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

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Geotextiles and Geomembranes xxx (xxxx) xxx

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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 Hydration Image analysis Particle shape Manufactured sand

Interface shear strength of geosynthetic clay liners (GCL) with the sand particles is predominantly influenced by the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. This study brings out the quantitative effects of particle shape on the interaction mechanisms and shear strength of GCL-sand interfaces. Interface direct shear tests are conducted on GCL in contact with a natural sand and a manufactured sand of identical gradation, eliminating the particle size effects. Results showed that manufactured 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 GCLsand 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 Geosynthetic clay liners (GCL) are geocomposites of unique nature which combine the characteristics of bentonite clay and polymer. They are proven to be effective hydraulic barriers. Based on their hydraulic performance in landfill liner systems, researchers demonstrated that GCLs of 6–10 mm thickness can replace compacted clay liners (CCLs) of 900 mm thickness in a landfill (Giroud et al., 1997; Didier et al., 2000; Lake and Rowe, 2000; Shackelford et al., 2000; Sivakumar Babu et al., 2001; Zhan et al., 2018). The economic implications of using GCLs in place of CCLs are well documented in literature. GCLs have an edge over CCLs because of their reduced thickness, easy installation properties, low cost, and good conformance with the differential settlements of the underlying soil (Bouazza, 2002). The self-healing property of GCLs provides a longer service life (Daniel, 2000). GCL is proven to be su­ perior to CCL in several aspects like quality assurance, availability, construction speed, durability to freeze-thaw and wet-dry cycles (Man­ assero et al., 2000). GCL Layers are effectively being used in baseliner and capping systems of engineered landfills. The surface of the GCL is a woven or a nonwoven geotextile, which is in contact with sand particles at different locations in a composite liner

system. The interface shear mechanisms of GCL surface and sand par­ ticles govern the mechanical design of these systems. In a cover system, the GCL is underlain by a sand layer and the interactions govern the sliding and other instabilities that might arise under the external and internal loading conditions. In the base layer, sand layer is usually a part of the compacted subgrade, which supports the GCL and the interface shear in such a case can govern the sliding of the liner. GCL-sand in­ terfaces are also used at the interlayers in a double-liner system. All these possible GCL-sand interfaces in a landfill are schematically shown in Fig. 1. Inadequate shear resistance at the interface of GCL and sand layer in these components is the primary cause of concern for the me­ chanical stability of a landfill. Hence accurate quantification of interface and internal shear strength of GCL is very important for the stability assessment of landfills. Interface shear studies in literature are mostly focused on soilgeosynthetic interfaces. Most of these studies are performed with a conventional direct shear box, with its bottom-half fitted with a rigid block covered with a geosynthetic layer, to facilitate shearing between the geosynthetic layer and the soil filled in the upper-half of the shear box (Zettler et al., 2000; Fleming et al., 2006; Jotisankasa and Rurg­ 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/© 2021 Elsevier Ltd. All rights reserved.

Please cite this article as: Anjali G. Pillai, Madhavi Latha Gali, Geotextiles and Geomembranes, https://doi.org/10.1016/j.geotexmem.2021.11.004

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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 understand the GCL-sand interface shear strength. Particle shape char­ acterization through visual observations and measurements is a well-established technique in literature (Krumbein and Sloss, 1963; Barrett, 1980; Cho et al., 2006) In recent years, image analysis technique is extensively used for the quantification of the shape and size param­ eters of sand particles (Altuhafi et al., 2013; Sochan et al., 2015; Araujo et al., 2017; Wu et al., 2021). Image based studies on GCLs are mainly focused on measurement of GCL thickness and understanding surface changes to GCLs. Yesiller and Cekic (2005) used images of geomembranes obtained through an optical microscope to determine the thickness and surface features of textured geomembranes. Rowe et al. (2017) used high resolution X-ray images of samples of virgin GCLs and GCLs exhumed from landfills to understand the defects and surface changes to GCLs and hydration effects in bentonite. Zangi and Likos (2016) used microscopic images of desic­ cated bentonite in GCLs to quantify shrinkage strains in GCLs through computational algorithms in MATLAB. The image-based edge detection technique developed by Maini and Aggarwal (2009) was used for

Fig. 1. Interfaces in a landfill.

geosynthetic-geosynthetic interface shear behavior using direct shear tests, with rigid blocks covered with geosynthetics and fitted in both the halves of the box (Giroud et al., 1993; Jones and Dixon, 1998; Bergado et al., 2006; Fowmes et al., 2017). A review of these different interface shear tests reveals that the variation in the interaction behavior between the surfaces and the overall interfacial shear strength is different for all these different interfaces because of the interactions that are responsible for the development of adhesion and friction between the layers. Studies available on sand-GCL interface shear behavior are limited (Marr, 2001; Viana and Palmeira, 2003; Chai and Saito, 2016) and most of them deal with dry sands. GCLs get hydrated during their service, as they are exposed to leachates and rainwater. The presence of bentonite layer in a GCL can pose the danger of internal shear failure in GCL because bentonite consists of expansive clay minerals, which cause swelling under hy­ drated conditions. The swelling of bentonite reduces the internal shear 2

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Table 1 Properties of the GCL. Property

Value 2

Mass per unit area (g/m ) Nominal thickness (mm) Permeability (cm/sec) Tensile strength (longitudinal) (kN/m)

4300 6.0 5 × 10− 11.5

9

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 sandpolymer 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 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

Fig. 2. Specimens of GCL (a) nonwoven surface (b) woven surface.

