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Interface friction of geonets: A literature survey
Geotextiles and Geomembranes 10 (1991) 549-558
Interface Friction of Geonets: A Literature Survey Larry D. Lydick Fluid Systems, Inc., 3201 Triangle Park Drive, Cincinnati, Ohio 45246, USA
& George A. Zagorski National Seal Co., Farnsworth Center, 1245 Corporate Boulevard, Aurora, Illinois 60504, USA
ABSTRACT A wealth of test data is available on the friction interface between geonets and other geosynthetics available from manufacturers, consulting engineers, third party agencies and owners of waste management facilities. However, these results can vary significantly, resulting in potential confusion for the designer. Since the geonet/geosynthetic interface is often the most critical, care must be taken to choose data correctly. Choosing an artificially high friction angle unintentionally erodes the safety factor. A lower than actual friction angle limits the design. This technical note examines friction angles of geonets and other geosynthetics. The variability of test results and how they relate to the test profile and to the shear box is also investigated.
INTRODUCTION Geonets are polymeric materials that replace or augment aggregates traditionally used to transmit liquids. For waste management applications, geonets are used in leachate collection systems, leak detection systems, methane gas collection systems and landfill cap drainage systems (Fig. 1). The planar flow structure of a geonet is a profiled mesh manufactured by extruding two sets of strands through a counter rotating die. This process 549 Geotextiles and Geomembranes 0266-1144/91/$03.50 (~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
Larry D. Lydick, George A. Zagorski
550
~-~1111-~1111-----IIII~ .
XXXXXX
.
.
.
.
.
.
.
.
.
.
.
.
.
GEOTEXTILE GEONET GEOMEMBRANE
(a)
(b)
Fig. 1. Use of geonets: (a) leachate collection systems; (b) landfill caps.
forms a three-dimensional net with diamond-shaped openings between adjacent sets of ribs. Geonets are produced of polyethylene and range in density from 0.935 to 0.942 g/cm 3. Geonets can have solid or foamed ribs and vary in thickness from 4 mm to 7.5 mm. ] The internal structure of the geonet has to be separated from the adjacent soil to prevent clogging. In this regard, the geotextile functions as both a separator and a filter. The majority of geotextiles used adjacent to geonets in waste management applications are nonwoven. Geonets were first used with high density polyethylene (HDPE) liners, but recently have been used with polyvinyl chloride (PVC) and very low density polyethylene (VLDPE) liners in caps or closure applications.
FRICTION A N G L E I M P O R T A N C E An accurate determination of the friction angle interface between various geosynthetics is imperative to t h e design of any waste management system. In order to maximize landfill volume, side slopes are typically as steep as possible while maintaining an appropriate factor of safety. The slope angle is controlled by the weakest frictional interface within the liner system profile. Depending on the profile, it would not be uncommon for a geonet to be a part of the weakest surface, although all interfaces need to be examined. The design procedure is relatively straightforward (see Fig. 2) 2
CONDITIONS A F F E C T I N G RESULTS Currently there is no specific ASTM Standard for interface friction between geosynthetics, although an ASTM committee is in the process of preparing a standard method. The test method most commonly used
Interface friction of geonets : a literature survey
551
W
.~ ~ ' ~ _ -, . - . , : ~.'.-:J-: •
1.. I $1:
~.~,
..~
Fig. 2. Shear stresses on geosynthetics along landfill slopes.
references A S T M D 3080, which is a direct shear test for soils. 3 Since there is no specific standard for friction testing between geosynthetics and m a n y factors are user specific, a broad range of friction angles can be found. Factors affecting geonet interfaces are as follows.
