Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade

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Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade☆ R. Kerry Rowea,∗, J.D.D. Garciaa, R.W.I. Brachmana, M.S. Hosneya,b a b

GeoEngineering Centre at Queen's-RMC, Queen's University, Ellis Hall, Kingston, ON, K7L 3N6, Canada Faculty of Engineering, Cairo University, Cairo, Egypt

ARTICLE INFO

ABSTRACT

Keywords: Geosynthetics Geosynthetic clay liner GCL Isothermal hydration Hydraulic conductivity Silty sand Water content Landfill Barrier

The effects of the silt aggregation, compaction density, and water content of the subgrade on the hydration of five different geosynthetic clay liner (GCL) products is reported based on a series of laboratory column experiments conducted over a six-year period. GCLs meeting typical specifications in terms of minimum hydraulic conductivity and swell index are hydrated to equilibrium from the same subgrade soil with sufficient cations to cause cation exchange during hydration. It is then shown that the GCL bentonite granularity and GCL structure can have a significant (~four orders of magnitude) effect on hydraulic conductivity under the same test conditions (from 8 × 10−12 m/s for one GCL to 6 × 10−8 m/s for another GCL product). The effect of subgrade water content on the hydraulic performance of GCLs are not self-evident and quite dependent on the bentonite granularity, GCL structure, and permeant. Varying the subgrade water content from 5 to 16% and allowing the GCL to hydrate to equilibrium before permeation led to up to 5-fold difference in hydraulic conductivity when permeated with tap water and up to 60-fold difference when the same product is permeated with synthetic municipal solid waste leachate. When permeated with synthetic leachate, increasing stress from 70 kPa to 150 kPa led to a slight (average 37%; maximum 2.7-fold) decrease in hydraulic conductivity due to a decrease in bulk void ratio. It is shown that hydraulic conductivity is lower for GCLs with a scrim-reinforced geotextile, and/ or with finer bentonite. It is shown that selecting a GCL based on the initial hydraulic conductivity and swell index in a manufacturers product sheet provides no assurance of good performance in field applications and it is recommended that designers pay more attention to selection of a GCL and preparation of the subgrade for important projects.

1. Introduction GCLs may be used alone in covers and as part of a barrier system in bottom liners for municipal solid waste (MSW) landfills. In covers/caps, it rests on a prepared subgrade with hydraulic conductivity, kfdn and is covered by a minimum of 0.3 m, but often 0.6–1.5 m, of cover soil to provide some stress and protection from wet-dry and/or freeze-thaw cycles. In a bottom liner, the GCL will usually be covered by a geomembrane and, in single composite liners, underlain by a prepared subgrade with hydraulic conductivity, kfdn, and water content wfdn. The subgrade is not considered part of the barrier system until, ks < 1 × 10−7 m/s and then it is only considered an attenuation layer (MOECC, 2012) until ks ≤ 1 × 10−9 m/s at which point it is considered a liner. As delivered, GCLs are initially at ~10% water content or, for

one manufacturer, ~25–30% water content. When placed on a typical granular subgrade (ks > 1 × 10−7 m/s) at a site, the subgrade generally acts as a primary source of moisture for hydration of the GCL. The suction gradient between the GCL and subgrade draws moisture from the subgrade to the GCL until suction equilibrium is reached. As the GCL takes up more moisture, the bentonite undergoes swelling. The amount of swelling and the hydraulic conductivity, k, will depend on a number of factors to be discussed later but, in particular, on the chemistry of the water with which they hydrate. In reality, the initial hydration from a subgrade is usually with pore water having some cations, but at a concentration well below that in a landfill leachate. With an unlimited amount of water readily available, the GCL will fully hydrate to a maximum gravimetric water content, wref, at a specific applied stress. When the GCL is hydrating from the subgrade pore

Dr. N. Touze acted as Editor for the review of this paper Corresponding author. E-mail addresses: [email protected] (R.K. Rowe), [email protected] (J.D.D. Garcia), [email protected] (R.W.I. Brachman), [email protected] (M.S. Hosney). ☆ *

https://doi.org/10.1016/j.geotexmem.2019.103486 Received 19 January 2019; Received in revised form 5 June 2019; Accepted 29 July 2019 0266-1144/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: R. Kerry Rowe, et al., Geotextiles and Geomembranes, https://doi.org/10.1016/j.geotexmem.2019.103486

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water, the GCL usually only hydrates to a gravimetric water content, w < wref, with an the apparent degree of saturation, w/wref, commonly in the range 35 < w/wref < 70% (Rayhani et al., 2011; Beddoe et al., 2011; Anderson et al., 2012; Sarabian and Rayhani, 2013; Hosney et al., 2016; Rowe, 2018). This raises the questions as to how the level hydration from the underlying subgrade, w and Sr, will affect k of the GCL when permeated with tap water or simulated MSW leachate. In this paper a GCL that has taken up water from a source other than the permeant before permeation is said to be “prehydrated”. It's water content prior to exposure to leachate will depend on the subgrade conditions (e.g., initial water content wfdn, grain size distribution, and smectite content), GCL properties, water retention curve, effective stress and time (e.g., Lu et al., 2017; Acikel et al., 2018a,b; Rowe, 2018; Gates et al., 2018; Ghavam-Nasiri et al. 2019a, 2019b). There has been considerable study of the effect of the aforementioned parameters on GCL hydration and a broad review of the literature is provided by Rowe (2018). The work most relevant to this paper is discussed in the following background section and the objectives of this paper were developed in that context. Despite the previous valuable work, there is still a paucity of data providing insight regarding how the prehydration factors and the chemical interaction associated with prehydration with pore fluid containing divalent cations affects the hydraulic performance of GCLs when permeated. There is a need for performance data to asses when greater GCL w is better. Thus, this paper extends the laboratory work of Rayhani et al. (2011) and Hosney et al. (2016) by examining the effect of previously unexplored factors on the performance of the GCLs prehydrated from “silty sand” extracted from the Queen's University Experimental Liner Test Site located in Godfrey, Ontario, Canada (QUELTS; Brachman et al., 2007). The objectives of this paper are twofold: First, to study, under isothermal conditions, the effect of subgrade preparation on the moisture uptake of the GCLs studied at QUELTS by varying the following compaction parameters: (1) the initial or dry subgrade grain size distribution (GSD) prior to wetting of the subgrade, (2) the number of compaction lifts (subgrade density), and (3) the conditioning time between compaction and placement of the GCL. Second, to assess the effect of prehydration on the performancebased parameters for five GCL products isothermally prehydrated under various conditions for up to six years as reported by Rayhani et al. (2011) and Hosney et al. (2016). The research presented herein will pay particular attention to the effect of prehydration on the exchangeable cations and swell index (SI) of the bentonite, and the hydraulic conductivity of the GCL for different initial (i.e., just prior to GCL prehydration): (1) subgrade water contents, (2) initial bentonite granularity, and (3) GCL structure. In addition, the effect of the hydraulic gradient, the effective stress, and the permeating liquid (i.e., tap water or synthetic municipal solid waste leachate) on the measured hydraulic conductivity of GCLs prehydrated from a given subgrade will be examined.

obtained at wfdn = 21% (field capacity). Hosney et al. (2016) reported a similar effect of wfdn for GCL6 with powdered bentonite tested under the same conditions. 2.2. Subgrade grain size distribution Several soil types including poorly graded sand (SP), silty sand (SM), clayey sand (SC), low-plasticity clay (CL) and high-plasticity clay (CH) have been used by multiple researchers to assess the effect of subgrade grain size distribution (GSD) and smectite content on GCL moisture uptake (Daniel et al., 1993; Eberle and von Maubege, 1998; Rayhani et al., 2011; Anderson et al. 2011; Sarabian and Rayhani, 2013; Bouazza et al., 2017a). Generally, a greater percentage of smaller-sized particles in the subgrade and a greater smectite content led to lower GCL water contents. However, none of these studies addressed the situation where the macrostructure of a soil changes depending on the water content and amount of crushing of aggregates during compaction. Aggregates may form as the soil dries due to an increase in suctions and, upon re-wetting and/or compaction of the soil, these aggregates may or may not break up. Thus, a dual porosity subgrade material is formed consisting of inter-aggregate and intra-aggregate pores. For example, at QUELTS, the subgrade GSD contained 32%–46% non-plastic fines by mass (passing 0.075 mm sieve) based on dry sieve analysis and has been classified as silty sand (Brachman et al., 2007; Rayhani et al., 2011; Hosney et al., 2016). However, Rowe et al. (2016a) reported far more variability with 33%–80% fines for soil samples taken across the site after wet sieve analyses during which the soil aggregates were broken down by washing. Thus, based on wet sieve analysis, the soil classification can vary between silty sand, sandy silt, or silt with sand and the implications for prehydration need to be assessed. The closest related study (Arvelo, 2004) obtained the compaction curves of four SM samples with a fines content of 15%, 20%, 25% and 30% by dry sieving and found that the maximum dry density increased with increasing dry fines content from 1983 kg/m3 to 2106 kg/ m3; however, Arvelo (2004) research did not address the effects of fines content and subgrade density on GCL prehydration. 2.3. GCL structure The structure of the GCL (i.e., type of bentonite, geosynthetic components, level of needle-punching, and absence or presence of thermal treatment fusing the needle-punched fibers to the carrier geotextile) have been shown to affect prehydration (Rowe, 2018). The maximum gravimetric water content, wref, achieved during prehydration varies depending on the structure of the GCL and is largely influenced by the level of confinement provided by the needle-punched fibres and the chemistry (e.g., ionic strength) of the hydrating liquid. Reported wref values for GCLs under 1–2 kPa confining stress with an unlimited supply of deionized water have ranged from 115% to 222% for the different GCLs tested and wref is expected to be lower at a higher stress or when hydrated with more aggressive liquid (Rayhani et al., 2011; Beddoe et al., 2011; Anderson et al., 2012; Sarabian and Rayhani, 2013; Hosney et al., 2016). GCLs with better anchorage of needle-punched fibres (e.g., by scrimreinforcement and thermal treatment) restrict bentonite swelling during prehydration and resulted in a lower wref (i.e., they were essentially fully hydrated at a lower bulk void ratio due to the fibres resisting the swelling pressure of the bentonite). It is known that, due to the smaller bulk void ratio, a lower wref can give a lower k (Lake and Rowe, 2000a; Lake and Rowe, 2000b; Petrov and Rowe, 1997). However, there is a paucity of k data comparing various GCLs hydrated with subgrade water and permeated with leachate. GCLs hydrated in the field from a subgrade are never fully saturated in the classical soil mechanics sense (a large backpressure, absent in the field, is needed to get full saturation). However, for a given stress and hydrating fluid, a GCL will not hydrate above wref and hence this will be

