- Email: [email protected]
Pull-out resistance of geogrid reinforcements
Geotextiles and Geomembranes 12 (1993) 133-159
Pull-Out Resistance of Geogrid Reinforcements Khalid Farrag Geosynthetic Engineering Research Laboratory (GERL), Louisiana Transportation Research Center (LTRC), Gourier Road, Baton Rouge, Louisiana 70803, USA
Yalcin B. Acar* Civil Engineering Department, Louisiana State University, Baton Rouge, Louisiana 70803, USA
&
Ilan Juran Department of Civil Engineering, Polytechnic University, 333 Jay Street, Brooklyn, New York, New York 11201, USA (Received 30 September 1991: accepted 25 November 1991)
ABSTRACT Testing equipment, specimen preparation and testing procedures are presented for load-controlled and displacement-rate-controlled pull-out tests for geosynthetic reinforcements in granular soils. The influence of the test type, confining pressure, soil density, boundary conditions, and geotextile characteristics on pull-out load-displacement response of selected geogrids embedded in sand are evaluated. Implications for testing procedure and analysis are discussed.
INTRODUCTION A wide variety o f geotextiles a n d geogrids are n o w a v a i l a b l e for civil e n g i n e e r i n g a p p l i c a t i o n s ( B o n a p a r t e et al., 1987; Koerner, 1990). In *To whom correspondence should be addressed. 133 GeotextUes and Geomembranes 0266--1144/93/$06.00 © 1993 Elsevier Science Publishers Ltd. England. Printed in Great Britain
134
Khalid Farrag. Yalcin B. Acar, llan Juran
selecting a specific geotextile or geogrid for reinforcement of embankments and slopes, the following performance aspects need to be assessed: (i) stress-strain-strain rate behavior of the confined reinforcementsoil composite system, (ii) pull-out performance and the associated load transfer mechanism. A standard testing procedure for confined properties ofgeosynthetics does not yet exist. The large number of factors that affect the interface properties of the confined reinforcement raise major difficulties in providing comparable test results. There is a wide scatter in the available pull-out test results. These differences in results are due to the use of different types of pull-out devices, the associated boundary effects, testing procedures and soil placement and compaction schemes (Juran et al., 1988). The increasing use of geosynthetics in soil reinforcement prompts the need to develop and standardize methods in evaluating the in-soil mechanical characteristics and interface properties of these materials. It becomes necessary to design adequate testing equipment, establish reliable testing procedures, and develop appropriate interpretation schemes. This paper presents the pull-out testing program implemented at the Geosynthetic Engineering Research Laboratory (GERL), a laboratory of Louisiana Transportation Research Center (LTRC) and Civil Engineering Department of Louisiana State University, to develop reliable testing procedures and interpretation schemes in evaluation of the short-term and long-term pull-out performance ofgeosynthetic reinforcements. The fundamental aspects of pull-out testing equipment and procedure are reviewed. A pull-out box that incorporates schemes which overcome most of the current limitations is presented. The factors that affect the measured interaction properties are evaluated by performance evaluation tests. Implications for engineering analysis are discussed.
FUNDAMENTAL ASPECTS OF PULL-OUT TESTING The shear stress-strain relationship developed along the soil-reinforcement interface is commonly tested in a direct shear box and/or a pull-out box. In the direct shear box, tests are usually conducted in accordance with the conventional procedure used in investigating interface properties (Acar et al., 1982). Commonly, a soil sample resting on a geotextile is sheared along the interface and the shear force-displacement behavior is recorded. In the pull-out box, a geotextile confined in soil is pulled out and often the pull-out load and the front displacement are recorded. In
Pull-out resistance of geogrid reinforcements
135
both tests, results are often expressed in terms of a friction ratio, tan 6/tan ~, (also called the efficiency factor) where 6 is the soil-reinforcement interface friction angle and O is the soil friction angle. Efficiency factors ranging from 0.6 to 1.0 for geotextiles and values larger than one for geogrids are reported (Juran et al., 1988; Koerner, 1990). Figure 1 presents a comparison of efficiency factors obtained from tests conducted using pull-out and direct shear testing equipment. The frictional resistances reported for different types of inclusions in dense sands are found to be greater in pull-out tests than those obtained in direct shear tests (Jewell, 1979; Schlosser & Guilloux, 1979). Ingold and Templeman (1979) found comparable values in both tests for geogrids tested under low confining pressures. However, under higher normal stresses, higher shear strength values were obtained in pull-out tests. Koerner (1986) reported higher shear resistance for geogrids in pull-out tests while, under higher confining pressures, direct shear tests gave higher shear resistance. Rowe et al. (1985) reported that both tests give approximately equal values of shear resistance for geotextiles tested in dense granular fill while for geogrids in a loose fill, pull-out tests rendered significantly lower values. Direct shear and pull-out tests are associated with different testing procedures, loading paths, failure mechanisms, and boundary conditions. Consequently, the interface frictional parameters obtained from both 3.5 ~ Direct Pull-out Reference Reinfon:emem Soil Shear Type Type t'-'O------'"+ lngot,J(l~83)TemmrSX-~ ~ S ~ ---4~--A Colins(1980) Nonwovm Grovel(cnu4t~l)
3.0
1
2.s 2.0
~
I---O--. + |........
~, ~
xo~,,~09s6) T ~ SX-2 mm s ~ x. . . . .9s6) ~z-100 D~S,~ Row,:(1985)
Tensar
$R-2 Loosesand
1.5
1.0
A. . . . . . . . L ~
.......
A
C]
o.sl ~ 0.0 ~ 0
50
100
150
200
250
300
NORMAL STRESS (kN/m2)
Fig. I. C o m p a r i s o n o f the efficiency factors o b t a i n e d for s o i l - g e o s y n t h e t i c systems in direct s h e a r a n d pull-out boxes.
136
Khalid Farrag, Yalcin B. Acar, llan Juran
tests can vary and often provide conflicting results. The reasons for the reported differences between the pull-out and direct-shear test results can be categorized as: (1) Restrained dilatancy: Cohesionless soils will dilate at low to medium confining pressures and at high densities. If this dilatancy is restrained in testing, the confining pressure along the interface will increase until a state is reached where failure can be achieved without any volume change (critical state). The amount of this restraint and the magnitude of the increase in confinement will depend upon the type and geometry of the imposed boundary conditions. These distances are often selected arbitrarily in the boxes used. Furthermore, the pull-out boxes are often designed with fixed boundaries while displacements are allowed in the top platten in the direct shear box. Fixed boundaries may promote restrained dilatancy and increased confinement. Therefore, even when the initial states are identical, different stress-deformation behavior will be developed due to confinement induced by boundary conditions. (2) Failure and interaction mechanism: The interaction mechanism is different in direct shear and pull-out. In direct shear tests, when the box dimension is large, the mobilized shear strain is postulated to be uniformly distributed along the soil-geosynthetic interface. While in the pull-out tests, mobilized strain is a combination of the interface shear strain and reinforcement extension. This coupled mechanism results in a non-uniform shear strain-stress distribution along the reinforcement. A realistic experimental model of pull-out behavior ofgeosynthetics in the field should incorporate extensibility. Pull-out boxes are more appropriate when extensibility is to be incorporated. A standard design for pull-out testing devices does not yet exist. Therefore, box dimensions and testing procedures differ from one box to the other. The dimensions of the box are usually selected as large as technically possible with the anticipation that boundary effects will be minimized. An evaluation of most of the available equipment and testing parameters demonstrates that the differences in testing equipment and procedures make comparisons of available test results extremely difficult (Juran et al., 1988; Farrag, 1990). The following is a list of desirable features in pull-out testing procedure and equipment: (1) The pull-out tests reported are often conducted at a controlled displacement rate. Few pull-out tests are reported under load-
PuU-out resistance of geogrid reinforcements
(2)
(3)
(4)
(5)
137
controlled mode (e.g., Tzong & Cheng-Kuang, 1987). Pull-out testing equipment should have the capability to provide loadcontrolled tests to facilitate investigation of time-dependent, in-soil creep behavior. In displacement-rate controlled tests, a range of pull-out rates are reported varying from 0.1 ram/rain to 20 mm/min. Myles (1982) studied the frictional resistance of the geotextile interface under different strain rates (10-75 mm/min) in a direct shear box and showed that results had little sensitivity to this range of displacement rates. In wide strip tests, Rowe and Ho (1986) showed that the geogrid tension strength varies with the applied rate of strain. It is necessary to establish the effect of pull-out displacement-rate on test results in different types of soils. The interaction between the soil and the side walls of the pull-out box can affect test results. Soil confinement is usually applied by means of flexible air-bags to insure uniform distribution of normal stress along a plane (Christopher, 1976; Palmeira & Milligan, 1989; Ingold, 1983). The applied confining stress is partially carried out by the side wall friction causing a reduction in the normal pressure applied at the reinforcement level. Some investigators inserted lubricated membranes along the walls in an attempt to provide a low friction boundary (Jewell, 1980). Alternatively, the specimen width/box width ratio may be chosen so as to minimize the effect of friction by side-walls. The interaction between the reinforcement-soil system and the rigid front wall can also influence test results. As the reinforcement is pulled out of the box, the lateral earth pressure developed on the front face can result in an increase in the pull-out resistance. Palmeira and Milligan (1989) used a front wall with different degrees of roughness to investigate the effect of friction on the front wall on pull-out test results. Christopher (1976) incorporated sleeves around the pull-out slot to transfer the point of application of the pull-out load far behind the rigid front wall. Other investigators (Williams & Houlihan, 1987) used flexible front faces to minimize the rigid front wall effect. The front wall interaction should be minimized in a pull-out box. The thicknesses of the soil above and below the reinforcement (referred as the soil thickness), differ according to the available clear height of the box. If soil thickness is small, the interaction between the soil-reinforcement system and the boundaries will significantly influence the shear resistance along the interface. Brand and Duffy (1987) studied the effect of soil thickness on pull-
138
Khalid Farrag, Yalein B. Acar, Ilan Juran
out resistance of a geogrid in clays. Their results demonstrate that as the soil thickness increases, pull-out resistance decreases until a minimum pull-out load is obtained. It is essential to decide upon the soil thickness in conducting a standard pull-out test. (6) Different specimen preparation and compaction procedures are utilized to insure uniform soil density. Soil compaction is carried out by means of an electric jack hammer (Johnston, 1985), standard proctor hammer (Saxena & Budiman, 1985), hand tamping devices (Elias, 1979) and by mechanical tamping (Anderson & Neilsen, 1985). A hopper with flexible tube is also used to insure uniform soil placement (Palmeira & Milligan, 1989; Jewell, 1980). Different specimen preparation procedures will result in differences in fabric leading to significantly different stress-deformation behavior. It then becomes essential to standardize specimen preparation procedures in commercial testing of confined behavior of geosynthetic reinforcements. (7) Several investigators (Christopher, 1976; Koerner, 1986; Brand & Duffy, 1987) clamped the reinforcement outside the box. This technique may result in an unconfined front portion of the reinforcement. Consequently, the effective interface area will vary and confined/unconfined properties of the geosynthetic will couple during the pull-out test. (8) The existing pull-out testing equipment are often instrumented to monitor only the displacement and the pull-out resistance at the front face. For extensible reinforcements such as geogrids and geotextiles, it is essential to monitor the displacements along the inclusion in order to interpret the load transfer mechanism and estimate the pull-out resistance in the field. The measured pull-out resistance is influenced by factors which include the details of testing equipment and procedures discussed above, and also the following compositional and environmental variables influencing the behavior of the composite system: (i)
the compositional characteristics of the geosynthetic reinforcement such as its type, geometry and configuration leading to different extensibility and load-strain-strain rate behavior, (ii) compositional characteristics of the reinforced soil such as its grain size distribution, and void ratio, (iii) environmental variables defining the initial state of the composite system such as the overburden pressure and the nature of the imposed deformation (load controlled or displacement rate controlled).