Testing device Friction angles are m e a s u r e d from two types of testing devices. O n e m e t h o d uses a tilt table. The questioned interface is placed on top of the tilt table and the plane is slowly inclined to determine the angle of frictional resistance of sliding. The second m e t h o d uses a direct shear box. The direct shear box consists of a fixed box above or below a sliding box. Layers of geosynthetics and soil are placed in a simulation of the field profile. A vertical force is applied, representing the o v e r b u r d e n pressure. The shear force and resulting shear deflection to move the box horizontally is m e a s u r e d and the friction angle is d e t e r m i n e d using standard MohrC o u l o m b graphical procedures. Shear boxes are m o r e c o m m o n than the tilt table and c o m e in various sizes. Typical shear box sizes are 1 0 . 2 x 10.2cm, 30.5 x 3 0 . 5 c m , and 45-7 × 45.7 cm. T h r o u g h conversations with various testing laboratories, the 30.5-cm square box seems to be most accurate for profiles which include a geonet. The surface area of 10.2 cm square box is felt to be too small to obtain accurate results. The 45.7 cm box seems to be too large to accurately control the test for certain profiles. Shear box size is a current topic of discussion in the A S T M committee.
Larry D. Lydick, George A. Zagorski
552
" "*" ,'. ':": :'t" ";:';.~ "." "'* ,~' ':"i :'t" "::'
l ::".,,'-:-k--'..~:..~'~: ,'-:~-'. J
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';;:kii};LL ; t~-:~:-.:h-:,...... I 5-.,i"..#~?Yl"::i:#::.~
;.~ .:~..:-:,~~;,#...:~i, i~i "::;:#~:.~ ' .: ......
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" z • ~1 = f. : , ~ . - t =;L~ ~-~ , ,'- - .... .•4 , ,'" z ='. ' "T-: " , ,'<
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• I..'.-t
~'~. ~'," " I, '.' , - t
"~" E L l
;.'~.:;::-~;:..~:;.fi;Z ~yi'::;:#::,~ , =-.." . . . . t.~ '.t~... • ..... , ,'. , ' ~ . : . ' . ,--;--, ,'. , - - : - :
FIXED
PARTIALLY
',¢, ,-.,:
f
FIXED
'. ,----
....
•
,--..-.-
of.
• "h;':' i-'t ":.z ;, ." "~.':"~"t ":':J ." :'i ,'-'-::.: I" ~'.:.:: .,," :( ,':-::.:" ::,5
FREE
Fig. 3. Types of shear box set-ups•
Geosynthetic attachment T h e geosynthetic with respect to the shear box can be either fixed (geosynthetic attached at both ends to the shear box base), partially fixed (geosynthetic attached to one end of shear box), or free (geosynthetic not attached to box at all) (see Fig. 3). Each type has its inherent disadvantages. T h e fixed box does not allow the geosynthetic to elongate. 4 In the field, some elongation is likely to occur, yielding s o m e w h a t different (but u n k n o w n ) friction angles. T h e geosynthetic can d e f o r m into the box of a partially fixed set-up thus having an u n e v e n plane of sliding. T h e geosynthetic in the free box may also d e f o r m or slide and 'bunch-up'. 4 D e f o r m a t i o n is likely to be different from test to test and from test to field, d e p e n d i n g on soil type, compaction, density and moisture content. If soil conditions are not consistent, the degree of d e f o r m a t i o n may differ, possibly resulting in varying friction angles.
Normal load Care must be taken in selecting the normal load so as to reflect field conditions. A t high normal loads, there is a possibility for the geosynthetics to impinge on each other (for example, a geotextile may impinge or 'grab' a geonet surface). At low normal loads (like a cap closure) this p h e n o m e n o n may not occur to the same extent. T h u s using high loads in a cap design may provide inaccurate factors-of-safety.
Condition of sample If the samples being tested are not in the as-received conditions (i.e. they are scratched, folded, torn, etc.), then nonrepresentative values m a y be
Interface friction of geonets: a literature survey
553
obtained. In this regard, a new set of surfaces should be required for each normal load test. Strain rate A direct shear strain rate of 5 mm/min is proposed by the ASTM committee for geosynthetic/geosynthetic interface shear evaluation. 3 Profile The profile can have a significant impact on the friction angles obtained. As discussed previously, clayey soils or flexible geomembranes (like V L D P E or PVC) can impinge into the net causing higher friction angles than might be obtained if only a rigid plate were attached. Therefore, site-specific profiles are imperative for simulated test. Type of geosynthetic Geomembranes consist of different polymers (i.e. H D P E , PVC), which give different results. Also similar liners can have different densities, smoothness, manufacturing processes and thickness, each somewhat affecting the results. Moisture conditions Wet samples may test slightly different than dry samples, the site-specific situation must be evaluated. Preloaded samples If a profile is preloaded it allows the interfaces to seat into each other. This can increase the friction results especially if one, or both, of the interfaces are flexible or if a compressible soil is involved.