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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.

Fig. 4. Gradations of river sand (RS) and manufactured sand (MS). Table 2 Properties of the sand. Property

River sand (RS)

Manufactured sand (MS)

Gradation Effective size, D10 (mm) Mean particle size, D50 (mm) D30 (mm) D60 (mm) Coefficient of uniformity, Cu Coefficient of curvature, Cc Specific gravity Maximum void ratio (emax) Minimum void ratio (emin) Permeability from constant head test (cm/s) Angle of internal friction from direct shear test

Poorly graded 0.18 0.59 0.35 0.70 3.885 0.972 2.69 0.69 0.61 2.24 × 10− 2

Well graded 0.15 1.06 0.54 1.39 9.266 1.39 2.57 0.54 0.39 1.45 × 10− 2

44.4◦

47◦

Descriptor

Formula

Parameter description

Sphericity (Wadell, 1932)

D Dcir

Roundness (Wadell, 1935)

∑N

Convexity (Preparata and Shamos, 1985)

A(T) A(T) + B

Aspect ratio (AR) ( Schneiderhohn, 1954) Elongation (Yangron, 2013) Roughness (Zheng and Hryciw, 2015)

XFmin XFmax

D - diameter of the circle with same projected area as that of the particle. Dcir - diameter of the smallest circumscribing circle of the particle. r - radius of the circle formed at corners of the projected area of particle N - number of identified corners of the projected area of the particle Rmax - radius of the largest inscribed circle within the particle A(T) - projected area of the particle B - area occupied by the convexity formed by the irregularity of the edge of the particle XFmin – minimum Feret distance XFmax – maximum Feret distance

1-AR

AR - aspect ratio of the particle

√̅̅̅̅ N 1∑ (yir − yis )2 N i=1

N- number of measurements yir - ith coordinate of the raw profile yis - ith coordinate of the smoothened profile

Normalized Roughness ( Vangla et al., 2018)

/ N

r i=1 i

Rmax

Rn =

Roughness L

L- length of the particle.

2.2. Sands Two different sands, namely, river sand (RS) of natural origin and manufactured sand (MS) from a quarry were used in this study. Photo­ graphs of these two sands are shown in Fig. 3. The grain size distribution of the sands is shown in Fig. 4. Based on the Unified Soil Classification System (USCS) described in ASTM D2487-17e1 (2020), the river sand and manufactured sand were classified as poorly graded sand (SP) well-graded sand (SW), respectively. Properties of the test sands are listed in Table 2. Interface shear tests planned in this study are intended to bring out the effects of particle shape on sand-GCL interaction mechanisms and interfacial shear strength. Since the gradations of RS and MS are different, using their original gradations in interface shear tests can impose particle size effects on results. Hence to eliminate the effects of 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

content of about 70%, with a water absorption capacity of about 650% and free swelling capacity higher than 24 ml/2 g. Fig. 2 shows the nonwoven and woven surfaces of GCL specimens. Properties of GCL are listed in Table 1.

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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 required proportions for both the sands separately.

2.2.1. Particle shape quantification through image analysis 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 5

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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 microscopic shear mechanisms in sand-GCL interfaces, it is important to quantify the shape of the sand grains. The available studies on particle shape are mostly centered around three different scales utilized for the characterization of two-dimensional projections of particles (Barett, 1980; Mitchell and Soga, 2005). Among these, macro-scale is used for measuring sphericity, meso-scale is used for measuring roundness and micro-scale is used for measuring for surface roughness. Starting with

the manual measurement of particle geometry aided by visual charts, the studies on sand particle shape quantification have evolved into ac­ curate measurement of particle shape through image analysis using so­ phisticated computational methods. The important shape parameters considered for grain shape quanti­ fication in this study are sphericity, roundness, and surface roughness. The well-established formulae used for quantifying these shape param­ eters are presented in Table 3. Several particles of different size ranges of 6

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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

MS 0.6–1.0 MS 0.6–2.0 MS 0.15–1.0 MS 0.15–2.0 RS 0.6–1.0 RS 0.6–2.0 RS 0.15–1.0 RS 0.15–2.0

N

Rmax (pixels)

A (T) (pixels)

B (pixels)

XFmin(pixels)

XFmax (pixels)

Roughness (pixels)

L (pixels)

6287.03 5896.86 1954.59

18 17 9

818.37 839.28 576.83′

3706535 3408988 1930508

1394669.21 977681.60 533446.03

1691.27 1770.92 1233.24

2695.69 2897.92 1723.60

2.734 2.939 1.200

1952.85 2672.45 2401.40

2022.00

2153.36

10

573.77

1930135

529889.25

1270.39

1785.51

2.244

5610.33

1629.20 1622.04 1610.04 1892.18

3174.40 3163.72 2424.68 2893.99

13 11 9 11

558.52 580.33 587.07 609.57

1521000 1507490 1470807 1775090

8578601.61 7206325.01 97719.18 190461.98

1375.04 1378.42 1248.18 1370.80

1618.83 1666.77 1676.98 1834.33

2.347 2.563 2.557 2.557

1806.07 2136.33 5115.20 8524.00

D (pixels)

