test method.

AUTHORS: P.C. Lopes, Ph.D. candidate, M.L. Lopes, Associate Professor, Depart-

ment of Civil Engineering, Geotechnical Division, University of Porto, Rua Dr. Roberto

Frias, 4200-465 Porto, Portugal, Telephone: 351/22 5081729, Telefax: 351/22 6053868,

E-mail: [email protected]; and M.P. Lopes, Ph.D. candidate, Civil Engineering Division,

University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal,

Telephone: 351/234 370941, Telefax: 351/234 370094, E-mail: [email protected].

PUBLICATION: Geosynthetics International is published by the Industrial Fabrics

Association International, 1801 County Road B West, Roseville, Minnesota 55113-

4061, USA, Telephone: 1/612-222-2508, Telefax: 1/612-631-9334. Geosynthetics

International is registered under ISSN 1072-6349.

DATE: Original manuscript submitted 4 April 2001, revised version received 25 July

2001, and accepted 28 July 2001. Discussion open until 1 April 2002.

REFERENCE: Lopes, P.C., Lopes, M.L., and Lopes, M.P., 2001, “Shear Behaviour

of Geosynthetics in the Inclined Plane Test – Influence of Soil Particle Size and Geo-

synthetic Structure”, Geosynthetics International , Vol. 8, No. 4, pp. 327-342.


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LOPES, LOPES, AND LOPES • Shear Behaviour of Geosynthetics in Inclined Plane Tests

328 GEOSYNTHETICS INTERNATIONAL • 2001, VOL. 8, NO. 4

1 INTRODUCTION

Inclined plane shear tests are used to characterise the interaction mechanism at soil-geosynthetic interfaces when the relative movement is mainly caused by shearing and

the geosynthetic is placed over an inclined surface.

With the objective of obtaining additional data on soil-geosynthetic interface

behaviour from inclined plane shear tests and the influence of soil particle size and test

methodology, the present paper studies the interaction behaviour of a high density

polyethylene smooth geomembrane, a high density polyethylene rough geomembrane,

a polypropylene nonwoven spun-bonded geotextile, a polypropylene woven geotex-

tile, a high density polyethylene uniaxial geogrid, and a polypropylene biaxial geogrid

embedded in two different granular soils, using an inclined plane shear test apparatus.

2 TEST APPARATUS

The inclined plane apparatus was developed according to test method prEN ISO

12957-2. The test can be carried out using the following two different methods:

1. with a rigid support for the geosynthetics; and

2. with the geosynthetic supported on a lower box that is filled with soil.

In general terms, the inclined plane apparatus (Figures 1 and 2) is structure that can

Figure 1. Inclined plane apparatus: (a) rigid base and upper box; (b) lower and upper

boxes.

(a) (b)


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be disassembled and includes:

• a rigid and smooth base that is 0.620 m long, 0.430 m wide, and 0.010 m high (the

geosynthetic is placed on this base for Test method 1);

• a rigid lower box, with internal dimensions of 0.510 m long, 0.350 m wide, and

0.080 m high that is filled with soil over which the geosynthetic is placed for Test

method 2;

• a rigid upper box, with internal dimensions of 0.300 m long, 0.300 m wide, and

0.080 m high, filled with soil that slides over the geosynthetics.

The rigid base can be raised at a rate of 0.5º/minute (test speed) and lowered, at the

end of the test, at the rate of 2º/minute. The apparatus includes three safety devices:

• one to stop the test when the displacement of the soil-filled upper box exceeds

0.050 m;

• the other two to stop the base when the maximum inclination of the apparatus is

reached and to stop the base at the horizontal position at the end of the test.

The displacement of the upper box is measured with a transducer. The inclination

inclinometer

upper box load application

load cell

transducer

geosynthetic

lower box

system of inclination

Figure 2. Schematic representation of the inclined plane apparatus: (a) rigid base and

upper box; (b) lower and upper boxes.

ometer

upper box load application

load cell

transducer

geosynthetic

rigid basesystemof inclination

Load application

Transducer

Load cell

Rigid baseInclination system

Inclinometer

Upper box

Geosynthetic

Upper box Load application

Load cell

Transducer

Geosynthetic

Lower box

Inclination system

(a)

(b)

Inclinometer


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of the rigid base is measured with an inclinometer, fixed to the base, and recorded in

terms of the angle of the inclined plane to the horizontal. This device also allows veri-

fication of the horizontality of the base at the start of the test.The geosynthetic is fixed using clamps that are placed outside the contact area and

fixed to the base by screws. When performing Test method 1 on more extensible geo-

synthetics, wrinkles appeared on the geosynthetic surface inhibiting the movement of

the upper box. Therefore, two lateral clamps were added although some restriction of

geosynthetic deformations at the border could still occur.

