Role of particle shape on the shear strength of sand-GCL interfaces under dry and wet conditions

20 Pages·2021·20.213 MB·English

by  Anjali G.Pillai and Madhavi Latha Gali

GeotextilesandGeomembranesxxx(xxxx)xxx
Contents lists available at ScienceDirect
Geotextiles and Geomembranes
journal homepage: www.elsevier.com/locate/geotexmem
Role of particle shape on the shear strength of sand-GCL interfaces under
dry and wet conditions
Anjali G. Pillai, Madhavi Latha Gali*
Department of Civil Engineering, Indian Institute of Science, Bangalore, 560012, India
A R T I C L E I N F O A B S T R A C T
Keywords: Interface shear strength of geosynthetic clay liners (GCL) with the sand particles is predominantly influenced by
Interface shear the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. This
Hydration study brings out the quantitative effects of particle shape on the interaction mechanisms and shear strength of
Image analysis
GCL-sand interfaces. Interface direct shear tests are conducted on GCL in contact with a natural sand and a
Particle shape
manufactured sand of identical gradation, eliminating the particle size effects. Results showed that manufactured
Manufactured sand
sand provides effective particle-fiber interlocking compared to river sand, due to the favorable shape of its grains.
Further, the role of particle shape on the hydration of GCL is investigated through interface shear tests on GCL-
sand interfaces at different water contents. Bentonite hydration is found to be less in tests with manufactured
sand, leading to better interface shear strength. Grain shape parameters of sands, surface changes related to
hydration and particle entrapment in GCL are quantified through image analysis on sands and tested GCL sur-
faces. It is observed that the manufactured sand provides higher interface shear strength and causes lesser hy-
dration related damages to GCL, owing to its angular particles and low permeability.
1. Introduction system. The interface shear mechanisms of GCL surface and sand par-
ticles govern the mechanical design of these systems. In a cover system,
Geosynthetic clay liners (GCL) are geocomposites of unique nature the GCL is underlain by a sand layer and the interactions govern the
which combine the characteristics of bentonite clay and polymer. They sliding and other instabilities that might arise under the external and
are proven to be effective hydraulic barriers. Based on their hydraulic internal loading conditions. In the base layer, sand layer is usually a part
performance in landfill liner systems, researchers demonstrated that of the compacted subgrade, which supports the GCL and the interface
GCLs of 6–10 mm thickness can replace compacted clay liners (CCLs) of shear in such a case can govern the sliding of the liner. GCL-sand in-
900 mm thickness in a landfill (Giroud et al., 1997; Didier et al., 2000; terfaces are also used at the interlayers in a double-liner system. All
Lake and Rowe, 2000; Shackelford et al., 2000; Sivakumar Babu et al., these possible GCL-sand interfaces in a landfill are schematically shown
2001; Zhan et al., 2018). The economic implications of using GCLs in in Fig. 1. Inadequate shear resistance at the interface of GCL and sand
place of CCLs are well documented in literature. GCLs have an edge over layer in these components is the primary cause of concern for the me-
CCLs because of their reduced thickness, easy installation properties, chanical stability of a landfill. Hence accurate quantification of interface
low cost, and good conformance with the differential settlements of the and internal shear strength of GCL is very important for the stability
underlying soil (Bouazza, 2002). The self-healing property of GCLs assessment of landfills.
provides a longer service life (Daniel, 2000). GCL is proven to be su- Interface shear studies in literature are mostly focused on soil-
perior to CCL in several aspects like quality assurance, availability, geosynthetic interfaces. Most of these studies are performed with a
construction speed, durability to freeze-thaw and wet-dry cycles (Man- conventional direct shear box, with its bottom-half fitted with a rigid
assero et al., 2000). GCL Layers are effectively being used in baseliner block covered with a geosynthetic layer, to facilitate shearing between
and capping systems of engineered landfills. the geosynthetic layer and the soil filled in the upper-half of the shear
The surface of the GCL is a woven or a nonwoven geotextile, which is box (Zettler et al., 2000; Fleming et al., 2006; Jotisankasa and Rurg-
in contact with sand particles at different locations in a composite liner chaisri, 2018; Shi et al., 2020). Some researchers have also studied the
* Corresponding author.
