Geotechnical characterization of peat-based landfill cover materials

Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

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Geotechnical characterization of peat-based landfill cover materials Afshin Khoshand a, Mamadou Fall b, * a b

Faculty of Engineering, K.N. Toosi University of Technology, Tehran, Iran Department of Civil Engineering, University of Ottawa, Ottawa K1N 6N5, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 March 2016 Received in revised form 5 May 2016 Accepted 17 May 2016 Available online xxx

Natural methane (CH4) oxidation that is carried out through the use of landfill covers (biocovers) is a promising method for reducing CH4 emissions from landfills. Previous studies on peat-based landfill covers have mainly focused on their biochemical properties (e.g. CH4 oxidation capacity). However, the utilization of peat as a cover material also requires a solid understanding of its geotechnical properties (thermal, hydraulic, and mechanical), which are critical to the performance of any biocover. Therefore, the objective of this context is to investigate and assess the geotechnical properties of peat-based cover materials (peat, peatesand mixture), including compaction, consolidation, and hydraulic and thermal conductivities. The studied materials show high compressibility to the increase of vertical stress, with compression index (Cc) values ranging from 0.16 to 0.358. The compressibility is a function of sand content such that the peatesand mixture (1:3) has the lowest Cc value. Both the thermal and hydraulic conductivities are functions of moisture content, dry density, and sand content. The hydraulic conductivity varies from 1.74
109 m/s to 7.35
109 m/s, and increases with the increase in sand content. The thermal conductivity of the studied samples varies between 0.54 W/(m K) and 1.41 W/(m K) and it increases with the increases in moisture and sand contents. Increases in sand content generally increase the mechanical behavior of peat-based covers; however, they also cause relatively high hydraulic and thermal conductivities which are not favored properties for biocovers. Ó 2016 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Landfill Geotechnical engineering Landfill cover Peat Compaction Compressibility Hydraulic and thermal conductivity

1. Introduction Greenhouse gases (GHGs) reabsorb infrared radiation which is reflected from the Earth and retain heat in the lower level of the atmosphere. GHGs have both natural and anthropogenic sources, but human induced GHG emissions have increased, more than emissions from most natural sinks, particularly after the Industrial Revolution (Albanna et al., 2011). The continuous increase in concentration of GHGs has caused an increase of over 0.5  C in the global surface temperature in the last 150 years (Wuebbles and Hayhoe, 2002), a phenomenon known as global warming. Global warming is one of the greatest environmental challenges in the 21st century, which has caused global and regional climate changes (Peixoto and Oort, 1992; Albanna et al., 2010). Methane (CH4) is a potent GHG with a global warming potential that is 25 times that of carbon dioxide (CO2) (IPCC, 2007). The

* Corresponding author. Fax: þ1 613 562 5173. E-mail address: [email protected] (M. Fall). Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.

anaerobic biodegradation of municipal solid waste (MSW) in landfills is one of significant global sources of anthropogenic CH4 emissions. The total global CH4 emissions from landfills almost doubled during the period from 1970 to 2010. It has also been estimated that 627.34 tonnes CO2e (CO2e or equivalent CO2 is the concentration of CO2 that causes the same level of radiative forcing as a given type and concentration of GHG) per year is generated in landfills worldwide, of which more than 85% is emitted into the atmosphere (IPCC, 2014). Therefore, mitigation actions are urgently required in light of the significant levels of CH4 found in the atmosphere (Stern and Kaufmann, 1996). The extraction and utilization of landfill gas (LFG) are commonly used to control CH4 emissions from landfills. However, there is evidence that a large amount of CH4 escapes from sites despite the use of extraction and utilization systems (Börjesson et al., 2007). Therefore, one of the most promising methods that would actually reduce CH4 emissions from landfills is the natural processing of microbial CH4 oxidation through active biological soil covers or biocovers (Scheutz et al., 2009a, 2011). This oxidation process principally relies on the activity of a group of bacteria known as methanotrophs, which are able to use molecular oxygen

http://dx.doi.org/10.1016/j.jrmge.2016.05.007 1674-7755 Ó 2016 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

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A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