The normal stress is applied by a rigid steel plate covering the entire internal area

of the upper box connected to a hanger with weights. The assurance that the normal

force approximately passes through the centre of gravity of the upper box is guaran-

teed by two wedges inclined 1:2 and placed on the front and back walls of this box. A

load cell, located between the load beam and the rigid plate, is used to measure the

applied normal force.

The upper box slides on rollers in laterally set runners. These lateral plates allow

adjustment of the height between the base of the upper box and the geosynthetic speci-

men such that the upper box itself does not load the specimen, minimising, however,

the loss of soil from the box during the test.

All measurements (i.e., displacement of the upper box, angle of the inclined plane

to the horizontal, and normal force) are recorded with time by an automatic data acqui-

sition system.

3 MATERIALS

3.1 Soils

Two sands, with the particle size distributions shown in Figure 3, were used in the

tests. These same soils were used in a previous study and are described in detail byLopes and Lopes (1999).

Soil 1 minimum and maximum unit weight values range from 15.0 to 17.9 kN/m 3

for soil particle diameter values that typically range from 0.0074 to 2.00 mm (Figure

3). Soil 2 minimum and maximum unit weight values range from 15.6 to 18.7 kN/m 3

for soil particle diameter values that typically range from 0.0074 to 9.54 mm (Figure

3). The physical properties of the two soils are given in Table 1.

The relative density values, I D , of Soils 1 and 2 are calculated using the following

expression:

(1)

where: γ min and γ max = minimum and maximum soil unit weight values; and γ = unitweight of the soil with a relative density of I D = 50%.

Both soils were compacted to a target relative density of I D = 50%. For I D = 50%, γ

is equal to 16.32 and 17.01 kN/m3 for Soils 1 and 2, respectively.

I D

γ max

γ -----------

γ γ mi n –

γ ma x γ mi n – ----------------------------⋅

100%×=


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Note: *Soil residual friction angle (at a vertical pressure of 10 kPa) in direct shear.

3.2 Geosynthetics

The following geosynthetics were used in the inclined plane test program:

1. high density polyethylene uniaxial geogrid (Geogrid GG1);

2. polypropylene biaxial geogrid (Geogrid GG2);

3. polypropylene nonwoven, spun-bonded geotextile (Geotextile GT1);

4. polypropylene woven geotextile (Geotextile GT2);

5. 2 mm-thick, high density polyethylene smooth geomembrane (Geomembrane GM1);

and

6. 2 mm-thick, high density polyethylene rough geomembrane (Geomembrane GM2).

The most significant geosynthetic properties for the present study are presented in

Tables 2 to 4 and in Figures 4 and 5. Figures 6 and 7 are photographs of the surfaces of

Geotextiles GT1 and GT2 and Geomembranes GM1 and GM2, respectively.

Table 1. Physical properties of the soils used in the test program.

Soil γ min

(kN/m3)

γ max(kN/m3)

γ (ID=50%)(kN/m3)

D50(mm)

D10(mm)

C u( D60 / D10)

φ *

(º)

Soil 1 15.0 17.9 16.32 0.43 0.18 2.94 36.2

Soil 2 15.6 18.7 17.01 1.3 0.44 3.64 49.5

Soil 1

Soil 2

Particle size (mm)

0.05 0.1 0.5 1 5 10

0

10

20

30

40

50

60

70

80

90

100

F r a c t i o n p a s s i n g ( % )

Figure 3. Particle size distributions for Soils 1 and 2.


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4 TEST METHOD

The tests were carried out using the following two methods:

1. with a rigid support for the geosynthetics (Test method 1); and

2. with the geosynthetic supported on a lower box filled with soil (Test method 2).