E-mail addresses: [email protected], [email protected] (A.G. Pillai).
https://doi.org/10.1016/j.geotexmem.2021.11.004
Received 9 July 2021; Received in revised form 3 November 2021; Accepted 23 November 2021
0266-1144/©2021ElsevierLtd.Allrightsreserved.
Pleasecitethisarticleas: AnjaliG.Pillai,MadhaviLathaGali,GeotextilesandGeomembranes,https://doi.org/10.1016/j.geotexmem.2021.11.004
A.G. Pillai and M.L. Gali G e o t e x t i l e s a n d G e o m e m b r a nesxxx(xxxx)xxx
strength of GCLs because of pullout and rupture of reinforcing fibers of
GCL (Ruedrich et al., 2011; Kong et al., 2017). Shear strength of hy-
drated GCL is influenced by the type of hydrating fluid, hydration time
and normal stress. Hence it becomes important to evaluate and under-
stand the internal shear mechanisms of geosynthetic clay liners. Studies
in this direction showed that the type of upper and carrier layers of
geotextile in a GCL and the type of bonding used for making the GCL
affect the internal shear strength of a GCL. Further, the peak and residual
shear strengths of reinforced GCLs were found to be influenced by the
swelling pressures developed in bentonite, leading to rupture and
pull-out of reinforcing needle punched fibres (Gilbert et al., 1996; Eid
and Stark, 1997; Müller et al., 2008; Bacas et al., 2013). Interface studies
between GCL and geomembrane demonstrated that the extruded
bentonite governs the shear resistance of a hydrated GCL (Lake and
Rowe, 2000; Fox and Stark, 2004; Vukeli´c et al., 2008; Ghazizadeh and
Bareither, 2018). The shear behavior of hydrated GCLs is mainly gov-
erned by the lubricating effect of the hydrating fluid at the interface,
method of hydration and the normal stress acting on the interface (Seo
et al., 2007; Zornberg and McCartney, 2009).
Effects of hydration on the performance of GCLs are well established
in literature. Yesiller et al. (2019) studied the effects of different
chemicals in hydrating water and the long-term exposure of GCLs to
moisture suction on the bentonite swelling in different types of needle
punched GCLs in the field. It was demonstrated that the surface texture
of GCLs plays an important role on the moisture suction and the lateral
swell of the bentonite in GCLs. The internal erosion of GCL under a
hydraulic gradient was studied by Rowe and Orsini (2003) in a fixed ring
hydraulic conductivity apparatus. The results demonstrated that for
GCLs with conventional woven and nonwoven carrier geotextile resting
on gravel or geonet, internal erosion may occur within the GCL,
resulting in an increase in the hydraulic conductivity by an order of at
least one magnitude. Whereas the GCLs resting on sand subgrades per-
formed effectively, with applied water heads of up to 70 m, without any
internal erosion. These studies emphasize the benefits of using sand as
the landfill base material.
When sand is used in the cover and liner systems of a landfill, sta-
bility of these systems depends on the internal and interface shear
strengths mobilized, which are mainly governed by the size and shape of
the sand grains and surface features of the GCL. The study of Li (2013)
showed that the constant volume shear strength of sands improves with
higher amounts of coarse fraction. Afzali-Nejad et al. (2017) carried out
interface shear tests on woven geotextile interfaced with angular fine
sand of uniform gradation and well-rounded fine glass beads. The study
highlighted the variation in peak and residual interface friction angles
between the geotextile and granular media with the change in particle
shape. For the angular sand-woven geotextile interface, increase in the
normal stress reduced both maximum dilation angle and peak friction
angle. Hence quantifying the shape of sand particles is important to
Fig. 1. Interfaces in a landfill.
understand the GCL-sand interface shear strength. Particle shape char-
acterization through visual observations and measurements is a
geosynthetic-geosynthetic interface shear behavior using direct shear
well-established technique in literature (Krumbein and Sloss, 1963;
tests, with rigid blocks covered with geosynthetics and fitted in both the
Barrett, 1980; Cho et al., 2006) In recent years, image analysis technique
halves of the box (Giroud et al., 1993; Jones and Dixon, 1998; Bergado
is extensively used for the quantification of the shape and size param-
et al., 2006; Fowmes et al., 2017). A review of these different interface
eters of sand particles (Altuhafi et al., 2013; Sochan et al., 2015; Araujo
shear tests reveals that the variation in the interaction behavior between
et al., 2017; Wu et al., 2021).