(O2) to oxidize CH4 into CO2. Biocovers are an alternative effective option for the mitigation of CH4 emissions where the implementation of LFG extraction and utilization systems is not technically or economically feasible (Scheutz et al., 2011). Previous studies have demonstrated that various organic soils (e.g. compost, peat, loam soil) can support the growth and activity of methanotrophic bacteria (Watson et al., 1997; Humer and Lechner, 1999; Stein and Hettiaratchi, 2001; Wilshusen et al., 2004; Einola, 2010; Gupta, 2011; He et al., 2012; Abushammala et al., 2014; Zainal and Buyong, 2015), and are suitable for practical applications in mitigating CH4 emissions. Peat is one of the more promising biocover materials. Indeed, peat is able to provide environmental conditions suitable for the proliferation and activity of methanotrophic bacteria (Stein and Hettiaratchi, 2001; Streese and Stegmann, 2003; Einola, 2010; Lu et al., 2011; Zainal and Buyong, 2015). Furthermore, many researchers (e.g. Stein and Hettiaratchi, 2001; Einola et al., 2009) have experimentally demonstrated that peat materials show a high CH4 oxidation rate (up to 90%) as illustrated in Table 1, which provides a summary of the CH4 oxidation rates of various biocover materials. It can be observed that the CH4 oxidation rate of peat (up to 90%) is close to that of compost (up to 100%) and much higher than that observed in other types of biocover materials (loam soil, topsoil, agricultural soil, and sand). However, for the peat biocover material to be of interest, aside from a high CH4 oxidation rate (Table 1), the material should demonstrate its geotechnical properties that are comparable to or better than those of existing biocover materials (particularly compost) used in construction practices. The geotechnical properties (mechanical, hydraulic, and thermal) of biocover materials are of prime importance for design, construction and maintenance of any biocover as will be discussed later. Absence of a proper understanding of the geotechnical properties of cover materials may lead to inaccurate biocover design and consequently, construction of inefficient biocovers. However, to date, the geotechnical characteristics of peat-based biocover material are not well understood. Therefore, the goal of this paper is to provide insight into the geotechnical properties of peat-based biocover materials and assess their suitability for use as biocover media from a geotechnical point of view. 2. Materials and methods 2.1. Material selection and characterization Mixing potential biocover materials with sand minimizes the settlement and compaction of biocovers (Powelson et al., 2006; Philopoulos et al., 2009; Scheutz et al., 2009a; Khoshand and Fall,

Table 1 Summary of CH4 oxidation rates of different organic soils. Biocover material

CH4 oxidation rate (%)

Reference

Loam soil

50 65 65 40 37 41 63 85 90 32 45 53 96 100

Stein and Hettiaratchi (2001) Scheutz et al. (2003) De Visscher et al. (1999) Kightley et al. (1995) Humer and Lechner (1999) Kightley et al. (1995) Powelson et al. (2006) Stein and Hettiaratchi (2001) Einola et al. (2009) Stein and Hettiaratchi (2001) De Visscher et al. (1999) Humer and Lechner (2001) Haubrichs and Widmann (2006) Philopoulos et al. (2009)

Topsoil Sand Peat Agricultural soil Compost

2014). Compaction is especially important when consideration is given to any field installations, as there will be some traffic on the surface of the medium (e.g. maintenance) (Philopoulos et al., 2009). Therefore, laboratory investigations have been conducted in this study on peat and peatesand mixture samples with ratios of 1:3, 1:1 and 3:1 (w/w). The aforementioned ratios are recommended in Pokhrel et al. (2011), a study on the CH4 oxidation capacity of different mixtures of potential biocover materials. Ottawa sand, obtained from Unimin Canada Ltd. is used in this study. The sand was oven-dried prior to use in the experiments in order to eliminate any methanotrophic bacteria that may be present in the sand. Also, the sand was free of organic content based on the results of laboratory tests performed in accordance with ASTM D2974-14 (2014). The peat soil samples were collected from the Moose Creek Bog in Moose Creek, Canada, which is owned and operated by Lafleche Environmental Inc. The peat samples were transported to a laboratory and stored at a temperature of 3  C before further characterization. The mineralogical compositions of the peat material and peate sand mixtures were determined by X-ray diffraction analyses and the results are presented in Table 2. The selected geotechnical properties of all the samples were determined in accordance with the procedures described by ASTM standards. A grain size analysis was performed in accordance with ASTM D422-63(1998) (1998). It can be seen in Fig. 1 that all of the samples have a grain size that ranges from 0.07 mm to 5 mm and the grain size distribution becomes coarser as the sand ratio is increased. The grain size distribution of the peat samples indicates that the percentage of grains that pass through the sieves Nos. 10, 40 and 100 is 79%, 22% and 7%, respectively. The pure peat sample in this study is classified as organic SW (well graded sand) and the rest of samples are organic SP (poorly graded sand) based on the Unified Soil Classification System (USCS). However, this classification system is not suited for organic soils because samples are only considered as peat when the organic content is more than 75%. Therefore, a classification system proposed by Wüst et al. (2003), based on the ash and organic contents of peats, is also used in this study. Based on this classification, peat and peatesand mixture with a mix ratio of 3:1 are considered as peat, while peatesand mixtures with mix ratios of 1:1 and 1:3 are considered as muck. A summary of the properties and pH value of the samples are shown in Table 3. The pH value of the peat sample is 6.72, which falls within the range quoted by Cola and Cortellazzo (2005). 2.2. Methods 2.2.1. Compaction test In order to experimentally determine the values of the optimum moisture content and corresponding maximum dry density of the studied materials, standard Proctor compaction tests were performed in accordance with ASTM D698-12 (2012). 2.2.2. Consolidation test Conventional consolidation tests were performed on the samples at the dry side of the optimum moisture content and the optimum moisture content in accordance with ASTM D2435/ D2435M-11 (2011) to determine the consolidation characteristics of the peat and its mixture samples. The dried samples were moistened and compacted to reach the desired densities for specific moisture contents that correspond to standard Proctor compaction test results. Each test consisted of five increments of loading (5 kPa, 10 kPa, 20 kPa, 40 kPa and 80 kPa) and the duration of each loading was 24 h to ensure that the long-term compressibility of the samples was properly simulated (Moo-Young and Zimmie, 1996). Each test was repeated twice.