Pull-out resistance of geogrid reinforcements
139
Two pull-out boxes which permit an evaluation of different factors influencing pull-out response are designed and constructed. A testing program is implemented in performance evaluation of the boxes.
TESTING PROGRAM The primary objective of the performance evaluation study was to assess the sensitivity of test results obtained in the designed equipment to the changes in the fundamental testing parameters (geogrid type, specimen width, sleeve length, soil thickness, displacement rate, soil density and confining pressure) and to establish a data base for development of reliable testing and interpretation procedures. Table 1 presents the implemented testing program.
Equipment A schematic diagram of the box and associated instrumentation are presented in Fig. 2. Figure 3 presents a view of the pull-out box and the sand hopper system. The different components and characteristics of the box are described below: (i)
The inner dimensions of the pull-out box are 1-52 m (60 in) long, 0-90 m (36 in) wide and 0.76 m (30 in) high. The box is constructed in modular units to allow changes in box dimensions and front wall opening size. A sleeve is used at the facing to transfer the interface pull-out load behind the rigid front wall. An air bag is used at the top to provide a uniformly distributed vertical pressure. (ii) Pull-out load is applied by a hydraulic loading system through clamping plates that extend inside the sleeve. This arrangement ascertains that the geosynthetic specimen remains confined throughout the test. The loading system can apply either a constant pull-out rate or a constant pull-out load. Load controlled mode is used to evaluate the creep behavior. (iii) The front displacements, the pull-out rate and the pull-out load are acquired/monitored at the clamping plates by means of a LVDT, velocity transducer and a load cel, respectively. Displacements along the reinforcement are measured by tell-tale wires connected to LVDTs. Two earth pressure cells are placed on the front wall.
Khalid Farrag, Yalein R Aear, llan Juran
140
Table I
Testing Program and Testing Parameters Purpose~
Geogrid type
Dimension L/W (m)
Repeatability
Tensar SR2 Conwed 9027
1.00/0.30 i.00/0-30
Specimen width
Tensar SR2
Unit weight (kN/m 3)
Displacement rate, 6 (mm/min)
48-2
16.5
6.0
1.00/0-30 1.00/1~45 i.00/0.60 1-00/0-75
48.2
16.7
4.0
Sleeve length Tensar SR2 (0, 20 and 30 cm)
!.00/0-30
48.2
16.4
20.0
Soil thickness (20, 40, 60 and 70 cm)
Tensar SR2
1.00/0.30
48.2
16.4
6-0
Displacement rate
Tensar SR2
1.00/ff30
48.2
16.4
4-0 6.0 10-0 20-0
Soil density
Tensar SR2
1.00/0.30
48.2
15.7 16.4 16.7 17-0
4-0
Confining pressure
Tensar SR2
1.00/1~30
Conwed 9027
1.00/0.30
34-0 48.2 69-0 48-2 96.0 140-0
16.5 16.5 16.5 16.7 16.7 16.7
6-0 6-0 6.0 4.0 4.0 4.0
Conwed 9027
1.00/0-30
48.2
16.7
2.0
Effect of transverse fibs
Confining pressure (kN/m 2)
~Unless otherwise noted, all tests are conducted at a displacement rate of 6 mm/min, at a soil thickness of 60 cm (30 cm top and 30 cm bottom) and a sleeve length of 30 cm.
(iv) A n e l e v a t e d s a n d h o p p e r w i t h v a c u u m e x t r a c t i o n is u s e d to facilitate sand placement and removal from the box. (v) A d a t a a c q u i s i t i o n s y s t e m m o n i t o r s t h e i n p u t p a r a m e t e r s a n d r e c o r d s t h e r e s p o n s e p a r a m e t e r s (e.g. d i s p l a c e m e n t rate, a p p l i e d
141
Pull-out resistance of geogrid reinforcements
I 30.5l
°m l 30.5l cm~
cm [AI~
iVl 5cm
T M
153cm
5 cm
Fig. 2. A schematic diagram of the pull-out box.
Fig. 3. A view of the box and the sand loading system.
~.~2
Khahd Farrag, Yalcin B. Acar, llan Juran
100 80
~-
20
q t
0 .01
o
n
......
I ~
.1
1
......
I
1
. . . . . . .
10
GRAIN SIZE (nun) Fig. 4. Grain size distribution of the sand used in the study.
vertical pressure, pull-out load, soil pressure at the walls, and displacements at the front and at locations along the reinforcement). Materials
A locally available commercial blasting sand is used. The grain size distribution of'this sand is presented in Fig. 4. This sand is poorly graded 9,it h a n eftective diameter, Duoof 0-26 m m and maximum and m i n i m u m densities of l 7.4 kN/m 3( 110-9 pcf) and 15.6 kN/m 3(99.0 pc0, respectively. l v¢t~ different types of geogrids, Tensar SR2 and Conwed-9027, are scleoed in perlormance assessment evaluation tests. Geogrid specimens ol 0.t5 rt~, 0~30 m, 0.45 m and 0.75 m in width and 0.90 m in length are ~c'~(t'd in ~he box. "~est procedure
i he sand i~ poured into the pull-out box from the elevated hopper ~hrough a flexible outlet. The elevated hopper moves along the box to : t*~am a relatively uniform sand fill. The sand is placed in four layers of ~ cm ~0 m) each, leveled and then compacted to the desired relative ~siv,, Compaction effort is imparted manually using a vibrating 'c~,,, hammer. After compaction, the density is measured with a . ~.~.. clc~s4ty gauge. When the sleeve elevation is reached, the ~:~, , w ~ ~ placed on the compacted bottom layer of sand. Geogrid
Pull-out resistance of geogrid reinforcements
143
specimens are bolted to the clamping plates which are then inserted through sleeves of 30 cm (1 ft) length on the front wall. The inextensible tell-tale wires used for displacement measurement along the reinforcement are connected to the five LVDTs placed on a rear end table. The wires are encased in a polyethylene tube placed along the reinforcement. Subsequently the top layer of sand is placed in layers of 15 cm and compacted. A standard test is defined as a test conducted on a Tensar SR2 geogrid of 30 cm (1 ft) width and 92 cm (3 ft) length at a confining pressure of 48.2 kN/m 2 (7 psi), sand density of 16.5 kN/m 3(105 pcf), soil thickness of 61 cm [30 cm placed on the top half and 30 cm placed on the bottom halt], a displacement-rate of 6 mm/min, and a sleeve length of 30 cm (1 ft).
ANALYSIS OF TEST RESULTS Repeatability of tests and load transfer mechanism The repeatability of the pull-out response in standard tests is demonstrated in Fig. 5a while the repeatability of displacements obtained at the front and rear ends of the reinforcement are displayed in Fig. 5b. Geosynthetics are extensible reinforcements. Consequently, methods developed in evaluating the shear resistance of rigid reinforcements cannot be applied in interpreting pull-out test results for geosynthetics. The displacement distribution along the geogrid is indispensable in interpretation of test results and in evaluation of pull-out resistance. The interface shear distribution along the confined geogrid is established by nodal-displacement measurements. The locations of the nodes for displacement measurement are displayed in Fig. 6. The displacement distribution along the geogrid is measured at different pull-out load levels. Figure 7 presents the development of displacements along the reinforcement with pull-out load. The progressive movement of the geogrid nodes during testing is demonstrated. The displacements recorded along the geogrids demonstrate the nonlinearity in the displacement distribution along the geogrid due to coupling of the shear strain at the interface with material elongation resulting in a higher extension at the front part. It is necessary to implement a load-transfer procedure to determine the confined extension properties and interface shear stress-strain behavior. Such a procedure is presented by Juran et al. (1990).
144
Khalid Farrag, Yalcin B. Acar, //an Juran 8O
E Z a ¢I 0.J
GEOGRID: TENSAR SR2 O'n = 4 8 k N / m 2 { 7 p s i ) Vn = 1 6 . 5 k N / m 3 { 1 0 5 p c f ) = 6 ram/rain
60
40
I-0
!
.J "J =) n
20
0
I 20
I 40 FRONT
I I 60 80 DISPLACEMENT
I I I00 120 (ram}
140
(a) GEOGRID: TENSAR SR2
140
o-n = 4 8 k N / m 2 { 7 p s i ) )'n = 16.5 k N / m 5 (105pcf)
E
120 IZ "~ I 0 0 I.iJ U .~ 80 .J n
or) .J .~ E3 0 Z
60 40 20 0
200
400
600 800 TIME (sec)
I000
1200
(b) Fig. 5. R e p e a t a b i l i t y of tests. (a) Pull-out load versus front d i s p l a c e m e n t , (b) displacements distribution along the r e i n f o r c e m e n t .
Geogrid specimen width The development of soil-wall friction along the box side walls can influence the test results. The applied confining pressure can be partially carded by the friction along the side walls. As a result, the confining pressure applied on the specimen will be reduced. This reduction will be
Pull-out resistance of geogrid reinforcements
145
Disolocement Meosurement Locotions
Fig. 6. Displacement measurement locations along a geogrid.
~ , ~
GEOGRID: TENSAR SR2 o"n = 48 kN/m 2 (7psi)
o
~-o
~C~\
~ , , ~
--Peok
Lood
tt.I ,~ U 0
==
I
2 3 GEOGRID NODES
4
Fig. 7. D i s t r i b u t i o n o f d i s p l a c e m e n t s a l o n g the geogrid.
more p r e d o m i n a n t near the side walls. Specimen width/box width ratio should be so selected as to keep the reinforcement and the side wall at a distance which will minimize this effect. In order to evaluate the effect of the side friction in the designed box, (a) load cells are placed at the center a n d close to the wall of the box at geogrid level and the vertical response to vertical pressure is recorded, and (b) pull-out tests are conducted using different specimen widths. Figure 8a demonstrates that lower cell pressures are recorded close to the wall than at the center. Figure 8(b) shows the results of tests on Tensar geogrid specimens of different widths. These tests were conducted u n d e r a confining pressure of 48-2 k N / m 2 (7 psi), soil density of 16.7 k N / m 3 (106 pcf) a n d pull-out rate of 4 m m / m i n . A significant reduction in the pull-out resistance is noted w h e n specimen width is increased to 76 cm (2.5 ft). Tests conducted with geogrid specimens of 30 cm to 61 cm (1 ft to
146
Khalid Farrag, Yalcin B. Acar, llan Juran 1.10 ~:-
34
@
kPa
48 kPa []
70 kPa
1.00
°i
i
g
0.90
BOX WIDTH
0.80
i 0
i
i
0.1
t
=
=
2L t
0.2
0.90 m
i
t
i
0.3
i
0.4
=
i
J
0.5
0.6
i 0.7
NORMALIZED DISTANCEFROM SIDE WALL, x/L
(a) 8O
"E
60
z
o ..J
Specimens with 0 . 5 0 , 0 . 4 5 8~ 6 0 m Width ~
~.
- ' - ~ - - - - ~//I
~---~-
40
t-
0.75m
Width
? _J J o.