RESULTS F R O M L I T E R A T U R E S U R V E Y Several information sources were contacted to provide data for this literature search. Response was via three testing laboratories, four manufacturers, and two end users. Also used were related published technical papers.
Larry D. Lydick, GeorgeA. Zagorski
554 20 18 16 14 IZ ¢n 10 8 6 4 2 0
SMOOTH HDPE
TEXTURED HDPE
PVC
VLDPE
GEOTEXTILE
Fig. 4. Average friction angles between geonets and other geosynthetics.
From these responses, information on the following interfaces were obtained: • four types of geonets • eleven types of geomembranes • five types of nonwoven geotextiles Each type being from a different manufacturer or consisting of a different polymer. A n average of all friction data obtained between geonet/geotextile and geonet/geomembrane interfaces is shown in Fig. 4. (A summary of individual test results appears in the Appendix.) These averages do not differentiate between any factors that could cause varied results. It is perhaps of greater interest to compare these average values with the specific results that make up the averages (see Fig. 5). This broad range of results proves variables exist. These differences must be considered when using friction angle test results. Clearly, average values do not necessarily provide for a conservative design. Figure 6 utilizes the same friction data from which Figs 4 and 5 were developed, but now differentiates according to shear box size. (Although the box size for every test was not available, those which did document the box size were included.) This figure shows that shear box size alone may have a tremendous effect on test results. This graphic difference is also seen in the data indicated in Table 1.
Interface friction of geonets: a literature survey
555
2 1 - 2 5 10"0°1° 16-20 40'0"/o
1 6 - 2 0 30"0°/o 11-15 40°1o
11-15 42'9 °1o
6-10
6 - 1 0 11.4°1o < 6 5'7°1o Smooth HDPE
11-15 100"0 °1o
20°1°
Textu r e d HDPE
//
1 6 - 2 0 12.5°1o 11-15 43.8 °1°
6 - 1 0 37"5°/o <: 6 6-2°/o
PVC
VLDPE 26-30
6'2°1°
2 1 - 2 5 37"5°1, Nonwoven geotextile
16-20
18"8"1o
11-15 31-2 °1o 6 - 1 0 6"2"/o
Fig. 5. Distribution of the friction results between geonets and other geosynthetics.
25 20
5 O
Smooth HDPE
Textured HDPE
VLDPE
PVC
Geotextile
Geosyntheti¢
Fig. 6. Average friction angle between geonets and other geosynthetics relative to shear box size. • 10.2 cm x 10-2 cm [ ] 30.5 cm x 30.5 cm ~ 45.7 cm x 45.7 cm
556
Larry D. Lydick, George A. Zagorski TABLE 1
Summary of Friction Angles of Geonet/Geosynthetic Interfaces Geosynthetic
Smooth HDPE Textured HDPE PVC VLDPE Nonwoven geotextile
Average friction angle ~
13-9 13.9 15-0 11.1 18.0
10.2 × 10.2 cm 30.5 × 30.5 cm 45.7 x 45.7 cm Box Box Box average average average
5-0 --5-5 --
13-4 17-1 -9.2 16.5
17.6 --12.6 23.0
aAverage includes tests which did not specify shear box size.