In Test method 1, the specimen is fixed on the rigid base. In Test method 2 the lower

box is filled with the same soil and the geosynthetic specimen is placed over the soil and

fixed. In both cases, the specimen must be flat and free from folds and wrinkles. The

Table 2. Characteristics of uniaxial Geogrid GG1.

Geosynthetic a L(mm)

aT (mm)

b B(mm)

b R(mm)

t B(mm)

t R(mm)

Tensile

strength

(kN/m)

Peak

strain

(%)

Tensile

stiffness

(kN/m)

Geogrid GG1 160.0 16.0 16.0 6.02.7 max

2.5 min0.9 55.0 11.5 478.3

Table 3. Characteristics of biaxial Geogrid GG2 (MD = machine direction, XMD = cross-

machine direction).

Geosynthetica L

(mm)

aT (mm)

b LR(mm)

bTR(mm)

t J (mm)

t LR(mm)

t TR(mm)

Tensile strength Peak

strain

(%)

Tensile

stiffness

(kN/m)MD

(kN/m)

XMD

(kN/m)

Geogrid GG2 33.0 33.0 2.2 2.5 5.8 2.2 1.4 40.0 40.0 11.5 347.8

Table 4. Characteristics of Geotextiles GT1 and GT2 (MD = machine direction).

GeosyntheticMass per unit area

(g/m2)

Thickness

(mm)

Tensile strength

(kN/m)

Peak strain, MD

(%)

Tensile stiffness

(kN/m)

Geotextile GT1 800.0 6 50.0 65.0 76.9

Geotextile GT2 600.0 1 70.0 11.5 608.7

t R

t B

aL

bB aT

bR

Figure 4. Geometry of uniaxial Geogrid GG1.


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geosynthetic specimens were 0.70 m long and 0.43 m wide for Test method 1 and 0.60

m long and 0.36 m wide for Test method 2.

In Test method 2, the soil was poured into the box from a constant height of 0.20 m

and placed in 0.020 m-thick layers. Each layer was levelled and compacted to the

required density using a light compacting hammer. Due to the dimensions of the box, it

was not possible to control the soil density with a nuclear densimeter. Instead, the soil

density was controlled by measuring the soil volume necessary to place the soil in the

box with the required unit weight and by defining the soil volume to be used in each layer.

After fixing the geosynthetic specimen, the upper box was assembled and aligned

in the starting position. The upper box was then filled with soil using procedures simi-

t LR aL bLR aT

bTR

t TR t J

Figure 5. Geometry of biaxial Geogrid GG2.

Figure 6. Surfaces of the woven geotextile, Geotextile GT2 (left), and of the nonwoven

geotextile, Geotextile GT1 (right).


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lar to the ones used for the lower box. The rigid plate was placed, the load beam was

positioned, and the weights were applied. The tests were carried out with a normal

stress value of 10kPa. Finally, the test rate was chosen, the horizontality of the base

was verified, and the transducer and the inclinometer were set to zero.The test program carried out to study the friction characteristics of the geosynthet-

ics is presented in Table 5. Although 24 tests are reported in Table 5, each one was

repeated twice and, thus, a total of 72 tests were carried out.

5 ANALYSIS OF TEST RESULTS

5.1 Introduction

Each test was repeated twice to determine a mean value and to ensure repeatability of

the test results. A satisfactory degree of repeatability of test results was achieved as

indicated in Figure 8, which shows the variation of the measured upper box displace-

ment with the inclination of the rigid base for the three tests conducted using Geomem-

brane GM1 specimens under the same conditions. For the three tests, the greatestvariation of the measured maximum inclination of the rigid base was 3.3% (0.3% and

3.0% for the other two tests). The maximum value for the coefficient of variation, for

all the tests carried out, was approximately 5%.

Figure 7. Surfaces of the smooth geomembrane, Geomembrane GM1 (top), and the

rough geomembrane, Geomembrane GM2 (bottom).


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The data collected from the inclined plane shear tests includes the variation of the

upper box displacement with the inclination of the rigid base. The inclination for the

maximum upper box displacement (50 mm) provides information regarding the slip-

ping angle of the upper box, β , which allows the derivation of the soil-geosynthetic

interface friction angle values, φ sg .

The normal stress, σ n , applied when the slipping angle of the upper box is equal to

β can be derived as follows:

(2)

where: F v = vertical force acting on the soil-geosynthetic interface; and A = soil-geo-

synthetic contact area.