20
0
GEOGRID: TENSAR SR2 o-n = 4 8 k N / m 2 ( T p s i ) ~n = 16.7 k N / m 3 ( 1 0 6 p c f ) = 4mm/min I
I
20 FRONT
40 DISPLACEMENT
I
6O
80
(ram)
(b) Fig. 8. (a) The change in vertical pressure ratio at the geogrid level along the width of the box. (hi The effect of geogrid specimen width on the pull-out response.
2 fl) rendered results within the variability of tests. It is concluded that a dJsta nee of at least 15.0 cm is left between the specimen edge and the box wall (i.e. a m a x i m u m specimen width 2 ft for the designed box) to minimize the side wall effect on test results.
Pull-out resistance of geogrid reinforcement,~
147
Sleeve length The interaction between the soil-inclusion system and the rigid front wall of the pull-out box can affect the measured pull-out resistance. As the reinforcement is pulled out of the box, lateral earth pressure develops against the rigid front face and results in an apparent increase in the geogrid pull-out resistance. The rigid boundary effect can be reduced by means of a sleeve incorporated around the slot on the front wall. Sleeves transfer the point of application of the pull-out load inside the soil mass far beyond the rigid front wall. The effect of sleeve length is investigated by tests conducted with no sleeve at the front face, and sleeve lengths of 20 cm (8 in) and 30.5 cm (12 in). Figure 9a shows the effect of sleeve length on the peak pull-out load. In these tests, the lateral pressures developed on the facing were also measured using the two earth pressure cells fixed on the rigid front wall of the box. The relationship between the sleeve length and the lateral pressure developed on the front wall is presented in Fig. 9b. Cell 01 is located higher than Cell 02 (see Fig. 2). Both cells display a decreasing lateral earth pressure as the sleeve length is increased. The results from both cells demonstrate that the increase in the sleeve length leads to a reduction in the earth pressure developed at the front rigid wail: consequently, a reduction in the pull-out resistance. The effecl of the rigid front wall is m i n i m u m when a sleeve length of at least 30 cm is used. It is decided to use a sleeve length of 30 cm in standard pull-out tesling.
Soil thickness The rigid boundaries above and below the reinforcement can affect the interaction mechanism between the soil and the geogrid. These boundaries can lead to increases in the normal stresses in the vicinity of the geogrid surface, specifically when the soil thickness is small and the soil dilatancy is restrained. Moreover, friction can develop between the soil and the bottom rigid boundary. This friction will also update the mobilized soil-geogrid shear stress at the interface. Tests are performed with different thickness of top and bottom soil. Top and bottom soil thicknesses of 10 cm, 30 cm and 40 cm are used. In one test, a top soil thickness of 30 cm and bottom soil thickness of 40 cm (i.e. a total soil thickness of 70 cm) is also used in an attempt to investigate the influence of different soil thicknesses. Tests are conducted with the Tensar geogrid under a confining stress of 48.2 kN/m 2 ('7 psi) and an average soil density of 16.4 k N / m 3 (104 pcf). The effect of soil thickness, on the pull-out response of the geogrid is shown in Fig. 10. The followi, : observations are made:
Khalid Farrag, Yalcin B. Acar, flan Juran
148 I00
GEOGRID: T E N S A R SR2 crn = 4 8 k N / m 2 ( 7 p s i ) Xn 16.4 k N / m 3 ( 1 0 4 p c f ) "~
8O
= 20 mm/rain
Z
o <~
60 -~ "
o J I:~ o
40
i
,,-I J
n
~ "
~
"-''-- - ~
No-sleeve
f
20
0
-m {8inl
Sleeve3 0 c m 1
I
I
50
I00
150
FRONT
DISPLACEMENT
{12in)
200
(mm)
(a)
i
120
,
"X 100| ~
GEOGRID:TENSAR$R2 ConfiningPressure:48.2kNIm2 Earth pressure attestcomple~n
--- .
~m
40"l-"J
f'
20t
0
,
,
l u 'CELL01/ •
,
CELL02 I
,
10
I
,
20
g
30
S L E E V E L E N G T H (cm)
(b) Fig. 9. T h e effect o f sleeve length on the pull-out response. (a) Pull-out load; (19) lateral earth pressure on the front wall.
(a) The decrease in soil thickness results in an apparent increase in the pull-out strength of the geogrid. (b) The increase in the soil thickness above the geogrid from 30 cm to 40 cm does not have any significant effect on the pull-out resistance of the geogrid.
Pull-out resistance of geogrid reinforcements
149
80
/ r . 2Oc m zJ¢
601-
/"
/ I //'//f
i
...--~--~
/ T = 40era ~'----../....
"~'.
°°°°
0
.J
k-
I#(// 11t7
0
.5 ._1
a.
20 [
I I 0
,.7°°.. .... GEOGRID:TE_NSARSR2 ~ . 4.8 kNlm~ 17p$i) . Y.n " 16,4 kN/mi (104pcf) 8 = 6ram/rain 50 IO0 150 FRONT DISPLACEMENT (ram)
200
Fig. 10. The effect of soil thickness on pull-out response.
These results suggest that a soil thickness of at least 30 cm (1 fi) above and below the reinforcement (a total of 2 ft soil thickness) is necessary in uniform sands in order to eliminate the effect of the boundary and confinement effects on the pull-out response.
Displacement rate ASTM D4595-86 recommends a standard rate of strain of 10 + 3% per min for wide strip method of test. However, this rate is established by experience obtained in unconfined tests. In confined tests, reinforcement experiences its peak pull-out strength at much lower levels of strain. Therefore, there is a need to establish the displacement-rate for confined tests. Tests are conducted to investigate the effect of displacement-rate on pull-out response using the Tensar geogrid. Figure 11 shows the pull-out response of the geogrid under four different pull-out displacement-rates (2 mm/min, 6 mm/min, 10 mm/min, and 20 mm/min). An order of magnitude increase in the displacement-rate from 2 mm/min to 20 mm/ min results in a reduction of 25% in the peak pull-out resistance. It is noted that higher displacement rates mobilize lower strains along the reinforcement. Pull-out load will mobilize both the interface shear along the geosynthetic-soil interface and the passive resistance along the transverse ribs. Lower strains along the reinforcement imply a higher contribution from the interface friction. Passive resistance along the transverse ribs will lead to higher pull-out resistance and mobilize higher strains along the reinforcement. The test results with this specific type of soil and the Tensar geogrid
150
Khalid Farrag, Yalcin B. Acar. 11an Juran 60
i
40 TENSAR SR2 Confining Pressure: 48.2 kN/m2 Soil D~Mty: 16.4 kN/m3
<
m
30
l
*
I
5
0
*
I
10
,
I
15
i
20
25
DISPLACEMENT RATE (ram/rain)
(a)
1.0l
•
•
.
.
.
.
.
~'~ma~
s~
'
"
Displacementscorrespond 0.8
0.4
0.2
I 20mm(m!n • 10 ram/rain •
0.0
~ --
6 ram/rain i
0
1
2
3
,
I
4
i
i
5
6
(b) Fig. II. The effect of displacement rate on the pull-out response. (a) Pull-out load: (b) displacements along the reinforcement.
indicate that displacement rates of less than 6 mm/min influence test results to a lesser degree. Displacement rates of less than 6 mm/min are recommended in standard pull-out testing of geotextiles in uniform sands.
Pull-out resistance of geogrid reinforcements
151
Soil density The frictional resistance along the inclusion is highly influenced by soil density. Dense soils, under moderate confining pressures, tend to dilate when shear stresses are mobilized along the reinforcement interface. The soil surrounding the soil-reinforcement interface develops a condition of restrained dilatancy as the reinforcement is pulled out. The magnitude of this restraint depends upon the type of test (displacement controlled or stress controlled), soil density, soil thickness and confining stress. When soil dilation is restrained, the confining stress at the interface increases and results in an apparent increase in the pull-out resistance. The effect of the relative soil density on the pull-out response are evaluated by tests conducted with soil compacted at different densities (15-7 kN/m 3, 16-4 kN/m 3 and 16.7 kN/m 3) and under identical confining pressures of 48.2 kN/m 2. Figure 12 shows the effect of soil density on Tensar geogrid. An increase in the soil density results in an increase in the peak pull-out resistance and significantly higher interface stiffness moduli are obtained. Soil compaction mobilizes the lateral earth pressure resistance on the geogrid and increases the frictional resistance mobilized at the interface. The restraint on geogrid movement and slippage increases the interface modulus and peak pull-out resistance. The increase in density concentrates the strains to the point of load application on the front face and consequently, the mode of failure can be due to breakage of the reinforcement rather than slippage along the reinforcement.
Confining pressure The effect of confining pressure on the frictional resistance of the reinforcement has been demonstrated by several investigators (McGown et al., 1982; Juran & Chen, 1989). For geogrid reinforcements, confined extension during pull-out testing is restrained by the passive resistance of the soil and particles interlocking within transversal geogrid elements. The confined extension results in an apparent increase in the tensile strength and stiffness modulus. Figure 13 shows the effect of the confining pressure on the pull-out response for Tensar geogrid measured under confining pressures of 34 kN/m 2 (5 psi) and 48-2 kN/m 2 (7 psi). Figure 14 presents test results on Conwed geogrid tested under confining pressures of 48.2 kN/m 2 (7 psi), 96.4 kN/m 2 (14 psi) and 140 kN/m 2 (20 psi). An increase in the confining pressure results in significant increases in
Khalid Farrag, Yalcin B Acar, 11an Juran
152
6oS° I ,
/
~.
GEOGRID: T E N S A R S R 2
Or. = 4 8 k N / m 3 ( 7 p s i ) = 4ram/rain
ur.u KNIm Yn = 1 6 . 6 k N / m 3 •,,~Yn = 16.4 k N / m 3
o_j 40 o
...........
Yn = 15.4 k N / m 3
20
I 20
Or
FRONT
I I 40 60 DISPLACEMENT
I 80
•
I00
(ram)
(a)
~ = 48 kPa IMspimmmmt Rate = 3.8 - 6 ram/rain
CONFINING
70
I" 4O
10
15
15.5
16
16.5
17
1"/.5
D R Y D E N S I T Y (kNlm3)
(b) the pull-out resistance of these geogrids. Although the increase in the confining pressure reduces soil tendency to dilate, it leads to an increase in the passive soil resistance on the transverse ribs: consequently, the geogrid pull-out resistance increases. The effect of the confining pressure on the mobilized load transfer along the reinforcement is also illustrated.
Pull-out resistance of geogrid reinforcements
153
GEOGRID TENSAR SR2
0"n = 4 8 k N / m 21,7psi) 0
0.8
fYn = 16.4 kN/m 3
ac~
,,6
//~Yn
0.6
~ 16.-6 kN/m3
Z
U ¢ t Z0 .J~ O. (/) Q
0.4
0.2
I
I
I
T
I
2
3
4
5
GEOGRID NODES
(c) Fig. 12. The effect of density on the pull-out response. (a) Development of pull-out load: (b) the change of peak pull-out resistance with soil density: (c) displacements along the reinforcement when the peak pull-out load is reached.