CONCLUSION A n important issue to assess regarding the variation of interface friction angles is its potential influence on the design process. If the profile includes only n o n w o v e n geotextile/soil interface, where the range of test results can be from 25 ° to 35 ° , there may be no serious consequences. Landfill and landfill closures are not typically designed with slopes steeper than 2:1 (26.6°). The natural soil may not have that degree of stability. H o w e v e r , if the interface includes a geonet, there may be serious consequences. The geonet/geotextile interface range from this study is from 6 ° to 25 °. Also note that the range is nearly as b r o a d for the g e o n e t / g e o m e m b r a n e interface. Nearly all landfills and landfill closures are designed s o m e w h e r e in this range. In order to eliminate or control the variability in test results, a standard test m e t h o d appears to be necessary. Careful d o c u m e n t a t i o n of variables using such a standard test m e t h o d is also required. O n e thing is certain. Accurate friction angle results d e m a n d specific test profiles with each variable reflecting site-specific field conditions as much as possible. Only then can confident design decisions b e c o m e possible.
REFERENCES AND BIBLIOGRAPHY
1. Zagorski, G. A. & Wayne, M. H., Geonet seams. The S e a m i n g o f G e o s y n t h e tics, 3rd GRI Seminar, Philadelphia, PA, ed. T. S. Ingold. Elsevier Applied Science, London, 1989, pp. 487-500.
Interface friction of geonets: a literature survey
557
2. Koerner, R. M., Designing with Geosynthetics, 2nd edn, Prentice Hall, 1990. 3. Bove, J. A., Direct shear friction testing for geosynthetics in waste containment. Geosynthetic Testing for Waste Containment Applications, A S T M STP 1081, ed. M. Koerner, American Society for Testing and Materials, Philadelphia, 1990. 4. Takasumi, D. L., Green, K. R. & Holtz, R. D., Soil-Geosynthetics Interface
Strength Characteristics: A Review of State-of-the-Art Testing Procedures. Industrial Fabrics Association International 1991, pp. 87-100. 5. Williams, N. D. & Houlihan, M., Evaluation of Friction Coefficients Between Geomembranes, Geotextiles and Related Products, Third International Conference on Geotextiles, Vienna, Austria, 1986, International Geotextile Society, 1986, pp. 891-6. 6. Martin, J. P., Koerner, R. M. & Whitty, J. E., Experimental Friction
Evaluation of Slippage Between Geomembranes, Geotextiles and Soils, International Conference on Geomembranes, Denver, USA. Industrial Fabrics Association International 1984, pp. 191-6. 7. Gourc, J. P., Gballou, J., Blivet, J. C., Puig, J. & Mathieu, G., Geosynthetic skin-friction: influence of the equipment on the measure, Geotextiles, Geomembranes and Related Products, ed. G. ,den Hoedt. Balkema, Rotterdam, 1990.
APPENDIX
Shear box size (cm)
Friction angle
Geonet versus smooth HDPE 10.2 x 10-2 30.5 × 30-5 45-7 × 45.7 Not documented
5 11, 11, 11.3, 13, 13, 14, 14, 14, 15, 15, 16.6 16, 16, 17, 17, 18, 18, 18, 18-5, 19, 19 5, 8, 9.1, 10, 10, 11, 11, 12, 12, 15, 17, 17-6, 19
Geonet versus textured HDPE 10.2 × 10-2 30-5 x 30.5 45-7 x 45-7 Not documented
12.5, 16, 22.8 6.7, 9.8, 11.2, 12.5, 15-3, 16, 16.3
Geonet versus PVC 10-2 × 10.2 30.5 x 30.5 45.7 × 45-7 Not documented
15
558
Larry D. Lydick, George A. Zagorski
Shear box size (cm)
Friction angle
Geonet versus VLDPE 10.2 x 10-2 30.5 × 30-5 45.7 x 45-7 Not documented
5,6 7-7, 9, 10.9 9, 9, 11, 11, 11, 12, 14, 15, 15, 16, 16
Geonet versus nonwoven geotextile 10.2 x 10.2 30.5 x 30.5 45.7 × 45.7 Not documented
m
11.8, 11.8, 14, 15.4, 17.6, 18, 27 23, 23 10, 14, 19, 20, 21, 21, 22
Interface Friction of Geonets: A Literature Survey Larry D. Lydick Fluid Systems, Inc., 3201 Triangle Park Drive, Cincinnati, Ohio 45246, USA
& George A. Zagorski National Seal Co., Farnsworth Center, 1245 Corporate Boulevard, Aurora, Illinois 60504, USA
ABSTRACT A wealth of test data is available on the friction interface between geonets and other geosynthetics available from manufacturers, consulting engineers, third party agencies and owners of waste management facilities. However, these results can vary significantly, resulting in potential confusion for the designer. Since the geonet/geosynthetic interface is often the most critical, care must be taken to choose data correctly. Choosing an artificially high friction angle unintentionally erodes the safety factor. A lower than actual friction angle limits the design. This technical note examines friction angles of geonets and other geosynthetics. The variability of test results and how they relate to the test profile and to the shear box is also investigated.
