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2346
OKU Fen Bilimleri Enstitüsü Dergisi
7(5): 2346-2363, 2024
OKU Journal of The Institute of Science and
Technology, 7(5): 2346-2363, 2024
Osmaniye Korkut Ata Üniversitesi
Fen Bilimleri Enstitüsü
Dergisi
Osmaniye Korkut Ata University
Journal of The Institute of Science
and Technology
Farklı Geomembran Astarlarının 2B Yüzey Topografyası Değişimleri: Danecik Şekli, Bağıl
Yoğunluk ve Yükleme Perspektifleri
Tanay KARADEMİR1*
1İstanbul Bilgi Üniversitesi, Mühendislik ve Doğa Bilimleri Fakültesi, İnşaat Mühendisliği Bölümü, 34060, İstanbul
1https://orcid.org/0000-0002-9689-2140
*Sorumlu yazar: tanay.karademir@bilgi.edu.tr
Araştırma Makalesi
ÖZ
Makale Tarihçesi:
Geliş tarihi: 04.01.2024
Kabul tarihi: 21.07.2024
Online Yayınlanma: 10.12.2024
Farklı geomembranların, 2 boyutlu yüzey topografisi değişiklikleri, daha önce
farklı bağıl yoğunluklarda (Dr) ve farklı danecik şekillerinde (yuvarlak veya
köşeli) ve ayrıca çeşitli yükleme koşullarında granüler kumun aşındırıcı
etkisine maruz bırakılmış değişik polimerik reçinelerden (HDPE, LLDPE,
PVC) üretilen farklı geomembran astar tabakalarının 2 boyutlu yüzey
topografyası karakteristik özelliklerini tespit etmek ve belirlemek için profil
kabarma ölçümleri gerçekleştirilerek deneysel olarak incelenmiştir.
Topografyalardaki tepe ve vadilerden oluşan malzeme dislokasyonlarının
boyutu, şekli ve aralıkları gibi pürüzlülük özellikleri de dahil olmak üzere
tespit edilen profillerde açıkça görülen yüzey topoğrafik özellikleri farklı ve
geomembran tipine özgüdür. Bu nedenle, HDPE, LLDPE, PVC sırasına göre
geomembran astar tabakası ne kadar yumuşak ve esnek olursa, yüzey
topoğrafyasının daha şiddetli tepeler ve vadiler göstermesi nedeniyle aşınma
o kadar fazla gelişir. Ayrıca, kum danelerinin şekilsel özellikleri, köşeli-sivri
parçacıkların geomembran astarının yüzeyine nüfuz edebilmesi ve dolayısıyla
yüzey boyunca bir yörünge çizebilmesi nedeniyle daha şiddetli aşındırıcı
etkinin harekete geçmesine yol açmıştır. Geomembranların ölçülen yüzey
topoğrafyaları için belirlenen ortalama pürüzlülüğün (Ra) hesaplanan değerleri
aracılığıyla farklı geomembrane astarların yüzey topoğrafyası değişiminin
niceliği, Ra'nın (yani, yüzeysel topografik çeşitliliğin) yük, bağıl yoğunluk,
dane şeklinin parçacık sivriliği, ve geomembran astar tabakasının
yumuşaklığının artışıyla arttığını ortaya çıkardı. Yuvarlak daneli kum sistemi
için, bağıl yoğunluk %45'ten %85'e ve normal stres 75 kPa’dan 150 kPa’a
çıktığında, Ra değeri HDPE, LLDPE, PVC geomembran astarları için sırasıyla
%129, %133, %137 arttı. Köşeli daneli kum sistemi için, bağıl yoğunluk
%45'ten %85'e ve normal stres 75 kPa’dan 150 kPa’a çıktığında, Ra değeri
HDPE, LLDPE, PVC geomembran astarları için sırasıyla %234, %242, %262
arttı.
Anahtar Kelimeler:
2B yüzey topografyası
Yüzey aşınması
Geomembran çeşitleri
Kum özellikleri
Danecik şekli
Bağıl yoğunluk
2D Surface Topography Alterations of Different Geomembrane Liners: Grain Shape, Relative
Density and Loading Perspectives
Research Article
ABSTRACT
Article History:
Received: 04.01.2024
Accepted: 21.07.2024
Published online: 10.12.2024
The 2D surface topography alterations of different geomembranes were
experimentally be studied by performing profile relief measurements for
detecting and determining 2D surface topographical characteristics of different
geomembrane liner sheets produced from distinctive polymeric resins (HDPE,
LLDPE, PVC) previously subjected to abrasive action of granular sand grains
at different relative densities (Dr) and dissimilar particle shape (rounded or
angular), and additionally at various loading conditions. The surface
topographical characteristics as evident on the detected profiles including
asperity features such as size, shape, and spacing of material dislocations
Keywords:
2D surface topography
Surface wear
Geomembrane types
Sand properties
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Grain shape
Relative density
comprised of peaks and valleys in the topographies were different and unique
to geomembrane type. As such, the softer and the more flexible the
geomembrane liner sheet in an order of HDPE, LLDPE, PVC becomes, the
greater the abrasion has developed in that the surface topography demonstrated
more severe peaks and valleys. Further, the angular features of sand grains led
to the mobilization of more violent abrasive action in that angular particles
were able to penetrate into the surface of geomembrane liner, and thus, gouge
on a trajectory along the surface. The quantification of surface topography
alterations of different geomembranes by means of the computed values of
average roughness (Ra) determined for the measured surface topographies of
the liners unveiled that the Ra (i.e. surficial topographical changes) increases
with an increase in load, relative density, particle angularity of grain shape,
and softness of geomembrane liner sheet. For the rounded sand system, the
value of Ra increased 129%, 133%, 137% for HDPE, LLDPE, PVC
geomembrane liners, respectively when the relative density arised from 45%
to 85% as well as the normal stress raised up from 75 kPa up to 150 kPa. For
the angular sand system, the value of Ra increased 234%, 242%, 262% for
HDPE, LLDPE, PVC geomembrane liners, respectively when the relative
density arised from 45% to 85% as well as the normal stress raised up from 75
kPa up to 150 kPa.