The soil-geogrid interface shear stress is more uniformly mobilized along the geogrid under low confining pressures. The increase in the confining pressure restrains the geogfid displacement and results in a higher mobilization of the soil-geogrid interface shear stresses near the pull-out application point, a lower mobilization of the shear stresses at the rear end, and consequently a need for shorter effective adherence length of reinforcement. These results suggest that the design criteria for geogrid reinforced soil structures should take into account the in-soil confined extension properties derived from pull-out tests rather than the material properties obtained from unconfined extension tests. The unconfined behavior of the geogrid is substantially more ductile and its strain level at peak load is higher than that obtained under confined conditions. The use of unconfined properties may result in over-conservative design values for admissible geogrid extension and corresponding structure displacements. Interaction mechanism
The magnitude of the mobilized shear resistance along the soilreinforcement interface depends upon the type of reinforcement. In case of geogrids, the interface shear strength is primarily mobilized by the skin friction and the passive resistance against transversal fibs. For
154
Khalid Faring Yaicin R Acar, Ilan Juran 80 GEOGRID TENSAR SR? 3 ?'n = 16.4 kN/m 3 (104 pcf) : 6 ram/rain E ~z
60
0 1 F-
40
/
"~".~....
O-n : 4 8 k N / m 2
/
o I
.J .J o.
20
I
I
I
50 I00 FRONT D I S P L A C E M E N T
150 (ram)
20~
(a) po" O
n = 54kN/m
2
0.8
0.6
N
0.2
G
~
I
0
I
p
R
e
oSkRp2u1,- o ut load
I
I
2 3 GEOGRID NODES
-
~
I
4
5
(b) Fig. 13. The effect of confining pressureon pull-out response ofTensar SR2. (a) Pull-out load: (b) displacements along the reinforcement. coarse grained soils, the openings in the geogrids may allow soil particles to interlock between the ribs; thus, increasing its shear strength. The interface friction between the soil and the geogrid depends on the type of soil and the surface roughness of the geogdd while the contribution of passive soil resistance along the transverse ribs to the overall pull-out strength depends on many factors such as confining pressure, geogrid
155
Pull-out resistance of geogrid reinforcements I00
= 140kN/m 2 °'n = 96kN/m2
,0-n
801 L
E z
~.. /"
% ~./~°'n
= 48kN/m2
E:] 0 _J
0
I0 20 30 FRONT DISPLACEMENT (ram)
40
(a)
/o
n = 48kN/m 2
oa:~ 0.8
0.4
0.2 0
GEOGRID: CONWED Ya = 16"7kN/m3 (106pcf) "- 2 ram/rain I I I I
2
3
I
4
5
GEOGRID NODES
(b) Fig. 14. The effect of confining pressure on pull-out response of Conwed 9027. (a) Pullout load: (b) displacements along the reinforcement.
geometry and the diameter ratio (the ratio of the mean grain size of the soil to the size of the geogrid opening). Figure 15 presents the results of pull-out tests on Conwed geogrids performed with and without their transversal ribs. The results provide a sense of the contribution of the passive resistance on the transverse ribs to the overall pull-out load. The frictional resistance along the
Khalid Farrag Yalcin B. Acar, nan Juran
156
GEOGRID: CONWED Yn = 16.7 k N / m 5 (1061:)cf)
E
= 96 kN/m 2
z Q
o
._1
o
~a
;
ego
oe o ~- O ~ o ~- o oo-
•
".
•
..~ 0
CTn = 4 8 k N / m 2
.5 _J o.
i
0
I
I0
o•
No T r a n s v e r s a l
o•
With T r a n s v e r s a l Ribs
I
I
Ribs
I
20
FRONT D I S P L A C E M E N T
I
30
I
4O
(rnm)
Fig. 15. The effect of transverse ribs on the pull-out response of Conwed geogrid.
longitudinal ribs constitutes an average of 75% of the total pull-out load for this specific geogrid. It is also noted that the passive resistance along the transverse ribs is mobilized at higher levels of strain resulting in a higher post-peak pull-out resistance. The strain level required to mobilize the passive resistance along the transverse elements would vary with the geogrid and the soil type. SUMMARY AND CONCLUSIONS There exists significant differences in pull-out or direct shear tests used in experimental modelling of soil-geosynthetic interaction mechanism and/or performance evaluation of in-soil properties of geosynthetics. Standard testing equipment and testing procedures are needed to obtain comparable results and to develop appropriate methodologies in modelling the load-transfer mechanism. Pull-out boxes are designed and constructed at GERL/LTRC-LSU. The components of these equipment and the procedure used in testing are presented. Tests conducted in performance assessment of this equipment employed a uniform sand, and TENSAR SR2, CONWED 9027 geogrids. The following conclusions are obtained for the uniform sand and the geogrids used in this study:
Pull-out resistance of geogrid reinforcements
157
(1) Side frictions on the side walls of the box decrease the peak pullout load. A minimum clearance of 15 cm (6 in) is necessary in order to avoid any side-wall effect in case the friction along the wall is not minimized by some other means. (2) The increase in sleeve length decreases the effect of front wall on the pull-out resistance. The tests conducted in this study demonstrate that sleeve lengths of at least 30-5 cm (12 in) of length are necessary. (3) Increased thickness of soil cushioning the geosynthetic decreases the effects of the top and bottom boundaries. A soil thickness of at least 30 cm above and 30 cm below the geogrid is needed to eliminate the influence of these boundaries. (4) High displacement rates affect both the pull-out load and the displacement distribution along the reinforcement. The displacement rate effects are minimized if rates of the order of less than 6 mm/min are used. (5) Increased densities lead to mobilization of the interlocking mechanism and consequently, result in a higher contribution of the passive resistance exerted on the transverse ribs. At higher densities, pull-out resistance increases and displacements move closer to the point of application of the pull-out load. (6) Similar to the effects of increased densities, confinement increases the peak pull-out resistance and moves the displacements along the reinforcement toward the point of application of the load. (7) Pull-out resistance is developed by a combination of the passive resistance developed along the transverse ribs and interface shear mobilized along the geogrid. (8) Methods developed and employed in evaluation of rigid reinforcements should not be used in determining the confined extension properties and shear stress-strain behavior ofgeogrids. The results presented in this study demonstrate the significant variability obtained in pull-out load-displacement response of geogrids due to differences in equipment and testing procedures. It is necessary to realize the influence of these parameters on pull-out response in standard commercial testing ofgeosynthetics and in interpretation of the load-transfer mechanism using pull-out test results. ACKNOWLEDGEMENTS Design, construction, and performance assessment of the pull-out testing facility at GERL are supported by funds awarded and provided to
158
Khalid Farrag Yaicin B. Acar, llan Juran
LSU and by the Louisiana Transportation Research Center of Louisiana Department of Transportation and Development, Federal Highway Administration, the Board of Regents of the State of Louisiana, Conwed Plastics Inc. and Civil Engineering Department of Louisiana State University. The funds awarded and provided by these agencies and institutions are gratefully acknowledged. Dr Ilan Juran of Polytechnic University has had significant contributions in establishment of GERL in cooperation and collaboration with the second author. Dr Juran's efforts and contributions are gratefully acknowledged by GERL. LTRC project officer, Mr Paul Griffin and Conwed Plastics Product Development Manager, Mr Lee Richmond are acknowledged for their collaboration and cooperation. We acknowledge and appreciate the efforts of Ex-director of LTRC, Mr Ara Arman, the current director, Dr Peter Stopher, ex-chairman of the Civil Engineering Department, Dr Roger IC Seals and the current chairman Dr Richard Avent in establishment and e n h a n c e m e n t of GERL. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.
REFERENCES Acar, Y. B., Durgunoglu, T. & Tumay, M. T. (1982). Interface properties of sand. ASCE, JGE, 108(GT3), 648-54. Anderson, L. R. & Nielsen, M. R. (1984). Pull-out resistance of wire mats embedded in soil. Report for the Hilfiker Company, Eureka, CA. Bonaparte, R., Holtz, R. D. & Giroud, J. P. (1987). Soil reinforcement design using geotextiles and geogrids. Geotextile Testing and the Design Engineer, ASTM STP 952, pp. 69-116. Brand, S. R. & Duffy, D. M. (1987). Strength and pullout testing of geogrids. Geosynthetics Conference, New Orleans, Vol. 1, pp. 226-36. Christopher, B. R. (1976). Tensar SS2 geogrid evaluation. Evaluation report to Tensar Corporation. Elias, V. (1979). Friction in reinforced earth utilizing fine grained backfills. International Conference on Soil Reinforcement, Paris, France, pp. 435-8. Farrag, K. (1990). Interaction properties of geogrids in reinforced soil walls -testing and analysis. Dissertation submitted to Graduate School of Louisiana State University, Baton Rouge, LA. lngold, T. S. & Templeman, J. (1979). The comparative performance of polymer net reinforcement. International Conference on Soil Reinforcement, Paris, Vol. 1. Ingold, T. S. (1983). Laboratory pull-out testing of grid reinforcement in sand. Geotech. Testing J., 6(3 ), I 01 - 11. Jewell, R. A. (1979). Discussion presented to Section 8, Proceedings of VII European Conference on Soil Mechanics and Foundation Engineering, Brighton, UK.
Pull-out resistance of geogrid reinforcements
159
Jewell, R. A. (1980). Some effects of reinforcement on the mechanical behavior of soils. Dissertation submitted to Cambridge University, Cambridge, UK. Juran, I., Knochenmus, G., Acar, Y. B. & Arman, A. (1988). Pull-out response of geotextiles and geogrids (Synthesis of Available Experimental Data). Proceedings of Symposium on Geotextiles for Soil Improvement, ASCE, Geotech. Special Publication 18, pp. 92-111. Juran, I., Guermazi, A., Chen, C. L. & Ider, M. H. (1988). Modeling and simulation of load transfer in reinforced soil: Part 1. Int. J. Numer. Anal. Meth. Geomech, 12, pp. 141-55. Juran, I., lder, H. & Farrag, Kh. (1990). Strain compatibility analysis for geosynthetic reinforced soil walls. ASCE, JGE, 116(2), 312-29. Koerner, R. M. (1986). Direct shear/pull-out tests on geogrids, Report No. 1, Dept. of Civil Eng., Drexel Univ., Philadelphia, PA. Koerner, R. M. (1990). Designing with Geosynthetics. 2nd edn, Prentice Hall, Englewood Cliffs, NJ. McGown, A., Andrawes, K. Z. & Kabir, M. H. (1982). Load-extension testing of geotextiles confined in-soil, 2nd Int. Conf. on Geotextiles. Las Vegas, Vol. 3, pp. 793-8. Myles, B. (1982). Assessment of soil fabric friction by means of shear, 2nd Int. Conf. on Geotextiles, Las Vegas, Vol. 3, pp. 787-91. Palmeira, E. & Milligan, G. (1989). Large scale direct shear tests on reinforced soil. Soil and foundations, Japan. Soc. Soil Mech. Found Eng., 29(1), 18-30. Rowe, R. K. & Ho, S. K. (1986). Determination of geotextile stress-strain characteristics using a wide strip test, 3rd Int. Conf. on Geotextiles, Vienna, pp. 885-90. Rowe, R. K., Ho, S. K. & Fisher, D. (1985). Determination of soil-geotextile interface strength properties, 2nd Canadian Symposium on Geotextiles, Montreal, Canada, pp. 25-34. Saxena, S. K. & Budiman, J. S. (1985). Interface response ofgeotextiles, Proc. 1 lth Int. Conference on Soil Mechanics and Foundation Engineering, San Francisco, Vol. 3, pp. 1801--4. Schlosser, F. & Guilloux, A. (1979). Friction between soil and strips in reinforced earth structures, Int. Conf. on Soil Reinforcement, Paris, France, Vol. 1. Tzong, W. H. & Cheng-Kuang, S. (1987). Soil-geotextile interaction mechanism in pull-out test, Geosynthetics Conference. New Orleans, Vol. 1, pp. 250-9. Williams, N. D. & Houlihan, M. F. (1987). Evaluation of interface friction properties between geosynthetics and soils, Geosynthetics Conference. New Orleans, Vol. 2, pp. 616-27.