2. Background 2.1. Subgrade water content Previous researchers (Rayhani et al., 2011; Anderson et al., 2012; Hosney et al., 2016) reported that, other things being equal, a higher subgrade water content (wfdn) led to greater moisture uptake by the GCL. Rayhani et al. (2011) examined the prehydration of three GCL products with coarse or fine granular bentonite resting on the same silty sand subgrade exhumed from QUELTS. All these GCLs attained a higher equilibrium w with increasing wfdn and approached saturation to the maximum extent practicable on this subgrade when the wfdn was at field capacity (wfdn ~21%). The minimum GCL w of 30%–35% (essentially all crystalline water) was obtained when the subgrade at wfdn = 5% (i.e., near residual water content). The maximum w of 116%–141% was 2

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regarded as fully hydrated (“saturated”) and be considered to correspond to (an apparent) degree of saturation (Sr) ≈ w/wref = 1.

of the effect of prehydration from subgrade on hydraulic conductivity. With cations in the pore water of the subsoil from which a GCL hydrates, cation exchange can be expected with sodium in the bentonite being replaced by divalent cations (Guyonnet et al., 2005; Bradshaw et al., 2013). Bradshaw et al. (2013) showed that cation exchange during prehydration from sand, silt and clay subgrades prepared at 1% wet of optimum water content had a minimal effect on hydraulic conductivity of GCL permeated with deionized water in the absence of desiccation. However, when permeated with a simulated municipal solid waste leachate, the hydraulic conductivity increased by up to 5.6fold (Bradshaw and Benson, 2013). Many investigators have studied the hydraulic conductivity of GCLs (Petrov et al., 1997; Petrov and Rowe, 1997; Rowe, 1998; Ruhl and Daniel, 1997; Shackelford et al., 2000; Bouazza et al., 2017; Chen et al., 2018; Ghazizadeh et al., 2018; Jo et al., 2004, 2005; Kul and Ören, 2018; Lee et al., 2005; Lee and Shackelford, 2005; Scalia IV and Benson, 2010; Setz et al., 2017; Vasko et al., 2001Guyonnet et al., 2009) and they have generally found that higher permeant concentrations and solutions containing more divalent cations (Ca2+) than monovalent cations (Na+ and K+) lead to higher hydraulic conductivity.

2.4. GCL bentonite granularity Katsumi et al. (2008) and Hosney et al. (2016) both studied the effect of bentonite granularity during prehydration of GCLs on various subgrades. Katsumi et al. (2008) studied the prehydration of GCLs resting on a sand and decomposed granite subgrade. Heterogeneous prehydration of the granular bentonite was reported to have led to a wide range in the measured hydraulic conductivity values (2.9 × 10−11 m/s ≤ k ≤ 1.5 × 10−8 m/s) when permeated with 0.1–0.5 M CaCl2 solutions under less than 30 kPa effective stress. In contrast, most GCLs tested with powdered bentonite achieved 1.0 × 10−12 m/s ≤ k ≤ 1.0 × 10−11 m/s under otherwise similar conditions. Hosney et al. (2016) reported results for two GCLs, with a woven carrier geotextile, exhumed from QUELTS after having been in contact with a subgrade of silty sand subgrade compacted at a water content of 16% for 40 weeks. One GCL (denoted as GCL6) containing powdered bentonite had the highest steady-state w ≈ 125% while the other (GCL1) with fine-granular bentonite reached w ≈ 100%, although both GCLs reached a similar degree of prehydration (i.e., Sr ≈ w/ wref ≈ 0.6–0.7). However, performance-based parameters such as hydraulic conductivity, swell index and cation exchange capacity of those prehydrated specimens were not discussed.

3. Materials 3.1. Foundation soil (subgrade) Silty sand (maximum dry density, ρmax, = 1865 kg/m3; optimum water content, wopt, = 11.4%; Rayhani et al., 2011) exhumed from the QUELTS was used as subgrade material. The pore water of the silty sand subgrade had 230 ± 24 mg/L Ca2+, 35 ± 4 mg/L Mg2+, 31 ± 16 mg/L Na+, and 7 ± 2 mg/L K+.

2.5. Hydraulic performance Full hydration (Sr ≈ 1) with clean water prior to permeation has been shown to improve hydraulic performance relative to GCLs hydrated with permeant (Daniel et al., 1993; Petrov and Rowe, 1997; Jo et al., 2004; Rowe, 2005, 2014). However, while it is tempting to generalize this to imply that a higher GCL water content, w, prior to permeation will lead to better hydraulic performance, there is a lack of data to corroborate this generalization. Most hydration studies focused on GCL water content (e.g., Rayhani et al., 2011; Anderson et al., 2012; Hosney et al., 2016) and very few (notably Katsumi et al., 2008 and Bradshaw and Benson, 2013) have performed a controlled examination

3.2. Geosynthetic clay liners Five sodium-bentonite GCLs (from three manufacturers) with virgin swell index values of 24 mL/2 g to 32 mL/2 g were examined (Table 1). These GCLs had all been used in previous studies of GCL shrinkage (Take et al., 2015a; Brachman et al., 2015, 2018; Rowe et al., 2018) and downslope erosion (Brachman et al., 2015, Take et al., 2015b, Rowe et al.,

Table 1 Virgin properties of needle-punched GCLs used in the study, following the nomenclature of GCLs tested in the Queen's Environmental Liner Test Site (QUELTS). W = woven; NW = nonwoven; NWSR = nonwoven scrim-reinforced. Component

Properties

Symbol

Units

GCL1

GCL2

GCL3

GCL4

GCL6

Bentonite

Granularity Grainsize distributiona

– D10 D30 D60 D90 MA SI CEC Na Ca – – MA – MA – – – wref kTW

– mm mm mm mm g/m2 ml/2 g cmol/kg % % % – g/m2 – g/m2 – mm % % m/s

Fine 0.1 0.28 0.35 0.65 4960 ( ± 400) 27 75 61 28 96 W 120 NW 230 Yes 6.3 5 151 ± 4 2.8 × 10−11

Fine 0.07 0.25 0.40 0.60 4510 ( ± 100) 28 78 67 24 96 NWSR 260 NW 230 Yes 6.5 6 119 ± 3 3.8 × 10−11

Coarse 0.4 0.65 1.10 1.70 5280 ( ± 100) 24 94 76 17 84 W 135 NW 280 No 7.2 18 212 ± 8 –

Coarse 0.4 0.65 1.10 1.70 4520 ( ± 400) 24 94 76 16 77 NW 210 NW 270 No 7.5 23 195 ± 6 3.3 × 10−11

Powder – – 0.075 0.125 4470 ( ± 550) 32 105 63 30 93 W 110 NW 220 Yes 6.8 9 222 ± 3 –

Carrier Geotextile Cover Geotextile GCL

Dry mass per unit aread Swell Index Cation Exchange Capacity Exchange Complex-sodium Exchange Complex-calcium Smectite Content Type Mass per unit area Type Mass per unit area Thermal treatment Off-roll thickness Off-roll water content Reference water contentb Hydraulic conductivityc

a

Tests for GCL1 and GCL4 were performed by M. Rayhani, Queen's University. GCL3 was not tested, inferred to be approximately equal to GCL4 since it had the same bentonite. b Tested at 2 kPa in DI water for 1 month. c Tested at 11–15 kPa effective stress and permeated with tap water. d Statistics (mean ± standard deviation) for samples used for this study. 3

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2016a, b; Mukunoki et al., 2019) at the QUELTS. Following the nomenclature used in the QUELTS papers cited above, these five GCLs are denoted as GCL1, GCL2, GCL3, GCL4 and GCL6. GCL5 used at QUELTS was not considered in this study but, to aid readers of the other cited papers, the numbering has been retained since the findings for this paper aid in the interpretation of the GCL behavior reported in those papers. The GCLs examined herein all had a needle-punched nonwoven cover geotextile but different carrier geotextiles and bentonite granularity. GCL1 had a slit-film woven and GCL2 had a scrim-reinforced nonwoven carrier geotextile and both contained fine granular bentonite (D60 = 0.35–0.40 mm). GCL3 had a slit-film woven and GCL4 had a nonwoven carrier geotextile and both contained coarse granular bentonite (D60 = 1.1 mm). GCL6 had a slit-film woven carrier geotextile and powdered bentonite (D60 = 0.075 mm). GCL1, GCL2 and GCL6 were thermally treated to bond the needle-punched fibres to the carrier geotextile. GCL3 and GCL4 were not thermally treated and relied totally on physical entanglement of the needle-punched fibres to provide anchorage. The mass per unit area bentonite, MA, in the samples tested is given in Table 1. They were generally very similar with an average of 4670 g/ m2. GCL6 had the lowest average (4470 ± 550 g/m2) but the highest standard deviation. GCL6, GCL2 (4510 ± 100 g/m2) and GCL4 (4520 ± 400 g/m2) had the essentially the same average (to within 1%) and were very similar to GCL6. GCL1 (4960 ± 400 g/m2) about 10% higher than the others. There was no statistically significant difference between MA of these four GCLs. MA for GCL3 (5280 ± 100 g/ m2) was the highest and the difference in averages from the others was statistically significant. The difference in MA between GCL3 and the others likely had some effect on the prehydration of the GCL that could not be isolated in this study, however very limited testing was conducted on GCL3 and the focus of the study was on the other four GCLs.

prepared for each test configuration to check the reproducibility of test results. The duplicate cells had the same height (i.e., 510 mm) but a smaller internal and external diameter (of 97 mm and 120 mm, respectively). 4. Experimental method 4.1. Setup of prehydration cells All the isothermal prehydration tests were conducted at room temperature and, unless otherwise noted, followed the procedure of Rayhani et al. (2011) and Hosney et al. (2016) as summarized below. First, the silty sand subgrade was oven-dried at 105 °C overnight, cooled and then mixed with tap water (characterized by pH = 7.4, Ca2+ = 39 mg/L, Mg2+ = 10 mg/L, Na+ = 19 mg/L, K+ = 2 mg/L, Zn2+ = 0.44 mg/L, S2− = 12.3 mg/L, Sr2+ = 0.2 mg/L, CO32− = 60 mg/L, Cl− = 31 mg/L, total dissolved solids = 173 mg/L; ionic strength = 6.38 mM, RMD = 0.02 M0.5) to a target water content (wfdn) and then compacted in a PVC cell to a total thickness of 450 mm (ρd = 1768 kg/m3; 96% of standard Proctor maximum). Following compaction and prior to placement of the GCL, the PVC pipe was sealed and the subgrade was left to condition (for one day, unless otherwise noted) to allow for moisture equilibration to be consistent with what was done in previous experiments (Rayhani et al., 2011; Hosney et al., 2016). Virgin GCL samples were cut and placed on the subgrade and covered with a 1.5 mm-thick intact HDPE geomembrane (GMB) to simulate a typical composite liner when the GCL is in contact with the GMB (not below a wrinkle); and finally a steel weight was placed above the GMB to applying a 2 kPa vertical stress to improve the contact between the GCL and subgrade without significantly affecting the GCL w (Rayhani et al., 2011). All PVC cells were then sealed and stored at room temperature for at least four weeks to allow the GCL sufficient time to reach a steady-state water content as moisture migrated upwards from the subgrade. The cells were opened periodically to measure the mass and thickness of the GCL.