the surfaces and the overall interfacial shear strength is different for all
Image based studies on GCLs are mainly focused on measurement of
these different interfaces because of the interactions that are responsible
GCL thickness and understanding surface changes to GCLs. Yesiller and
for the development of adhesion and friction between the layers. Studies
Cekic (2005) used images of geomembranes obtained through an optical
available on sand-GCL interface shear behavior are limited (Marr, 2001;
microscope to determine the thickness and surface features of textured
Viana and Palmeira, 2003; Chai and Saito, 2016) and most of them deal
geomembranes. Rowe et al. (2017) used high resolution X-ray images of
with dry sands.
samples of virgin GCLs and GCLs exhumed from landfills to understand
GCLs get hydrated during their service, as they are exposed to
the defects and surface changes to GCLs and hydration effects in
leachates and rainwater. The presence of bentonite layer in a GCL can
bentonite. Zangi and Likos (2016) used microscopic images of desic-
pose the danger of internal shear failure in GCL because bentonite
cated bentonite in GCLs to quantify shrinkage strains in GCLs through
consists of expansive clay minerals, which cause swelling under hy-
computational algorithms in MATLAB. The image-based edge detection
drated conditions. The swelling of bentonite reduces the internal shear
technique developed by Maini and Aggarwal (2009) was used for
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Table 1
Properties of the GCL.
Property Value
Mass per unit area (g/m2) 4300
Nominal thickness (mm) 6.0
Permeability (cm/sec) 5 ×10(cid:0)9
Tensile strength (longitudinal) (kN/m) 11.5
measuring the crack width in a bentonite layer. One of the important
observations in this study was that the main reasons for difference in
cracking pattern and the hydraulic conductivity of bentonite in GCLs are
the internal factors including the fibers in bentonite rather than external
stress conditions. By quantifying the surface changes in the sheared
GCLs through microstructural investigations and relating them to the
shape parameters of sands, significant information on the microlevel
interactions in sand-GCL interfaces can be obtained. Studies in this di-
rection are limited and the present work focuses on this aspect.
Studies revealed that sliding and rolling of sand particles over
polymeric sheets are the two main shearing mechanisms of sand-
polymer interfaces (Dove et al., 2006). Some researchers have re-
ported damage to the woven geotextile during interface direct shear
tests (Stark et al., 1996; Ghazizadeh and Bareither, 2018). Through a
series of interface direct shear tests on GCL and texture geomembrane
composite systems, Ghazizadeh and Bareither (2018) concluded that
GCLs with woven geotextiles are not recommended for situations that
create high stress conditions in landfills. Hence there is a need to study
the interface shear response of woven and nonwoven surfaces of GCLs
with sand under dry and wet conditions. A part of the present study
focuses on this aspect.
Feasibility of using manufactured sand and other alternate materials
for various civil engineering constructions is being investigated by
several researchers (Nanthagopalan and Santhanam, 2011; Yamei and
Lihua, 2017; Zhao et al., 2017; Zhang et al., 2020). Such replacement
needs proven studies to understand the physical, chemical, and me-
chanical properties of manufactured sand along with its ability to pro-
vide adequate frictional strength at the interface. In the present study,
both natural river sand and manufactured sand are used as interfacing
materials with GCL, to compare their interface shear strength under dry
and wet conditions through modified direct shear tests. Precise particle
shape quantifications and analysis of interface shear strength under dry
and hydrated conditions are planned to bring out the effect of shape
parameters of sand particles on the GCL-sand interface shear strength.
To demonstrate the comparative surface changes to GCL due to shearing
under various test conditions, image analysis is carried out on original
and sheared specimens of GCLs. The objective of this study is to bring
out the quantitative effects of sand particle size and shape on the shear
behavior of dry and hydrated sand-GCL interfaces.