Pull-Out Resistance of Geogrid Reinforcements Khalid Farrag Geosynthetic Engineering Research Laboratory (GERL), Louisiana Transportation Research Center (LTRC), Gourier Road, Baton Rouge, Louisiana 70803, USA
Yalcin B. Acar* Civil Engineering Department, Louisiana State University, Baton Rouge, Louisiana 70803, USA
&
Ilan Juran Department of Civil Engineering, Polytechnic University, 333 Jay Street, Brooklyn, New York, New York 11201, USA (Received 30 September 1991: accepted 25 November 1991)
ABSTRACT Testing equipment, specimen preparation and testing procedures are presented for load-controlled and displacement-rate-controlled pull-out tests for geosynthetic reinforcements in granular soils. The influence of the test type, confining pressure, soil density, boundary conditions, and geotextile characteristics on pull-out load-displacement response of selected geogrids embedded in sand are evaluated. Implications for testing procedure and analysis are discussed.
INTRODUCTION A wide variety o f geotextiles a n d geogrids are n o w a v a i l a b l e for civil e n g i n e e r i n g a p p l i c a t i o n s ( B o n a p a r t e et al., 1987; Koerner, 1990). In *To whom correspondence should be addressed. 133 GeotextUes and Geomembranes 0266--1144/93/$06.00 © 1993 Elsevier Science Publishers Ltd. England. Printed in Great Britain
134
Khalid Farrag. Yalcin B. Acar, llan Juran
selecting a specific geotextile or geogrid for reinforcement of embankments and slopes, the following performance aspects need to be assessed: (i) stress-strain-strain rate behavior of the confined reinforcementsoil composite system, (ii) pull-out performance and the associated load transfer mechanism. A standard testing procedure for confined properties ofgeosynthetics does not yet exist. The large number of factors that affect the interface properties of the confined reinforcement raise major difficulties in providing comparable test results. There is a wide scatter in the available pull-out test results. These differences in results are due to the use of different types of pull-out devices, the associated boundary effects, testing procedures and soil placement and compaction schemes (Juran et al., 1988). The increasing use of geosynthetics in soil reinforcement prompts the need to develop and standardize methods in evaluating the in-soil mechanical characteristics and interface properties of these materials. It becomes necessary to design adequate testing equipment, establish reliable testing procedures, and develop appropriate interpretation schemes. This paper presents the pull-out testing program implemented at the Geosynthetic Engineering Research Laboratory (GERL), a laboratory of Louisiana Transportation Research Center (LTRC) and Civil Engineering Department of Louisiana State University, to develop reliable testing procedures and interpretation schemes in evaluation of the short-term and long-term pull-out performance ofgeosynthetic reinforcements. The fundamental aspects of pull-out testing equipment and procedure are reviewed. A pull-out box that incorporates schemes which overcome most of the current limitations is presented. The factors that affect the measured interaction properties are evaluated by performance evaluation tests. Implications for engineering analysis are discussed.
FUNDAMENTAL ASPECTS OF PULL-OUT TESTING The shear stress-strain relationship developed along the soil-reinforcement interface is commonly tested in a direct shear box and/or a pull-out box. In the direct shear box, tests are usually conducted in accordance with the conventional procedure used in investigating interface properties (Acar et al., 1982). Commonly, a soil sample resting on a geotextile is sheared along the interface and the shear force-displacement behavior is recorded. In the pull-out box, a geotextile confined in soil is pulled out and often the pull-out load and the front displacement are recorded. In
Pull-out resistance of geogrid reinforcements
135
both tests, results are often expressed in terms of a friction ratio, tan 6/tan ~, (also called the efficiency factor) where 6 is the soil-reinforcement interface friction angle and O is the soil friction angle. Efficiency factors ranging from 0.6 to 1.0 for geotextiles and values larger than one for geogrids are reported (Juran et al., 1988; Koerner, 1990). Figure 1 presents a comparison of efficiency factors obtained from tests conducted using pull-out and direct shear testing equipment. The frictional resistances reported for different types of inclusions in dense sands are found to be greater in pull-out tests than those obtained in direct shear tests (Jewell, 1979; Schlosser & Guilloux, 1979). Ingold and Templeman (1979) found comparable values in both tests for geogrids tested under low confining pressures. However, under higher normal stresses, higher shear strength values were obtained in pull-out tests. Koerner (1986) reported higher shear resistance for geogrids in pull-out tests while, under higher confining pressures, direct shear tests gave higher shear resistance. Rowe et al. (1985) reported that both tests give approximately equal values of shear resistance for geotextiles tested in dense granular fill while for geogrids in a loose fill, pull-out tests rendered significantly lower values. Direct shear and pull-out tests are associated with different testing procedures, loading paths, failure mechanisms, and boundary conditions. Consequently, the interface frictional parameters obtained from both 3.5 ~ Direct Pull-out Reference Reinfon:emem Soil Shear Type Type t'-'O------'"+ lngot,J(l~83)TemmrSX-~ ~ S ~ ---4~--A Colins(1980) Nonwovm Grovel(cnu4t~l)
3.0
1
2.s 2.0
~
I---O--. + |........
~, ~
xo~,,~09s6) T ~ SX-2 mm s ~ x. . . . .9s6) ~z-100 D~S,~ Row,:(1985)
Tensar
$R-2 Loosesand
1.5
1.0
A. . . . . . . . L ~
.......
A
C]
o.sl ~ 0.0 ~ 0
50
100
150
200
250
300
NORMAL STRESS (kN/m2)
Fig. I. C o m p a r i s o n o f the efficiency factors o b t a i n e d for s o i l - g e o s y n t h e t i c systems in direct s h e a r a n d pull-out boxes.
136
Khalid Farrag, Yalcin B. Acar, llan Juran
tests can vary and often provide conflicting results. The reasons for the reported differences between the pull-out and direct-shear test results can be categorized as: (1) Restrained dilatancy: Cohesionless soils will dilate at low to medium confining pressures and at high densities. If this dilatancy is restrained in testing, the confining pressure along the interface will increase until a state is reached where failure can be achieved without any volume change (critical state). The amount of this restraint and the magnitude of the increase in confinement will depend upon the type and geometry of the imposed boundary conditions. These distances are often selected arbitrarily in the boxes used. Furthermore, the pull-out boxes are often designed with fixed boundaries while displacements are allowed in the top platten in the direct shear box. Fixed boundaries may promote restrained dilatancy and increased confinement. Therefore, even when the initial states are identical, different stress-deformation behavior will be developed due to confinement induced by boundary conditions. (2) Failure and interaction mechanism: The interaction mechanism is different in direct shear and pull-out. In direct shear tests, when the box dimension is large, the mobilized shear strain is postulated to be uniformly distributed along the soil-geosynthetic interface. While in the pull-out tests, mobilized strain is a combination of the interface shear strain and reinforcement extension. This coupled mechanism results in a non-uniform shear strain-stress distribution along the reinforcement. A realistic experimental model of pull-out behavior ofgeosynthetics in the field should incorporate extensibility. Pull-out boxes are more appropriate when extensibility is to be incorporated. A standard design for pull-out testing devices does not yet exist. Therefore, box dimensions and testing procedures differ from one box to the other. The dimensions of the box are usually selected as large as technically possible with the anticipation that boundary effects will be minimized. An evaluation of most of the available equipment and testing parameters demonstrates that the differences in testing equipment and procedures make comparisons of available test results extremely difficult (Juran et al., 1988; Farrag, 1990). The following is a list of desirable features in pull-out testing procedure and equipment: (1) The pull-out tests reported are often conducted at a controlled displacement rate. Few pull-out tests are reported under load-
PuU-out resistance of geogrid reinforcements
(2)
(3)
(4)
(5)
137
controlled mode (e.g., Tzong & Cheng-Kuang, 1987). Pull-out testing equipment should have the capability to provide loadcontrolled tests to facilitate investigation of time-dependent, in-soil creep behavior. In displacement-rate controlled tests, a range of pull-out rates are reported varying from 0.1 ram/rain to 20 mm/min. Myles (1982) studied the frictional resistance of the geotextile interface under different strain rates (10-75 mm/min) in a direct shear box and showed that results had little sensitivity to this range of displacement rates. In wide strip tests, Rowe and Ho (1986) showed that the geogrid tension strength varies with the applied rate of strain. It is necessary to establish the effect of pull-out displacement-rate on test results in different types of soils. The interaction between the soil and the side walls of the pull-out box can affect test results. Soil confinement is usually applied by means of flexible air-bags to insure uniform distribution of normal stress along a plane (Christopher, 1976; Palmeira & Milligan, 1989; Ingold, 1983). The applied confining stress is partially carried out by the side wall friction causing a reduction in the normal pressure applied at the reinforcement level. Some investigators inserted lubricated membranes along the walls in an attempt to provide a low friction boundary (Jewell, 1980). Alternatively, the specimen width/box width ratio may be chosen so as to minimize the effect of friction by side-walls. The interaction between the reinforcement-soil system and the rigid front wall can also influence test results. As the reinforcement is pulled out of the box, the lateral earth pressure developed on the front face can result in an increase in the pull-out resistance. Palmeira and Milligan (1989) used a front wall with different degrees of roughness to investigate the effect of friction on the front wall on pull-out test results. Christopher (1976) incorporated sleeves around the pull-out slot to transfer the point of application of the pull-out load far behind the rigid front wall. Other investigators (Williams & Houlihan, 1987) used flexible front faces to minimize the rigid front wall effect. The front wall interaction should be minimized in a pull-out box. The thicknesses of the soil above and below the reinforcement (referred as the soil thickness), differ according to the available clear height of the box. If soil thickness is small, the interaction between the soil-reinforcement system and the boundaries will significantly influence the shear resistance along the interface. Brand and Duffy (1987) studied the effect of soil thickness on pull-
138
Khalid Farrag, Yalein B. Acar, Ilan Juran
out resistance of a geogrid in clays. Their results demonstrate that as the soil thickness increases, pull-out resistance decreases until a minimum pull-out load is obtained. It is essential to decide upon the soil thickness in conducting a standard pull-out test. (6) Different specimen preparation and compaction procedures are utilized to insure uniform soil density. Soil compaction is carried out by means of an electric jack hammer (Johnston, 1985), standard proctor hammer (Saxena & Budiman, 1985), hand tamping devices (Elias, 1979) and by mechanical tamping (Anderson & Neilsen, 1985). A hopper with flexible tube is also used to insure uniform soil placement (Palmeira & Milligan, 1989; Jewell, 1980). Different specimen preparation procedures will result in differences in fabric leading to significantly different stress-deformation behavior. It then becomes essential to standardize specimen preparation procedures in commercial testing of confined behavior of geosynthetic reinforcements. (7) Several investigators (Christopher, 1976; Koerner, 1986; Brand & Duffy, 1987) clamped the reinforcement outside the box. This technique may result in an unconfined front portion of the reinforcement. Consequently, the effective interface area will vary and confined/unconfined properties of the geosynthetic will couple during the pull-out test. (8) The existing pull-out testing equipment are often instrumented to monitor only the displacement and the pull-out resistance at the front face. For extensible reinforcements such as geogrids and geotextiles, it is essential to monitor the displacements along the inclusion in order to interpret the load transfer mechanism and estimate the pull-out resistance in the field. The measured pull-out resistance is influenced by factors which include the details of testing equipment and procedures discussed above, and also the following compositional and environmental variables influencing the behavior of the composite system: (i)
the compositional characteristics of the geosynthetic reinforcement such as its type, geometry and configuration leading to different extensibility and load-strain-strain rate behavior, (ii) compositional characteristics of the reinforced soil such as its grain size distribution, and void ratio, (iii) environmental variables defining the initial state of the composite system such as the overburden pressure and the nature of the imposed deformation (load controlled or displacement rate controlled).