Table 5. Summary of the tests carried out in the inclined plane shear test program.

Test Geosynthetic Soil Test method

T1 Geogrid GG1

Soil 1 1

T2 Geogrid GG2

T3 Geotextile GT1

T4 Geotextile GT2

T5 Geomembrane GM1

T6 Geomembrane GM2

T7 Geogrid GG1

Soil 1 2

T8 Geogrid GG2

T9 Geotextile GT1

T10 Geotextile GT2

T11 Geomembrane GM1T12 Geomembrane GM2

T13 Geogrid GG1

Soil 2 1

T14 Geogrid GG2

T15 Geotextile GT1

T16 Geotextile GT2

T17 Geomembrane GM1

T18 Geomembrane GM2

T19 Geogrid GG1

Soil 2 2

T20 Geogrid GG2

T21 Geotextile GT1

T22 Geotextile GT2T23 Geomembrane GM1

T24 Geomembrane GM2

σ n

F v β cos

A-------------------=


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The shear stress, τ , at the sliding surface is defined as:

(3)

where f ( β ) is the force required to restrain the empty upper box when the tilt table is

inclined at the angle β .

The soil-geosynthetic interface friction angle values, φ sg , can be calculated using

the following equation:

(4)

5.2 Influence of Geosynthetic Structure

To evaluate the influence of the geosynthetic structure, the results corresponding to

tests carried out with Soil 1 and Test method 1 (Tests T1 to T6 in Table 5) were consid-

ered (Table 6).

Table 6. Average soil-geosynthetic interface friction angle values obtained using Soil 1

and Test method 1 (Tests T1 to T6 in Table 5).

GeosyntheticGeogrid

GG1

Geogrid

GG2

Geotextile

GT1

Geotextile

GT2

Geomembrane

GM1

Geomembrane

GM2

φ sg (º) 27.6 30.1 32.2 30.5 21.4 31.2

τ F v β sin f β ( )+

A--------------------------------=

φ sg tan τ

σ n------ φ sg

τ

σ n------atan=⇒=

0 5 10 15 20 25

0

20

40

60

Inclination (º)

D i s p l a c e m e n t ( m m )

Figure 8. Inclined plane shear test results for three tests using Geomembrane GM1.


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When comparing the results obtained for the two geomembranes, a difference of

approximately 46% in φ sg values was measured when using a rough material

(Geomembrane GM2, with a soil-geosynthetic interface friction angle value of 31.2º)instead of a smooth material (Geomembrane GM1, with a soil-geosynthetic interface

friction angle value of 21.4º). The roughness of the surface is responsible for the

increasing resistance.

For the considered geotextiles, the lowest soil-geosynthetic interface friction value

was obtained for the woven material (Geotextile GT2), due to its smoother surface, which

is a result of the manufacturing process (where filaments are arranged in the machine

direction and in the cross-machine direction). The difference between the soil-geotextile

interface friction values for Geotextiles GT1 and GT2 was approximately 6%.

The following are interaction mechanisms that can be mobilised in the soil-geogrid

interface: (i) skin friction over the planar geogrid surface; (ii) soil-soil friction through

the geogrid apertures; and (iii) passive resistance of the geogrid bearing members.

When the soil-geogrid interface pullout interaction mechanism occurs, the third mech-

anism is very important, as shown by Lopes and Lopes (1999). Before analysing the

geogrid test results, it is important to consider that, for the inclined plane shear test (as

well in the direct shear test), it is not possible to consider the contribution of that mech-

anism on the soil-geogrid interface resistance. However, the inclined plane shear test

provides information on the interface shear resistance mobilized at the soil-geogrid

interface when shear is the predominant mechanism of interaction.

The soil-geogrid interface friction angle values of 27.6º and 30.1º, which were

obtained for Geogrids GG1 and GG2, respectively, indicate that the friction mobilised

on the interface is higher for the biaxial (Geogrid GG2) than for the uniaxial geogrid

(Geogrid GG1). Geogrid GG1 has a higher lateral solid area when compared with

Geogrid GG2; therefore, the more favoured behaviour of Geogrid GG2 can be the

result of more effective mobilisation of the friction in the soil underneath, through the

biaxial geogrid apertures.Comparing all of the materials tested, the lowest soil-geosynthetic interface friction

angle value obtained was 21.4º, which corresponds to Geomembrane GM1 (continuous

material with the smoothest surface), while the highest value was 32.2º, which was

obtained for Geotextile GT1 (having the roughest surface). Therefore, the structure of

the geosynthetic, particularly its roughness, plays a very important role in the soil-geo-

synthetic interface resistance.