INTRODUCTION Geonets are polymeric materials that replace or augment aggregates traditionally used to transmit liquids. For waste management applications, geonets are used in leachate collection systems, leak detection systems, methane gas collection systems and landfill cap drainage systems (Fig. 1). The planar flow structure of a geonet is a profiled mesh manufactured by extruding two sets of strands through a counter rotating die. This process 549 Geotextiles and Geomembranes 0266-1144/91/$03.50 (~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
Larry D. Lydick, George A. Zagorski
550
~-~1111-~1111-----IIII~ .
XXXXXX
.
.
.
.
.
.
.
.
.
.
.
.
.
GEOTEXTILE GEONET GEOMEMBRANE
(a)
(b)
Fig. 1. Use of geonets: (a) leachate collection systems; (b) landfill caps.
forms a three-dimensional net with diamond-shaped openings between adjacent sets of ribs. Geonets are produced of polyethylene and range in density from 0.935 to 0.942 g/cm 3. Geonets can have solid or foamed ribs and vary in thickness from 4 mm to 7.5 mm. ] The internal structure of the geonet has to be separated from the adjacent soil to prevent clogging. In this regard, the geotextile functions as both a separator and a filter. The majority of geotextiles used adjacent to geonets in waste management applications are nonwoven. Geonets were first used with high density polyethylene (HDPE) liners, but recently have been used with polyvinyl chloride (PVC) and very low density polyethylene (VLDPE) liners in caps or closure applications.
FRICTION A N G L E I M P O R T A N C E An accurate determination of the friction angle interface between various geosynthetics is imperative to t h e design of any waste management system. In order to maximize landfill volume, side slopes are typically as steep as possible while maintaining an appropriate factor of safety. The slope angle is controlled by the weakest frictional interface within the liner system profile. Depending on the profile, it would not be uncommon for a geonet to be a part of the weakest surface, although all interfaces need to be examined. The design procedure is relatively straightforward (see Fig. 2) 2
CONDITIONS A F F E C T I N G RESULTS Currently there is no specific ASTM Standard for interface friction between geosynthetics, although an ASTM committee is in the process of preparing a standard method. The test method most commonly used
Interface friction of geonets : a literature survey
551
W
.~ ~ ' ~ _ -, . - . , : ~.'.-:J-: •
1.. I $1:
~.~,
..~
Fig. 2. Shear stresses on geosynthetics along landfill slopes.
references A S T M D 3080, which is a direct shear test for soils. 3 Since there is no specific standard for friction testing between geosynthetics and m a n y factors are user specific, a broad range of friction angles can be found. Factors affecting geonet interfaces are as follows.