Pull-out resistance of geogrid reinforcements
139
Two pull-out boxes which permit an evaluation of different factors influencing pull-out response are designed and constructed. A testing program is implemented in performance evaluation of the boxes.
TESTING PROGRAM The primary objective of the performance evaluation study was to assess the sensitivity of test results obtained in the designed equipment to the changes in the fundamental testing parameters (geogrid type, specimen width, sleeve length, soil thickness, displacement rate, soil density and confining pressure) and to establish a data base for development of reliable testing and interpretation procedures. Table 1 presents the implemented testing program.
Equipment A schematic diagram of the box and associated instrumentation are presented in Fig. 2. Figure 3 presents a view of the pull-out box and the sand hopper system. The different components and characteristics of the box are described below: (i)
The inner dimensions of the pull-out box are 1-52 m (60 in) long, 0-90 m (36 in) wide and 0.76 m (30 in) high. The box is constructed in modular units to allow changes in box dimensions and front wall opening size. A sleeve is used at the facing to transfer the interface pull-out load behind the rigid front wall. An air bag is used at the top to provide a uniformly distributed vertical pressure. (ii) Pull-out load is applied by a hydraulic loading system through clamping plates that extend inside the sleeve. This arrangement ascertains that the geosynthetic specimen remains confined throughout the test. The loading system can apply either a constant pull-out rate or a constant pull-out load. Load controlled mode is used to evaluate the creep behavior. (iii) The front displacements, the pull-out rate and the pull-out load are acquired/monitored at the clamping plates by means of a LVDT, velocity transducer and a load cel, respectively. Displacements along the reinforcement are measured by tell-tale wires connected to LVDTs. Two earth pressure cells are placed on the front wall.
Khalid Farrag, Yalein R Aear, llan Juran
140
Table I
Testing Program and Testing Parameters Purpose~
Geogrid type
Dimension L/W (m)
Repeatability
Tensar SR2 Conwed 9027
1.00/0.30 i.00/0-30
Specimen width
Tensar SR2
Unit weight (kN/m 3)
Displacement rate, 6 (mm/min)
48-2
16.5
6.0
1.00/0-30 1.00/1~45 i.00/0.60 1-00/0-75
48.2
16.7
4.0
Sleeve length Tensar SR2 (0, 20 and 30 cm)
!.00/0-30
48.2
16.4
20.0
Soil thickness (20, 40, 60 and 70 cm)
Tensar SR2
1.00/0.30
48.2
16.4
6-0
Displacement rate
Tensar SR2
1.00/ff30
48.2
16.4
4-0 6.0 10-0 20-0
Soil density
Tensar SR2
1.00/0.30
48.2
15.7 16.4 16.7 17-0
4-0
Confining pressure
Tensar SR2
1.00/1~30
Conwed 9027
1.00/0.30
34-0 48.2 69-0 48-2 96.0 140-0
16.5 16.5 16.5 16.7 16.7 16.7
6-0 6-0 6.0 4.0 4.0 4.0
Conwed 9027
1.00/0-30
48.2
16.7
2.0
Effect of transverse fibs
Confining pressure (kN/m 2)
~Unless otherwise noted, all tests are conducted at a displacement rate of 6 mm/min, at a soil thickness of 60 cm (30 cm top and 30 cm bottom) and a sleeve length of 30 cm.
(iv) A n e l e v a t e d s a n d h o p p e r w i t h v a c u u m e x t r a c t i o n is u s e d to facilitate sand placement and removal from the box. (v) A d a t a a c q u i s i t i o n s y s t e m m o n i t o r s t h e i n p u t p a r a m e t e r s a n d r e c o r d s t h e r e s p o n s e p a r a m e t e r s (e.g. d i s p l a c e m e n t rate, a p p l i e d
141
Pull-out resistance of geogrid reinforcements
I 30.5l
°m l 30.5l cm~
cm [AI~
iVl 5cm
T M
153cm
5 cm
Fig. 2. A schematic diagram of the pull-out box.
Fig. 3. A view of the box and the sand loading system.
~.~2
Khahd Farrag, Yalcin B. Acar, llan Juran
100 80
~-
20
q t
0 .01
o
n
......
I ~
.1
1
......
I
1
. . . . . . .
10
GRAIN SIZE (nun) Fig. 4. Grain size distribution of the sand used in the study.
vertical pressure, pull-out load, soil pressure at the walls, and displacements at the front and at locations along the reinforcement). Materials
A locally available commercial blasting sand is used. The grain size distribution of'this sand is presented in Fig. 4. This sand is poorly graded 9,it h a n eftective diameter, Duoof 0-26 m m and maximum and m i n i m u m densities of l 7.4 kN/m 3( 110-9 pcf) and 15.6 kN/m 3(99.0 pc0, respectively. l v¢t~ different types of geogrids, Tensar SR2 and Conwed-9027, are scleoed in perlormance assessment evaluation tests. Geogrid specimens ol 0.t5 rt~, 0~30 m, 0.45 m and 0.75 m in width and 0.90 m in length are ~c'~(t'd in ~he box. "~est procedure
i he sand i~ poured into the pull-out box from the elevated hopper ~hrough a flexible outlet. The elevated hopper moves along the box to : t*~am a relatively uniform sand fill. The sand is placed in four layers of ~ cm ~0 m) each, leveled and then compacted to the desired relative ~siv,, Compaction effort is imparted manually using a vibrating 'c~,,, hammer. After compaction, the density is measured with a . ~.~.. clc~s4ty gauge. When the sleeve elevation is reached, the ~:~, , w ~ ~ placed on the compacted bottom layer of sand. Geogrid
Pull-out resistance of geogrid reinforcements
143
specimens are bolted to the clamping plates which are then inserted through sleeves of 30 cm (1 ft) length on the front wall. The inextensible tell-tale wires used for displacement measurement along the reinforcement are connected to the five LVDTs placed on a rear end table. The wires are encased in a polyethylene tube placed along the reinforcement. Subsequently the top layer of sand is placed in layers of 15 cm and compacted. A standard test is defined as a test conducted on a Tensar SR2 geogrid of 30 cm (1 ft) width and 92 cm (3 ft) length at a confining pressure of 48.2 kN/m 2 (7 psi), sand density of 16.5 kN/m 3(105 pcf), soil thickness of 61 cm [30 cm placed on the top half and 30 cm placed on the bottom halt], a displacement-rate of 6 mm/min, and a sleeve length of 30 cm (1 ft).
ANALYSIS OF TEST RESULTS Repeatability of tests and load transfer mechanism The repeatability of the pull-out response in standard tests is demonstrated in Fig. 5a while the repeatability of displacements obtained at the front and rear ends of the reinforcement are displayed in Fig. 5b. Geosynthetics are extensible reinforcements. Consequently, methods developed in evaluating the shear resistance of rigid reinforcements cannot be applied in interpreting pull-out test results for geosynthetics. The displacement distribution along the geogrid is indispensable in interpretation of test results and in evaluation of pull-out resistance. The interface shear distribution along the confined geogrid is established by nodal-displacement measurements. The locations of the nodes for displacement measurement are displayed in Fig. 6. The displacement distribution along the geogrid is measured at different pull-out load levels. Figure 7 presents the development of displacements along the reinforcement with pull-out load. The progressive movement of the geogrid nodes during testing is demonstrated. The displacements recorded along the geogrids demonstrate the nonlinearity in the displacement distribution along the geogrid due to coupling of the shear strain at the interface with material elongation resulting in a higher extension at the front part. It is necessary to implement a load-transfer procedure to determine the confined extension properties and interface shear stress-strain behavior. Such a procedure is presented by Juran et al. (1990).
144
Khalid Farrag, Yalcin B. Acar, //an Juran 8O
E Z a ¢I 0.J
GEOGRID: TENSAR SR2 O'n = 4 8 k N / m 2 { 7 p s i ) Vn = 1 6 . 5 k N / m 3 { 1 0 5 p c f ) = 6 ram/rain
60
40
I-0
!
.J "J =) n
20
0
I 20
I 40 FRONT
I I 60 80 DISPLACEMENT
I I I00 120 (ram}
140
(a) GEOGRID: TENSAR SR2
140
o-n = 4 8 k N / m 2 { 7 p s i ) )'n = 16.5 k N / m 5 (105pcf)
E
120 IZ "~ I 0 0 I.iJ U .~ 80 .J n
or) .J .~ E3 0 Z
60 40 20 0
200
400
600 800 TIME (sec)
I000
1200
(b) Fig. 5. R e p e a t a b i l i t y of tests. (a) Pull-out load versus front d i s p l a c e m e n t , (b) displacements distribution along the r e i n f o r c e m e n t .
Geogrid specimen width The development of soil-wall friction along the box side walls can influence the test results. The applied confining pressure can be partially carded by the friction along the side walls. As a result, the confining pressure applied on the specimen will be reduced. This reduction will be
Pull-out resistance of geogrid reinforcements
145
Disolocement Meosurement Locotions
Fig. 6. Displacement measurement locations along a geogrid.
~ , ~
GEOGRID: TENSAR SR2 o"n = 48 kN/m 2 (7psi)
o
~-o
~C~\
~ , , ~
--Peok
Lood
tt.I ,~ U 0
==
I
2 3 GEOGRID NODES
4
Fig. 7. D i s t r i b u t i o n o f d i s p l a c e m e n t s a l o n g the geogrid.
more p r e d o m i n a n t near the side walls. Specimen width/box width ratio should be so selected as to keep the reinforcement and the side wall at a distance which will minimize this effect. In order to evaluate the effect of the side friction in the designed box, (a) load cells are placed at the center a n d close to the wall of the box at geogrid level and the vertical response to vertical pressure is recorded, and (b) pull-out tests are conducted using different specimen widths. Figure 8a demonstrates that lower cell pressures are recorded close to the wall than at the center. Figure 8(b) shows the results of tests on Tensar geogrid specimens of different widths. These tests were conducted u n d e r a confining pressure of 48-2 k N / m 2 (7 psi), soil density of 16.7 k N / m 3 (106 pcf) a n d pull-out rate of 4 m m / m i n . A significant reduction in the pull-out resistance is noted w h e n specimen width is increased to 76 cm (2.5 ft). Tests conducted with geogrid specimens of 30 cm to 61 cm (1 ft to
146
Khalid Farrag, Yalcin B. Acar, llan Juran 1.10 ~:-
34
@
kPa
48 kPa []
70 kPa
1.00
°i
i
g
0.90
BOX WIDTH
0.80
i 0
i
i
0.1
t
=
=
2L t
0.2
0.90 m
i
t
i
0.3
i
0.4
=
i
J
0.5
0.6
i 0.7
NORMALIZED DISTANCEFROM SIDE WALL, x/L
(a) 8O
"E
60
z
o ..J
Specimens with 0 . 5 0 , 0 . 4 5 8~ 6 0 m Width ~
~.
- ' - ~ - - - - ~//I
~---~-
40
t-
0.75m
Width
? _J J o.
20
0
GEOGRID: TENSAR SR2 o-n = 4 8 k N / m 2 ( T p s i ) ~n = 16.7 k N / m 3 ( 1 0 6 p c f ) = 4mm/min I
I
20 FRONT
40 DISPLACEMENT
I
6O
80
(ram)
(b) Fig. 8. (a) The change in vertical pressure ratio at the geogrid level along the width of the box. (hi The effect of geogrid specimen width on the pull-out response.