Dcir (pixels)

2260.91 2087.54 1572.68

2998.08 2840.80 2024.60

1575.95 1393.77 1386.21 1371.75 1523.20

N ∑

ri

i=1

(pixels)

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 MS 0.6–2 MS 0.15–1 MS 0.15–2 RS 0.6–1 RS 0.6–2 RS 0.15–1 RS 0.15–2

0.7542 0.7347 0.7768 0.7794 0.8555 0.8546 0.8520 0.8050

0.4268 0.4133 0.3765 0.3753 0.4372 0.4956 0.4589 0.4316

0.7266 0.7789 0.7835 0.7846 0.8199 0.8722 0.9377 0.9031

0.6274 0.6117 0.7155 0.7115 0.8494 0.8270 0.7443 0.7473

0.3726 0.3723 0.2845 0.2885 0.1506 0.1730 0.2557 0.2527

0.0014 0.0011 0.0005 0.0004 0.0013 0.0012 0.0005 0.0003

sands used in this study are analyzed for their shape through image analysis. Images of sand particles are procured in multiple orientations and analyzed individually. The microscopic images were taken using Nikon 80i optical microscope with Q-Imaging Micropublisher to capture images at different magnifications. To obtain the geometrical

information, the images were converted to binary images and their respective shape parameters were analyzed based on an algorithm coded in MATLAB 2021a. Fig. 5 presents a flowchart of key steps in particle shape analysis.

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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 MS

0.8155 0.77085

0.4001 0.3898

0.7985 0.7411

0.7792 0.7054

0.2208 0.2946

0.0010 0.0024

0.3

RS MS

0.8207 0.7817

0.3818 0.3740

0.8817 0.8111

0.7430 0.6659

0.257 0.3341

0.0009 0.0015

0.15

RS MS

0.8689 0.7912

0.4556 0.4017

0.9051 0.8105

0.7532 0.6001

0.2468 0.3999

0.0008 0.0009

0.075

RS MS

0.8842 0.8003

0.4340 0.3971

0.8852 0.7805

0.8086 0.6426

0.1914 0.3574

0.0021 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 particle can be approximated to a sphere. Wadell (1932) proposed the most widely used definition of sphericity, based on the projected area of a particle on its stable axis. It is defined as the ratio of the diameter of the circle having an area equal to the largest projected area of the particle to the diameter of the smallest circle that circumscribes the projection. The region properties function of the image processing toolbox of MATLAB is used to obtain the area and perimeter of the particle (Step S1). The methodology proposed by Zheng and Hryciw (2015) is used for obtaining the minimum circumscribing circle (Step S2). The procedure requires finding out the minimum number of points needed to create the particle boundary, without losing any corners. The first trial diameter for the circumscribing circle is obtained by joining the two farthest points based on distance calculation. If the circle connects all the 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 previous points to form the new circle.

diameter of the largest inscribed circle of the particle. The algorithm and the key steps shown in the flowchart are based on the computational geometry method described by Zheng and Hryciw (2015). The method has three important steps, discretization of particle boundary without missing any curvatures (Step R1), identifying the potential corner points (Step R2) and fitting best fit circles into the identified corners (Step R3). 2.2.1.3. Convexity. Convexity describes the compactness of the particle shape. Convexity is defined as the ratio of the projected area of the particle and the convex polygon formed along the outline of the particle, which is termed as the convex hull. The convex polygon formed around the silhouette of particle is termed as the convex hull. Convexity is computed in one (Step C1) using the region properties function of image processing toolbox in MATLAB 2021a. 2.2.1.4. Aspect ratio and elongation. Aspect ratio is the ratio of width to length of the particle. Aspect ratio can be computed as the ratio of the minimum Feret distance to the maximum Feret distance. Feret distance is the distance between two tangents to the contour of the particle. The region properties function is used in the algorithm to compute the ferret distances and aspect ratio in a single step (A1). The elongation, computed as 1- aspect ratio (Step E1), is a measure of the proportion of

2.2.1.2. Roundness. Roundness is a measure for the smoothness of the particle boundary. Roundness calculations as per Wadell (1935) require identification of all-important corners of the particle, computing the average radius of circles fitting into the corners and measuring the 8

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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 particle, using conventional root mean square formula. To obtain the roughness of a sand particle, the raw profile of the particle is initially obtained as the outline of the boundary points, using the polar co­ ordinates (Step SR1). To obtain the smoothened profile, locally weighted scatter plot smoothing (LOESS) method suggested by Zheng and Hryciw (2015) is used (Step SR2). The deviation of the original raw profile from the filtered roughness-free profile gives a measure of the particle roughness and is computed as shown in Table 3 (Step SR3). Roughness is a scale dependent parameter, the value of which changes with the resolution and scan length of the measuring instrument. To eliminate the particle size effects, a parameter called normalized roughness (Rn) given by Vangla et al. (2018) is obtained based on the length of the particle along the major axis of the particle.