2. Materials used
2.1. Geosynthetic clay liner (GCL)
The commercial product name of the geosynthetic clay liner (GCL)
used in the study is Macline GCL-W. It consists of a layer of sodium
bentonite placed between a nonwoven geotextile as the basal layer and a
woven geotextile as the upper layer. Nonwoven geotextiles consist of
irregular randomly oriented polymeric fibres, which are bonded by
mechanical or thermal means. The fibres in the nonwoven geotextile
used in this study are needle punched. Woven geotextiles consist of two
sets of orthogonally interlaced polymeric filaments, woven into a fabric
Fig. 2. Specimens of GCL (a) nonwoven surface (b) woven surface.
with regular openings. Generally, nonwoven geotextiles have low me-
chanical strength and are used as functional components in drainage and
filtration applications. Woven geotextiles have relatively higher me-
chanical strength and are used for soil reinforcement. These geotextiles
are made of Polypropylene. Bentonite in the GCL has montmorillonite
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Fig. 3. Samples of sand (a) manufactured sand (MS) (b) river sand (RS).
Table 3
Morphological descriptors and their mathematical formulae.
Descriptor Formula Parameter description
Sphericity (Wadell, D D - diameter of the circle with same
1932) Dcir projected area as that of the
particle.
Dcir - diameter of the smallest
circumscribing circle of the
particle.
∑ /
Ro1u9n3d5n)e ss (Wadell, Ni=1ri N rc o- rrnaedrisu os fo tfh teh ep rcoijrecclete fdo ramreead o aft
Rmax particle
N - number of identified corners of
the projected area of the particle
Rmax - radius of the largest inscribed
circle within the particle
Convexity (Preparata A(T) A(T) - projected area of the particle
and Shamos, 1985) A(T)+B B - area occupied by the convexity
formed by the irregularity of the
edge of the particle
Aspect ratio (AR) ( XFmin XFmin – minimum Feret distance
Schneiderhohn, XFmax XFmax – maximum Feret distance
1954)
Fig. 4. Gradations of river sand (RS) and manufactured sand (MS). Elongation (Yangron, 1-AR AR - aspect ratio of the particle
2013) √̅̅̅̅
Table 2 RoHurgyhcniwes,s 2(0Z1h5e)n g and N1i∑=N1(yir(cid:0) yis)2 Nyir- -n iuthm cboeorr odfi nmateea souf rtehme eranwts profile
Properties of the sand. yis - ith coordinate of the
smoothened profile
Property R(RivSe) r sand M(MaSn)u factured sand NoVrmanaglilzae det R aol.u,g 2h0n1es8s) ( Rn = RougLh n ess L- length of the particle.
Gradation Poorly graded Well graded
Effective size, D10 (mm) 0.18 0.15 2.2. Sands
Mean particle size, D50 (mm) 0.59 1.06
D30 (mm) 0.35 0.54
D60 (mm) 0.70 1.39 Two different sands, namely, river sand (RS) of natural origin and
Coefficient of uniformity, Cu 3.885 9.266 manufactured sand (MS) from a quarry were used in this study. Photo-
Coefficient of curvature, Cc 0.972 1.39 graphs of these two sands are shown in Fig. 3. The grain size distribution
Specific gravity 2.69 2.57
of the sands is shown in Fig. 4. Based on the Unified Soil Classification
Maximum void ratio (emax) 0.69 0.54
Minimum void ratio (emin) 0.61 0.39 System (USCS) described in ASTM D2487-17e1 (2020), the river sand
Permeability from constant head test 2.24 ×10(cid:0)2 1.45 ×10(cid:0)2 and manufactured sand were classified as poorly graded sand (SP)
(cm/s) well-graded sand (SW), respectively. Properties of the test sands are
Angle of internal friction from direct 44.4◦ 47◦
listed in Table 2.
shear test
Interface shear tests planned in this study are intended to bring out
the effects of particle shape on sand-GCL interaction mechanisms and
content of about 70%, with a water absorption capacity of about 650% interfacial shear strength. Since the gradations of RS and MS are
and free swelling capacity higher than 24 ml/2 g. Fig. 2 shows the different, using their original gradations in interface shear tests can
nonwoven and woven surfaces of GCL specimens. Properties of GCL are impose particle size effects on results. Hence to eliminate the effects of
listed in Table 1. particle size, a synthesized gradation, which lies between the gradations
of RS and MS as shown in Fig. 4 is targeted. Samples of RS and MS with
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Fig. 5. Flowchart of the grain shape characterization algorithm.