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

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Table 2 Chemical compositions of peat and its mixtures used in this study. Material

Contents (%)

Peat Peatesand (3:1 w/w) Peatesand (1:1 w/w) Peatesand (1:3 w/w)

LOI (%)

SiO2

Al2O3

Fe2O3

Na2O

MgO

CaO

K2O

TiO2

P2O5

Cr2O3

MnO

V2O5

11.7 31.51 51.2 70.79

2.49 3.25 3.97 4.71

1.04 0.8 0.57 0.35

0.62 0.65 0.67 0.69

0.85 0.66 0.45 0.24

5.13 3.96 2.76 1.56

0.68 1.15 1.61 2.08

0.09 0.07 0.06 0.04

0.3 0.22 0.15 0.08

<0.01 <0.01 <0.01 <0.01

0.03 0.02 0.01 <0.01

<0.01 <0.01 <0.01 <0.01

76.1 57.16 38.21 19.27

Note: LOI is the loss in ignition (%).

3. Results and discussion

90 80

3.1. Compaction characteristics

Peat

70

Peat-sand (3:1 w/w)

60

Peat-sand (1:1 w/w) Peat-sand (1:3 w/w)

50 40 30 20 10 0 0.01

0.1

1

10

Grain size (mm) Fig. 1. Grain size distribution of peat and its mixtures.

2.2.3. Hydraulic conductivity test Hydraulic conductivity tests were performed on all of the samples in accordance with the procedures that are outlined in ASTM D5084-10 (2010). Hydraulic conductivity was measured by using a flexible wall permeameter on the samples (with dimensions of 115 mm in height and 50 mm in diameter) at the optimum, dry side of the optimum, and wet side of the optimum moisture contents and related densities based on the compaction curves. In order to prevent anomalies during the flow rate measurements, a low hydraulic gradient (approximately 10) was maintained in all of the tests (Benson and Othman, 1993). Also, for each sample, hydraulic conductivity tests were repeated until three values were derived which fell within 10% of each other (minimum three and maximum five replicates for each sample). The average value was considered as the saturated hydraulic conductivity of the sample tested.

Fig. 2 illustrates the compaction curves of peat and its mixtures. The maximum dry density of the peat and peatesand mixtures with mix ratios of 3:1, 1:1 and 1:3 is 402 kg/m3, 570 kg/m3, 709 kg/ m3 and 1004 kg/m3, and the corresponding optimum moisture content is 96%, 82%, 77% and 64%, respectively. The maximum dry density value of the peat is approximately 42%, 76% and 150% lower than that of the peatesand mixtures with mix ratios of 3:1, 1:1 and 1:3, respectively. Said and Taib (2009) also reported similar compaction behavior (similar ranges of maximum dry density and optimum moisture content) for peaty soils. The maximum dry density increases when the sand content in the mixtures is increased. Pure peat has the lowest maximum dry

1050 1000

Dry density (kg/m3)

Percentage of finer by weight (%)

100

Peat

950

Peat-sand (3:1 w/w)

900

Peat-sand (1:1 w/w)

850

Peat-sand (1:3 w/w)

800 750 700 650 600 550

2.2.4. Thermal conductivity test The materials were first air-dried and then kept dry for 10 d to reach thermal equilibrium. Afterwards, the samples were moistened and compacted to reach the desired densities in accordance with the compaction curves. The thermal conductivity of the samples was determined based on the non-steady state method under isothermal conditions by using the KD2 device (Decagon Devices, Inc., 2006). Each thermal conductivity test was performed two to four times to ensure the repeatability of the results.