3.3. Prehydration cell The prehydration cells used were the same as used by Rayhani et al. (2011), Rowe et al. (2011), and Hosney et al. (2016). The cells were 510 mm high polyvinyl chloride (PVC) columns with an internal diameter of 145 mm and an external diameter of 167 mm. Duplicate pipes were

4.2. Effect of subgrade preparation on GCL prehydration Various compaction parameters were investigated for the case of

Table 2 GCL6 hydration test configurations with varying subgrade properties (initial grain size distribution, number of compaction lifts and conditioning time) and the resulting subgrade dry density and GCL water content at 4 weeks. Subgrade is silty sand at 16% water content. Parameter Investigated

Drying Method GSD (low density) GSD (high density) GSD (same energy) Number of lifts Increasing density for GSD1 Increasing density for GSD2 Conditioning Time a b c d

Test ID

T1 T2 T2 T3 T4 T5 T7 T6 T4 T7 T3 T6 T4 T1 T2 T7 T4 T1 T8

Initial Grain Size Distributiona

Wet Fines (%)

# Lifts

Compaction Energy (blows/lift)b

Conditioningc (days)

GSD2 GSD2d GSD2d GSD1 GSD2 GSD1 GSD2 GSD1 GSD2 GSD2 GSD1 GSD1 GSD1 GSD2 GSD2d GSD2 GSD2 GSD2 GSD2

45 59 59 60 56 56 50 54 56 50 60 54 56 45 59 50 56 45 60

3 3 3 3 5 5 3 3 5 3 3 3 5 3 3 3 5 3 3

20–25 20–25 20–25 20–25 20–25 10–15 20–25 20–25 20–25 20–25 20–25 20–25 10–15 20–25 20–25 20–25 20–25 20–25 20–25

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 20

After oven-drying and prior to re-wetting and compaction. GSD1 = 30–45% dry fines; GSD2 = 5–20% dry fines. Standard Proctor hammer. Post-Compaction (before GCL placement). Airdried to w = 2.5% prior to re-wetting and compaction. 4

Dry density, ρd (kg/m3)

GCL6 at 4 weeks w (%)

w/wref (−)

1650 1650 1650 1650 1764 1768 1727 1688 1764 1727 1650 1688 1768 1650 1650 1727 1764 1650 1650

96 105 105 87 110 134 91 91 110 91 87 91 134 96 105 91 110 96 72

0.43 0.47 0.47 0.39 0.50 0.60 0.41 0.41 0.50 0.41 0.39 0.41 0.60 0.43 0.47 0.41 0.50 0.43 0.32

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GCL6 that was left in contact with the subgrade at a water content of wfdn = 16% (field average) for 4 weeks (time to approach suction equilibrium). The initial subgrade macrostructure (prior to wetting and compaction), number of compaction lifts, and post-compaction conditioning time were varied individually as shown in Table 2 and the change in GCL water content with time was recorded.

(typically 20–25 blows/lift for three lifts or 10–15 bows/lift for five lifts to reach a total of 450 mm thickness), however in both cases it is expected that there is a local variation in density such that the upper portion of a layer will experience more overall compaction and become denser than the bottom of the layer.

4.2.1. Oven-drying The exhumed soil was dried in the oven to a constant mass (0% water content) prior to re-wetting and compaction. To check that oven drying did not alter the subgrade soil properties and was representative of actual field conditions after the soil had undergone significant drying, tests were performed for two preparation conditions. In one, GCL6 was placed on a subgrade that had been oven-dried (T1; Table 2) while in the other the subgrade had been air-dried to a water content of 2.5% (T2; Table 2).

4.2.4. Conditioning time Typically, the compacted subgrade was left to condition for 1 day prior to placement of the GCL. The conditioning time was then varied to 20 days to examine its effect on prehydration. 4.3. Effect of GCL product and prehydration on GCL performance Based on field data from QUELTS (Brachman et al., 2007), three initial subgrade water contents, wfdn, were selected: 5% (residual), 10% (just below the 11.4% optimum), and 16% (field average water content when QUELTS was constructed). To examine the effect of wfdn on chemical interaction and GCL hydraulic conductivity after prehydration (Tables 3 and 4), all five GCLs were placed on the subgrade left for between 87 and 328 weeks. On exhumation, the effect of wfdn on GCL bentonite swell index, cation exchange capacity, and hydraulic conductivity, k, were investigated. Bentonite specimens were extracted from the GCL samples at the end of prehydration (i.e., before any hydraulic conductivity test) and upon termination of the hydraulic conductivity test. Subsequently, the swell index was measured as per ASTM D5890 except that the bentonite was air dried instead of oven-dried to avoid changes to the bentonite mineralogy. The cation exchange capacity and exchangeable cations were also measured (ASTM D7503). The Falling-Head Rising-Tailwater Method (ASTM D5084) was used to calculate the hydraulic conductivity of 70 mm-diameter specimens taken from GCL samples prehydrated to the water content consistent with suction equilibrium on the particular subgrade being considered. Backpressure saturation was omitted to preserve the GCL's prehydrated condition and avoid altering the pore structure and pore water chemistry prior to permeation and hence to best represent likely field conditions. After reaching hydraulic and chemical equilibrium (i.e., when the effluent and influent had the same electrical conductivity and there was no further change in hydraulic conductivity), blue dye was added to the permeant to examine for any preferential flow paths in the GCL. The hydraulic performance of GCLs can depend on testing conditions such as the effective stress, hydraulic gradient, and permeating fluid. The conditions tested examined are described below.

4.2.2. Subgrade macrostructure The prehydration from two initial silty sand macrostructures (prior to wetting and mixing) were examined. Soil exhumed from previous prehydration cells was oven-dried until gravel-sized aggregates formed (Fig. 1a). Two macrostructures were then obtained by crushing the soil to the desired aggregation, herein referred to as aggregated grain size distribution (GSD). One macrostructure had minimal aggregates (GSD1 in Fig. 1b; T3 in Table 2) and the other had sand-sized aggregates (GSD2 in Fig. 1c; T1 in Table 2). These two were then compared with the soil from the same site tested by Rayhani et al. (2011; Fig. 2). GSD1 represented the reasonably typical condition with minimal clods as observed in the field during construction of the embankment at QUELTS (i.e., where soil was excavated, moved, spread, and quickly compacted to form the embankment without any significant change in water content or grain size distribution before it was compacted). Dry sieve analyses gave 30%–45% fines passing 0.075 mm sieve although it had about 15% less medium sand and 10% less fine sand than the soil tested by Rayhani et al. (2011; Fig. 2). The difference between GSD1 and the Rayhani et al. (2011) soil represent typical field variability at this site. GSD2 represented a non-typical case at the QUELTS site, but a feasible situation if this same silty sand was excavated, moved, spread, and allowed to dry before rewetting and compaction. In this case, the drying led to a change in macrostructure with the aggregation of silt particles into sand size agglomerates before it was brought to a desired water content and compacted. Dry sieve analyses gave 8%–20% fines. Wet sieve analyses were also performed according to ASTM D1140 to obtain the fines content when the aggregates were broken down completely. With wet sieving, there were 45%–60% fines regardless of the initial GSD. In reality, the actual GSD during prehydration would be achieved after compaction and would have a fines content between the dry sieve fines (lower bound) and wet sieve fines (upper bound) depending on compactive effort and water content.

4.3.1. Effect of GCL prehydration on k permeated by low ionic strength water (TW): cover tests Cover applications were also considered where the GCL had prehydrated from the silty sand subgrade after it was covered by about 1 m of cover soil and heavy rainfall had permeated through the GCL at a low head (Table 3). In these “cover tests”, k was assessed for the initially prehydrated GCLs at a vertical effective stress σv’ = 15 kPa when permeated with low ionic strength (tap) water (pH = 7.4, Ca2+ = 39 mg/L, Mg2+ = 10 mg/L, Na+ = 19 mg/L, K+ = 2 mg/L, Zn2+ = 0.44 mg/L, S2− = 12.3 mg/L, Sr2+ = 0.2 mg/L, CO32− = 60 mg/L, Cl− = 31 mg/L, total dissolved solids = 173 mg/L; ionic strength = 6.38 mM, RMD = 0.02 M0.5).

4.2.3. Compaction lifts (layer thickness) A study was conducted to assess the effect of the number of lifts used during compaction. After the desired soil macrostructure was obtained, the subgrade was compacted in the prehydration cell in either three 150 mm-thick lifts or five 90 mm-thick lifts to a total of 450 mm thickness. The number of blows was kept constant per unit volume

Fig. 1. Photos of dry QUELTS silty sand (a) immediately after oven-drying with natural gravelsized aggregates (b) after crushing to GSD1 with minimal aggregates and (c) after crushing to GSD2 with notable sand-sized aggregates.