Table 7 Details of the interface shear tests. Soil type

Normal stress (kPa)

Interface

Water content in soil (%)

River sand (RS)

7

NGCL-RS WGCL-RS NGCL-RS WGCL-RS NGCL-RS WGCL-RS NGCL-RS WGCL-RS NGCL-RS WGCL-RS NGCL-MS WGCLMS NGCL-MS WGCLMS NGCL-MS WGCLMS NGCL-MS WGCLMS NGCL-MS WGCLMS

0 0 0 0 0 0 0 0 0, 6, 12, 25 0 0 0

15 30 60 100 Manufactured sand (MS)

7 15 30 60 100

2.2.2. Results of shape analysis Fig. 6 and Fig. 7 show the typical microscopic images of MS and RS particles, respectively, obtained at 20× magnification and segmented with image processing by converting to binary images. For the mea­ surement of sphericity, roundness, convexity, aspect ratio, elongation and roughness, images of individual particles captured in two different orientations were analyzed, as shown in Fig. 8. 200 particles of each dominant size of sands (0.6 mm, 0.3 mm, 0.15 mm, and 0.075 mm) were analyzed. 100× magnification is used for particles of smaller size (0.15 mm and 0.075 mm) and 40× magnification is used for particles of larger 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 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. Aspect ratio analysis using Feret measurements for typical MS and RS particles are shown in Fig. 9. The final quantification of each shape

0 0 0 0 0 0 0, 6, 12, 25 0

the particle. 2.2.1.5. Surface roughness. Surface roughness is a measure of the un­ evenness of a surface. Roughness of the particle surface is computed as 9

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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 particles of each size and type fraction, analyzed for two different ori­ entations. Geometric parameters computed for the shape characteriza­ 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 captured in different orientations as shown in Fig. 8 are presented in Table 5. Average shape parameters for different size fractions of RS and MS are given in Table 6. RS particles have higher sphericity and higher roundness compared to MS particles. Convexity is a measure for surface regularity with regular particles having higher convexity. Aspect ratio represents the shape of the particle, with regular shape having higher aspect ratio. MS particles have lower aspect ratio and lower convexity for all size ranges, leading to higher irregularities.

3. Testing methodology 3.1. Test setup The direct shear setup complemented with digital recording abilities developed by Vangla and Latha (2015, 2016) is used for the evaluation of interface shear strength parameters in this study. The test setup was modified to perform the interface shear tests by designing the lower frame in a way that planar geosynthetic sample is affixed to the steel platform of dimensions 180 mm × 180 mm using gripper plates and bolts. The test setup allows digital recording of the interface shear tests data with sand and geosynthetic materials like GCL, to evaluate the 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 10

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4. Results and discussion Results from various interface shear tests are presented and discussed in the following sections. 4.1. Sand-GCL interface shear tests Studies by earlier researchers (Holtz et al., 1981; Santamarina and Cho, 2001) highlighted that at the same void ratio, the particle size did not affect the peak friction angle of sands with similar shape charac­ teristics. To eliminate the particle size effects and to study the individual effects of shape parameters, the direct shear tests in this study were performed on sand specimens prepared at target gradation by carefully adjusting the proportions to match the average gradation of river sand and manufactured sand. The high relative density of 80% used in the tests led to comparatively similar void ratios in specimens of river sand and manufactured sand. The variation in the friction angles can be attributed to the dissimilar particle shape, as suggested by Santamarina and Cho (2001). 4.1.1. Interface shear strength under dry conditions Effect of normal stress and the type of interfacing geosynthetic sur­ face on the shear behavior of GCL-sand interfaces is analyzed from the results of interface shear tests conducted under normal stresses of 7, 15, 30, 60 and 100 kPa. Tests were carried out with nonwoven surface of GCL (NGCL) and woven surface of GCL (WGCL) interfacing with sand. The interfaces in the shear tests are denoted as NGCL-RS (nonwoven geotextile side of GCL interfacing with river sand), NGCL-MS (nonwoven geotextile side of GCL interfacing with manufactured sand), WGCL-RS (woven geotextile side of GCL interfacing with river sand), WGCL-MS (woven geotextile side of GCL interfacing with manufactured sand). The response for NGCL-RS and NGCL-MS interfaces is presented in Fig. 12. For both NGCL-MS and NGCL-RS interfaces, increase in normal stress increased the peak interface shear stress because of increased inter­ locking of particles with the fibres of nonwoven GCL surface under high normal stresses. Further it is observed that the shear strength of NGCLMS interfaces showed higher interface shear resistance compared to NGCL-RS interfaces under similar testing conditions. MS particles have greater surface area compared to RS particles, due to the lower aspect ratio and higher elongation of their shape. Hence for the same sized MS and RS particles, the shearing area for an MS particle is statistically greater than that of an RS particle in an interface shear test. Also, the MS particles have rougher boundary compared to RS particles. In all the interface tests, a significant increase in shear stress for NGCL-MS inter­ face was seen beyond a shear displacement of 2 mm. The stress-strain responses at different normal stresses are not similar because the nonwoven geotextile has randomly oriented fibers and it is impossible to have identical specimens for testing. However, the peak shear stress obtained in case of NGCL-MS interfaces is always higher than that ob­ tained in case of NGCL-RS. The better performance of NGCL-MS in­ terfaces indicates efficient interlocking of particles of manufactured sand with the randomly oriented nonwoven fibers. Particles with higher elongation, lower convexity and higher surface roughness will have higher surface irregularities, which give them a tendency to interlock easily than particles with smooth surface. Because of their higher ir­ regularities, MS particles get interlocked easily with the fibres of GCL surface, developing more interface friction compared to RS particles. As observed, MS particles have higher surface roughness compared to RS particles, indicating that the surface of MS particles is more uneven compared to RS particles. The unevenness of the surface results in higher friction with the interfacing material. The overall shape of the MS par­ ticles, which is a combination of less sphericity, less roundness, less convexity, less aspect ratio, more elongation, and more roughness compared to RS particles, is the reason for enhanced fibre-particle interaction, leading to relatively higher shear resistance at the