Fig. 6. Microscopic images of typical MS particles at 20×magnification; grayscale (left) and segmented (right).
Fig. 7. Microscopic images of typical RS particles at 20×magnification; grayscale (left) and segmented (right).
target gradation are created by scalping specific size fractions in 2.2.1. Particle shape quantification through image analysis
required proportions for both the sands separately. The constituent particle shape is known to influence the internal and
interface shear behavior, as suggested by several studies (Wadell, 1932;
Riley, 1941; Jensen et al., 1999, 2001; Vangla and Latha, 2014, 2016; Su
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Fig. 8. Sand particle shape characterization through image analysis (images taken at 40x and 100×magnifications, as indicated).
et al., 2018; Vangla et al., 2018; Liu et al., 2021). To understand the the manual measurement of particle geometry aided by visual charts,
microscopic shear mechanisms in sand-GCL interfaces, it is important to the studies on sand particle shape quantification have evolved into ac-
quantify the shape of the sand grains. The available studies on particle curate measurement of particle shape through image analysis using so-
shape are mostly centered around three different scales utilized for the phisticated computational methods.
characterization of two-dimensional projections of particles (Barett, The important shape parameters considered for grain shape quanti-
1980; Mitchell and Soga, 2005). Among these, macro-scale is used for fication in this study are sphericity, roundness, and surface roughness.
measuring sphericity, meso-scale is used for measuring roundness and The well-established formulae used for quantifying these shape param-
micro-scale is used for measuring for surface roughness. Starting with eters are presented in Table 3. Several particles of different size ranges of
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Fig. 9. Images of particles of 0.6 mm size analyzed for Feret measurements.
Table 4
Geometric parameters computed from the image analysis of typical sand particles for shape characterization (Refer to Fig. 8 and Table 3).
Particle D(p ixels) D(pciirx els) i∑=N1ri N R(pmixaxe ls) A(p i(xTe)l s) B (pixels) XFmin(pixels) X(pFimxaexl s) R(poiuxeglhsn) ess L (pixels)
(pixels)
MS 0.6–1.0 2260.91 2998.08 6287.03 18 818.37 3706535 1394669.21 1691.27 2695.69 2.734 1952.85
MS 0.6–2.0 2087.54 2840.80 5896.86 17 839.28 3408988 977681.60 1770.92 2897.92 2.939 2672.45
MS 1572.68 2024.60 1954.59 9 576.83′ 1930508 533446.03 1233.24 1723.60 1.200 2401.40
0.15–1.0
MS 1575.95 2022.00 2153.36 10 573.77 1930135 529889.25 1270.39 1785.51 2.244 5610.33
0.15–2.0
RS 0.6–1.0 1393.77 1629.20 3174.40 13 558.52 1521000 8578601.61 1375.04 1618.83 2.347 1806.07
RS 0.6–2.0 1386.21 1622.04 3163.72 11 580.33 1507490 7206325.01 1378.42 1666.77 2.563 2136.33
RS 0.15–1.0 1371.75 1610.04 2424.68 9 587.07 1470807 97719.18 1248.18 1676.98 2.557 5115.20
RS 0.15–2.0 1523.20 1892.18 2893.99 11 609.57 1775090 190461.98 1370.80 1834.33 2.557 8524.00
Table 5
Results of shape analysis of typical sand particles (Refer to Fig. 8).
Sand particle Sphericity Round-ness Convexity Aspect ratio Elongation Normalized roughness
MS 0.6–1 0.7542 0.4268 0.7266 0.6274 0.3726 0.0014
MS 0.6–2 0.7347 0.4133 0.7789 0.6117 0.3723 0.0011
MS 0.15–1 0.7768 0.3765 0.7835 0.7155 0.2845 0.0005
MS 0.15–2 0.7794 0.3753 0.7846 0.7115 0.2885 0.0004
RS 0.6–1 0.8555 0.4372 0.8199 0.8494 0.1506 0.0013
RS 0.6–2 0.8546 0.4956 0.8722 0.8270 0.1730 0.0012
RS 0.15–1 0.8520 0.4589 0.9377 0.7443 0.2557 0.0005
RS 0.15–2 0.8050 0.4316 0.9031 0.7473 0.2527 0.0003
sands used in this study are analyzed for their shape through image information, the images were converted to binary images and their
analysis. Images of sand particles are procured in multiple orientations respective shape parameters were analyzed based on an algorithm coded
and analyzed individually. The microscopic images were taken using in MATLAB 2021a. Fig. 5 presents a flowchart of key steps in particle
Nikon 80i optical microscope with Q-Imaging Micropublisher to capture shape analysis.
images at different magnifications. To obtain the geometrical
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Table 6
Average particle shape parameters of MS and RS.