Testing device Friction angles are m e a s u r e d from two types of testing devices. O n e m e t h o d uses a tilt table. The questioned interface is placed on top of the tilt table and the plane is slowly inclined to determine the angle of frictional resistance of sliding. The second m e t h o d uses a direct shear box. The direct shear box consists of a fixed box above or below a sliding box. Layers of geosynthetics and soil are placed in a simulation of the field profile. A vertical force is applied, representing the o v e r b u r d e n pressure. The shear force and resulting shear deflection to move the box horizontally is m e a s u r e d and the friction angle is d e t e r m i n e d using standard MohrC o u l o m b graphical procedures. Shear boxes are m o r e c o m m o n than the tilt table and c o m e in various sizes. Typical shear box sizes are 1 0 . 2 x 10.2cm, 30.5 x 3 0 . 5 c m , and 45-7 × 45.7 cm. T h r o u g h conversations with various testing laboratories, the 30.5-cm square box seems to be most accurate for profiles which include a geonet. The surface area of 10.2 cm square box is felt to be too small to obtain accurate results. The 45.7 cm box seems to be too large to accurately control the test for certain profiles. Shear box size is a current topic of discussion in the A S T M committee.
Larry D. Lydick, George A. Zagorski
552
" "*" ,'. ':": :'t" ";:';.~ "." "'* ,~' ':"i :'t" "::'
l ::".,,'-:-k--'..~:..~'~: ,'-:~-'. J
" "'~.":"I-"L ";-'i,,' '.'" 't?':'" "'( ";:
';;:kii};LL ; t~-:~:-.:h-:,...... I 5-.,i"..#~?Yl"::i:#::.~
;.~ .:~..:-:,~~;,#...:~i, i~i "::;:#~:.~ ' .: ......
.t "":
" z • ~1 = f. : , ~ . - t =;L~ ~-~ , ,'- - .... .•4 , ,'" z ='. ' "T-: " , ,'<
•
• I..'.-t
~'~. ~'," " I, '.' , - t
"~" E L l
;.'~.:;::-~;:..~:;.fi;Z ~yi'::;:#::,~ , =-.." . . . . t.~ '.t~... • ..... , ,'. , ' ~ . : . ' . ,--;--, ,'. , - - : - :
FIXED
PARTIALLY
',¢, ,-.,:
f
FIXED
'. ,----
....
•
,--..-.-
of.
• "h;':' i-'t ":.z ;, ." "~.':"~"t ":':J ." :'i ,'-'-::.: I" ~'.:.:: .,," :( ,':-::.:" ::,5
FREE
Fig. 3. Types of shear box set-ups•
Geosynthetic attachment T h e geosynthetic with respect to the shear box can be either fixed (geosynthetic attached at both ends to the shear box base), partially fixed (geosynthetic attached to one end of shear box), or free (geosynthetic not attached to box at all) (see Fig. 3). Each type has its inherent disadvantages. T h e fixed box does not allow the geosynthetic to elongate. 4 In the field, some elongation is likely to occur, yielding s o m e w h a t different (but u n k n o w n ) friction angles. T h e geosynthetic can d e f o r m into the box of a partially fixed set-up thus having an u n e v e n plane of sliding. T h e geosynthetic in the free box may also d e f o r m or slide and 'bunch-up'. 4 D e f o r m a t i o n is likely to be different from test to test and from test to field, d e p e n d i n g on soil type, compaction, density and moisture content. If soil conditions are not consistent, the degree of d e f o r m a t i o n may differ, possibly resulting in varying friction angles.
Normal load Care must be taken in selecting the normal load so as to reflect field conditions. A t high normal loads, there is a possibility for the geosynthetics to impinge on each other (for example, a geotextile may impinge or 'grab' a geonet surface). At low normal loads (like a cap closure) this p h e n o m e n o n may not occur to the same extent. T h u s using high loads in a cap design may provide inaccurate factors-of-safety.
Condition of sample If the samples being tested are not in the as-received conditions (i.e. they are scratched, folded, torn, etc.), then nonrepresentative values m a y be
Interface friction of geonets: a literature survey
553
obtained. In this regard, a new set of surfaces should be required for each normal load test. Strain rate A direct shear strain rate of 5 mm/min is proposed by the ASTM committee for geosynthetic/geosynthetic interface shear evaluation. 3 Profile The profile can have a significant impact on the friction angles obtained. As discussed previously, clayey soils or flexible geomembranes (like V L D P E or PVC) can impinge into the net causing higher friction angles than might be obtained if only a rigid plate were attached. Therefore, site-specific profiles are imperative for simulated test. Type of geosynthetic Geomembranes consist of different polymers (i.e. H D P E , PVC), which give different results. Also similar liners can have different densities, smoothness, manufacturing processes and thickness, each somewhat affecting the results. Moisture conditions Wet samples may test slightly different than dry samples, the site-specific situation must be evaluated. Preloaded samples If a profile is preloaded it allows the interfaces to seat into each other. This can increase the friction results especially if one, or both, of the interfaces are flexible or if a compressible soil is involved.