To Cite: Karademir T. 2D Surface Topography Alterations of Different Geomembrane Liners: Grain Shape, Relative Density
and Loading Perspectives. Osmaniye Korkut Ata Üniversitesi Fen Bilimleri Enstitüsü Dergisi 2024; 7(5): 2346-2363.
1. Introduction
Surface topography is of importance in playing a major role for the mobilized mechanical behavior, and
hence, the strength-stability performance of composite layered systems containing synthetic
geomembrane liner and natural granular sand. The interaction of one material with the other one is
investigated within the scope of contact mechanics. The interaction of synthetic geotechnical materials
(e.g. geomembranes) with a natural construction material (e.g. sand) result in alteration of surface
topography of synthetic geomembranes produced from different base polymers (e.g. high-density
polyethylene (HDPE), linear-low-density polyethylene (LLDPE), polyvinylchloride (PVC)). This is due
to inherent softness nature of polymeric geomembrane liner sheets as well as the abrasive texture (i.e.
micro-structure) of natural granular material sand. For this reason, the index and physical properties of
sand (e.g. relative density, grain shape) and their role as well as the capability in altering surface
topography of counterface geomembrane when employed as a composite layer system, adjacent to each
other, in infrastructural applications for geotechnical projects including embankments, landfills, dams
should be examined in detail and determined in relative terms. In this way, the mechanical behavior of
those geo-materials can be engineered by capturing the degree for the importance of manufacturing
material characteristics of synthetic geomembrane liners as per core polymer type (HDPE, LLDPE,
PVC) and/or the degree for the influence of physical and index properties of natural granular material
sand as per relative density and grain shape. Since this interaction and contact behavior can impact the
performance and constructability of the aforementioned infrastructural applications in which the
geosynthetic liners, comprised of geomembrane layers, interact and contact with granular sands. To this
end, a comprehensive experimental study was conducted to investigate and evaluate the resultant
surficial wear quantitatively induced on the geomembrane liner surfaces due to abrasive action of
granular sand particles based on diversifying boundary conditions (i.e. loading conditions),
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geomembrane core material properties (i.e. HDPE, LLDPE, PVC), counterface particulate material
physical properties (i.e. sand grain shape (rounded or angular), relative density). This current study is
unique in terms of the type of base/core material of the selected geomembranes such as LLDPE, PVC
to detect surface topographical alterations as well as in terms of the relatively large range of relative
densities from 45% up to 85% examined throughout the laboratory testing program. Additionally, the
tested sand specimens were intentionally selected with different morphologies including distinct
morphological properties such as roundness, sphericity, angularity, and regularity. In earlier research
studies, the HDPE geomembrane type was generally preferred to be a liner material as well as similar
granular materials possessing identical morphological characteristics were in general selected to be
utilized in the laboratory experimental programs. On the other hand, the geomembrane liners produced
from the base polymeric materials such as LLDPE and PVC have recently utilized widely in both
geotechnical and geoenvironmental projects including infrastructural facilities (e.g. embankments,
dams) and environmental applications (e.g. landfills). Further, the influence of grain shape of
counterface particulate materials being roundness or angularity of soil particles is necessarily required
to be analyzed in detail by varying or preserving the other physical characteristics of the selected sand
test specimens such as mean grain size, specific gravity in order to discern the effect of sand
morphological properties.
2. Surface Topography Characterization and The Relevant Previous Research
Surface topography is of importance in playing a major role for the mobilized mechanical behavior, and
hence, the strength-stability performance of composite layered systems containing synthetic
geomembrane liner and natural granular sand that are placed as a counterface to each other. Among
various surface topography determination parameters developed to characterize topographical
alterations, the most commonly utilized parameter to quantify and quantize those alterations in surficial
topography is the average roughness parameter (Ra) (Ward, 1982) that could be computed as follows:
(1)
Where:
L: Assessment Length
z(x): Height of the Profile from the mean line
Moreover, a different concept, called normalized roughness (Rn) to characterize surface topographical
characteristics of continuum materials such as geomembranes, was proposed by Uesugi and Kishida
(1986). The Rn parameter accounts also for the relative aspect of the roughness such that the ratio of
maximum roughness (Rmax) with respect to the mean grain size (D50) of counterface granular material
(sand). Furthermore, DeJong and Frost (2002) conducted a comprehensive research study and unveiled
the relevance of the relative aspect of surface roughness. As such, when a spherical particle is travelling
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on a simplified rough surface consisting of simple surficial topographical features of peaks and valleys,
the influence of peaks and valleys on the trajectory of the spherical particle is not the same (dissimilar)
with the path over which the centroid trace of that individual particle. Further, the vertical deviation
from the centroid trace is distinct for different sizes of particles.