Particle size (mm) Sand type Sphericity Roundness Convexity Aspect ratio Elongation Normalized roughness
0.6 RS 0.8155 0.4001 0.7985 0.7792 0.2208 0.0010
MS 0.77085 0.3898 0.7411 0.7054 0.2946 0.0024
0.3 RS 0.8207 0.3818 0.8817 0.7430 0.257 0.0009
MS 0.7817 0.3740 0.8111 0.6659 0.3341 0.0015
0.15 RS 0.8689 0.4556 0.9051 0.7532 0.2468 0.0008
MS 0.7912 0.4017 0.8105 0.6001 0.3999 0.0009
0.075 RS 0.8842 0.4340 0.8852 0.8086 0.1914 0.0021
MS 0.8003 0.3971 0.7805 0.6426 0.3574 0.0050
Fig. 10. Schematic diagram of the interface direct shear setup.
2.2.1.1. Sphericity. Sphericity is the degree to which the shape of a diameter of the largest inscribed circle of the particle. The algorithm and
particle can be approximated to a sphere. Wadell (1932) proposed the the key steps shown in the flowchart are based on the computational
most widely used definition of sphericity, based on the projected area of geometry method described by Zheng and Hryciw (2015). The method
a particle on its stable axis. It is defined as the ratio of the diameter of the has three important steps, discretization of particle boundary without
circle having an area equal to the largest projected area of the particle to missing any curvatures (Step R1), identifying the potential corner points
the diameter of the smallest circle that circumscribes the projection. The (Step R2) and fitting best fit circles into the identified corners (Step R3).
region properties function of the image processing toolbox of MATLAB is
used to obtain the area and perimeter of the particle (Step S1). The 2.2.1.3. Convexity. Convexity describes the compactness of the particle
methodology proposed by Zheng and Hryciw (2015) is used for shape. Convexity is defined as the ratio of the projected area of the
obtaining the minimum circumscribing circle (Step S2). The procedure particle and the convex polygon formed along the outline of the particle,
requires finding out the minimum number of points needed to create the which is termed as the convex hull. The convex polygon formed around
particle boundary, without losing any corners. The first trial diameter the silhouette of particle is termed as the convex hull. Convexity is
for the circumscribing circle is obtained by joining the two farthest computed in one (Step C1) using the region properties function of image
points based on distance calculation. If the circle connects all the processing toolbox in MATLAB 2021a.
boundary points, then it is fixed as the circumscribing circle. In sce-
narios, where points lie outside the circle, the farthest point is added to 2.2.1.4. Aspect ratio and elongation. Aspect ratio is the ratio of width to
previous points to form the new circle. length of the particle. Aspect ratio can be computed as the ratio of the
minimum Feret distance to the maximum Feret distance. Feret distance
2.2.1.2. Roundness. Roundness is a measure for the smoothness of the is the distance between two tangents to the contour of the particle. The
particle boundary. Roundness calculations as per Wadell (1935) require region properties function is used in the algorithm to compute the ferret
identification of all-important corners of the particle, computing the distances and aspect ratio in a single step (A1). The elongation,
average radius of circles fitting into the corners and measuring the computed as 1- aspect ratio (Step E1), is a measure of the proportion of
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Fig. 11. Plan view of the components of the interface shear setup.
the difference between the raw profile and the smoothened profile of the
Table 7 particle, using conventional root mean square formula. To obtain the
Details of the interface shear tests. roughness of a sand particle, the raw profile of the particle is initially
Soil type Normal stress Interface Water content in soil obtained as the outline of the boundary points, using the polar co-
(kPa) (%) ordinates (Step SR1). To obtain the smoothened profile, locally
weighted scatter plot smoothing (LOESS) method suggested by Zheng
River sand (RS) 7 NGCL-RS 0
WGCL-RS 0 and Hryciw (2015) is used (Step SR2). The deviation of the original raw
15 NGCL-RS 0 profile from the filtered roughness-free profile gives a measure of the
WGCL-RS 0 particle roughness and is computed as shown in Table 3 (Step SR3).