5

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Fig. 2. Grain size distribution of QUELTS silty sand as per Rayhani et al. (2011), GSD1 with minimal aggregates and GSD2 with notable sand-sized aggregates. Table 3 Cover liner test configurations of various GCLs prehydrated on silty sand subgrade at 2 kPa at a range of subgrade water contents (wfdn) and the resulting GCL water contents (w) at the end of the hydration period and the hydraulic conductivity (kTW) after permeation with tap water (TW) at effective stress 15 kPa effective stress. Hydration tests were prepared by M. Rayhani et al. (2011). GCL

Subgrade

Hydration Test

Hydraulic Conductivity Tap water at 15 kPa

At 30 weeks (Rayhani et al., 2011) wfdn

Ref Test ID

% 1

5 16 5 5 16

2 4 a

PM14 PM5 PM16 PM4 PM2

At termination

At termination

w

w/wrefa

time

w

w/wrefa

kTW

%

%

weeks

%

%

m/s

34 102 40 83 114

23 68 34 43 58

246 306 246 315 328

28 95 33 61 103

19 63 28 31 53

1.6 × 10−11 7.8 × 10−11 1.9 × 10−11 3.6 × 10−11 9.5 × 10−11

wref at 2 kPa in DI water.

ionic concentrations of 660 mg/L Na+, 190 mg/L K+, 380 mg/L Ca2+, and 190 mg/L Mg2+ (Cl− = 2420 mg/L, total dissolved solids = 3840 mg/L; ionic strength = 85.4 mM, RMD = 0.26 M0.5). This leachate was somewhat similar to, but a little weaker than, the “strong” synthetic leachate used by Bradshaw and Benson (2013) and reported to have 760 mg/L Na+, 191 mg/L K+, 454 mg/L Ca2+, and 540 mg/L Mg2+ ions (pH = 7.4; total dissolved solids = 5584 mg/L; ionic

4.3.2. Effect of GCL prehydration on k permeated by synthetic leachate (SSL): MSW bottom liner tests A series of tests were conducted to simulate the situation where a GCL in a bottom liner in a municipal solid waste (MSW) landfill had had time to hydrate to the extent possible for a given GCL and subgrade before coming into contact with leachate in the early stage of landfill development. The permeant used, denoted as SSL, had a pH of 7.4 and

Table 4 Basal liner test configurations of various GCLs prehydrated on silty sand subgrade at 2 kPa at a range of subgrade water contents (wfdn) and the resulting GCL water contents (w) and hydraulic conductivity (kSSL) after permeation with synthetic municipal solid waste leachate (SSL) at 70 kPa and 150 kPa effective stress. GCL

1 2 3 4 6

a b c d

Hydration Test

Hydraulic Conductivity (Leachate)

wfdn

w

w/wrefa

Hydration period

SI after hydration

kSSLb 70 kPa

%

%

%

weeks

mL/2 g

m/s

17 12 19 12 13 15 10 17 11 11

d

5 16 5 16 16 5 16 5 10 16

55 85 58 104 118 52 93 61 83 105

37 56 49 88 56 27 48 28 38 47

87 157 179 164 157 87 187 102 207 102

kSSLc 150 kPa m/s −10

2.0 × 10 8.5 × 10−11 3.1 × 10−11 6.7 × 10−11 2.1 × 10−10 6.0 × 10−8 5.1 × 10−10 8.4 × 10−12 3.6 × 10−11 4.5 × 10−11

kSSL(70kPa) —————————————————kSSL(150kPa)

wf

−10

1.9 × 10 3.2 × 10−11 2.8 × 10−11 4.7 × 10−11 1.1 × 10−10 5.4 × 10−8 3.1 × 10−10 8.0 × 10−12 – 2.5 × 10−11

%

(−)

67 70 72 83 82 78 73 72 80 77

1.05 2.7 1.1 1.4 1.9 1.1 1.6 1.04 – 1.8

wref at 2 kPa in DI water. Hydraulic conductivity to synthetic municipal solid waste leachate (SSL) at 70 kPa and Δh = 0.35–0.70 m Hydraulic conductivity to synthetic municipal solid waste leachate (SSL) at 150 kPa and Δh = 0.35–0.70 m Hydraulic conductivity of a similar non-prehydrated (virgin) GCL permeated with SSL at 70 kPa is 2.3 × 10−11 m/s (Bradshaw and Benson, 2013). 6

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strength = 134 mM, RMD = 0.21 M0.5). Once chemical and hydraulic equilibrium had been reached, some specimens were then continued at a higher stress, as noted below, and for some blue dye was added to the permeant to examine for any preferential flow paths in the GCL. A number of variables were examined as indicate in the subsections below.

most direct comparison of the effect of bentonite granularity, although GCL3 is not thermally treated.

4.3.2.1. Effect of increasing hydraulic gradient on k. Generally, when applying Darcy's Law, the hydraulic conductivity is considered independent of hydraulic gradient and this is generally true for tests conducted on virgin GCLs (Shackelford et al., 2000). However, for needle-punched GCLs that have experienced significant cation exchange, this is not necessarily true (see Rowe et al., 2017 for a study of field samples from QUELTS). This study investigated the effect of cation exchange during prehydration and gradient on hydraulic conductivity for a bottom liner with only about 1 m–1.5 m of waste directly over the liner. This provides an indication of how effective a liner may or may not be in the very early stages. After prehydration on a subgrade, a k test was conducted at σv’ = 15 kPa for a range of differential heads across GCL4. GCL4 was examined because Rowe et al. (2017) had found that, of the five GCLs, it was most prone to this effect due to a combination of large needle-punched bundles and coarse granular bentonite. The k test was first conducted to equilibrium at a differential head across the specimen of 0.07 m, and then the process repeated as the head was increased to 0.14 m, 0.21 m, 0.49 m and 1.20 m. The permeant used was SSL, discussed above, to assess the effect of leachate seepage. At each stage, the hydraulic conductivity was allowed to reach equilibrium prior to increasing the differential head. For all other GCLs tested, the k tests were conducted at an initial hydraulic head differential of 0.35 m. In some cases with low hydraulic conductivity, this was increased to 0.6–0.7 m to accelerate the time to equilibrium.

Eight prehydration tests were conducted for GCL6 placed on silty sand subgrade with varying macrostructure (due to silt aggregation), compaction layer thickness, and conditioning time (number of days after compaction and before GCL placement on the subgrade). Table 2 gives the GCL water content after 4 weeks (at or near equilibrium). The effect of the subgrade compaction parameters on the prehydration of GCLs is discussed in the following subsections.

5. Results and discussion 5.1. Effect of subgrade preparation on GCL prehydration

5.1.1. Effect of oven-drying soil at 105 °C before compaction GCL6 was prehydrated from both oven-dried GSD2 soil (Test T1; Table 2) and on GSD2 soil that had been air-dried to w = 2.5% before rewetting (Test T2; Table 2) compacted at wfdn = 16% to ρd = 1650 kg/ m in the same manner. The initial rate of uptake was the same (both at w = 68% at 1 week) and water contents were relatively similar at 4 weeks (w = 96% for oven-dried T1 and w = 105% for air-dried Test T2). Thus, oven-drying of GSD2 prior to rewetting and compaction had at most a small effect on prehydration from the same nominal subgrade since the difference is more likely due to subtle differences in the subgrade grainsize distribution than the method of drying. 5.1.2. Effect of subgrade macrostructure (silt aggregation) To investigate the effect of silt aggregation during drying, GCL6 was placed on a GSD1 subgrade with minimal aggregates (Test T3 and T5) and on a GSD2 subgrade (Test T2 and T4) at wfdn = 16%. Despite the difference in dry sieve GSD (36% fines for GSD1 and 19% fines for GSD2; Fig. 1), the GSD was the same when reduced to the basic particles by wet sieving with both having 59–60% fines for T2 and T3 and 56% fines for T5 and T6. For a target compacted density of ρd = 1650 kg/m3 the same amount of energy was needed (20–25 blows/lift in three lifts) for both GSDs however after 4-weeks prehydration GCL6 on the coarser GSD2 was 105% for T2 compared to 87% for T3 with the finer GSD1. Thus, at this level of compaction, the dried structure of the soil appears to have been largely retained. To achieve a higher target compacted density of ρd = 1766 kg/m3, on finer GSD1 it took with 10–15 blows/lift in five 90 mm-thick lifts (a total of 50–75 blows) to get ρd = 1768 kg/m3 (Test T5, Table 2) while for coarser GSD2 after 20–25 blows/lift in five 90 mm-thick lifts (a total of 100–125 blows) the density was essentially the same at ρd = 1764 kg/m3 although it required about 80% more compactive energy (Test T4, Table 2). Despite similar subgrade densities, there was a notable difference in the resulting prehydration, with GCL6 on the more aggregated, coarser GSD2 at ρd = 1764 kg/m3 (T4) only reaching w = 110% (w/wref = 0.5) while on the finer GSD1 at ρd = 1768 kg/m3 (T5) hydrated to w = 134% (w/wref = 0.6). This suggests that the initial silt aggregates in the GSD2 subgrade did not fully break down during compaction. When the two GSD soils were compacted with the same energy (a total of 60–75 blows over 3 lifts) the coarser GSD2 (Test T7) compacted to ρd = 1727 kg/m3 while the finer GSD1 (Test T6) compacted to ρd = 1688 kg/m3 and in both cases after 4-weeks GCL6 had hydrated to w = 91%. The difference in the prehydration of finer GSD1 and coarse (more aggregated) GSD2 at the two densities is notable. At a low density (ρd = 1650 kg/m3), GSD1 gave rise to less prehydration (w = 87%, T3) than the coarse GSD2 (w = 96% in T1 and 105% in T2). While at the higher density (ρd ~ 1766 kg/m3), for GSD1 with less initial aggregates, the soil particles arranged more effectively during compaction and

4.3.2.2. Effect of increasing effective stress on k. To assess the effect of effective stress as the landfill waste thickness increased, tests were conducted to simulate conditions when the waste was only 5–6 m thick (σv’ = 70 kPa; Table 4) and when its thickness increased to 11–13 m (vertical stress, σv’ = 150 kPa). A k test was first conducted at σv’ = 70 kPa and, after reaching chemical and hydraulic equilibrium, σv’ was increased to 150 kPa and the test continued to equilibrium at that stress. The stress level of σv’ = 70 kPa was selected to allow for direct comparisons with results published by Bradshaw and Benson (2013). 4.3.2.3. Effect of GCL scrim-reinforcement on k. GCL1 and GCL2 both have the same fine granular bentonite and needle-punched nonwoven cover geotextile and are thermally-treated. They differ in that GCL1 has slit-film woven carrier geotextile while GCL2 has a scrim-reinforced (woven and nonwoven needle-punched together) carrier geotextile. The latter has been shown to provide better attachment of the needlepunched fibres (Lake and Rowe, 2000). To establish the effect the hydraulic effect of this difference, k tests with SSL as the permeant were conducted on specimens of GCL1 and GCL2 samples pre-hydrated on silty sand at wfdn = 5% and wfdn = 16%. 4.3.2.4. Effect of bentonite granularity on k. All GCLs are needlepunched and have a needle-punched nonwoven cover geotextile. GCL1 and GCL6 both have a slit-film woven carrier geotextile and are thermally treated. They differ in that GCL1 has fine granular bentonite and GCL6 has powdered bentonite. GCL3 (woven carrier) and GCL4 (nonwoven carrier) both have coarse granular bentonite. To establish the hydraulic effect of this difference, k tests with SSL as the permeant were conducted on specimens of GCL1, GCL2, GCL3, and GCL4 samples pre-hydrated on silty sand at wfdn = 5% and wfdn = 16%. A comparison between GCL1, GCL3 and GCL6 (all with a woven carrier) provides the 7