The alterations in surface topography of such a soft, flexible synthetic material geomembrane is caused
by the penetration of relatively harder counterface granular sand particles into the softer surface of
geomembrane liners, and thus, ploughing on a trajectory along the surface. This incident and/or this
observed mechanism results in abrasion in geomembrane, and hence, decreases the durability leading to
deterioration of endurance properties due to induced damage by sand particles. The severity and the
significance of the abrasion produced on the surface of the liner is attributed to the material properties
of core polymer of the geomembrane (i.e. HDPE, LLDPE, PVC) as well as the particulate characteristics
and index properties of the granular sand (i.e. relative density, particle shape). Related research by Frost
et al. (2002) numerically investigated that the hardness/softness properties of the geomembrane liners
can be linked and coupled to the alterations in surficial topographical features generated by counterface
sand grains. In order to demonstrate this, they performed discrete element modeling (DEM) for the
distinct particulate versus continuum material interfaces such that the DEM provided insight on the
behavior at global level (i.e. macro level), and additionally, extend the understanding on the mechanism
regarding local particle response at micro level in penetrating and ploughing along the surface of
counterface geomembrane leading to the changes in topography.
Furthermore, another research study on the quantitative measurement of induced surface changes in the
geomembranes due to shearing effect by Vangla and Gali (2016) revealed that the shearing mechanism
at the interface governed by critical normal stress level dependent on both granular material and
geomembrane characteristics plays a vital role on the resultant mutual interaction developed between
soil and synthetic polymeric materials. Moreover, Araujo et al. (2022) published the result of a specific
study concerned with the geomembrane inherent surface roughness at production stage instead of
induced abrasive wear at post-production stage or in-application (in-employment) phase. Their findings
showed that the mean height of profile elements on the geomembrane liner surface and the liner core
(base) material volume presented stronger correlations with the resultant generated mutual interaction
along with counterface materials. Further, the change pattern of geomembrane surface roughness for
textured geomembranes was examined by Xu et al. (2023). It was shown that the application of texture
on the geomembrane surface improves frictional performance. On the other hand, the variation in the
roughness of textured surface affects the surface deformation characteristics of geomembrane liner, and
thus, induced wear on the liner surface controlled by the asperity height of the textural elements on the
geomembrane surface.
A different study on the geomembrane mutual interaction along with a dissimilar counterface material
being geosynthetic clay liners (GCL) was carried out by Feng et al. (2022) to observe the topographical
alterations on the surface of geomembrane liners when being in direct contact with GCLs employing
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multi-functional laboratory apparatus. It was investigated that the surface roughness of geomembrane
liners and the hydration condition of geosynthetic clay liners prominently influence the mechanical
interaction between the geomembrane and the GCL as well as the resulting surficial topographical
changes at the contact surface. Furthermore, Adeleke et al. (2021) published the influence of the
resulting asperities located on the geomembrane surface based on roughness and topographical
alterations on the surficial features of the liners. It was detected that as the surface texturing was
increased, a more pronounced wear/abrasion occurred that resulted in observing deeper mechanical
interaction.
Xia et al. (2024) reported the experimental and numerical results of a geoenvironmental study concerned
with the geomembrane liners subjected to mutual interaction with municipal solid waste (MSW) samples
of different ages in landfill applications. It was shown that the sliding surface of geomembrane-lined
landfills is discontinuous at the lining interface, which can delay the penetration of slip surfaces and
block the formation of slip, and thus, prevent substantial abrasive wear induced on the geomembrane
liners. Although they revealed the mutual interaction between geomembranes and MSW of different
ages, and the resulting wear on the liner surface, they didn’t either measure the occurrence of surficial
wear or quantify the degree and severity of this induced abrasive action and the resultant surface
topographical changes developed on the liner surface of geomembranes produced from distinct
base/core polymer resins such as HDPE, LLDPE, and PVC. In this regard, the current study presented
in this paper could supplement the previous study of Xia et al. (2024) by extending understanding in
terms of the detection and the quantification of surface topographical alterations on the geomembrane
liners manufactured from various base polymeric materials including HDPE, LLDPE, PVC due to
induced abrasive action, and hence, the generated surficial wear.