500 450 400 350 20

40

60

80

100

120

140

160

Water content (%) Fig. 2. Compaction curves of peat and peatesand mixtures.

Table 3 Properties and pH value of peat and its mixtures. Material

Initial moisture content, w (%)

Organic content (%)

pH (in H2O)

Specific gravity, Gs

Dry bulk density (kg/m3)

WHC (%)

Peat Peatesand (3:1 w/w) Peatesand (1:1 w/w) Peatesand (1:3 w/w)

195.48 145.43 98.63 48.93

69.73 51.92 35.35 19.12

6.72 6.75 6.81 6.91

1.65 1.9 2.15 2.4

375.4 659.5 943.7 1227.8

218.86 166.67 137.08 94.41

Note: WHC is the water holding capacity.

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

3.2. Consolidation characteristics Consolidation can significantly change the porosity of biocovers. The gas transport process in biocovers (e.g. advection flux of CH4 (upward) and diffusion flux of oxygen (downward) into the CH4 oxidation zone) and consequently the performance (CH4 oxidation capacity) of biocovers is strongly affected by their porosity (Pedersen et al., 2011). Moreover, the strain due to consolidation could also translate into crack development in the biocovers which can affect their physical stability (Bajwa, 2012) as well as the amount of CH4 emitted into the atmosphere. These cracks may provide the preferential pathways for the escape of CH4 into the atmosphere. Thus, there is a need to understand the consolidation characteristics of peat-based biocover materials. Here, the compressibility of the studied materials is categorized as primary consolidation and secondary consolidation (Mesri and Ajlouni, 2007). 3.2.1. Primary consolidation Primary consolidation is defined by the variation of the void ratio and a logarithm of the consolidation pressure as shown in Fig. 4. It is clear from Fig. 4 that the void ratio of samples is reduced when there is an increase in the moisture content and consolidation pressure. The lowest void ratio values (2.57, 1.87, 1.61 and 1.11 for peat and peatesand mixtures with mix ratios of 3:1, 1:1 and 1:3, respectively) can be observed at higher moisture contents and a consolidation pressure of 80 kPa. The calculation of the coefficient of consolidation (Cv) was carried out based on the square root of time (Taylor’s) method. It is evident from Table 4 that there is a significant reduction in the Cv value of the samples with an increase in consolidation pressure, and this reduction is more pronounced in the samples with less sand content. The compression index (Cc) and recompression index (Cr) values are presented in Table 5. The Cc value, which reflects the compressibility of the materials, ranges from 0.358 to 0.134 in this study. This is a lower range compared with those obtained by Kazemian et al. (2011) and Ulusay et al. (2010) because the pre-compaction of the studied

80

(a)

1

50

40 30

0.6

40

0.4

30

Sr

50

20 10

0 40

50

Sr FAS 70

80

Water content (%)

70

80

90

100 110

90

100

1

90

(d)

80

0.8

70 60

0.6

50

Sr

(c)

60

60

Water content (%) 100 90 80 70 60 50 40 30 20 10 0

FAS (%)

Sr

Water content (%)

50

10

0

30 40 50 60 70 80 90 100 110 120 130

40

20

Sr FAS

0.2

0

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

60

0.8

60

Sr FAS

70

(b)

70

FAS (%)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

FAS (%)

Sr

density value in comparison to the other mixtures which can be attributed to its lower specific gravity (1.65). During the compaction process, samples with lower sand content required the addition of more water but were still compactable while those with higher sand content required less water and were less compactable. Compared to pure peat, the peatesand mixtures have lower optimum moisture contents and are less compactable (Stone and Ekwue, 1993). In biocovers, the CH4 oxidation rate is significantly influenced by oxygen penetration. The parameters that can control oxygen penetration and therefore the CH4 oxidation rate are related to the free air space (FAS) and the degree of saturation (Sr) of biocover media. The diffusivity of biocover media is regulated by the degree of saturation (Sr) (Gebert et al., 2011) because saturation creates water menisci which cause discontinuous diffusion pathways (Moldrup et al., 2001). Therefore, by varying the FAS and Sr with moisture content for the studied materials, the significance of these parameters is taken into account, as shown in Fig. 3. This figure shows that increases in moisture content reduce the FAS to a value close to zero at the wet side of the optimum moisture content. Consequently, the air phase in the pores becomes occluded (Nagaraj et al., 2006) and the gases have to diffuse in the liquid phase. The gaseous diffusion in the liquid phase is much less than that in air (Gebert et al., 2011), which slows the diffusive migration of oxygen through the biocover media and greatly reduces the CH4 oxidation rate. The O2 consumption rate reaches the maximum value when the FAS is between 20% and 35% (Haug, 1993). So, it can be concluded from Fig. 3 that the maximum O2 consumption rate (and thereby the highest CH4 oxidation rate) in the peat and peatesand samples with mix ratios of 3:1, 1:1 and 1:3 should be expected when the moisture content is 85%e110%, 70%e85%, 80%e95% and 65%e 75%, respectively. It can be clearly observed that the range of moisture content associated with the maximum rate of O2 consumption in the studied samples generally decreases with the increase in sand content.