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macrostructure had a minimal effect on prehydration at this level of compaction (although it did affect the density achieved). In contrast, with five layers of compaction and sufficient compaction to achieve the same dry density (ρd ~ 1766 kg/m3) the GSD did make a difference with the GCL reaching w = 134% (w/wref = 0.6) for the finer GSD1 (T5) and w = 110% (w/wref = 0.5) for the coarser (due to silt aggregation) GSD2 (T4, Table 2). For a GSD1 there was non-linear increase in GCL hydrated water content, w, with increasing density (Tests T3, T6 & T4 near bottom of Table 2). For GSD2, at lower densities (1650 ≤ ρd ≤ 1730 kg/m3; near bottom of Table 2), there was not sufficient energy to break of the sand size silt aggregates and w depended mostly on the details of the grain size distribution of the soil in the upper lift closer to the GCL, with no clear effect of ρd. However, there was an increase in w when more energy was used when compacting in five lifts to ρd = 1764 kg/m3 (Test T4; bottom of Table 2).

Fig. 3. Photos of notable silt aggregates on a GSD2 silty sand subgrade compacted to 1727 kg/m3 in three 15 cm-thick lifts, at the end of a hydration test (T8). The dark spots are the sand sized, silt clods up to 3 mm–4 mm in diameter.

resulted in fewer and smaller inter-aggregate pores (giving w = 134% in T5 for GSD1 and 110% in T4 for GSD2). Conversely, some of the sand-sized aggregates in GSD2 remained after compaction (e.g., Fig. 3), forming a dual porosity material with larger inter-aggregate pores. The GSD1 subgrade with less air voids would have a higher degree of saturation and lead to greater moisture uptake. It is evident from the forgoing that for the same GCL and fundamentally the same soil the prehydration of the GCL will be a function of a relative complex interaction between the particle size distribution of the soil (which may just depend on its wetting and drying history) and the compaction density. When the data discussed above is arranged in order of density for each GSD as in the lower portion of Table 2 there is a clear pattern for the finer GSD1 and a more complex pattern for the aggregated GSD2. However, in both case the densest soil compacted in five lifts gave the highest GCL water contents.

5.1.4. Effect of conditioning time Another factor that could affect GCL prehydration from compacted aggregated silty-sand or sand subgrade is the potential for water to drain from its large pores. Considering that the typical in-situ natural water content at the QUELTS site was 16%, tests T1 and T8 for GSD2 were both compacted at wfdn = 16% to ρd = 1650 kg/m3 in three lifts and T1 was allowed to sit after compaction for 1 day while T8 was left for 20 days prior to placement of the GCL6. The resulting GCL6 water contents after 4 weeks of prehydration were w = 96% for T1 and 72% for T8. This suggests that drainage in the soil prior to placements of the GCL did have an effect on the final 4-week hydrated GCL. The drainage of water from the larger subgrade pores was also apparent in microscope photos of the subgrade taken after 0 days, 1 day and 7 days of conditioning after compaction (Fig. 4). Thus, the time between soil placement or wetting time prior to GCL placement can be expected to affect GCL prehydration—especially in areas where the water table is low and the main source of moisture for the subgrade is external (rainfall or manually added).

5.1.3. Effect of compacted lift thickness and subgrade density GCL6 was placed on two subgrades of GSD2: one compacted with three 150 mm-thick lifts (Test T7) and one compacted in five 90 mmthick lifts (Test T4). The same 20–25 blows/lift was used in both cases. After 4-weeks on the two subgrades, GCL6 prehydrated to w = 91% (w/ wref = 0.41) in Test T7 (GSD2; three 150 mm-thick lifts; a total of 60–75 blows; ρd = 1727 kg/m3) and a higher w = 110% (w/wref = 0.5) in Test T4 (GSD2; five 90 mm-thick lifts; a total of 100–125 blows; ρd = 1764 kg/m3). Thus, the moisture uptake was significantly inhibited by the combination of aggregation and lower density and hence degree of saturation (due to less compaction). In the T7 GSD2 subgrade, the silt aggregates were still visible after compaction (Fig. 3) but they were not visible at the higher-density T4 (it appears that more compaction broke down more aggregates at this water content). Silt aggregates were not apparent for the GSD1 subgrades. For the finer GSD1 subgrade, when compacted in three 150 mmthick lifts for Test T6 (a total of 60–75 blows) to ρd = 1688 kg/m3, after 4-weeks prehydration GCL6 reached w = 91% (w/wref = 0.5) as compared to w = 134% when compacted in five 90 mm-thick lifts for Test T5 (a total of 100–125 blows) to ρd = 1768 kg/m3. Comparing the results for three 150 mm-thick lifts on GSD1 T6 (ρd = 1688 kg/m3) and GSD2 T7 (ρd = 1727 kg/m3) for which GCL6 achieved w = 91% in both cases, it appears that the details of subgrade

5.1.5. Implications of different level of GCL prehydration The forgoing discussion of difference in prehydration depending on subgrade grain size distribution and compaction of the same soil at the same water content raises the question: “does the level of initial prehydration really matter?” This question will now be explored for five GCLs. 5.2. Effect of GCL prehydration on k permeated by tap water (TW) at 15 kPa: cover tests The prehydration tests on the QUELTS silty sand (Fig. 2) reported by Rayhani et al. (2011) were continued for up to six years as part of the present study to assess the effect of long-term prehydration at various subgrade water contents on GCL performance parameters upon permeation. These tests are listed in Table 3 with reference to the test numbers (PM14, PM5, PM16, PM4 and PM2) used previously by Rayhani et al. (2011).

Fig. 4. Microscope photos of the silty sand subgrade with GSD2 [Test T3] at ρd = 1650 kg/m3. (a) after compaction (no conditioning), (b) after 1-day conditioning, and (c) after 7-days conditioning. 8

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Fig. 5. Hydraulic conductivity of GCL1, GCL2 and GCL4 under virgin conditions and after prehydration on silty sand at 5% and 16% subgrade water contents (wfdn) when permeated (a) with tap water at 15 kPa, and (b) with SLL at 70 kPa.

As discussed by Rayhani et al. (2011), a higher subgrade water content during prehydration gave rise to greater GCL water contents. For example, after 30 weeks at wfdn = 5%, the GCL water contents were w = 34% (w/wref = 23%) for GCL1 (PM14) and w = 83% (w/ wref = 43%) for GCL4 (PM4). At wfdn = 16%, the water contents were w = 102% (w/wref = 68%) for GCL1 (PM5) and w = 114% (w/ wref = 58%) for GCL4 (PM2). At termination, after 246–328 weeks, the trend remained the same although the GCL water contents decreased by 6–7% for GCL1 and GCL2 and 13–22% for GCL4. This long-term decrease may likely be due to the depletion of water in the subgrade over time changing the suction equilibrium and hence the rate of uptake and/or due to moisture redistribution in the GCL cause a change in its suction. When the GCL specimens prehydrated as described in the previous paragraph were permeated with tap water, the equilibrium hydraulic conductivity (kTW; Table 3; Fig. 5a) was 4.9-fold (GCL1) to 2.4-fold (GCL4) lower for the samples hydrated on the subgrade at wfdn = 5% (1.6 × 10−11 m/s ≤ k ≤ 3.6 × 10−11 m/s; Table 3) than those hydrated on the subgrade at wfdn = 16% (7.8 × 10−11 m/ s ≤ k ≤ 9.5 × 10−11 m/s; Table 3). Furthermore, even the un-prehydrated (virgin) samples had a lower hydraulic conductivity to tap water (2.8 × 10−11 m/s ≤ k ≤ 3.8 × 10−11 m/s; Table 1) than those hydrated at wfdn = 16%. The difference is attributed to the greater cation exchange and consequent lower swell index for the GCLs hydrated on the subgrade at wfdn = 16% (10≤ SI ≤ 13; Table 4) than those hydrated on the subgrade at wfdn = 5% (15 ≤ SI ≤19; Table 4 and Fig. 6a) and the virgin samples (24 ≤ SI ≤ 28; Table 1). This trend in swell index plotted in Fig. 6a, where the decrease in swell index with prehydration can be seen more clearly, is also true for the basal liner

prehydration tests (Table 4 and Table 5; discussed in the next subsection). The GCL hydration to w of 40% or less that occurred at the low subgrade water content (wfdn = 5%,) were largely achieved by vapourdominated transport which is the primary mode of transport below the GCL water-entry value. Hence, very little cation exchange occurred since vapour transport could not transfer cations and the cations that did reach the GCL bentonite were due to that small amount of water above the water entry value. Thus, in this case, a higher water content and more prehydration prior to permeation was actually detrimental (contrary to common wisdom) but makes sense since the pore water of the prehydration soil had considerably more cations (~230 mg/L Ca2+) than the TW permeant (~30 mg/L Ca2+). 5.3. Effect of gradient on k permeated SSL at 15 kPa: MSW bottom liner tests Specimens of GCL4 hydrated on wfdn = 5% were subjected to five applied differential heads (Δh) at σv’ = 15 kPa and permeated with SSL. Starting at a differential head of Δh = 0.07 m (gradient, i ~ 7), Δh was increased to 0.14 m, 0.21 m, 0.49 m, and 1.20 m after reaching equilibrium at each stage. At a very low gradient of 7, the k was 5 × 10−9 m/s. At Δh = 0.14 m and higher (i ≥ 14), 1 × 10−7 ≤ k ≤ 2 × 10−7 m/s. These results are within the range of k values reported by Rowe et al. (2017) for GCL4 samples exhumed from a test cover at QUELTS after 5 and 7 years and permeated with a 10 mM CaCl2 solution under the same differential heads. The closest values are for the 5-year specimens with cracks and roots which were reported as 1.3 × 10−9 m/s at Δh = 0.07 m and 1.1 × 10−7 ≤ k ≤ 2.0 × 10−7 m/s at Δh = 0.14 m–1.20 m. Rowe et al. (2017) attributed the large increase in k at differential heads of 0.14 m and 9