Using micro computed tomography and shear band analysis; soil and geosynthetic material interaction
was studied to extend multi-scale understanding by Khan and Latha (2023). They were interested in
shape parameters of sand particles including convexity, aspect ratio, and roughness that were quantified
at different scales. The current study presented in this paper that deals with sand particle roundness or
angularity and their quantification will complement the earlier study of Khan and Latha (2023) in this
origin. Besides, one-dimensional surface profile measurements performed in this current study will aid
the readers in comprehending the developed form and the generated pattern of surface topographical
changes along with further engineering quantification by means of quantitative parameters such average
roughness (Ra).
As evidently seen from the relevant studies published in the literature and discussed earlier, an
experimental study is necessarily required to fill the gap in terms of the assessment of 2D surface
topography alterations of different geomembrane liners produced from distinct base polymeric materials
including HDPE, LLDPE, PVC due to abrasive action (i.e. wear) induced by granular counterface
materials (i.e. sandy soils) at different grain shapes (i.e. rounded or angular), at various relative densities
(Dr: 45%, 65%, 85%), and at a range of loading conditions from 75 kPa up to 150 kPa. Further, the
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detected surficial topographical changes on the geomembrane liners are required to be quantitatively
evaluated by means of roughness parameters such as average roughness (Ra) in order to comprehend the
degree, magnitude and significance of surficial wear induced on the geomembrane liners due to abrasive
action of granular particulate materials (i.e. sand) that are in direct contact and interaction with
geomembranes employed in geotechnical and geoenvironmental projects in the infrastructural field
applications including embankments, landfills, artificial ponds. The engineering quantification and the
comparative analysis conducted in the current study and presented in this paper as well as the test results
and the experimental findings of the laboratory program not only fill the gap in the literature regarding
the detection and the quantitative evaluation of the resultant polymeric material condition due to abrasive
wear but also will provide a comprehensive understanding for the engineers in design and the contractor
practitioners in construction sites in terms of material selection, the resulting mutual compatibility of
the preferred materials, and the consequential changes exhibited on the physical characteristics,
mechanical properties of the selected materials in construction as well as operation stages. In this way,
the design engineers and the contractor practitioners would be able to estimate stable durability and
secure lifespan of the utilized polymeric geomembranes and particulate granular soils as well as their
safe and secure mutual interaction in the multi-layered composite systems typically applied in
infrastructural facilities including embankments, landfills and artificial ponds.
3. Geomembrane Liner Types and Granular Materials
3.1. Types of Geomembrane Liners
The geomembrane liner sheets utilized throughout the testing program consist of three different types
produced from distinct base polymer resins including high-density-polyethylene (HDPE), linear-low-
density-polyethylene (LLDPE) and polyvinylchloride (PVC) to investigate the influence of base
polymer type, and hence, the softness/hardness characteristics of the lining sheets on the alterations of
surficial topographical features. All the selected geomembrane liners possess thickness of 1 mm (40
Mil). The specific gravity (Gs) of HDPE, LLDPE, and PVC geomembranes are 0.94, 093, and 1.20,
respectively. Those three types of liner sheets are widely preferred, commonly utilized geomembranes
in geotechnical infrastructural applications and geoenvironmental projects owing to the enhanced
strength properties particularly for the geomembranes produced from HDPE base polymer and owing
to the superior flexibility characteristics especially for the liners manufactured from LLDPE as well as
PVC base polymeric materials.
3.2. Granular Materials
In the experimental program, two different types of sand were used to examine the influence of particle
shape such that the one comprised of rounded grains whereas the other composed of angular grains.
Additionally, the sand specimens were prepared at three different relative densities (Dr) including Dr:
45%, 65%, and 85% to evaluate the effect of an important physical index property of granular materials
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on the resulting abrasion induced into counterfaced geomembrane liner sheets. In order to evaluate the
principal role of only the shape (i.e. angularity versus roundness) of sand grains (i.e. particles), the
testing materials were purposefully selected in such a way that they possess similar index properties
including average particle size, identical soil particle gradation with an only exception of grain shape.
The index properties are presented in Table 1 below. The mean particle sizes of the two sand specimens
tested in the comprehensive experimental program were selected to be similarly identical in an intention
to examine the predominant influence of sand grain shape (i.e. rounded or angular) on the resultant
abrasion observed on the polymeric liner surfaces. The sand specimens prepared at different relative
densities ranging from 45% up to 85% are expected to be exhibiting different mutual interaction with
the counterface continuum materials being geomembrane liners possessing distinctive core polymeric
resins and having different softness or hardness characteristics as well as having distinct flexibility or
stiffness.