RESULTS F R O M L I T E R A T U R E S U R V E Y Several information sources were contacted to provide data for this literature search. Response was via three testing laboratories, four manufacturers, and two end users. Also used were related published technical papers.
Larry D. Lydick, GeorgeA. Zagorski
554 20 18 16 14 IZ ¢n 10 8 6 4 2 0
SMOOTH HDPE
TEXTURED HDPE
PVC
VLDPE
GEOTEXTILE
Fig. 4. Average friction angles between geonets and other geosynthetics.
From these responses, information on the following interfaces were obtained: • four types of geonets • eleven types of geomembranes • five types of nonwoven geotextiles Each type being from a different manufacturer or consisting of a different polymer. A n average of all friction data obtained between geonet/geotextile and geonet/geomembrane interfaces is shown in Fig. 4. (A summary of individual test results appears in the Appendix.) These averages do not differentiate between any factors that could cause varied results. It is perhaps of greater interest to compare these average values with the specific results that make up the averages (see Fig. 5). This broad range of results proves variables exist. These differences must be considered when using friction angle test results. Clearly, average values do not necessarily provide for a conservative design. Figure 6 utilizes the same friction data from which Figs 4 and 5 were developed, but now differentiates according to shear box size. (Although the box size for every test was not available, those which did document the box size were included.) This figure shows that shear box size alone may have a tremendous effect on test results. This graphic difference is also seen in the data indicated in Table 1.
Interface friction of geonets: a literature survey
555
2 1 - 2 5 10"0°1° 16-20 40'0"/o
1 6 - 2 0 30"0°/o 11-15 40°1o
11-15 42'9 °1o
6-10
6 - 1 0 11.4°1o < 6 5'7°1o Smooth HDPE
11-15 100"0 °1o
20°1°
Textu r e d HDPE
//
1 6 - 2 0 12.5°1o 11-15 43.8 °1°
6 - 1 0 37"5°/o <: 6 6-2°/o
PVC
VLDPE 26-30
6'2°1°
2 1 - 2 5 37"5°1, Nonwoven geotextile
16-20
18"8"1o
11-15 31-2 °1o 6 - 1 0 6"2"/o
Fig. 5. Distribution of the friction results between geonets and other geosynthetics.
25 20
5 O
Smooth HDPE
Textured HDPE
VLDPE
PVC
Geotextile
Geosyntheti¢
Fig. 6. Average friction angle between geonets and other geosynthetics relative to shear box size. • 10.2 cm x 10-2 cm [ ] 30.5 cm x 30.5 cm ~ 45.7 cm x 45.7 cm
556
Larry D. Lydick, George A. Zagorski TABLE 1
Summary of Friction Angles of Geonet/Geosynthetic Interfaces Geosynthetic
Smooth HDPE Textured HDPE PVC VLDPE Nonwoven geotextile
Average friction angle ~
13-9 13.9 15-0 11.1 18.0
10.2 × 10.2 cm 30.5 × 30.5 cm 45.7 x 45.7 cm Box Box Box average average average
5-0 --5-5 --
13-4 17-1 -9.2 16.5
17.6 --12.6 23.0
aAverage includes tests which did not specify shear box size.