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Table 5 Comparison of swell index (SI) values of the bentonite in the GCL after off-roll sampling (virgin), prehydration (hydrated) and hydraulic conductivity test (final) and the cation exchange capacity (CEC) and bound concentrations of sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg) after prehydration. Final CEC data could not be obtained due to limited sample mass. GCL

1 2 3 4 6 a b c

wfdn

Virgin SI

Hydrated SIa

Final SIb

Hydrated CECa

Hydrated Bound Naa

Hydrated Bound Ka

Hydrated Bound Caa

Hydrated Bound Mga

%

mL/2 g

mL/2 g

mL/2 g

Cmol/kg

%

%

%

%

5 16 5 16 16 5 16 5 16

26 26 25 25 24 24 24 32 32

17 12 19 12 13 15 10 17 11

9 10 16 8 9 6 6 11 7

79.4 77.9 68.2 89.5 93.7 91.1 72.9 81.6 88.9

61 33 62 43 32 63 26 68 22

3 2 2 2 19c 2 2 2 2

24 46 24 40 38 21 51 17 53

11 18 12 15 11 13 21 12 23

After hydration (before hydraulic conductivity test). After permeation with SSL (after hydraulic conductivity test). Anomalous: Based on value reported in ICP analysis but suspect.

higher to preferential flow through fibre bundles. The design head for landfills is often 0.3 m and for this case for GCL4 k ~1.5 × 10−7 m/s. Bundle flow was most prevalent for GCL4 which, amongst the GCLs tested (GCL1 to GCL4), had the largest bundle size and percentage area of bundles and also coarse granular bentonite. Although the k test was performed at σv’ = 15 kPa to simulate resistance to leachate very early in the landfill operations, it also represent a “best case” scenario of the GCL below a wrinkle at the bottom of a landfill where the σv’~0 and the design head is usually 0.3 m. Thus, for GCL4 with its coarse bentonite and the large needle-punched bundles, the hydraulic conductivity below a wrinkle would be for k ≥ 2 × 10−7 m/s.

5.4. Effect of effective stress on k permeated SSL: MSW bottom liner tests When GCLs hydrated in the same manner as those discussed above were permeated with SSL at 70 kPa, the differences in the hydraulic performance between GCL1 and GCL4 was substantially more than when permeated with tap water (TW). While TW permeation through a GCL prehydrated on a given subgrade water content were modest (e.g., for wfdn = 5%, kTW (GCL1) = 1.6 × 10−11 m/s; kTW (GCL4) = 3.6 × 10−11 m/s), when permeated with synthetic MSW leachate (SSL) the difference was more than two orders of magnitude (e.g., for wfdn = 5%, kSSL (GCL1) = 2.0 × 10−10 m/s; kSSL (GCL4) = 6.0 × 10−8 m/s). This is evident in Fig. 5b where there is a greater spread of data points between the

Fig. 6. (a) Swell index and (b) exchangeable cation percentage of GCL1, GCL2, GCL4 and GCL6 during virgin conditions and immediately at the end of hydration period on 5% and 16% subgrade water contents. 10

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More importantly, for those cases where the prehydration and GCL was such that the k > 1 × 10−10 m/s and up to 6 × 10−8 m/s, the increase in σv’ had a minor effect in decreasing the hydraulic conductivity; if it was high at 70 kPa it remained high at 150 kPa. 5.5. Effect of GCL scrim-reinforcement on k permeated SSL: MSW bottom liner tests GCL1 and GCL2 were both thermally-treated, and both contained essentially identical fine granular bentonite and nonwoven cover geotextiles. The primary difference between these two products from the same manufacturer was that GCL1 had a woven carrier geotextile while GCL2 had a scrim-reinforced nonwoven carrier geotextile. After prehydration at wfdn = 5%, GCL1 reached w = 55% (w/wref = 0.37) and GCL2 reached a similar w = 58% but higher Sr (w/wref = 0.49). At wfdn = 16%, GCL1 reached w = 85% (w/wref = 0.56) and GCL2 reached w = 104% (w/wref = 0.88). Thus, although the gravimetric water content, w, was similar for the two GCLs, the higher confinement at low prehydration stress provided by the better anchorage provided by the scrim reinforced carrier means a much better degree of saturation prior to permeation under a higher stress. When hydrated from wfdn = 5% at low stress and then permeated with SSL at 70 kPa, GCL2 (k = 3.1 × 10−11 m/s) had an almost order of magnitude lower hydraulic conductivity than GCL1 (k = 2.0 × 10−10 m/s; Table 3). The difference was much less (a factor of 1.3) when hydrated from wfdn = 16% (GCL2 k = 6.7 × 10−11 m/s vs GCL1 k = 8.5 × 10−11 m/s; Table 3). Given the typical of subgrade density and water content variability at a site (including at the QUELTS site; Rowe et al. 2016a,b); there are clear benefits from the more robust scrim reinforced GCL (despite the slightly higher price to pay for the extra geotextile) on projects where minimizing leakage of contaminants is important. Consistent with previous studies, this is likely because the scrim-reinforcement restricted bentonite swelling and decreased bulk void ratio; thereby, reducing hydraulic conductivity (Lake and Rowe, 2000). Thus, in this particular case, scrim-reinforcement improved both the degree of prehydration from the subgrade and the k of the GCL to SSL.

Fig. 7. Cross-section photos of (a) GCL6 showing a more continuous macrostructure than (b) GCL1 at the end of hydration on silty sand at 5% subgrade water content (wfdn). At wfdn = 16% (c and d), both GCLs show a gel-like texture. Scale in mm.

GCLs, especially at wfdn = 5%, compared to Fig. 5a. In this case, the impact of increasing wfdn was found to be both negative and positive depending on the GCL. As was the case for TW, even with SSL, the k was a little higher at wfdn = 16% than at 5% for GCL2 and GCL6. Specifically, for GCL2, kSSL was twice as high when hydrated from wfdn = 16% (6.7 × 10−11 m/s) as when hydrated from wfdn = 5% (3.1 × 10−11 m/s) while for GCL6, kSSL was 4.5 × 10−11 m/s when hydrated from wfdn = 16%, 3.6 × 10−11 m/s when hydrated from wfdn = 10%, and 0.8 × 10−11 m/ s when hydrated from wfdn = 5%. However, all values are considered relatively low being less than 7 × 10−11 m/s at 70 kPa and less than 5 × 10−11 m/s at 150 kPa (Table 4). In stark contrast to GCLs 2 and 6 discussed above, for:

• GCL4, • •

k was 120-fold higher when prehydrated at wfdn = 5% (kSSL = 6.0 × 10−8 m/s at 70 kPa, more than three orders of magnitude higher than kTW) than when better-hydrated at wfdn = 16% (kSSL = 5.1 × 10−10 m/s at 70 kPa, five times higher than kTW); GCL1 k was 2.4 fold higher when prehydrated at wfdn = 5% (kSSL = 2.0 × 10−10 m/s at 70 kPa, an order of magnitude higher than kTW and similar to the virgin kSSL = 1.9 × 10−10 m/s) than when hydrated at wfdn = 16% (kSSL = 8.5 × 10−11 m/s at 70 kPa, only 9% higher than kTW); and, GCL3 when prehydrated at wfdn = 16%, kSSL = 2.1 × 10−10 m/s at 70 kPa was less than that for GCL4 but notably higher than for the other three GCLs at wfdn = 16% (Table 3).

5.6. Effect of bentonite granularity on k permeated SSL: MSW bottom liner tests GCL1 and GCL6 both had woven carrier and nonwoven cover geotextiles and were thermally-treated but GCL1 contained fine granular bentonite and GCL6 contained powdered bentonite. At wfdn = 5%, GCL6 reached w = 61% (w/wref = 0.28) and hydrated to a more continuous macrostructure (Fig. 7a) giving kSSL = 8 × 10−12 m/s; whereas, GCL1 reached w = 55% (w/wref = 0.37) but retained a granular macrostructure (Fig. 7b) at the end of prehydration and gave kSSL = 2 × 10−10 m/s. At wfdn = 16%, the difference in grain size was not as significant as both GCLs were well-hydrated to a gel-like texture (Fig. 7c and d) at the end of prehydration with GCL6 reaching w = 105% (w/wref = 0.47 and kSSL = 4.5 × 10−12 m/s at 70 kPa) and GCL1 reaching w = 85% (w/wref = 0.58 and kSSL = 8.5 × 10−11 at 70 kPa). Therefore, in the case of a comparison between different grain sizes of bentonite, it is the water content that is more important than the degree of saturation when the difference in w/wref is relatively small. Both GCLs underwent considerable cation exchange during prehydration with a swell index of 17 ml/2 g at wfdn = 5% and 11–12 ml/ 2 g at wfdn = 16%. Under all conditions tested, GCL6 had a lower hydraulic conductivity than GCL1 despite similar swell index values before permeation with leachate. The larger surface area of initially powdered bentonite particles in GCL6 allowed more water molecules to attach to the bentonite which resulted in a higher w (but a lower Sr) and more swelling than GCL1. Considering the three GCLs with a woven carrier (albeit not thermally treated for GCL3), even under relatively ideal prehydration

There were two distinct differences between GCL3/GCL4 and the other three GCLs. First the latter GCLs were thermally treated to give better fibre anchorage and less needle-punching and fewer fibres were needed to get an adequate peel and shear strength. Rowe et al. (2017) demonstrated that at QUELTS the size and percentage of the total area occupied by needle-punched fibres made a substantial difference to the hydraulic conductivity when the GCL has been subjected to cation exchange (SI ≤ 15 mg/L in present tests after prehydration; Table 4) on the same subgrade over 5–7 years in the field. Second, GCL3 & GCL4 had coarse granular bentonite; whereas, GCL1 & GCL2 had fine granular bentonite and GCL6 had powdered bentonite. These differences are significant. If GCL4 were in direct contact with the GMB, then if hydrated from this silty sand at wfdn = 5% under a head differential of 0.35 m, GCL4 k ~1.5 × 10−7 m/s at 15 kPa and 6 × 10−8 m/s at both 70 and 150 kPa. If hydrated at wfdn = 16% then 6 × 10−10 ≤ k ≤ 3 × 10−10 m/s under 70 and 150 kPa). This 2000-fold range is a clear warning of the importance of (a) choice of GCL, (b) prehydration conditions, and (c) effective stress on the selection of relevant design parameters. Increasing the σv’ from 70 kPa to 150 kPa led to a modest reduction in hydraulic conductivity (less than 2-fold in all but one case; Table 4). 11