40

0.4

30

Sr FAS

0.2

FAS (%)

4

20 10

0

0 30

40

50

60

70

80

90

Water content (%)

Fig. 3. Relationship between degree of saturation, free air space and moisture content: (a) peat; (b) peatesand (3:1 w/w); (c) peatesand (1:1 w/w); and (d) peatesand (1:3 w/w).

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

3.00 2.95

2.15

Void ratio, e

2.85 2.80 2.75 2.70

2.10 2.05 2.00

2.65

1.95

2.60

1.90

2.55

1.85 1

2.00

10 Consolidation pressure (kPa)

(c)

1.95

100

1

1.35

w=59% w=77%

1.90

10 Consolidation pressure (kPa)

1.85 1.80 1.75 1.70 1.65

100

w=50%

(d)

w=64%

1.30

Void ratio, e

Void ratio, e

w=62% w=82%

(b)

2.20

w=96%

2.90

Void ratio, e

2.25

w=70%

(a)

5

1.25 1.20 1.15

1.60 1.10

1.55 1

10 Consolidation pressure (kPa)

100

1

10 Consolidation pressure (kPa)

100

Fig. 4. Consolidation curves of (a) peat, (b) peatesand (3:1 w/w), (c) peatesand (1:1 w/w), and (d) peatesand (1:3 w/w).

peat material to the dry side of the optimum moisture content has significantly reduced its initial void ratio, which is significantly lower than that of the peat material investigated in previous studies (e.g. Ulusay et al., 2010; Kazemian et al., 2011). Table 4 Coefficients of consolidation of peat and its mixtures. Consolidation pressure (kPa)

5 10 20 40 80

Peat

Peatesand (3:1)

Peatesand (1:1)

Peatesand (1:3)

w (%)

Cv (m2/yr)

w (%)

Cv (m2/yr)

w (%)

Cv (m2/yr)

w (%)

Cv (m2/yr)

70 96 70 96 70 96 70 96 70 96

3.34 3.24 3.06 2.89 2.79 2.70 2.65 2.50 2.34 2.25

62 82 62 82 62 82 62 82 62 82

3.09 3.01 2.75 2.69 2.59 2.52 2.42 2.34 2.28 2.19

59 77 59 77 59 77 59 77 59 77

2.92 2.83 2.71 2.54 2.55 2.42 2.31 2.23 2.17 2.06

50 64 50 64 50 64 50 64 50 64

2.64 2.52 2.40 2.26 2.15 2.10 2.01 1.95 1.91 1.83

The primary consolidation and associated settlement can be explained in terms of the Cc value, in that a higher Cc value results in larger primary settlement. It can be clearly observed from Table 5 that there is a general trend of Cc decreasing with increasing sand content. The Cc value of the peat samples is the highest among all of the studied materials (Cc of the peat samples is up to 124% higher than that of peatesand samples) (Table 5) and subsequently, the peat samples experience larger primary settlement that could increase the risk of crack formation and/or significantly reduce the FAS, and thereby reduce the efficiency of CH4 oxidation. It is clear from Table 5 that the Cc values generally increase with an increase in initial moisture content because the high moisture content of peat is related to more water in the pores (Huat et al., 2011). However, the pore water can be easily dissipated during consolidation due to the increase in vertical stress. As shown in Table 5, the Cr values of the samples show that the swelling potential of the materials is low for the studied materials. These Cr values are also consistent with those reported by Mesri and Ajlouni (2007). It is also clear that the Cr value decreases as the sand content is increased, which means that the samples with a higher sand content have less swelling potential.