Fig. 13. Results from repeated shear tests on NGCL-MS interfaces.

acting on the interface is recorded by the load cell connected to the worm gear. The steel shear box is modified with one transparent acrylic sidewall to visualize of the movement of sand particles during the test. Fig. 10 gives a schematic diagram of the test setup. 3.2. Sample preparation Geosynthetic clay liner (GCL) specimens were cut to 180 mm × 180 mm and fixed to the steel platform. To fix the specimen, clamps and gripper plates were designed (Fig. 11). Usage of clamps and gripper plates allows the extension of fabric and shear-induced deformations during the test. Errors due to the use of adhesives to fix the geosynthetic specimens are minimized in this type of arrangement. Once the GCL specimen was fixed to the steel platform, the shear box was placed on top and filled with required quantity of sand. In all the interface shear tests, sand was compacted to 80% relative density using hand compac­ tion. The maximum and minimum unit weights of the sands were determined as per ASTM D4253-16e1(2016). The maximum unit weight is determined through the vibratory table test and the minimum unit weight is determined by pouring the sand into the mould of the vibra­ tory table in its loosest state. The relative density of the sand samples at the target gradation is fixed as 80%, and the weight of sand to fill the shear box at that density is calculated from the maximum and minimum unit weights and the volume of the shear box. The required amount of sand is filled into the shear box in three layers, each layer compacted 25 times using a hand compactor. 3.3. Interface direct shear testing Interface direct shear tests were carried out on sand-GCL interfaces to investigate the effects of normal stress, initial water content in sands and shape of sand grains on the mobilization of interface shear strength. Interface shear tests with dry sand were conducted at normal stresses of 7, 15, 30, 60 and 100 kPa at a displacement rate of 1.0 mm/min. To ensure repeatability, three trials were carried out for most of the tests. 3.4. Test matrix The details of the interface shear tests are given in Table 7. Tests were carried out on river sand and manufactured sand with different water contents and different normal stresses. Tests were conducted with both woven and non-woven interfaces of the GCL. 11

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Fig. 14. Response of WGCL-RS and WGCL-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.

interfaces. Since the gradation of sand specimens was same in all these tests, the effects of particle size are eliminated, thereby highlighting the individual effects of particle shape. At normal stresses beyond 30 kPa, the peak stress was not reached within the displacement limits of the test and hence the strain-softening is not seen for specimens at higher normal stresses. Interface shear tests on NGCL-MS specimens were repeated to ensure the repeatability of test results and the results from the repeated tests at different normal stresses are shown in Fig. 13. Difference in peak shear stresses of repeated tests is within ±5 kPa, which could be attributed to the slight variations in the fibre quality of GCL specimens cut from a full roll. Comparison of interface shear responses of WGCL-MS and WGCL-RS specimens is shown in Fig. 14 for different normal stresses. As observed, WGCL-MS interfaces showed generally better interface shear strength compared to WGCL-RS interfaces, because of better interlocking of particles with the asperities in the woven geotextile. At higher normal

stresses, the peak stress was not reached within the displacement limits of the experiment and hence the strain-softening is not seen for speci­ mens tested at higher normal stresses. The interface shear strength parameters were obtained from the plots of normal stress vs. shear stress plotted from the interface shear tests, as shown in Fig. 15. The values of interface friction angle (φp) and interface adhesion (ap) are listed in Table 8. The interface friction angles of NGCL and WGCL surfaces with both the type of sands are almost the same. However, the adhesion exhibited by NGCL surfaces is much higher compared to WGCL surfaces. Since the fibres in a nonwoven geotextile are randomly oriented, the entrapment of particles in between the fibres is more, causing higher adhesion. The surface of nonwoven geotextile is rough and hence it also generates slightly higher interface friction. This result is consistent with the findings from previous studies on interface shear strength of GCL. The earlier studies are focused on evaluating the interface shear strength of geomembrane-GCL interfaces. McCartney and Swan (2002, 2009) observed that textured geomembrane interfaced 12

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Fig. 15. Failure envelopes in interface shear tests (a) NGCL-sand interfaces (b) WGCL-sand interfaces. Fig. 16. Response of NGCL-sand interfaces in shear tests with change in water content.