5.3 Influence of Soil Particle Size

Soil particle size is one of the factors affecting the soil-geosynthetic interface behav-

iour and has been studied by Jewell et al. (1984), Palmeira and Milligan (1989), Jewell

(1990, 1996), Boyle and Holtz (1994), Chen and Chen (1994), Forsman and Slunga

(1994), and Lopes and Lopes (1999).As in the study presented by Lopes and Lopes (1999), because two different soils

are considered, parameters, such as soil unit weight, are distinct and affect the

soil-geosynthetic interface behaviour. To minimize this problem, the two soils were

placed in the box with the same relative density by adjusting the compaction process.


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The results obtained in the tests carried out on all the geosynthetics, using Test

method 1, with Soils 1 and 2 (Tests T1 to T6 and T13 to T18 in Table 5) are presented

in Table 7.Geogrids GG1 and GG2 exhibit different behaviour when in contact with either

Soils 1 or 2. For Geogrid GG1 in contact with Soil 1, the soil-geosynthetic interface

friction angle is 27.6º, while for the Geogrid GG1-Soil 2 interface the value is 29.5º.

The Geogrid GG2-Soil 1 interface friction angle is 30.1º, while for the Geogrid GG2-

Soil 2 interface the value is 33.0º. For both Geogrids GG1 and GG2, there is an

increase in the soil-geosynthetic interface friction angle in the tests performed with

Soil 2 instead of Soil 1: 6.9 and 9.6%, respectively. This difference is due to the wider

soil particle size distribution of Soil 2, which increases the soil-geosynthetic contact

surface, and the higher friction angle of Soil 2. The increase of the soil-geogrid inter-

face resistance is larger for Geogrid GG2, due to the reasons discussed in Section 5.2.

As in the case of geogrids, there is an increase of the soil-geotextile interface resis-

tance when Geotextiles GT1 and GT2 are in contact with Soil 2 instead of Soil 1. This

difference increases with a decreasing roughness of the geotextile surface: 2.2% for

Geotextile GT1 and 10.2% for Geotextile GT2.

Geomembranes GM1 and GM2 exhibit different behaviours when in contact with

either Soils 1 or 2. For the Geomembrane GM1-Soil 1 interface, the soil-geosynthetic

interface friction angle is 21.4º, while for the Geomembrane GM1-Soil 2 interface the

value is 24.5º. For the Geomembrane GM2-Soil 1 interface, the soil-geosynthetic

interface friction angle is 31.2º, while for the Geomembrane GM2-Soil 2 interface the

value is 32.9º. For Geomembrane GM1, there is a 14.5% increase in the soil-geosyn-

thetic interface friction angle for the tests performed with Soil 2 instead of Soil 1. This

difference is due to the wider soil particle size distribution of Soil 2, which increases

the soil-geosynthetic contact surface. The meaningless difference measured for

Geomembrane GM2 is justified by the roughness of the geosynthetic surface. The geo-

synthetic with the large increase in the soil-geosynthetic interface resistance isGeomembrane GM1 (14.5%), the one with the smoothest surface.

It is also clear that the less resistant interfaces are Geomembrane GM1 and Geogrid

GG1, independent of the soil considered. The most resistant interfaces are Geogrid

GG2-Soil 2 and Geotextile GT2-Soil 2 (Soil 2 has a wider soil particle size distribution

and a large average soil particle size).

Table 7. Average soil-geosynthetic interface friction angle values obtained using Test

method 1 and Soils 1 and 2 (Tests T1 to T6 and T13 to T18 in Table 5).

Parameter Soil

Geosynthetic

Geogrid

GG1

Geogrid

GG2

Geotextile

GT1

Geotextile

GT2

Geomembrane

GM1

Geomembrane

GM2

φ sg (º)Soil 1 27.6 30.1 32.2 30.5 21.4 31.2

Soil 2 29.5 33.0 32.9 33.6 24.5 32.9

∆φ sg (%) 6.9 9.6 2.2 10.2 14.5 5.4


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5.4 Influence of Test Method

To evaluate the influence of the test method, the test results for Soil 1 and Test Meth-ods 1 and 2 (Tests T1 to T12 in Table 5) are considered (Table 8).