Table 5 Compression and recompression indices of peat and its mixtures. Sample

w (%)

Cc

Cr

Peat

70 96 62 82 59 77 50 64

0.308 0.358 0.312 0.249 0.231 0.272 0.134 0.16

0.034 0.063 0.053 0.056 0.044 0.048 0.029 0.021

Peatesand (3:1) Peaesand (1:1) Peatesand (1:3)

3.2.2. Secondary consolidation Secondary consolidation is the ratio of the coefficient of the secondary consolidation (Ca) to Cc (Mesri and Ajlouni, 2007). The variations in Ca/Cc for peat and peatesand mixtures at different stress levels (5e80 kPa) are shown in Fig. 5, which indicates that the values of Ca/Cc for the studied samples decrease when the consolidation pressure is increased. Ulusay et al. (2010) also reported similar observations for peat materials. However, most of the previous studies obtained Ca/Cc values of 0.06  0.01 or even

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

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0.065

Peat - w=70%

0.060

Peat - w=96%

0.055 Peat-sand (3:1) - w=62%

Cα /Cc

0.050

Peat-sand (3:1) - w=82%

0.045 0.040

Peat-sand (1:1) - w=59%

0.035

Peat-sand (1:1) - w=77%

0.030

Peat-sand (1:3) - w=50%

0.025

Peat-sand (1:3) - w=64%

0.020 0

10

20

30 40 50 60 Consolidation pressure (kPa)

70

80

90

Fig. 5. Variations in Ca/Cc with consolidation pressure for the studied peat and peatesand mixtures.

higher (up to 0.1) for peat materials (Mesri and Ajlouni, 2007; Ulusay et al., 2010). Duraisamy et al. (2007) reported even lower values of Ca/Cc (0.027 and 0.038) for some of their peat samples. In the current study, the values of Ca/Cc for the peat samples range from 0.048 to 0.061, so the obtained values are close to those of Mesri and Ajlouni (2007). The values of Ca/Cc for different geotechnical materials vary in a narrow range of 0.01e0.1 (Ulusay et al., 2010). The magnitude of Ca/Cc can be used to explain the deformability and compressibility of soils (Mesri and Ajlouni, 2007). In the current study, the peat samples consist of deformable grains and thereby are highly compressible and have the highest values of Ca/Cc. Increases in the sand content result in a lower range of Ca/Cc values because granular sand materials such as sands are less deformable, with Ca/Cc ¼ 0.02  0.01 (Mesri and Vardhanabhuti, 2009). Secondary compression not only has a negative effect on the gas exchange process and thereby the CH4 oxidation potential of biocovers, but also affects the pore structure (Bajwa, 2012) and consequently the physical stability of biocovers. Based on the obtained values of Ca/Cc in the current study, the peat experiences more secondary settlement among the studied materials which could increase the risk of the physical failure of peat biocovers mainly due to crack development and changes in the pore structure. 3.3. Hydraulic conductivity Biocovers should also have an appropriate hydraulic conductivity to minimize rainfall infiltration and subsequent biocover saturation, which can result in a drastic reduction in the CH4 oxidation rate (Pokhrel, 2006; Scheutz et al., 2009a,b; Khoshand and Fall, 2014). Therefore, changes in the hydraulic conductivity of biocovers have important effects on the CH4 oxidation rate and water intrusion. Fig. 6 shows the test results of saturated hydraulic conductivity for peat and its mixtures in the current study. It can be observed that the hydraulic conductivity values range from 1.74
109 m/s to 7.35
109 m/s while the moisture contents vary in an approximate range of 45%e132%. As illustrated in Fig. 6, the hydraulic conductivity of the specimens is reduced when the moisture content and dry density are increased until reaching the optimum moisture content and the maximum dry density. Afterward, the hydraulic conductivity is increased for moisture contents that exceed the optimum one. The minimum hydraulic conductivities (1.74
109 m/s, 2.62
109 m/s, 4.45
109 m/s and 6.26
109 m/s for peat and

Fig. 6. Hydraulic conductivity versus moisture content.

peatesand mixtures with mix ratios of 3:1, 1:1 and 1:3, respectively) were achieved at the optimum moisture content. Moo-Young and Zimmie (1996) studied the geotechnical behavior of potential landfill covers and reported a similar trend in the variation of the hydraulic conductivity with moisture content. The hydraulic conductivity of peat is a function of the void ratio and the size and shape of the flow channels (Mesri and Ajlouni, 2007). The grains of peat are naturally porous and subsequently have a large pore size which results in a high initial hydraulic conductivity at low dry density (Huat et al., 2011). The dry density is increased due to compaction until the maximum dry density is obtained. This causes smaller pores and tortuous flow channels through the samples which eventually result in a significant reduction in the hydraulic conductivity to a comparable value of clay soils (Huat et al., 2011). The hydraulic conductivity of peat varies in the range of 1.74
109e2.33
109 m/s while the hydraulic conductivity of

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

0.7

395

0.6

390 0.5

385

Dry density Thermal conductivity

380 375

20

Dry density (kg/m3)

715

40

60

80 100 120 Water content (%)

0.4

140

160

1.2

(c)