Table 8 Interface shear strength parameters. Interface

Interface friction angle φ (degrees)

Interface adhesion ap (kPa)

NGCL-MS NGCL-RS WGCL-MS WGCL-RS

36.67 31.38 35.8 31.4

32.07 19.89 20.53 18.11

4.1.2. Interface shear strength under hydrated conditions Bentonite, the key component of GCL, exhibits a volumetric expan­ sion of 900% upon hydration. The impact of hydration of GCL on interface shear behavior is generally underestimated. Most of the earlier researchers have ignored the effects of hydration and reported the shear strength of interfaces under dry conditions (Stark and Eid, 1996; Merrill and O’Brien, 1997; Thielmann et al., 2013; Kebaili et al., 2016). In field, the moisture content of the subgrade and cover materials continuously fluctuate due to changes in the groundwater level and infiltration of precipitation, which can result in hydration of the GCL by suction at the GCL-soil interface. In this study, the effects of hydration of GCL due to suction from the base material on the interface shear behavior is analyzed. Dry specimens of GCL are used in these interface shear tests with water content of sand varied in different tests. The hy­ dration of GCL due to suction occurs during the progression of the shear test. This simulates the field condition, where GCL gets hydrated pro­ gressively, due to the suction of water from the subgrade. By capturing

with nonwoven side of GCL showed about 10◦ higher interface friction angle compared to woven side of GCL in interface direct shear tests. Triplett and Fox (2001) showed that a textured geomembrane develops higher shear strength when it is sheared against nonwoven side of a GCL compared to the case when it is sheared against the woven side of the GCL. In the present study, compared to river sand, manufactured sand showed better adhesion, because the particles in manufactured sand are more angular and they get easily entrapped into the surfaces, providing resistance to shearing.

13

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Fig. 18. Variation of peak interface shear stress with water content in sand.

present study get hydrated before the application of shear stress in the test. Interface shear tests with wet sand were carried out at moisture contents of 6%, 12% and 25% by weight of sand under a high normal stress of 100 kPa, to initiate favorable conditions for suction. Since nonwoven interfaces showed better interface shear strength, they are chosen for this part of the study. Stress-displacement plots from shear tests on hydrated NGCL-RS and NGCL-MS interfaces are shown in Fig. 16a and Fig. 16b, respectively. Results show a significant decrease in peak interface shear stress for both interfaces with increase in water content. The decrease in peak interface shear stress is attributed to the swelling of bentonite, which results in the reduction of internal strength of GCL, leading to pull-out of reinforcing fibres, as suggested by Suzuki et al. (2017). The variation in the modulus of deformation for different interfaces with varying water content at a normal stress of 100 kPa is shown in Fig. 17. The modulus of deformation is computed as the ratio of horizontal stress and horizontal strain at different strain levels and is presented in Table 9. As observed from Fig. 17 and Table 9, initially, the deformation modulus of MS interfaces is higher than RS interfaces at all water contents because of higher interlocking due to the favorable shape of MS particles. However, the modulus degradation seems to happen quickly in MS interfaces compared to RS interfaces. The reason for this response could be the rupture of fibers at the interface in case of MS, because the MS particles are angular and rough. The high-resolution camera images of tested GCL surfaces were not able to identify such rupture. Optical microscopic image analysis of tested GCL surfaces can be the future scope of this work to identify the fabric rupture to correlate with the modulus degradation. At higher strain levels, effects of water content are not significant on the modulus degradation. Variation of peak interface shear stress with increase in water con­ tent in sand is shown in Fig. 18. A significant reduction in peak shear stress is observed with the increase in water content up to 12%. The reason for this reduction is the swelling of bentonite, which causes the bentonite to get extruded through the asperities of GCL onto the inter­ face. Because of this, the sand particles interact with the slimy coat of bentonite on the GCL surface instead of interacting with the fibres of geotextile, thereby offering less resistance to shear. Beyond 12% water content, the swelling of bentonite is already completed and hence further reduction in shear strength is insignificant, as seen from the plot. The effects of hydration seem to be different for NGCL interfacing with RS and MS, as seen from Fig. 18. With the increase in water content from 0 to 6% in the bentonite layer, the peak shear stress reduced by about 15 kPa in NGCL-MS and it reduced by about 9.5 kPa in case of NGCL-RS.

Fig. 17. Degradation of deformation modulus for NGCL interfaces with varying water content of sand (a) NGCL-RS (b) NGCL-MS.

Table 9 Variation of modulus of deformation of GCL-Sand interfaces under dry and wet conditions. Interface NGCL-RS NGCL-RS NGCL-MS NGCL-MS

Water content (%) 0 25 0 25

Modulus of deformation (kPa) 0.5% strain

1.5% strain

2.5% strain

3750 1400 4250 3000

974 296 956 116

100–500 100–500 100–1000 100–900

images of GCL cross sections of hydrating GCLs for every 5 s, Scalia and Benson (2011) demonstrated that it takes less than a minute for the water to migrate from the bottom to the top of the GCL through the fi­ bres and then the bentonite in the GCL gets hydrated quickly through radial flow from the fibres and vertical flow from the geotextiles. Hence the virgin GCL specimens used above the wet sand subgrade in the 14

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Fig. 19. Results of image analysis of untested NGCL (a) Photograph (b) Grayscale image (c) Binary image.