Table 1. Index Properties of Granular Materials Used In Experimental Program
Granular Material
D50 (mm)
Cu
Cc
Gs
Rounded Sand
0.72
1.39
0.88
2.67
Angular Sand
0.75
1.34
0.71
2.67
4. 2D Surface Topography and Profile Relieves
The relevance in between surface topography of continuum materials as well as alterations in surficial
topographical characteristics and frictional mechanism, mechanical behavior as well as strength
properties has been emphasized by various researchers including Potyondy (1961), Brumund and
Leonards (1973), Uesugi and Kishida (1986), Paikowski et al. (1995), Frost et al. (2002), Vangla and
Gali (2016), Araujo et al. (2022), Khan and Latha (2023), Xu et al. (2023), and Xia et al. (2024). To this
end, the two dimensional (2D) surface topography alterations of a synthetic geo-material
(geomembrane) and its different types manufactured from distinct base polymers that are commonly
utilized in typical geotechnical applications including the infrastructural projects such as embankments,
landfills, dams was intended to experimentally be studied by the author. This was achieved by
performing profile relief measurements for detecting and determining 2D surface topographical
characteristics of different geomembrane liner sheets produced from distinctive polymeric resins
(HDPE, LLDPE, PVC) previously subjected to abrasive action of granular sand grains at different
relative density and dissimilar particle shape (rounded or angular), and additionally at various loading
conditions. In this way, the degree of influence of geomembrane polymeric material characteristics and
sand physical index properties as well as the state of the composite system – comprised of sand and
geomembrane – due to diverse loading situations on the resulting alterations in surface topography as
per surficial profile variations in terms of generated peaks and valleys as a result of material dislocations
will be investigated by means of a testing program conducted in the laboratory using stylus profilometer
(Figure 1).
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Figure 1. Stylus Profilometer to Quantify Surface Topography
In light of experimental surfacial topographical detections of geomembrane specimens through
profilometer measurements, the characteristics of 2D surface topography alterations of the specimens
were quantified and quantized using a universally recognized surface topography parameter being
average roughness (Ra) (Equation 1). As such, this quantification based on an important roughness
parameter will evidently aid the researchers to obtain a comparative analysis for the resultant wear
generated on the surface of polymeric geomembrane liners due to sand abrasion.
4.1. 2D Surface Topographies
The performed comprehensive testing program in the laboratory by means of stylus profilometer
consists of 36 surface topography quantification measurements to detect one-dimensional surface
topographies of geomembrane liners manufactured from three different core polymers including high-
density polyethylene (HDPE), linear-low-density polyethylene (LLDPE) and polyvinylchloride (PVC)
and subjected to various loading conditions ranging from 75 kPa up to 150 kPa and subsequent shearing
against different granular sands having dissimilar grain shape (rounded or angular) and distinct relative
densities ranging from 45% up to 85% (Table 2). In this way, it was intended to investigate the influence
of base polymer of geomembrane liner and relative density, and grain shape of sand particles as well as
the effect of loading conditions on the resultant abrasive wear induced on geomembrane surface. The
profilometer device utilized in the experimental program to measure and evaluate geomembrane
surficial topographical variations is a computer-automated and controlled testing system as well as
connected to a data acquisition system in order to log measurement data during the tests.
The quantification of 36 surface topographical alterations were evidently sufficient in order to
comprehend the behavioral changes, the generated patterns, and the developed forms of surficial wears
mobilized due to abrasive action of sand particles being in direct contact with the surface of
geomembrane liners manufactured from various core material polymer resins being relatively hard, and
stiff (e.g. HDPE) or contrarily being relatively soft, and flexible (e.g. PVC).
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Table 2. Laboratory Testing Program
In addition to qualitative distinctions, quantitative differences as well as visual variations were
investigated from surficial topographical profiles of different geomembranes. The surface topographies
of HDPE, LLDPE and PVC geomembrane liner sheets, quantified for the loading condition of 75 kPa
normal stress level and relative density of 45%, and subjected to abrasive action of rounded or angular
sands, are shown in Figure 2 for clear presentation of experimental data and explicit clarification of
surficial topographical profiles.
As per Figure 2, the surface topographies detected over a projected profile segment of 20 mm on three
different types of geomembranes produced from distinct polymeric resins including HDPE, LLDPE,
PVC subjected to abrasive action of rounded or angular sand particles are evidently demonstrate the
influence of the grain shape of sandy soil. The higher the angularity of sand particles the larger and
severer abrasion and wear induced on the geomembrane liners regardless of base polymer type of the
geomembrane that could be either HDPE, LLDPE, or PVC. Furthermore, the effect of geomembrane
base polymer is evidently observed from the resultant surface topographies developed as a result of
particulate material (i.e. sand) abrasive action such that as the geomembrane becomes relatively harder
and less flexible in an order from PVC to LLDPE, and then to HDPE, the abrasive wear induced on the
geomembrane liner decreases. As such, the lower the abrasion generated in which the surface
topography demonstrated smaller ups and downs in terms of minor peaks and valleys. In this regard, the
most intense (i.e. violent) surficial abrasion was exhibited in the softest and the most flexible
geomembrane manufactured from PVC and subjected to the abrasive action of angular sand, while the
most mild (i.e. gentle) abrasive wear was displayed in the hardest and the most stiff (inflexible)
geomembrane produced from HDPE and exposed to the abrasive action of rounded sand.