The results obtained using Geogrid GG2 show a slight variation of the soil-geosyn-

thetic interface friction angle (increase of 2.3%) when using Test method 2 instead of

Test method 1. For Geogrid GG1, the increase of the soil-geogrid interface friction

angle is higher, approximately 15%, when using Test method 2. These results can be

explained as follows:

1. The relationship between aperture area and solid surface area of the two geogrids is

larger for Geogrid GG2 than for Geogrid GG1.

2. The joints of Geogrid GG2 exhibit a rebound in the lateral superior and inferior

surfaces, while the joints of Geogrid GG1 are perfectly smooth.

3. Using Test method 1 on Geogrid GG2, the rebound in the joints on the lateral infe-

rior surface and the significant lateral open area allow the existence of a thin layer of soil beneath the geogrid. This is similar to the test conditions using Test method

2; when testing Geogrid GG1, this layer of soil is not as significant.

4. The mobilized friction on the geogrid apertures is much lower using Test method 1

on Geogrid GG1 than Test method 2, due to the formation of a soil-polished metal

interface and soil-soil interface, respectively.

As discussed in Section 5.2, considering Test method 1, the friction angle of the

soil-geogrid interface is higher for Geogrid GG2 than for Geogrid GG1. However, it

was verified that Test method 2 is more adequate to study soil-geogrid interface resis-

tance, and the conclusion referred in Section 5.2 has to be revised. In fact, using Test

method 2, the geogrid that allows the mobilization of higher interface strength is Geo-

grid GG1 (31.8º) and not Geogrid GG2 (30.8º).

The influence of the test method on the results obtained for the geotextiles tested is

small. There is an increase in the soil-geotextile interface friction angle value of 3%, for

both Geotextiles GT1 and GT2, when using Test method 2 as an alternative to Test method 1.

The sensitivity of the geomembranes to the test method is very minimal, decreasing

with the roughness of the geomembrane. In fact, there is a 3.7% and 0.3% decrease in

the soil-geomembrane interface friction angle for Geomembranes GM1 and GM2,

respectively, when Test method 2 is used instead of Test method 1.

Table 8. Average soil-geosynthetic interface friction angle values obtained using Soil 1

and Test methods 1 and 2 (Tests T1 to T12 in Table 5).

Parameter Test

method

Geosynthetic

Geogrid

GG1

Geogrid

GG2

Geotextile

GT1

Geotextile

GT2

Geomembrane

GM1

Geomembrane

GM2

φ sg (º)1 27.6 30.1 32.2 30.5 21.4 31.2

2 31.8 30.8 33.2 31.4 20.6 31.1

∆φ sg (%) 15.2 2.3 3.1 3.0 -3.7 -0.3


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As a final remark, it is important to consider the following factors:

1. The influence of the test method is not significant for bi-dimensional continuous

geosynthetics structures (geotextiles and geomembranes).

2. In the case of geogrids, with significantly large aperture areas, other issues can con-

trol the validity of testing these materials using Test method 1. Therefore, site or

project specific characteristics are the most important factors to consider when

choosing the most appropriate test method.

6 CONCLUSIONS

The present study considers the influence of geosynthetic structure, soil particle size,

and test method on the behaviour of geosynthetics in the inclined plane shear test. The

following are the most significant conclusions of this study:

• Geosynthetic structure has an important influence on the soil-geosynthetic inter-face friction angle.

• Geosynthetic surfaces with significantly large apertures (e.g., geogrids) are associ-

ated with the mobilisation of soil-soil friction; therefore, higher soil-geosynthetic

interface friction angle values are obtained.

• Geosynthetic surfaces allowing the penetration of soil particles into the geosyn-

thetic (e.g., nonwoven, spun-bonded geotextiles) are associated with higher

soil-geosynthetic interface friction angle values.

• Geosynthetic surface roughness (e.g., geomembranes) is associated with higher

soil-geosynthetic interface friction angle values.

• Soil particle size appears to have an important influence on soil-geogrid interface

friction angle values.