Further increase in water content from 6 to 12% caused a peak shear stress reduction of 6 kPa in case of NGCL-MS and 11 kPa in NGCL-RS. These results indicate that the hydration effects were quicker in NGCL-MS initially but then slowed down to reach the stable value at 12% water content. At any specific water content. hydrated NGCL-MS surfaces showed higher peak shear strength compared to NGCL-RS surfaces because of the higher friction generated at the interface due to their particle shape. With the increase in water content, extrusion of bentonite causes reduction in friction. This reduction is more pro­ nounced in NGCL-MS up to 6% water content and then stabilize at 12% water content. In case of NGCL-RS surface, the friction developed at the interface is relatively less and hence the reduction is almost linear from 0 to 12% water content. To understand the mechanisms for the difference in hydration effects in MS and RS more clearly, image analysis of GCL surfaces before and after the shear tests was carried out.

quantify the entrapment of particles, which varied substantially upon hydration. The tested and untested surfaces of GCL specimens were analyzed to understand the surface changes in terms of the area occu­ pied by the entrapped particles. The images of each tested specimen of the GCL after shear tests with moisture content of 0%, 12% and 25% in sand were captured under constant source of illumination with Sony HDR XR550 camera and analyzed in MATLAB 2021a. Fig. 19 and Fig. 20 show the results of image analyses on untested and tested NGCL surfaces, respectively. Fig. 19a shows the photograph of the NGCL surface before the test and Fig. 19b shows the grey scale image of the same. Fig. 19c shows the binary image of the untested GCL after the grey scale image is analyzed in MATLAB using thresholding technique. Thresholding is employed to segment an image based on the intensity values. The pixels with intensity value above a threshold are set to the foreground value and all remaining pixels are set to the back­ ground values. The pixels within the identified range are set to 1 (white) and remaining were set to 0 (black). Hence the threshold segmentation converts the grayscale image to a binary image. The threshold is set to identify voids in nonwoven surface because of the random orientation of fibres. In this image, the white spots indicate voids between the fibres.

4.2. Surface changes of GCL specimens The sheared surfaces of GCL were analyzed to understand and 15

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Fig. 20. Results of image analysis of NGCL-RS after the test (a) Photograph (b) Grayscale image (c) Binary image.

Fig. 20a shows the photograph of the NGCL-RS after a typical interface shear test. Fig. 20b shows the greyscale image of the same. Fig. 20c is the binary image of tested NGCL-RS generated from the greyscale image, using the thresholding technique explained earlier. Fig. 21a shows the photo a of NGL-MS interface after a typical shear test. Fig. 21b shows the greyscale image of the same. Fig. 21c is the binary image of tested NGCL generated from the greyscale image. The purpose of the analysis is to quantify the change to the surface of GCL specimens in terms of entrapped sand particles in the voids and entangled in fibres. The difference in grayscale intensity range of sand particles and fibres forms the base of the analysis. To generate Figs. 20c and 21c, the grayscale intensity range of sand particles is read through the data tips function from the toolbox of MATLAB 2021a. This value is used for thresholding the image and converting it to a binary image. Thresh­ olding is done at different values for RS and MS particles, depending on the grayscale intensity of sand particles. The white spots in Figs. 20c and 21c represent the entrapped sand particles. The density of white spots in Fig. 20c is much higher than in Fig. 21c, indicating higher entrapped sand particles in GCL interfaced with RS. There could be some voids, which are also covered in the white spots. However, their area of

coverage is very insignificant and hence neglected in the computations. Similarly, binary images are generated for specimens of NGCL tested along with RS and MS in interface shear tests at different moisture conditions of sand. From these images, the percentage area of coverage of sand particles within the sheared area of GCL surface is computed in MATLAB 2021a using the region properties function. Such computations are carried out for tests with different moisture contents and the results are presented in Table 10. Results showed that when tested in dry conditions (0% moisture content of base material), the entrapment of particles is comparable for both NGCL-RS and NGCL-MS interfaces. Thus, the higher shear strength exhibited by the MS in the dry condition can be corelated to the fric­ tional resistance developed by the better interlocking of MS particles with the fibres, due to their shape. The increase in moisture content was ensued by the hydration of NGCL. In the untested NGCL, bentonite is completely dry. After the interface shear test, water content analysis is carried out on bentonite taken from the tested NGCL by oven-drying. Water contents of bentonite were found to be 14.04% and 22.6% for NGCL specimens interfaced with MS and RS, respectively, proving that wet RS hydrates the NGCL easily than wet MS. The findings are 16

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Fig. 21. Results of image analysis of NGCL-MS after the test (a) Photograph (b) Grayscale image.