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conditions with wfdn = 16%, GCL3 with coarse granular bentonite hydrated to w = 118% (w/wref = 0.56) and had kSSL = 2.1 × 10−10 m/s (at 70 kPa), while GCL1 with fine granular bentonite hydrated to w = 85% (w/wref = 0.56) and had kSSL = 8.5 × 10−11 m/s (at 70 kPa), and GCL6 with powdered bentonite hydrated to w = 105% (w/ wref = 0.47) and had kSSL = 4.5 × 10−11 m/s (at 70 kPa). With 85% ≤ w ≤ 118%, after prehydration under very favourable wfdn of the silty sand, the degree of saturation varied over a relatively narrow range (0.47 ≤ w/wref ≤ 0.56) yet the value of k decreased up to about 5-fold from 2.1 × 10−10 m/s for coarse granular bentonite, to 8.5 × 10−11 m/s for fine granular bentonite, to 4.5 × 10−11 m/s for powdered bentonite. Thus, for a very practical range of degrees of saturation between 47% and 55% it was the grain size of the bentonite (with all acceptable bentonites) that governed the hydraulic conductivity to SSL much more than the gravimetric water contents (indeed GCL3 with the higher w = 118% performed worse than GCL1 with w = 85%). Under less ideal prehydration conditions with wfdn = 5%, GCL4 with coarse granular bentonite had w = 38% (w/wref = 0.2) and had kSSL = 6.0 × 10−8 m/s (at 70 kPa), GCL2 with fine granular bentonite w = 58% (w/wref = 0.49) had kSSL = 2.8 × 10−11 m/s (at 70 kPa), and GCL6 with powered bentonite w = 61% (w/wref = 0.28) and had kSSL = 8.4 × 10−12 m/s (at 70 kPa). GCL6 with powdered bentonite had the highest w and lowest k, while GCL4 with the lowest w had the highest k (by a factor of over 7000!) despite the same prehydration conditions. This shows that in real situations, the granularity of the bentonite and needle punching matter a great deal; but why? To examine the explanation for the difference in performance of the GCLs after prehydration from the QUELTS silty sand, blue dye was injected into the permeant prior to test termination and then at termination the GCLs were inspected to locate the permeant flow path through the GCL. At wfdn = 5%, all the granular GCLs (GCL1-GCL4) showed some preferential flow through the bundle fibres (Figs. 8–11) where blue dye was seen through the needle-punched holes at the bottom of the sample (permeation is from top to bottom), although the effect was least for GCL2, however for GCL6 (Fig. 12) the bentonite had a uniform colour and no visible preferential flow around the fibres. This is evidence of two things: (i) the benefit of powdered bentonite for hydrating and sealing around the needle-punched fibres under realistic prehydration conditions, and (ii) the value of minimizing the amount of needle-punching required to provide good bentonite confinement under typical low prehydration stresses and adequate internal shear strength that is provided by thermal treatment and even more so by a scrimreinforced carrier geotextile as well. At wfdn = 16%, preferential flow was still observed in GCL3 and GCL4 (the coarse granular GCLs) but was no longer observed in GCL1

Fig. 9. Cross-section (left) and bottom view (right) of GCL2 (wfdn = 5%) after a hydraulic conductivity test with synthetic MSW leachate (SSL) and blue dye injected during the test. The carrier geotextile was included in the bottom view as it showed the blue dye more conspicuously that was not seen in the crosssection photo. Note the blue colour around the needle-punched bundle holes indicating preferential flow through fibres.

Fig. 10. Cross-section (left) and bottom view (right) of the bentonite in GCL3 (wfdn = 5%) after a hydraulic conductivity test with synthetic MSW leachate (SSL) and blue dye injected during the test. Note the blue colour around the needle-punched bundle holes indicating preferential flow through fibres.

Fig. 11. Cross-section (left) and bottom view (right) of the bentonite in GCL4 (wfdn = 5%) after a hydraulic conductivity test with synthetic MSW leachate (SSL) and blue dye injected during the test. Note the blue colour around the needle-punched bundle holes indicating preferential flow through fibres.

Fig. 8. Cross-section (left) and bottom view (right) of the bentonite in GCL1 (wfdn = 5%) after a hydraulic conductivity test with synthetic MSW leachate (SSL) and blue dye injected during the test. The GCL is permeated from the top. Note the blue colour around the needle-punched bundle holes indicating preferential flow through fibres.

Fig. 12. Cross-section (left) and bottom view (right) of the bentonite in GCL6 (wfdn = 5%) after a hydraulic conductivity test with synthetic MSW leachate (SSL) and blue dye injected during the test. Note the uniform colour throughout the bentonite indicating uniform flow through the bentonite.

12

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and GCL2 (the fine granular GCLs) which now had uniformly coloured bentonite like GCL6. In fact, the effluent from the tests where no preferential flow was observed was only very lightly coloured indicating permeant flow through the bentonite in contrast to the highly coloured effluent where there was preferential flow. Thus, with decreasing bentonite granularity and higher subgrade water contents, the ability of the bentonite to seal around the fibres and create a continuous macrostructure increased.

occur when reworking and drying silty sand for compaction, on GCL prehydration is both complex and interrelated with other soil properties such as compaction density and water content. More silt aggregates led to lower densities for a given compaction energy, less GCL prehydration, and an increased risk of drainage of water over time. 2. Prehydration of the GCL was generally better the greater the dry density of the soil at a given moisture content. The best prehydration was achieved for a dry density greater than 95% standard Proctor maximum density and the higher the water connect of the subgrade at the time the GCL was placed over it. 3. The GCL hydraulic conductivity to synthetic leachate (kSSL) was greater than the hydraulic conductivity to tap water (kTW) but the difference depended on subgrade water content (wfdn) during prehydration. The greatest difference of four orders of magnitude (kTW = 1.6 × 10−11 and kSS = 1.2 × 10−7 m/s) was for GCL4 which had coarse granular bentonite, large needle-punched fibre bundles, and was not thermally treated. 4. The effect of wfdn on k depends on both the GCL and the permeant. a. For permeation of subgrade prehydrated GCLs with tap water, higher wfdn generally resulted in a higher kTW, due to increased cation exchange with higher foundation water content and higher cation concentration in the pore water than in the tap water. b. For permeation of subgrade prehydrated GCL2 and GCL6, wfdn = 16% (w = 104 and 105%, respectively, after prehydration) led to somewhat higher kSSL (6.7 × 10−11 and 4.5 × 10−11 m/s, respectively) than at wfdn = 5% (w = 58 and 61%, respectively, after prehydration) with kSSL (3.1 × 10−11 and 0.8 × 10−11 m/s, respectively); but all values were low! In contrast c. For permeation of subgrade prehydrated GCL4 with synthetic landfill leachate (SSL), wfdn = 16% led to a lower kSSL = 5.1 × 10−10 m/s (w = 93% after prehydration) than at wfdn = 5% (kSSL = 6.0 × 10−8 m/s, w = 38% after prehydration) since at wfdn = 16% the bentonite was approaching a gel like structure with limited hydrated granules surrounded by more hydrated bentonite while for wfdn = 5% there many large bentonite granules surrounded by voids when it encountered the SSL. d. For permeation of subgrade prehydrated GCL1 with SSL, wfdn = 16% led to a smaller kSSL = 8.5 × 10−11 m/s (w = 85% after prehydration) than at wfdn = 5% (kSSL = 2.0 × 10−10 m/s, w = 55% after prehydration) but unlike GCL4 there was no visible large pores and kSSL was 300-fold lower for GCL1 than GCL4 under the same hydrating conditions.

5.7. Comparison with field-hydrated samples (Rowe et al., 2017) Section 5.3 examined the hydraulic conductivity of laboratory-hydrated GCL4 permeated with SSL under 15 kPa and compared them to the hydraulic conductivity values reported by Rowe et al. (2017) for field-exhumed GCL4 specimens that had not been damaged by stones or roots after 5 and 7 years in a test cover and permeated with 10 mM CaCl2 solution under the same gradients and effective stress. This section will now compare the k data of laboratory-prehydrated GCLs 1 to 4 permeated with SSL under 70 kPa (basal liner) and Δh = 0.35 m–0.70 m (Table 4) to those reported by Rowe at al. (2017) for exhumed field specimens permeated with 10 mM CaCl2 at 15 kPa and Δh = 0.49–0.63 m. Comparison will also be made with values reported by Bradshaw and Benson (2013) for a GCL with a similar structure to GCL3 with a granular bentonite having D50~ 0.2 mm (likely more similar to the fine than coarse bentonite examined in this study as in GCL1 which has a similar structure but was thermally treated) and an initial SI = 28 ml/2 g, hydrated on a Red Wing clay at wfdn = 14% and pore water much less aggressive than Godfrey silty sand. Their GCL hydrated to w = 66% and had a SI~20 mL/2 g after prehydration. For the field-hydrated GCL1 after 5 and 7 years, Rowe et al. (2017) reported a range of 4.0 × 10−11 m/s ≤ k ≤ 1.9 × 10−10 m/s. The value of k = 8.5 × 10−11 m/s (Table 4) obtained for GCL1 hydrated on a wfdn = 16% subgrade (after significant cation exchange; SI = 12 ml/ 2 g) falls well within this range. Bradshaw and Benson (2013) reported k = 5.6 × 10−11 m/s and a SI~ 15 after permeation for their somewhat similar GCL which is very close to the low end of the range reported by Rowe et al. (2017) and a little below the value obtained in this study, which makes sense given the higher SI. For the field-hydrated GCL2, Rowe et al. (2017) reported a range of 1.1 × 10−9 m/s ≤ k ≤ 4.8 × 10−9 m/s. However, even the lowest value is still considerably higher than the largest-measured k for the laboratory-hydrated GCL2 of 6.7 × 10−11 m/s at wfdn = 16%. For the field-hydrated GCL3, k varied between 1.3 × 10−9 m/s and 6.6 × 10−8 m/s which were up to two orders of magnitude higher than the laboratory-hydrated GCL3 at k = 2.1 × 10−10 m/s. Similarly, the field-hydrated GCL4 (1.2 × 10−7 m/s ≤ k ≤ 4.0 × 10−7 m/s) was one to three orders of magnitude higher than the laboratory-hydrated GCL4 (5.1 × 10−10 m/s ≤ k ≤ 6.0 × 10−8 m/s). Except for GCL1, the laboratory prehydration procedure described resulted in lower k values than the samples recovered from the field. This difference is attributed to the absence of wet-dry in the laboratory tests that may severely inhibit prehydration and can impact performance. This aspect requires further research.