• Broadly graded soils with larger average soil particle sizes allow an increase inthe soil-geosynthetic interface resistance that is more significant to smooth geo-

synthetic surfaces.

• The choice of test method does not affect the soil-geomembrane or soil-geotextile

interface friction angle values obtained (geosynthetics with continuous surfaces).

• The validity of evaluating the soil-geogrid interface resistance using Test method 1

depends on the structure of the geogrid. It is suggested that site or project specific

characteristics are the most important factors to consider when closing the most

appropriate test method.

REFERENCES

Boyle, S.R. and Holtz, R.D., 1994, “Deformation Characteristics of Geosynthetic-Reinforced Structures”, Proceedings of the Fifth International Conference on Geo-

textiles, Geomembranes and Related Products, Vol. 1, Singapore, pp. 361-364.

Chen, R.-H. and Chen, C.-C., 1994, “Investigation of Pull-Out Resistance of Geo-


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grids”, Proceedings of the Fifth International Conference on Geotextiles, Geomem-

branes and Related Products, Vol. 1, Singapore, pp. 461-464.

Forsman, J. and Slunga, E., 1994, “The Interface Friction and Anchor Capacity of Syn-

thetic Georeinforcements”, Proceedings of the Fifth International Conference on

Geotextiles, Geomembranes and Related Products, Vol. 1, Singapore, pp. 405-410.

Jewell, 1990, “Reinforced Bond Capacity”, Geotechnique, Vol. 40, No. 3, pp. 513-518.

Jewell, 1996, “Soil Reinforcement with Geotextiles”, CIRIA and Thomas Telford Ltd,

London, United Kingdom, 332 p.

Jewell, R.A., Milligan, G.W.E., Sarsby, R.W., and Dubois, D., 1984, “Interaction

Between Soil and Geogrids”, Polymer Grid Reinforcement , Thomas Telford Ltd.,

Proceedings of a conference held in London, United Kingdom, March 1984, pp.

18-30.

Lopes, M.J. and Lopes, M.L., 1999, “Soil-Geosynthetic Interaction – Influence of Soil

Particles Size and Geosynthetic Structure”, Geosynthetics International , Vol. 6, No. 4, pp. 261-282.

Palmeira, E.M. and Milligan, G.W.E., 1989, “Scale and Other Factors Affecting the

Results of Pull-Out Tests of Grids Buried in Sand”, Geotechnique, Vol. 39, No. 3,

pp. 511-524.

prEN ISO 12957-2, “Geotextiles and geotextiles-related products - Determination of

friction characteristics – Part 2: inclined plane method ”.

NOTATIONS

Basic SI units are given in parentheses.

A = soil-geosynthetic contact area (m2)

a L = distance between uniaxial geogrid bearing members, or distance

between biaxial geogrid ribs (m)

aT = aperture width of uniaxial geogrids, or transverse distance between

biaxial geogrid ribs (m)

b B = width of uniaxial geogrid bearing members (m)

b LR = width of biaxial geogrid longitudinal members (m)

b R = width of uniaxial geogrid longitudinal members (m)

bTR = width of biaxial geogrid transverse members (m)

C u = coefficient of uniformity (dimensionless) D10 = particle diameter corresponding to 10% by weight of finer particles (m)

D50 = particle diameter corresponding to 50% by weight of finer particles (m)


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D60 = particle diameter corresponding to 60% by weight of finer particles (m)

F v = vertical force acting on soil-geosynthetic interface (N) f ( β ) = force required to restrain the empty upper box at inclination of β (N)

I D = relative density of soil (%)

t B = thickness of uniaxial geogrid bearing members (m)

t J = thickness of biaxial geogrid junctions (m)

t LR = thickness of biaxial geogrid longitudinal ribs (m)

t R = thickness of uniaxial geogrid ribs (m)

t TR = thickness of biaxial geogrid transverse ribs (m)

β = slipping angle of upper box (º)

φ = internal friction angle of soil (º)

φ sg = friction angle of soil-geosynthetic interface (º)

γ = unit weight of soil (N/m3)

γ (ID = 50%) = unit weight of soil at relative density of 50% (N/m3)

γ max = maximum unit weight of soil (N/m3)

γ min = minimum unit weight of soil (N/m3)

σ n = normal stress (N/m2)

τ = shear stress (N/m2)

shear behaviour of geosynthetics in the inclined plane test –

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