710

1.1

705

1

700

0.9

695

0.8

690 685 680

0.7

Dry density Thermal conductivity

30

40

50

60 70 80 90 Water content (%)

0.6 100 110

0.5

0.9

(b)

570 0.8 565 0.7

560 555

Dry density Thermal conductivity

550

0.3

Thermal conductivity, k (W/(m K))

Dry density (kg/m3)

400

575

545

40

1010

60

0.6

80 100 Water content (%)

120

0.5

1.7

(d)

1.6

1000

1.5

990

1.4

980

1.3

970

1.2 1.1

960

940

1

Dry density Thermal conductivity

950 30

40

Thermal conductivity, k (W/(m K))

0.8

(a)

Dry density (kg/m3)

405

Thermal conductivity, k (W/(m K))

Knowledge of the thermal conductivity of the biocover material is essential for assessing and predicting the evolution and distribution of temperature in biocovers. The simultaneous variation of thermal conductivity and dry density with moisture content is shown in Fig. 7. The thermal conductivity of the peat samples varies from 0.54 W/(m K) to 0.71 W/(m K) when the moisture content ranges from 60% to 132% (Fig. 7). The obtained results are close to those reported by Dissanayaka et al. (2012) for pure peat in wet conditions (0.1e0.6 W/(m K)). It can be seen in Fig. 7 that the thermal conductivity of the studied peat-based materials is influenced by the moisture content such that the thermal conductivity consistently increases as the moisture content increases. It can also be seen from Fig. 7 that the minimum thermal conductivities (0.54 W/(m K), 0.7 W/(m K), 0.87 W/(m K) and 1.17 W/(m K) for peat and peatesand mixtures with mix ratios of 3:1, 1:1 and 1:3,

50

60 70 80 Water content (%)

0.9 90

100

0.8

Thermal conductivity, k (W/(m K))

3.4. Thermal conductivity

respectively) are obtained at the minimum moisture contents because the thermal conductivity of water (0.59 W/(m K)) is more than 20 times greater than that of air (0.025 W/(m K)) (Holman, 2002). By increasing the moisture content, the volume fraction of air is decreased and the proportion of the pore spaces filled with water is increased (Ahn et al., 2009). So the water films completely surround the soil grains and improve the contact between them which results in increases in thermal conductivity (Abu-Hamdeh and Reeder, 2000). A similar trend in the variation of the thermal conductivity with moisture content has been reported in previous studies (Ahn et al., 2009; Chandrakanthi et al., 2005). Furthermore, Fig. 7 shows that the thermal conductivity increases with the increases in the dry density of the samples because generally, when the density of the soil samples increases to the maximum dry density due to compaction, the degree of inter-particle contact increases and contact between the individual grains becomes more intimate (Al Nakshabandi and Kohnke, 1965), which facilitates heat movement through the soil and results in an increase in thermal conductivity. After the maximum density is reached, the excess water causes higher pore water pressure that results in less compactability and subsequently a reduction in density (Ekwue et al., 2011). However, as the density is reduced, the moisture contents still increase and there is less contact between the individual grains due to the presence of water between the soil grains. Whereas water has a thermal conductivity greater than that of air and considerably lower than that of mineral soil particles (e.g. 2.9 W/(m K) for silt and clay, 3 W/(m K) for sandstone, and 3.8 W/(m K) for dolostone (Ramires et al., 1995; Côté and Konrad, 2005)), so the thermal conductivity continues to increase slightly after the optimum moisture content is reached (Fig. 7). Ekwue et al. (2011) also reported that the effect of density variation on the thermal conductivity is considerably less than that of moisture variation.

Dry density (kg/m3)

peatesand mixture with a mix ratio of 1:3 ranges from 6.29
109 m/s to 7.35
109 m/s. The hydraulic conductivity of peat is controlled by the physical and structural arrangement of the constituent grains (Edil, 2003), which can affect the pore size distribution and the size and shape of the flow channels. Increasing the sand content of the samples causes larger void ratios, pores and straight flow channels through the samples, which result in higher hydraulic conductivity. The interlinked fibrous structure of pure peat, which has a clogging effect (Edil, 2003), is another possible reason accounting for the lower hydraulic conductivity of peat in comparison to that of peatesand mixtures. However, an increase in sand content increases the hydraulic conductivity and consequently promotes the gas exchange process within peat-based biocovers. Nevertheless, the hydraulic conductivity of biocovers needs to be low enough to prevent water infiltration.

7

Fig. 7. Changes in thermal conductivity and dry density with water content: (a) peat; (b) peatesand (3:1 w/w); (c) peatesand (1:1 w/w); and (d) peatesand (1:3 w/w).