Geomembrane
Type
Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85
Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85
Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85 Dr: 45 Dr: 65 Dr: 85
σ = 75 kPa
σ = 150 kPa
HDPE
Rounded Sand
Angular Sand
Rounded Sand
Angular Sand
LLDPE
Rounded Sand
Angular Sand
Rounded Sand
Angular Sand
Angular Sand
PVC
Rounded Sand
Angular Sand
Rounded Sand
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(a) HDPE Geomembrane: Rounded Sand Abrasion
(c) LLDPE Geomembrane: Rounded Sand Abrasion
(e) PVC Geomembrane: Rounded Sand Abrasion
(b) HDPE Geomembrane: Angular Sand Abrasion
(d) LLDPE Geomembrane: Angular Sand Abrasion
(f) PVC Geomembrane: Angular Sand Abrasion
Figure 2. Surface Topographies of HDPE, LLDPE, PVC Geomembrane Liners (σ: 75 kPa and Dr: 45%)
Furthemore, in addition to the effect of granular soil particle shape, it was seen that the softness, hardness
characteristics of the geomembrane liners based on the flexibility, stiffness properties also strongly
influence and prominently control the degree, magnitude and significance of surficial wear induced due
to abrasive action of granular soils (i.e. sandy soils). When the hardness of geomembranes used in the
study including HDPE, LLDPE, PVC liners are compared with that of sandy soils utilized in the testing
program, the geomembranes possess relatively softer nature. On the other hand, the three distinct types
2356
of geomembranes were selected intentionally to investigate the effect of liner flexibility such that the
PVC geomembranes serve relatively higher flexibility while the HDPE geomembranes show relatively
larger stiff nature and the flexibility properties of LLDPE is being in between those two geomembrane
types aforementioned.
The fluctuations in the data in Figure 2 are attributed to the variations in surface topographical
characteristics of the geomembrane liners. Considering surface topographies as demonstrated in Figure
2 where the traverses are evident on the profiles, asperity features such as size, shape, and spacing of
material dislocations including peaks and valleys in the topographies were different and unique to
geomembrane type. As such, the softer the geomembrane liner sheet (HDPE LLDPE PVC)
becomes, the greater the abrasion has developed in that the surface topography demonstrated more
severe peaks and valleys (i.e. ups and downs). Furthermore, the angular features of sand grains led to
the mobilization of more violent (i.e. aggressive) abrasive action in that angular particles were able to
invade and penetrate into the surface geomembrane liner, and thus, gouge and plough on a trajectory
along the surface. This resulted in observing material dislocations at greater intensities such that the
surface topography exhibited both the peaks and the valleys not only at higher amplitudes but also
displaying the traverses at larger size as well as spacing. That is to say, the dimensions and spacing of
peaks and valleys were small-scale in relatively stiffer (harder) geomembrane liner sheet of HDPE in
which the surface topography did not possess sharp corners such that the transitions from peaks to
valleys to peaks were smooth and rounded as compared to that of LLDPE as well as PVC geomembranes
where sharp and rough transitions from peaks to valleys to peaks exhibited. Consequently, the surface
topography of inherently harder HDPE liner sheet depicted fewer ups and downs as well as smoother
and unsharpened returns from peaks and valleys. The fluctuations in the surface topography data were
considerably less owing to relatively firm and intensified core material characteristics of HDPE liner in
comparison to that of LLDPE and PVC geomembranes. To sum up, the dimensions and spacing of peaks
and valleys for the surface topography were small-scale in relatively stiffer geomembrane liner sheet of
HDPE in which the surface topography did not possess sharp corners such that the transitions from peaks
to valleys to peaks were smooth and rounded as compared to that of LLDPE as well as PVC
geomembranes where sharp and rough transitions from peaks to valleys to peaks exhibited.
4.2. Quantification of Surface Topography Alterations
A proper engineering analysis of surface topography involves in accurate quantification of topographical
alterations on the geo-material surface. Thereafter, the detected surface profiles require to be
numerically quantified to be able to perform further comparative analysis among the distinct
characteristic surfaces of different geo-materials. To this end, the quantification of alterations in
topography on the surfaces of geomembranes were determined in a quantitative manner by means of the
average roughness parameter (Ra) so as that the topographical changes were quantized to characterize
and to compare the alterations in surficial topography of different geomembranes manufactured from
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distinct core polymeric materials including HDPE, LLDPE and PVC. The changes in the values of Ra
with respect to several essential factors involved in the testing program such as the grain shape and/or
the density of granular material, loading condition and geomembrane base polymer are presented in
Figure 3.
The comparative analysis in Figure 3 exhibits that the average roughness (Ra) increases with an increase
in normal load from 75 kPa up to 150 kPa as well as with an increase in relative density (Dr) of granular
sand. The larger the loading the higher the contact stresses will develop at the contact surface in between
sand grains and continuum geomembrane leading to severe and harsh indentation of grains into the liner
material that causes significant alterations in surface topographical characteristics. Likewise, as the
density of granular material (Dr) becomes larger, the number of particles that exist at the contact surface
in between sand and geomembrane increases resulting in greater amount of grains to penetrate into
counterfaced polymeric material, and thus, plough on a trajectory along the surface by inducing and
triggering considerably major variations in surface topography. This is associated with larger magnitude
of topographical changes such that the polymeric-material dislocations mobilized on the surface of
geomembrane develops at higher intensities. Additionally, the higher values of average roughness (Ra)
were attained for the angular sand regardless of geomembrane type which demonstrates that the greater
aggressive abrasion is induced into the surface of geomembrane resulting in the substantial alteration of
surface topography. This is attributed to the angular features of sand grains having sharp corners such
that the invasion and penetration of angular particles into the surface of geomembrane liner at greater
extremity, and thus, gouging surface resulting in exceptional surficial topographical changes. Further,
the greatest abrasion – regardless of the grain shape of sand particles (rounded or angular) – has been
induced to the softest PVC geomembrane liner such that the softer the geomembrane liner becomes in
an order of HDPE, LLDPE, PVC, the significance of abrasive action of granular material on the liner
sheet has happened to be evident and vital as demonstrated from the detected increase in the values of
average roughness (Ra). This shows that the base polymer type from which the geomembrane has been
manufactured plays a crucial role such that the physical material characteristics of polymeric continuum
liner sheet are of importance regarding the resultant surficial topographical changes.