2 fl) rendered results within the variability of tests. It is concluded that a dJsta nee of at least 15.0 cm is left between the specimen edge and the box wall (i.e. a m a x i m u m specimen width 2 ft for the designed box) to minimize the side wall effect on test results.
Pull-out resistance of geogrid reinforcement,~
147
Sleeve length The interaction between the soil-inclusion system and the rigid front wall of the pull-out box can affect the measured pull-out resistance. As the reinforcement is pulled out of the box, lateral earth pressure develops against the rigid front face and results in an apparent increase in the geogrid pull-out resistance. The rigid boundary effect can be reduced by means of a sleeve incorporated around the slot on the front wall. Sleeves transfer the point of application of the pull-out load inside the soil mass far beyond the rigid front wall. The effect of sleeve length is investigated by tests conducted with no sleeve at the front face, and sleeve lengths of 20 cm (8 in) and 30.5 cm (12 in). Figure 9a shows the effect of sleeve length on the peak pull-out load. In these tests, the lateral pressures developed on the facing were also measured using the two earth pressure cells fixed on the rigid front wall of the box. The relationship between the sleeve length and the lateral pressure developed on the front wall is presented in Fig. 9b. Cell 01 is located higher than Cell 02 (see Fig. 2). Both cells display a decreasing lateral earth pressure as the sleeve length is increased. The results from both cells demonstrate that the increase in the sleeve length leads to a reduction in the earth pressure developed at the front rigid wail: consequently, a reduction in the pull-out resistance. The effecl of the rigid front wall is m i n i m u m when a sleeve length of at least 30 cm is used. It is decided to use a sleeve length of 30 cm in standard pull-out tesling.
Soil thickness The rigid boundaries above and below the reinforcement can affect the interaction mechanism between the soil and the geogrid. These boundaries can lead to increases in the normal stresses in the vicinity of the geogrid surface, specifically when the soil thickness is small and the soil dilatancy is restrained. Moreover, friction can develop between the soil and the bottom rigid boundary. This friction will also update the mobilized soil-geogrid shear stress at the interface. Tests are performed with different thickness of top and bottom soil. Top and bottom soil thicknesses of 10 cm, 30 cm and 40 cm are used. In one test, a top soil thickness of 30 cm and bottom soil thickness of 40 cm (i.e. a total soil thickness of 70 cm) is also used in an attempt to investigate the influence of different soil thicknesses. Tests are conducted with the Tensar geogrid under a confining stress of 48.2 kN/m 2 ('7 psi) and an average soil density of 16.4 k N / m 3 (104 pcf). The effect of soil thickness, on the pull-out response of the geogrid is shown in Fig. 10. The followi, : observations are made:
Khalid Farrag, Yalcin B. Acar, flan Juran
148 I00
GEOGRID: T E N S A R SR2 crn = 4 8 k N / m 2 ( 7 p s i ) Xn 16.4 k N / m 3 ( 1 0 4 p c f ) "~
8O
= 20 mm/rain
Z
o <~
60 -~ "
o J I:~ o
40
i
,,-I J
n
~ "
~
"-''-- - ~
No-sleeve
f
20
0
-m {8inl
Sleeve3 0 c m 1
I
I
50
I00
150
FRONT
DISPLACEMENT
{12in)
200
(mm)
(a)
i
120
,
"X 100| ~
GEOGRID:TENSAR$R2 ConfiningPressure:48.2kNIm2 Earth pressure attestcomple~n
--- .
~m
40"l-"J
f'
20t
0
,
,
l u 'CELL01/ •
,
CELL02 I
,
10
I
,
20
g
30
S L E E V E L E N G T H (cm)
(b) Fig. 9. T h e effect o f sleeve length on the pull-out response. (a) Pull-out load; (19) lateral earth pressure on the front wall.
(a) The decrease in soil thickness results in an apparent increase in the pull-out strength of the geogrid. (b) The increase in the soil thickness above the geogrid from 30 cm to 40 cm does not have any significant effect on the pull-out resistance of the geogrid.
Pull-out resistance of geogrid reinforcements
149
80
/ r . 2Oc m zJ¢
601-
/"
/ I //'//f
i
...--~--~
/ T = 40era ~'----../....
"~'.
°°°°
0
.J
k-
I#(// 11t7
0
.5 ._1
a.
20 [
I I 0
,.7°°.. .... GEOGRID:TE_NSARSR2 ~ . 4.8 kNlm~ 17p$i) . Y.n " 16,4 kN/mi (104pcf) 8 = 6ram/rain 50 IO0 150 FRONT DISPLACEMENT (ram)
200
Fig. 10. The effect of soil thickness on pull-out response.
These results suggest that a soil thickness of at least 30 cm (1 fi) above and below the reinforcement (a total of 2 ft soil thickness) is necessary in uniform sands in order to eliminate the effect of the boundary and confinement effects on the pull-out response.
Displacement rate ASTM D4595-86 recommends a standard rate of strain of 10 + 3% per min for wide strip method of test. However, this rate is established by experience obtained in unconfined tests. In confined tests, reinforcement experiences its peak pull-out strength at much lower levels of strain. Therefore, there is a need to establish the displacement-rate for confined tests. Tests are conducted to investigate the effect of displacement-rate on pull-out response using the Tensar geogrid. Figure 11 shows the pull-out response of the geogrid under four different pull-out displacement-rates (2 mm/min, 6 mm/min, 10 mm/min, and 20 mm/min). An order of magnitude increase in the displacement-rate from 2 mm/min to 20 mm/ min results in a reduction of 25% in the peak pull-out resistance. It is noted that higher displacement rates mobilize lower strains along the reinforcement. Pull-out load will mobilize both the interface shear along the geosynthetic-soil interface and the passive resistance along the transverse ribs. Lower strains along the reinforcement imply a higher contribution from the interface friction. Passive resistance along the transverse ribs will lead to higher pull-out resistance and mobilize higher strains along the reinforcement. The test results with this specific type of soil and the Tensar geogrid
150
Khalid Farrag, Yalcin B. Acar. 11an Juran 60
i
40 TENSAR SR2 Confining Pressure: 48.2 kN/m2 Soil D~Mty: 16.4 kN/m3
<
m
30
l
*
I
5
0
*
I
10
,
I
15
i
20
25
DISPLACEMENT RATE (ram/rain)
(a)
1.0l
•
•
.
.
.
.
.
~'~ma~
s~
'
"
Displacementscorrespond 0.8
0.4
0.2
I 20mm(m!n • 10 ram/rain •
0.0
~ --
6 ram/rain i
0
1
2
3
,
I
4
i
i
5
6
(b) Fig. II. The effect of displacement rate on the pull-out response. (a) Pull-out load: (b) displacements along the reinforcement.
indicate that displacement rates of less than 6 mm/min influence test results to a lesser degree. Displacement rates of less than 6 mm/min are recommended in standard pull-out testing of geotextiles in uniform sands.
Pull-out resistance of geogrid reinforcements
151
Soil density The frictional resistance along the inclusion is highly influenced by soil density. Dense soils, under moderate confining pressures, tend to dilate when shear stresses are mobilized along the reinforcement interface. The soil surrounding the soil-reinforcement interface develops a condition of restrained dilatancy as the reinforcement is pulled out. The magnitude of this restraint depends upon the type of test (displacement controlled or stress controlled), soil density, soil thickness and confining stress. When soil dilation is restrained, the confining stress at the interface increases and results in an apparent increase in the pull-out resistance. The effect of the relative soil density on the pull-out response are evaluated by tests conducted with soil compacted at different densities (15-7 kN/m 3, 16-4 kN/m 3 and 16.7 kN/m 3) and under identical confining pressures of 48.2 kN/m 2. Figure 12 shows the effect of soil density on Tensar geogrid. An increase in the soil density results in an increase in the peak pull-out resistance and significantly higher interface stiffness moduli are obtained. Soil compaction mobilizes the lateral earth pressure resistance on the geogrid and increases the frictional resistance mobilized at the interface. The restraint on geogrid movement and slippage increases the interface modulus and peak pull-out resistance. The increase in density concentrates the strains to the point of load application on the front face and consequently, the mode of failure can be due to breakage of the reinforcement rather than slippage along the reinforcement.
Confining pressure The effect of confining pressure on the frictional resistance of the reinforcement has been demonstrated by several investigators (McGown et al., 1982; Juran & Chen, 1989). For geogrid reinforcements, confined extension during pull-out testing is restrained by the passive resistance of the soil and particles interlocking within transversal geogrid elements. The confined extension results in an apparent increase in the tensile strength and stiffness modulus. Figure 13 shows the effect of the confining pressure on the pull-out response for Tensar geogrid measured under confining pressures of 34 kN/m 2 (5 psi) and 48-2 kN/m 2 (7 psi). Figure 14 presents test results on Conwed geogrid tested under confining pressures of 48.2 kN/m 2 (7 psi), 96.4 kN/m 2 (14 psi) and 140 kN/m 2 (20 psi). An increase in the confining pressure results in significant increases in
Khalid Farrag, Yalcin B Acar, 11an Juran
152
6oS° I ,
/
~.
GEOGRID: T E N S A R S R 2
Or. = 4 8 k N / m 3 ( 7 p s i ) = 4ram/rain
ur.u KNIm Yn = 1 6 . 6 k N / m 3 •,,~Yn = 16.4 k N / m 3
o_j 40 o
...........
Yn = 15.4 k N / m 3
20
I 20
Or
FRONT
I I 40 60 DISPLACEMENT
I 80
•
I00
(ram)
(a)
~ = 48 kPa IMspimmmmt Rate = 3.8 - 6 ram/rain
CONFINING
70
I" 4O
10
15
15.5
16
16.5
17
1"/.5
D R Y D E N S I T Y (kNlm3)
(b) the pull-out resistance of these geogrids. Although the increase in the confining pressure reduces soil tendency to dilate, it leads to an increase in the passive soil resistance on the transverse ribs: consequently, the geogrid pull-out resistance increases. The effect of the confining pressure on the mobilized load transfer along the reinforcement is also illustrated.
Pull-out resistance of geogrid reinforcements
153
GEOGRID TENSAR SR2
0"n = 4 8 k N / m 21,7psi) 0
0.8
fYn = 16.4 kN/m 3
ac~
,,6
//~Yn
0.6
~ 16.-6 kN/m3
Z
U ¢ t Z0 .J~ O. (/) Q
0.4
0.2
I
I
I
T
I
2
3
4
5
GEOGRID NODES
(c) Fig. 12. The effect of density on the pull-out response. (a) Development of pull-out load: (b) the change of peak pull-out resistance with soil density: (c) displacements along the reinforcement when the peak pull-out load is reached.
The soil-geogrid interface shear stress is more uniformly mobilized along the geogrid under low confining pressures. The increase in the confining pressure restrains the geogfid displacement and results in a higher mobilization of the soil-geogrid interface shear stresses near the pull-out application point, a lower mobilization of the shear stresses at the rear end, and consequently a need for shorter effective adherence length of reinforcement. These results suggest that the design criteria for geogrid reinforced soil structures should take into account the in-soil confined extension properties derived from pull-out tests rather than the material properties obtained from unconfined extension tests. The unconfined behavior of the geogrid is substantially more ductile and its strain level at peak load is higher than that obtained under confined conditions. The use of unconfined properties may result in over-conservative design values for admissible geogrid extension and corresponding structure displacements. Interaction mechanism
The magnitude of the mobilized shear resistance along the soilreinforcement interface depends upon the type of reinforcement. In case of geogrids, the interface shear strength is primarily mobilized by the skin friction and the passive resistance against transversal fibs. For
154
Khalid Faring Yaicin R Acar, Ilan Juran 80 GEOGRID TENSAR SR? 3 ?'n = 16.4 kN/m 3 (104 pcf) : 6 ram/rain E ~z
60
0 1 F-
40
/
"~".~....