CONCLUSION A n important issue to assess regarding the variation of interface friction angles is its potential influence on the design process. If the profile includes only n o n w o v e n geotextile/soil interface, where the range of test results can be from 25 ° to 35 ° , there may be no serious consequences. Landfill and landfill closures are not typically designed with slopes steeper than 2:1 (26.6°). The natural soil may not have that degree of stability. H o w e v e r , if the interface includes a geonet, there may be serious consequences. The geonet/geotextile interface range from this study is from 6 ° to 25 °. Also note that the range is nearly as b r o a d for the g e o n e t / g e o m e m b r a n e interface. Nearly all landfills and landfill closures are designed s o m e w h e r e in this range. In order to eliminate or control the variability in test results, a standard test m e t h o d appears to be necessary. Careful d o c u m e n t a t i o n of variables using such a standard test m e t h o d is also required. O n e thing is certain. Accurate friction angle results d e m a n d specific test profiles with each variable reflecting site-specific field conditions as much as possible. Only then can confident design decisions b e c o m e possible.
REFERENCES AND BIBLIOGRAPHY
1. Zagorski, G. A. & Wayne, M. H., Geonet seams. The S e a m i n g o f G e o s y n t h e tics, 3rd GRI Seminar, Philadelphia, PA, ed. T. S. Ingold. Elsevier Applied Science, London, 1989, pp. 487-500.
Interface friction of geonets: a literature survey
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2. Koerner, R. M., Designing with Geosynthetics, 2nd edn, Prentice Hall, 1990. 3. Bove, J. A., Direct shear friction testing for geosynthetics in waste containment. Geosynthetic Testing for Waste Containment Applications, A S T M STP 1081, ed. M. Koerner, American Society for Testing and Materials, Philadelphia, 1990. 4. Takasumi, D. L., Green, K. R. & Holtz, R. D., Soil-Geosynthetics Interface
Strength Characteristics: A Review of State-of-the-Art Testing Procedures. Industrial Fabrics Association International 1991, pp. 87-100. 5. Williams, N. D. & Houlihan, M., Evaluation of Friction Coefficients Between Geomembranes, Geotextiles and Related Products, Third International Conference on Geotextiles, Vienna, Austria, 1986, International Geotextile Society, 1986, pp. 891-6. 6. Martin, J. P., Koerner, R. M. & Whitty, J. E., Experimental Friction
Evaluation of Slippage Between Geomembranes, Geotextiles and Soils, International Conference on Geomembranes, Denver, USA. Industrial Fabrics Association International 1984, pp. 191-6. 7. Gourc, J. P., Gballou, J., Blivet, J. C., Puig, J. & Mathieu, G., Geosynthetic skin-friction: influence of the equipment on the measure, Geotextiles, Geomembranes and Related Products, ed. G. ,den Hoedt. Balkema, Rotterdam, 1990.
APPENDIX
Shear box size (cm)
Friction angle
Geonet versus smooth HDPE 10.2 x 10-2 30.5 × 30-5 45-7 × 45.7 Not documented
5 11, 11, 11.3, 13, 13, 14, 14, 14, 15, 15, 16.6 16, 16, 17, 17, 18, 18, 18, 18-5, 19, 19 5, 8, 9.1, 10, 10, 11, 11, 12, 12, 15, 17, 17-6, 19
Geonet versus textured HDPE 10.2 × 10-2 30-5 x 30.5 45-7 x 45-7 Not documented
12.5, 16, 22.8 6.7, 9.8, 11.2, 12.5, 15-3, 16, 16.3
Geonet versus PVC 10-2 × 10.2 30.5 x 30.5 45.7 × 45-7 Not documented
15
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Larry D. Lydick, George A. Zagorski
Shear box size (cm)
Friction angle
Geonet versus VLDPE 10.2 x 10-2 30.5 × 30-5 45.7 x 45-7 Not documented
5,6 7-7, 9, 10.9 9, 9, 11, 11, 11, 12, 14, 15, 15, 16, 16
Geonet versus nonwoven geotextile 10.2 x 10.2 30.5 x 30.5 45.7 × 45.7 Not documented
m
11.8, 11.8, 14, 15.4, 17.6, 18, 27 23, 23 10, 14, 19, 20, 21, 21, 22