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(a) σ = 75 kPa & Dr = 45%
(c) σ = 75 kPa & Dr = 65%
(e) σ = 75 kPa & Dr = 85%
(b) σ = 150 kPa & Dr = 45%
(d) σ = 150 kPa & Dr = 65%
(f) σ = 150 kPa & Dr = 85%
Figure 3. Comparison of Average Roughness (Ra) of Different Geomembrane Liners for
Different Relative Density and Loading Conditions
The variation of average roughness (Ra) with respect to the change in relative density (Dr) as well as
normal stress (σ) and grain shape (particle angularity/roundness) for the geomembrane liners produced
from HDPE, LLDPE, and PVC base polymeric materials are presented in Figures 4a, 4b, and 4c,
respectively. Regardless of the type of base polymer, the Ra increased with an increase in Dr and/or σ.
In addition, when the counterface soil particle shape turns into more angular, the increase in the resultant
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measured value of Ra becomes relatively larger for the same identical geomembrane liner as compared
to the counterface soils having more roundness features in grain shape. Further, the largest increment in
the magnitude of Ra was detected for the PVC geomembrane, whereas the smallest rise in the value of
Ra was observed in the HDPE geomembrane. The increase in the Ra was medium by the value for the
LLDPE geomembrane.
Furthermore, the rate (i.e. slope) of increase in Ra was lower from Dr = 45% to 65%, and then, beyond
the relative density of 65%, the rate of increase became larger particularly for the HDPE geomembrane
in all loading conditions (i.e. 75 kPa, 150 kPa) for both rounded and angular sand systems; as well as
for the LLDPE geomembrane in all loading conditions (i.e. 75 kPa, 150 kPa) but for only rounded sand
system. On the other hand, the LLDPE geomembrane for angular sand system didn’t exhibit a change
in the rate of increase throughout the entire range of relative density tested from 45% up to 85%.
Moreover, the PVC geomembrane didn’t display a discernable and noteworthy change in the rate of
increase regardless of loading condition or granular soil grain shape such as sand particle angularity or
roundness.
In light of comparative analysis presented in Figure 4, it is further noted that the softer and the more
flexible the geomembrane liner becomes, the higher the increase in the detected magnitude of Ra is
explored. Therefore, the hardness/softness of the geomembrane plays an important role for the resultant
alterations in surface topography of the liners, and thus, for the resulting surficial abrasion or wear
mobilized on the geomembrane counterfaced with granular materials including sandy soils possessing
different grain shape features such as distinct particle angularity/roundness. Moreover, the rate of
increase in the detected value of Ra becomes higher for the counterface soils possessing angular grain
properties, while the rate of that increase is displayed relatively lower for the counterface soils having
rounded particle features. Consequently, it is further highlighted that the angular soil grains lead to the
development of more abrasive wear, and thus, greater alterations in surface topographical features of
geomembrane liners that are in direct contact with soil particles under imposed loads/forces, and hence,
under induced stresses at different modes and directions.
To sum up, surficial topographical alterations of geomembranes are controlled by the liner
hardness/softness characteristics as per polymeric core material flexibility/stiffness properties as well as
granular soil particle shape such as roundness, angularity, relative density and loading conditions. In
this regard, the greatest increase in the value/magnitude of Ra was observed for angular sand systems in
largest loading case (i.e. 150 kPa) whereas the smallest increase in the value/magnitude of Ra was seen
for rounded sand systems in lowest loading situation (i.e. 75 kPa) regardless of the type of the
geomembrane liner. Consequently, the surficial wear induced on the liner surface due to abrasion is
strongly governed by the morphology/shape of the sand particles (i.e. rounded, angular). Further,
sufficient magnitude of loading is necessarily required for the sand particles to implement and excite
abrasive action in order to penetrate and generate abrasion on the liner surface. Therefore, it is evident
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that different surface topographical changes could be observed depending on geomembrane or sand
material selection as well as based on their mutual interaction over the entire extent of the contact area.
