This study evaluated the effect of geomembrane surface asperities on the shear behavior of polypropylene and polyester geotextile-geomembrane interfaces. Tests were conducted on smooth and textured geomembranes with varying asperity heights and densities. Results showed that textured geomembranes had higher peak and large displacement friction angles compared to smooth geomembranes. Polyester interfaces exhibited higher shear strength than polypropylene interfaces. Increasing asperity volume from 2.39 to 4.01 resulted in an average 19% increase in friction angle. The findings can help increase the design life of landfill liners by improving interface shear strength.

1. Asperities effect on polypropylene &
polyester geotextile-geomembrane
interface shear behaviour
D. Adeleke, D. Kalumba and J. Oriokot
Department of Civil Engineering
University of Cape Town
South Africa

2. • Introduction
• Materials and Methodology
• Results
• Conclusions
• Acknowledgement
• References
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
Outline
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
1

3. Introduction
• Waste generation is expected to increase as
the world population grows and its
management would be an intergral
component of smart cities (DWAF, 1998
Bacas et al, 2015).
• Among other methods of managing waste,
landfilling has proved to be one of the most
used, as 90% generated waste are disposed
off at landfill (DWAF, 1998).
• Landfill liners system are critical determinate
of its design life, especially at the slope.
• To further improve the design life of
landfills, geosynthetics are added for various
function.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
2

4. • Geosynthetics are polymeric planar product
used mostly with soil and rock, and acts as an
integral part of many engineering project and
structure (ASTM D5321, 2014).
• Common geosynthetics used in landfills liners
are geomembranes and geotextiles. They can
perform functions such as liquid barriers,
filtration, separation, reinforcement, and
drainage.
• Smooth geomembranes are prone to low shear
resistance at slope interface, hence, the need for
surface texturing.
Introduction
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
3

5. • As it is believed that
introducing asperities can
increase friction angle, this
has resulted into the
availability of geomembrane
with different asperity
parameters such as height,
density, spacing and
configuration.
• The question remains, how
much increase in shear
strength is obtainable with
varying asperity parameters.?
•
Introduction
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
4

6. Materials and Methodology
• Two different geotextiles
(polypropylene and polyester) and
three geomembranes with varying
asperity height and density were the
geosynthetics used in this study to
carry out 30 interfaces shear test.
• The polypropylene and polyester
geotextiles were labelled as GTXP
and GTXE respectively. While the
HDPE geomembranes are labelled
as GMBSN, GMBTL, and GMBTH.
• Details of the geomembrane surface
properties are given below.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
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7. Materials and Methodology
Asperities Properties
Surface parameters GMBSN GMBTL GMBTH
GMB Thickness, mm 1.5 1.5 1.5
Height (mm) 0 0.7 1.9
Density (No per mm2) 0 3.42 2.11
Spacing (MD/XMD) N/A 5/5 11/5
Configuration N/A Straight Inclined
Asperity volume 0 2.39 4.01
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
6

8. Materials and Methodology
• Test method GRI GM13 with the aid of
a dial gauge was used to measure the
average asperity heights at 10 widely
distributed point (Koerner & Koerner
2015).
• Average asperity density was measured by
counting spikes pec 100 mm2 from two
different locations.
• Direct shear tests according to ASTM
D5321 (2014) were performed under dry
conditions. The shear rate, consolidation
time, and applied normal stress are 1
mm/min, 10 min and 50 - 400 kPa
respectively.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
7

9. Effect of asperity volume on shear displacement before failure
Results
0
2
4
6
8
10
0 100 200 300 400
Displacementbeforefailure,mm
Normal stress, kPa
Smooth GMB/PET Low GMB/PET High GMB/PET
Smooth GMB/PP Low GMB/PP High GMB/PP
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
8

10. Effect of asperity volume on peak and large displacement
friction angle
Results
0
5
10
15
20
25
30
35
40
0 0,5 1 1,5 2 2,5 3 3,5
Fictionangle
Smooth GMB low asperity GMB, high asperity GMB
GMB/PET-Peak GMB/PP-Peak GMB/PET-LD GMB/PP-LD
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
9

11. Effect of asperity volume on polyester peak & LD shear failure
envelope
Results
y = 0,3745x
y = 0,6363x
y = 0,712x
0
50
100
150
200
250
300
350
0 100 200 300 400 500
Interfaceshearstrength,kPa
Normal stress, kPa
smooth Low asp. Vol. High asp. Vol.
Peak
y = 0,1922x
y = 0,2697x
y = 0,3693x
0
20
40
60
80
100
120
140
0 100 200 300 400 500
Interfaceshearsrenght,kPa
Normal stress, kPa
smooth Low asp. Vol. High asp. Vol.
Large displacement
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
10

12. Effect of asperity volume on polypropylene peak & LD shear
failure envelope
Results
y = 0,176x
y = 0,6333x
y = 0,6511x
0
50
100
150
200
250
300
350
0 100 200 300 400 500
Interfaceshearstrength,kPa
Normal stress, kPa
smooth Low asp. Vol. High asp. Vol.
Peak
y = 0,1617x
y = 0,2546x
y = 0,3369x
0
20
40
60
80
100
120
140
160
0 100 200 300 400 500
Interfaceshearsrenght,kPa
Normal stress, kPa
smooth Low asp. Vol. High asp. Vol.
Large displacement
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
11

13. Results
Peak friction angle LD friction angle
Polyester
(PET)
GMBSN GMBTL GMBTH GMBSN GMBTL GMBTH
20,5 32,5 35,5 10,9 15,1 20,3
Polypropylene
(PP)
GMBSN GMBTL GMBTH GMBSN GMBTL GMBTH
10,0 32,4 33,1 9,2 14,3 18,6
12
• A consistent increase in the peak and large displacement friction
angles was observed for both geotextile with increase in
geomembrane surface asperity.
• Specifically, surface asperities yielded higher friction angles in
polyester (PET) geotextile than polypropylene (PP) geotextile
interface. In other words, PET friction angle on the average is
14.65% higher than PP friction angle.
• When asperity volume increases from 2.39 to 4.01, there was an
average increase of 19% in the friction angle.

14. Conclusion
• Textured geomembranes yield higher peak and LD shear strengths than
smooth geomembranes.
• Polyester geotextile interface exhibit higher interface shear characteristics
than the polypropylene geotextile, hence, its more suitable for ultimate
limit state design.
• Geomembranes with low asperity height and high asperity density (low
asperity volume) results in excessive post-peak shear reduction. With the
reduction been more pronounced in polypropylene geotextile.
• Polypropylene geotextile has more functional before failure than polyester
geotextile when asperities are involved, hence, it is preferred for
serviceability limit state designs.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
13

15. ……… It is anticipated that the successful implementation of
the findings of this study in site-specific conditions will
increase the design life of landfill liners.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
15

16. Acknowledgements
• A/Prof. Kalumba and the Geotechnical Research Division,
University of Cape Town, Cape Town, South Africa.
• Sponsorship and support by the MasterCard Foundation at the
University of Cape Town.
• Mr. Charles and other Civil Engineering Workshop Staff
• Mr. Hardie (AKS Liner) and Mr. Johnny (Fibertex SA)
• Mrs. Adeleke, siblings and friends.
Geotechnical Engineering Research Group
Department of Civil Engineering
University of Cape Town
7th International Symposium on Deformation
Characteristics of Geomaterials (IS-
GLASGOW 2019)
16