O-n : 4 8 k N / m 2
/
o I
.J .J o.
20
I
I
I
50 I00 FRONT D I S P L A C E M E N T
150 (ram)
20~
(a) po" O
n = 54kN/m
2
0.8
0.6
N
0.2
G
~
I
0
I
p
R
e
oSkRp2u1,- o ut load
I
I
2 3 GEOGRID NODES
-
~
I
4
5
(b) Fig. 13. The effect of confining pressureon pull-out response ofTensar SR2. (a) Pull-out load: (b) displacements along the reinforcement. coarse grained soils, the openings in the geogrids may allow soil particles to interlock between the ribs; thus, increasing its shear strength. The interface friction between the soil and the geogrid depends on the type of soil and the surface roughness of the geogdd while the contribution of passive soil resistance along the transverse ribs to the overall pull-out strength depends on many factors such as confining pressure, geogrid
155
Pull-out resistance of geogrid reinforcements I00
= 140kN/m 2 °'n = 96kN/m2
,0-n
801 L
E z
~.. /"
% ~./~°'n
= 48kN/m2
E:] 0 _J
0
I0 20 30 FRONT DISPLACEMENT (ram)
40
(a)
/o
n = 48kN/m 2
oa:~ 0.8
0.4
0.2 0
GEOGRID: CONWED Ya = 16"7kN/m3 (106pcf) "- 2 ram/rain I I I I
2
3
I
4
5
GEOGRID NODES
(b) Fig. 14. The effect of confining pressure on pull-out response of Conwed 9027. (a) Pullout load: (b) displacements along the reinforcement.
geometry and the diameter ratio (the ratio of the mean grain size of the soil to the size of the geogrid opening). Figure 15 presents the results of pull-out tests on Conwed geogrids performed with and without their transversal ribs. The results provide a sense of the contribution of the passive resistance on the transverse ribs to the overall pull-out load. The frictional resistance along the
Khalid Farrag Yalcin B. Acar, nan Juran
156
GEOGRID: CONWED Yn = 16.7 k N / m 5 (1061:)cf)
E
= 96 kN/m 2
z Q
o
._1
o
~a
;
ego
oe o ~- O ~ o ~- o oo-
•
".
•
..~ 0
CTn = 4 8 k N / m 2
.5 _J o.
i
0
I
I0
o•
No T r a n s v e r s a l
o•
With T r a n s v e r s a l Ribs
I
I
Ribs
I
20
FRONT D I S P L A C E M E N T
I
30
I
4O
(rnm)
Fig. 15. The effect of transverse ribs on the pull-out response of Conwed geogrid.
longitudinal ribs constitutes an average of 75% of the total pull-out load for this specific geogrid. It is also noted that the passive resistance along the transverse ribs is mobilized at higher levels of strain resulting in a higher post-peak pull-out resistance. The strain level required to mobilize the passive resistance along the transverse elements would vary with the geogrid and the soil type. SUMMARY AND CONCLUSIONS There exists significant differences in pull-out or direct shear tests used in experimental modelling of soil-geosynthetic interaction mechanism and/or performance evaluation of in-soil properties of geosynthetics. Standard testing equipment and testing procedures are needed to obtain comparable results and to develop appropriate methodologies in modelling the load-transfer mechanism. Pull-out boxes are designed and constructed at GERL/LTRC-LSU. The components of these equipment and the procedure used in testing are presented. Tests conducted in performance assessment of this equipment employed a uniform sand, and TENSAR SR2, CONWED 9027 geogrids. The following conclusions are obtained for the uniform sand and the geogrids used in this study:
Pull-out resistance of geogrid reinforcements
157
(1) Side frictions on the side walls of the box decrease the peak pullout load. A minimum clearance of 15 cm (6 in) is necessary in order to avoid any side-wall effect in case the friction along the wall is not minimized by some other means. (2) The increase in sleeve length decreases the effect of front wall on the pull-out resistance. The tests conducted in this study demonstrate that sleeve lengths of at least 30-5 cm (12 in) of length are necessary. (3) Increased thickness of soil cushioning the geosynthetic decreases the effects of the top and bottom boundaries. A soil thickness of at least 30 cm above and 30 cm below the geogrid is needed to eliminate the influence of these boundaries. (4) High displacement rates affect both the pull-out load and the displacement distribution along the reinforcement. The displacement rate effects are minimized if rates of the order of less than 6 mm/min are used. (5) Increased densities lead to mobilization of the interlocking mechanism and consequently, result in a higher contribution of the passive resistance exerted on the transverse ribs. At higher densities, pull-out resistance increases and displacements move closer to the point of application of the pull-out load. (6) Similar to the effects of increased densities, confinement increases the peak pull-out resistance and moves the displacements along the reinforcement toward the point of application of the load. (7) Pull-out resistance is developed by a combination of the passive resistance developed along the transverse ribs and interface shear mobilized along the geogrid. (8) Methods developed and employed in evaluation of rigid reinforcements should not be used in determining the confined extension properties and shear stress-strain behavior ofgeogrids. The results presented in this study demonstrate the significant variability obtained in pull-out load-displacement response of geogrids due to differences in equipment and testing procedures. It is necessary to realize the influence of these parameters on pull-out response in standard commercial testing ofgeosynthetics and in interpretation of the load-transfer mechanism using pull-out test results. ACKNOWLEDGEMENTS Design, construction, and performance assessment of the pull-out testing facility at GERL are supported by funds awarded and provided to
158
Khalid Farrag Yaicin B. Acar, llan Juran
LSU and by the Louisiana Transportation Research Center of Louisiana Department of Transportation and Development, Federal Highway Administration, the Board of Regents of the State of Louisiana, Conwed Plastics Inc. and Civil Engineering Department of Louisiana State University. The funds awarded and provided by these agencies and institutions are gratefully acknowledged. Dr Ilan Juran of Polytechnic University has had significant contributions in establishment of GERL in cooperation and collaboration with the second author. Dr Juran's efforts and contributions are gratefully acknowledged by GERL. LTRC project officer, Mr Paul Griffin and Conwed Plastics Product Development Manager, Mr Lee Richmond are acknowledged for their collaboration and cooperation. We acknowledge and appreciate the efforts of Ex-director of LTRC, Mr Ara Arman, the current director, Dr Peter Stopher, ex-chairman of the Civil Engineering Department, Dr Roger IC Seals and the current chairman Dr Richard Avent in establishment and e n h a n c e m e n t of GERL. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.
REFERENCES Acar, Y. B., Durgunoglu, T. & Tumay, M. T. (1982). Interface properties of sand. ASCE, JGE, 108(GT3), 648-54. Anderson, L. R. & Nielsen, M. R. (1984). Pull-out resistance of wire mats embedded in soil. Report for the Hilfiker Company, Eureka, CA. Bonaparte, R., Holtz, R. D. & Giroud, J. P. (1987). Soil reinforcement design using geotextiles and geogrids. Geotextile Testing and the Design Engineer, ASTM STP 952, pp. 69-116. Brand, S. R. & Duffy, D. M. (1987). Strength and pullout testing of geogrids. Geosynthetics Conference, New Orleans, Vol. 1, pp. 226-36. Christopher, B. R. (1976). Tensar SS2 geogrid evaluation. Evaluation report to Tensar Corporation. Elias, V. (1979). Friction in reinforced earth utilizing fine grained backfills. International Conference on Soil Reinforcement, Paris, France, pp. 435-8. Farrag, K. (1990). Interaction properties of geogrids in reinforced soil walls -testing and analysis. Dissertation submitted to Graduate School of Louisiana State University, Baton Rouge, LA. lngold, T. S. & Templeman, J. (1979). The comparative performance of polymer net reinforcement. International Conference on Soil Reinforcement, Paris, Vol. 1. Ingold, T. S. (1983). Laboratory pull-out testing of grid reinforcement in sand. Geotech. Testing J., 6(3 ), I 01 - 11. Jewell, R. A. (1979). Discussion presented to Section 8, Proceedings of VII European Conference on Soil Mechanics and Foundation Engineering, Brighton, UK.
Pull-out resistance of geogrid reinforcements
159
Jewell, R. A. (1980). Some effects of reinforcement on the mechanical behavior of soils. Dissertation submitted to Cambridge University, Cambridge, UK. Juran, I., Knochenmus, G., Acar, Y. B. & Arman, A. (1988). Pull-out response of geotextiles and geogrids (Synthesis of Available Experimental Data). Proceedings of Symposium on Geotextiles for Soil Improvement, ASCE, Geotech. Special Publication 18, pp. 92-111. Juran, I., Guermazi, A., Chen, C. L. & Ider, M. H. (1988). Modeling and simulation of load transfer in reinforced soil: Part 1. Int. J. Numer. Anal. Meth. Geomech, 12, pp. 141-55. Juran, I., lder, H. & Farrag, Kh. (1990). Strain compatibility analysis for geosynthetic reinforced soil walls. ASCE, JGE, 116(2), 312-29. Koerner, R. M. (1986). Direct shear/pull-out tests on geogrids, Report No. 1, Dept. of Civil Eng., Drexel Univ., Philadelphia, PA. Koerner, R. M. (1990). Designing with Geosynthetics. 2nd edn, Prentice Hall, Englewood Cliffs, NJ. McGown, A., Andrawes, K. Z. & Kabir, M. H. (1982). Load-extension testing of geotextiles confined in-soil, 2nd Int. Conf. on Geotextiles. Las Vegas, Vol. 3, pp. 793-8. Myles, B. (1982). Assessment of soil fabric friction by means of shear, 2nd Int. Conf. on Geotextiles, Las Vegas, Vol. 3, pp. 787-91. Palmeira, E. & Milligan, G. (1989). Large scale direct shear tests on reinforced soil. Soil and foundations, Japan. Soc. Soil Mech. Found Eng., 29(1), 18-30. Rowe, R. K. & Ho, S. K. (1986). Determination of geotextile stress-strain characteristics using a wide strip test, 3rd Int. Conf. on Geotextiles, Vienna, pp. 885-90. Rowe, R. K., Ho, S. K. & Fisher, D. (1985). Determination of soil-geotextile interface strength properties, 2nd Canadian Symposium on Geotextiles, Montreal, Canada, pp. 25-34. Saxena, S. K. & Budiman, J. S. (1985). Interface response ofgeotextiles, Proc. 1 lth Int. Conference on Soil Mechanics and Foundation Engineering, San Francisco, Vol. 3, pp. 1801--4. Schlosser, F. & Guilloux, A. (1979). Friction between soil and strips in reinforced earth structures, Int. Conf. on Soil Reinforcement, Paris, France, Vol. 1. Tzong, W. H. & Cheng-Kuang, S. (1987). Soil-geotextile interaction mechanism in pull-out test, Geosynthetics Conference. New Orleans, Vol. 1, pp. 250-9. Williams, N. D. & Houlihan, M. F. (1987). Evaluation of interface friction properties between geosynthetics and soils, Geosynthetics Conference. New Orleans, Vol. 2, pp. 616-27.