30 NGCL-RS 0
Roughness is a scale dependent parameter, the value of which changes
WGCL-RS 0
60 NGCL-RS 0 with the resolution and scan length of the measuring instrument. To
WGCL-RS 0 eliminate the particle size effects, a parameter called normalized
100 NGCL-RS 0, 6, 12, 25 roughness (Rn) given by Vangla et al. (2018) is obtained based on the
WGCL-RS 0 length of the particle along the major axis of the particle.
Manufactured sand 7 NGCL-MS 0
(MS) WGCL- 0
MS 2.2.2. Results of shape analysis
15 NGCL-MS 0 Fig. 6 and Fig. 7 show the typical microscopic images of MS and RS
WGCL- 0 particles, respectively, obtained at 20× magnification and segmented
MS
with image processing by converting to binary images. For the mea-
30 NGCL-MS 0
WGCL- 0 surement of sphericity, roundness, convexity, aspect ratio, elongation
MS and roughness, images of individual particles captured in two different
60 NGCL-MS 0 orientations were analyzed, as shown in Fig. 8. 200 particles of each
WGCL- 0 dominant size of sands (0.6 mm, 0.3 mm, 0.15 mm, and 0.075 mm) were
MS analyzed. 100×magnification is used for particles of smaller size (0.15
100 NGCL-MS 0, 6, 12, 25
WGCL- 0 mm and 0.075 mm) and 40×magnification is used for particles of larger
MS size (0.6 mm and 0.3 mm). In Fig. 8, MS 0.6–1 and MS 0.6–2 refer to the
images of typical manufactured sand particles of 0.6 mm size captured in
two different orientations. Similarly, RS 0.15–1 and RS 0.15–2 refer to
the particle.
the images of typical river sand particles of size 0.15 mm captured in two
different directions. Similar notation applies to other images in Fig. 8.
2.2.1.5. Surface roughness. Surface roughness is a measure of the un-
Aspect ratio analysis using Feret measurements for typical MS and RS
evenness of a surface. Roughness of the particle surface is computed as
particles are shown in Fig. 9. The final quantification of each shape
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A.G. Pillai and M.L. Gali G e o t e x t i l e s a n d G e o m e m b r a nesxxx(xxxx)xxx
Fig. 12. Response of NGCL-RS and NGCL-MS interfaces in shear tests under different normal stresses (a) 7 kPa (b) 15 kPa and (c) 30 kPa (d) 60 kPa and (e) 100 kPa.
parameter was done by averaging the results obtained for 200 individual 3. Testing methodology
particles of each size and type fraction, analyzed for two different ori-
entations. Geometric parameters computed for the shape characteriza- 3.1. Test setup
tion of typical particles shown in Fig. 8 are given in Table 4. Shape
characteristics of images of MS and RS particles of different sizes The direct shear setup complemented with digital recording abilities
captured in different orientations as shown in Fig. 8 are presented in developed by Vangla and Latha (2015, 2016) is used for the evaluation
Table 5. Average shape parameters for different size fractions of RS and of interface shear strength parameters in this study. The test setup was
MS are given in Table 6. RS particles have higher sphericity and higher modified to perform the interface shear tests by designing the lower
roundness compared to MS particles. Convexity is a measure for surface frame in a way that planar geosynthetic sample is affixed to the steel
regularity with regular particles having higher convexity. Aspect ratio platform of dimensions 180 mm × 180 mm using gripper plates and
represents the shape of the particle, with regular shape having higher bolts. The test setup allows digital recording of the interface shear tests
aspect ratio. MS particles have lower aspect ratio and lower convexity data with sand and geosynthetic materials like GCL, to evaluate the
for all size ranges, leading to higher irregularities. interface friction and adhesion parameters. The bearings facilitate the
movement of steel platform placed on the top, in line with the groves
provided underneath the platform. The platform carries the attached
geosynthetic sample. Displacements to the steel platform are monitored
by the LVDTs (linear variable differential transformers). The shear force
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