The differences are likely due to the combined effect of bentonite granularity and effectiveness of needle-punching. 5. Increasing effective stress from 70 kPa to 150 kPa led to a slight decrease in kSSL due to a decrease in bulk void ratio; however, the previous history of hydrating conditions under low stress was far more critical than increasing the vertical applied stress above 70 kPa at the time of leachate permeation. 6. Gradient had a minimal effect on kSSL except for GCL4 at an effective stress of 15 kPa where k = 5 × 10−9 m/s at Δh = 0.07 m increased to ~1–2 x 10−7 m/s at Δh ≥ 0.14 m. 7. A scrim-reinforced geotextile improved fibre anchorage of the GCL and reduced kSSL. 8. Finer bentonite grains resulted in lower kSSL. GCL6 which contained powdered bentonite had a kSSL that was an order of magnitude lower than GCL1 with a similar structure but fine-granular bentonite. Preferential flow through fibres was also more prevalent in granular GCL1, GCL2, GCL3 and GCL4 when hydrated for a subgrade at wfdn = 5% and for GCL3 and GCL4 when hydrated for a subgrade at wfdn = 16% due to inability to self-heal around the fibres whereas the powdered GCL (GCL6) maintained a continuous macrostructure

6. Summary and conclusions 6.1. Summary Five different GCL products of varying bentonite granularity and structure were prehydrated on the same silty sand subgrade, removed from the subgrade, and tested for hydraulic conductivity to both tap water (TW; simulated rainwater with a brief residency time in a silty sand overburden) and simulated municipal solid waste leachate (SSL). For the GCLs and conditions examined, it was found that: 1. The effect of silt aggregation into sand size particles, which can 13

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• More research is needed to assess the desired subgrade water con-

at all subgrade water contents. GCL2 with fine granular bentonite and a scrim-reinforced and thermally treated carrier was close in terms of performance.

•

7. Conclusions

•

Based on the information provided in the preceding sections of this paper and in the cited literature, it is concluded that:

• GCLs

•

•

with similar nominal specifications (e.g., ASTM D5887 & D5890) in terms of minimum k and SI under standard index conditions can perform extremely differently when hydrated from real soil and permeated with a municipal solid waste leachate. For the conditions tested, the most robust GCL in terms of performance was GCL6 with powdered bentonite and thermally treated, followed by GCL2 with fine grained bentonite and a scrim reinforced and thermally treated carrier geotextile, followed, less reliably, by GCL1 with fine grained bentonite and woven and thermally treated carrier geotextile. GCL4 with coarse bentonite, large needle-punched bundles and not thermally treated was the worst performer, especially with respect to synthetic landfill leachate. It can be inferred that increasing bentonite granularity, larger fibre bundle size, cation exchange before exposure to SSL all reduce the ability of the bentonite to seal around needle-punched fibres in the presence of synthetic municipal solid waste landfill leachate (SSL). GCLs placed on a subgrade at or above residual water content can be expected to hydrate from the subgrade. The rate of prehydration will depend on the particular subgrade and GCL but the GCLs examined in this study were generally within 10% of the final equilibrium within 20–30 days (sometimes earlier). Thus, in the field, one can expect that a GCL will experience most of its prehydration prior to exposure to the permeant it is to contain, at relatively low stress. If there is cation exchange in the bentonite from the pore water it has up-taken from the subgrade, as in this study, then the level of prehydration, cation exchange, and the bulk void ratio of the GCL have been shown to affect the hydraulic conductivity of the GCL with respect to a given permeant, with the effect potentially being different for different permeants. It follows that the objective of subgrade preparation and GCL selection for GCL in a bottom liner should be to maximize GCL prehydration with time by appropriate compaction of the subgrade and selection of a GCL that will restrict bentonite swelling (to minimize the void ratio) while hydrating to a water content in excess of 100% and an apparent degree of saturation w/wref ~ 50% or higher.

Acknowledgements This research was funded by NAUE GmbH who also provided their GCL. The North American GCLs were provided by Terrafix Geosynthetics and CETCO. The data and opinions expressed herein are solely those of the authors and were formed without the input from any external individuals including the suppliers and funding agency noted above. References Acikel, A.S., Gates, W.P., Singh, R.M., Bouazza, A., Rowe, R.K., 2018a. Insufficient initial hydration of GCLs from some subgrades: factors and causes. Geotext. Geomembranes 46 (6), 770–781. Acikel, A.S., Gates, W.P., Singh, R.M., Bouazza, A., Fredlund, D.G., Rowe, R.K., 2018b. Time-dependent unsaturated behaviour of geosynthetic clay liners. Can. Geotech. J. 55, 1824–1836. https://doi.org/10.1139/cgj-2017-0646. 2018. Anderson, R., Rayhani, M.T., Rowe, R.K., 2012. Laboratory investigation of GCL hydration from clayey sand subsoil. Geotext. Geomembranes 31, 31–38. https://doi.org/ 10.1016/j.geotexmem.2011.10.005. Arvelo, A., 2004. Effects of the Soil Properties on the Maximum Dry Density Obtained from the Standard Proctor Test. Electronic Theses and Dissertations. University of Central Florida. Beddoe, R.A., Take, W.A., Rowe, R.K., 2011. Water-retention behavior of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 137 (11), 1028–1038. https://doi.org/10. 1061/(ASCE)GT.1943-5606.0000526. Bouazza, A., Ali, M.A., Gates, W.P., Rowe, R.K., 2017a. New insight on geosynthetic clay liner hydration: the key role of subsoils mineralogy. Geosynth. Int. 24 (2), 139–150. https://doi.org/10.1680/jgein.16.00022. Bouazza, A., Rouf, M.A., Singh, R.M., Rowe, R.K., Gates, W.P., 2017b. Gas advectiondiffusion in geosynthetic clay liners with powder and granular bentonites. Geosynth. Int. 24 (6), 607–614. https://doi.org/10.1680/jgein.17.00027. Brachman, R.W.I., Rowe, R.K., Take, W.A., Arnepalli, N., Chappel, M.J., Bostwick, L.E., Beddoe, R., 2007. Queen's composite geosynthetic liner experimental site. In: 60th Canadian Geotechnical Conference, Ottawa, October 2007, pp. 2135–2142. Brachman, R.W.I., Rentz, A., Rowe, R.K., Take, W.A., 2015. Classification and quantification of downslope erosion from a GCL when covered only by a black geomembrane. Can. Geotech. J. 52 (4), 395–412. https://doi.org/10.1139/cgj-2014-0241. Brachman, R.W.I., Rowe, R.K., Take, W.A., 2018. Reductions in GCL overlap beneath an exposed geomembrane. ASCE J. Geotech. Geoenviron. Eng. 144 (12), 04018094 1–9. https://doi.org/10.1061/(ASCE)GT.1943-5606.0001985. Bradshaw, S.L., Benson, C.H., 2013. Effect of municipal solid waste leachate on hydraulic conductivity and exchange complex of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 140 (4), 04013038. https://doi.org/10.1061/(ASCE)GT.19435606.0001050. Bradshaw, S.L., Benson, C.H., Scalia, J., 2013. Hydration and cation exchange during subgrade hydration and effect on hydraulic conductivity of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 139 (4), 526–538. https://doi.org/10.1061/(ASCE)GT. 1943-5606.0000793. Chen, J.N., Benson, C.H., Edil, T.B., 2018. Hydraulic conductivity of geosynthetic clay liners with sodium bentonite to coal combustion product leachates. J. Geotech. Geoenviron. Eng. 1443, 04018008. https://doi.org/10.1061/ASCE.GT.1943-5606. 0001844. 2018. Daniel, D.E., Shan, H.Y., Anderson, J.D., 1993. Effects of partial wetting on the performance of the bentonite component of a geosynthetic clay liner. Geosynthetics ’93 3, 1482–1496 January 1993. Eberle, M.A., von Maubeug, K., 1997. Measuring the in-situ moisture content of geosynthetic clay liners (GCLs) using time domain reflectometry (TDR). 6th Int. Conf. on Geosynthetics, Atlanta 1, 205–210. Gates, W.P., Dumadah, G., Bouazza, A., 2018. Micro X-ray visualisation of the interaction of geosynthetic clay liner components after partial hydration. Geotext. Geomembranes 46, 739–747. Ghavam-Nasiri, A., El-Zein, A., Airey, D., Rowe, R.K., 2019a. Water Retention of

Thus, assuming the liner will be covered in a timely manner and not subject to thermal cycles due to the sun, it is tentatively recommended that for silty sand subgrade containing divalent cations:

• The subgrade should be compacted to give an upper lift thickness

• •

tent and the influence of daily thermal cycles to this conclusion if the GCL is in a composite liner that will be left exposed to the sun. Construction quality control inspectors ensure that the subgrade is compacted in accordance with specifications that will allow successful GCL prehydration. The GCL be selected with a bentonite grain size no coarser than shown for “fine” bentonite in Table 1, the needle-punch bundle diameter ≤1.1 ± 0.5 mm and a total area of bundles < 10% (steep side slopes may need a different GCL with greater needle punching). The use of a scrim-reinforced carrier and/or thermal treatment allows better fibre anchorage with less needle-punching and hence better confinement of the bentonite at low stress prehydration but is less critical with powdered bentonite and with granular bentonite.

not exceeding 150 mm at a dry density, ρd ≥ 95% standard Proctor optimum, at a water content, wfdn, greater than standard Proctor optimum, wopt, (wopt < wfdn < min{w95 or wfield capacity}) where the upper limit is the minimum of: (a) the upper water content, w95, in standard Proctor compaction test to give ρd = 95% standard Proctor optimum, and (b) field capacity for this soil (wfield capacity). The upper subgrade lift should be placed at a dry density high enough to prevent significant rutting of the subgrade at the time of covering with the GCL Given the potential for moisture loss due to evaporation before the GCL is placed, the upper lift should be placed just before the GCL is to be placed. If that is not possible and the water content drops below wopt, the water content can be increased by watering several times, allowing just enough time to infiltrate before rewatering, and then covering as soon as any puddles from watering have gone.

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