swelling of bentonite upon hydration and extrusion of bentonite through the pores of geotextile, forming a slimy layer at the interface. The values are also indicative of the increase in hydration of the specimen when interfaced with river sand. Conclusively, the reduction in interface shear strength in case of NGCL-RS interfaces can be explained by the pull-out of reinforcing fibres under the swelling pressure of bentonite, which is significantly higher for NGCL-RS interfaces. The benefits of replacing natural sand with manufactured sand are twofold. By virtue of their angularity, roughness and surface irregularity, MS particles provide higher interface shear resistance in dry conditions. Under wet condi­ tions, the hydration of bentonite in GCL is comparatively less when interfacing with MS because of the relatively less permeability of MS subgrade, which reduces sucking of water into the GCL. With less hy­ dration, the swelling pressures of bentonite also reduce when MS is used and hence the entrapment of particles onto the GCL surface also is less and hence the shearing resistance under wet conditions also is higher with manufactured sand. Fig. 22 shows the variation in the interface adhesion and friction values for different interfaces with change in water content, change in

Table 10 Surface changes in GCL after sand particle entrapment. Moisture Content (%)

RS Coverage area (%)

MS Coverage area (%)

0 12 25

3.44 33.13 35.55

2.29 14.3 20.8

complemented by the results of constant head permeability tests on target gradation of RS and MS particles compacted to the same density as the specimens used in the interface shear tests. Permeability of RS was found to be 2.24 × 10− 2 cm/s, which is higher than the permeability of MS, which was measured to be 1.45 × 10− 2 cm/s. Because of its comparatively low permeability, MS allowed lesser water to enter the NGCL for hydration and hence the shear resistance offered by NGCL-MS specimens was found to be always higher. The values listed in Table 10 highlight a significant increase in the entrapment of particles on the surface of NGCL during the interface shear test of NGCL-RS as compared to NGCL-MS. This was due to the 17

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Fig. 22. Correlation between the particle shape parameters and GCL-sand interface shear strength.

the type of geotextile and change in the type of sand. The shape pa­ rameters shown in the plot are the average shape parameters for the sample gradation, computed from the parameters for different sizes listed in Table 6. As illustrated in Fig. 22, RS particles, which have higher roundness, higher sphericity, lower elongation, and lower roughness, showed lower interface shear strength parameters. MS par­ ticles, which have relatively lower roundness, lower sphericity, higher elongation, and higher roughness, showed higher interface shear strength parameters. Increase in water content decreased the shear strength parameters, the effect being more significant in RS interfaces because of quick hydration. Further, the shape parameters of the man­ ufactured sand particles can be conveniently fixed during quarrying, to obtain much better interface friction. Hence manufactured sand can be effectively used to replace natural sand in landfill designs, to solve the issues of non-availability of natural sand for constructions. Such replacement provides further stability benefits to the landfill slopes, as observed from the present study.

(RS) and a manufactured sand (MS) of identical gradation and system­ atic image analyses to quantify the shape parameters of particles and surface changes to GCL specimens subjected to shear tests, the following major conclusion are drawn. (1) Shear resistance offered by the nonwoven side of GCL (NGCL) is significantly higher compared to the shear resistance offered by the woven side of the GCL (WGCL) in sand-GCL interface shear tests. Nonwoven geotextile has randomly oriented polymeric fi­ bres, which provide better interlocking, leading to better shear resistance. (2) Interface shear tests performed on river sand (RS) and manufac­ tured sand (MS) with NGCL showed a higher peak shear stress for NGCL-MS interface as compared to NGCL-RS interface, the dif­ ference in their performance being significant at higher displacement values. Better performance of manufactured sand is attributed to the favorable shape characteristics of its grains. (3) Shape characterization of sand particles using microscopic image analysis showed that the MS particles are more elongated and have higher surface irregularities compared to RS particles that

5. Conclusions From the series of interface direct shear tests on a natural river sand 18

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A.G. Pillai and M.L. Gali

(4)

(5) (6)

(7)

undergo natural weathering and erosion, resulting in higher frictional resistance. Water in sand layer causes hydration of bentonite in GCL, fol­ lowed by its swelling. Increase in the swelling pressure due to increase in the water content results in damage to GCL specimens by pullout of reinforcing fibres, thereby reducing the interface shear strength. Further, swollen bentonite extrudes onto the interface and forms a lubricating slimy surface, reducing the friction at the interface. Effects of hydration are more when GCL specimens are in contact with RS, due to the high permeability of RS. Image analysis of NGCL specimens before and after the tests revealed that the sand particle entrapment in nonwoven geo­ textile is comparable for both RS and MS in dry tests, indicating that the better performance of MS is due to the favorable shape of its grains. In wet tests, the particle entrapment is significantly more in case of RS compared to MS, confirming that the hydra­ tion effects are more pronounced in case of RS. Better perfor­ mance of MS in wet tests is due to favorable shape of its grains and less hydration and swelling of bentonite, resulting in less reduction in friction at the interface. Manufactured sand provided higher interface shear strength and lesser damage to GCL in terms of pullout of reinforcing fibers caused by bentonite swelling, due to less hydration by suction. Hence the natural sand interfacing with GCL in liners and capping components of landfills can be replaced with manufactured sand, with added benefits.

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