(a) HDPE Geomembrane
(b) LLDPE Geomembrane
(c) PVC Geomembrane
Figure 4. The variation of Average Roughness (Ra) with respect to the change in Relative Density (Dr) as well as
Normal Stress (σ) and Grain Shape (Particle Angularity/Roundness)
5. Conclusions
In light of experimental findings/results of the current research study presented in the paper, the surface
topographical characteristics as evident on the detected profiles including asperity features such as size,
shape, and spacing of material dislocations comprised of peaks and valleys in the topographies were
different and unique to geomembrane type. As such, the softer the geomembrane liner sheet (from HDPE
to LLDPE then to PVC) becomes, the greater the abrasion has developed in that the surface topography
demonstrated more severe peaks and valleys. Further, the angular features of sand grains led to the
mobilization of more violent abrasive action in that angular particles were able to penetrate into the
surface geomembrane liner, and thus, gouge on a trajectory along the surface. The dimensions and
spacing of peaks and valleys for the surface topography were small-scale in relatively stiffer
geomembrane liner sheet of HDPE in which the surface topography did not possess sharp corners such
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that the transitions from peaks to valleys to peaks were smooth and rounded as compared to that of
LLDPE as well as PVC geomembranes where sharp and rough transitions from peaks to valleys to peaks
exhibited. As such, the fluctuations in the surface topography data were considerably less owing to
relatively firm and intensified core material characteristics of HDPE liner in comparison to that of
LLDPE and PVC geomembranes. Consequently, the quantification of surface topography alterations of
different geomembranes by means of the computed values of average roughness (Ra) determined for the
measured surface topographies of the liners unveiled that the Ra (i.e. surficial topographical changes)
increases with an increase in load, relative density, particle angularity of grain shape and softness of
geomembrane liner sheet. As such, for the rounded sand system, the value of Ra increased 129%, 133%,
137% for HDPE, LLDPE, PVC geomembrane liners, respectively when the relative density arised from
45% to 85% as well as the normal stress raised up from 75 kPa up to 150 kPa. For the angular sand
system, the value of Ra increased 234%, 242%, 262% for HDPE, LLDPE, PVC geomembrane liners,
respectively when the relative density arised from 45% to 85% as well as the normal stress raised up
from 75 kPa up to 150 kPa. Therefore, it is evidently seen that the grain shape (roundedness versus
angularity) of particulate materials (i.e. sand) plays a significant role for the induced surface wear onto
geomembrane liner regardless of its type or its core polymer (i.e. HDPE, LLDPE, PVC) due to greater
magnitude of abrasive action mobilized by the sharp features of angular sand particles. For this reason,
a higher increase in the value of Ra was displayed for the angular sand system as compared to that of the
rounded sand system. Furthermore, when the increase in the value of Ra as the Dr arised from 65% to
85% is compared with the increase in the value of Ra as the Dr arised from 45% to 65%, it was obviously
realized that a greater rise was identified beyond Dr = 65% up until Dr = 85% substantially in particular
for the HDPE geomembrane, considerably for the LLDPE geomembrane, and marginally for the PVC
geomembrane system.
To sum up, the alteration of surface topography is not only a function of geomembrane material
properties including softness/hardness characteristics but also controlled by counterface material
physical properties, shape features as well as boundary conditions including the magnitude of loading,
the severity of external forces, and induced stresses. For this reason, the selection of materials and the
design of multi-layered infrastructural application plays an important and critical role for the sake of
safety and stability in a typical composite system designed and constructed for a geotechnical or
geoenvironmental facility including more than one distinct materials which are counterfacing each other
through a contact surface along with a direct interaction due to imposed external loads/forces and
induced resultant stresses. Consequently, the design engineers shall necessarily pay special attention to
mutual interactive behavior and engineering properties of materials in construction including strength
and durability characteristics in addition to single/unique material strength and durability response
against imposed loads and forces in any mode of application such as compression, tension or shear
induced singly of jointly/collectively.
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In conclusion, the detected surficial topographical changes on the geomembrane liners were
quantitatively evaluated by means of a common roughness parameter (i.e. average roughness, Ra) in
order to comprehend the degree, magnitude and significance of surficial wear induced on the
geomembrane liners due to abrasive action of granular particulate materials (i.e. sand) that are in direct
contact and interaction with geomembranes employed in geotechnical and geoenvironmental projects in
the infrastructural field applications including embankments, landfills, artificial ponds. The engineering
quantification and the comparative analysis conducted in the current study and presented in this paper
as well as the test results and the experimental findings of the laboratory program not only filled the gap
in the literature regarding the detection and the quantitative evaluation of the resultant polymeric
material condition due to abrasive wear but also provided a comprehensive understanding for the
engineers in design and the contractor practitioners in construction sites in terms of material selection,
the resulting mutual compatibility of the preferred materials, and the consequential changes exhibited
on the physical characteristics, mechanical properties of the selected materials in construction as well
as operation stages. In this way, the design engineers and the contractor practitioners would be able to
estimate stable durability and secure lifespan of the utilized polymeric geomembranes and particulate
granular soils as well as their safe and secure mutual interaction in the multi-layered composite systems
typically applied in infrastructural facilities including embankments, landfills and artificial ponds.
Statement of Conflict of Interest
No conflict of interest is declared such that no known competing financial interests or personal
relationships exist which could have appeared to influence the work reported in this paper.
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