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007

8

A. Khoshand, M. Fall / Journal of Rock Mechanics and Geotechnical Engineering xxx (2016) 1e9

1.6

Peat Peat-sand (3:1 w/w)

Thermal conductivity, k (W/(m K))

1.4

Peat-sand (1:1 w/w) Peat-sand (1:3 w/w)

1.2 1 0.8 0.6 0.4 0.2

0.2

0.4

0.6 Degree of saturation, Sr

0.8

1

Fig. 8. Changes in thermal conductivity with degree of saturation.

Fig. 8 shows the variation of thermal conductivity as a function of Sr. It can be seen that in peat-based biocovers, the thermal conductivity increases with the increase in Sr. As mentioned above, the thermal conductivity of water is significantly greater than that of air. At a low value of Sr, there are insufficient water molecules to fill the air gaps between the grains and the thermal conductivity is low. However, by increasing Sr, the air gaps are filled with water and the contact between the grains is increased and subsequently the thermal conductivity is increased (Al Nakshabandi and Kohnke, 1965). The obtained results indicate that the thermal conductivity of the studied materials is strongly influenced by the sand content such that the thermal conductivity significantly increases with increasing sand content. This is caused by the lower thermal conductivity of peat materials in comparison to that of mineral soils (e.g. 0.05e0.6 W/(m K) (Dissanayaka et al., 2012), and 0.2e0.52 W/ (m K) (Kujala et al., 2008) for peat versus 2.9 W/(m K) for silt and clay, 3 W/(m K) for sandstone, and 3.8 W/(m K) for dolostone (Côté and Konrad, 2005)). It can be concluded that the peat material as opposed to peate sand mixtures conducts less heat and thereby provides better temperature insulation, which can ensure an appropriate and stable temperature for methanotrophic bacteria within the biocover, an especially important factor for cold climates when the atmospheric temperature is low. 4. Conclusions In this context, a comprehensive assessment and comparison of compaction and consolidation behaviors, and hydraulic and thermal conductivities of peat-based materials, i.e. peat and peatesand mixtures with a mix ratio of 1:3, 1:1, and 3:1, respectively, for biocovers are made. The key conclusions based on the analyses of the obtained results are drawn as follows. The optimum moisture content of the peat-based materials varies from 64% to 96% while the maximum dry density ranges from 402 kg/m3 to 1004 kg/m3. The maximum dry density and compactability decrease with increasing sand content. Moreover, the variations in FAS with moisture content suggests that the maximum O2 consumption rate (thereby the highest CH4 oxidation potential) in the peat and peatesand samples with mix ratios of 3:1, 1:1 and 1:3 can be realized when the moisture content is 85%e 110%, 70%e85%, 80%e95% and 65%e75%, respectively.

The studied peat-based materials are compressible. The samples show high compressibility to the increase in consolidation pressure, and higher initial moisture contents and organic contents result in higher compressibility. It should be noted that after loading, the primary consolidation of the studied samples is completed in a relatively short time, followed by secondary consolidation. The value of Ca/Cc of the samples is in the range of 0.061e0.03. This parameter is a function of the consolidation pressure and organic content. The obtained values of Ca/Cc suggest that pure peat has the largest secondary settlement among the studied materials (peatesand mixtures) which results in higher potential of failure of the pure peat-based biocovers due to the development of cracks or changes in the pore structure. However, this can be mitigated by increasing the sand content. The hydraulic conductivity values range from 1.74
109 m/s to 7.35
109 m/s while the moisture contents vary in an approximate range of 45%e132%. The minimum hydraulic conductivity is achieved at the optimum moisture content and decreases when the moisture content is increased to the wet side of the optimum moisture content. The thermal conductivity increases with increasing bulk density, and moisture and sand contents. However, the effect of the variation in bulk density on thermal conductivity is considerably less than that of the moisture content variation. It is found that pure peat materials can provide better temperature insulation and consequently a more stable temperature within biocovers than the peatesand biocover materials due to their low thermal conductivity which is particularly important in cold climates. Conflict of interest The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. References Abu-Hamdeh NH, Reeder RC. Soil thermal conductivity: effects of density, moisture, salt concentration, and organic matter. Soil Science Society of America Journal 2000;64(4):1285e90. Abushammala MFM, Basri NEA, Irwan D, Younes MK. Methane oxidation in landfill cover soils: a review. Asian Journal of Atmospheric Environment 2014;8(1):1e14. Ahn HK, Sauer TJ, Richard TL, Glanville TD. Determination of thermal properties of composting bulking materials. Bioresource Technology 2009;100(17):3974e81. Al Nakshabandi G, Kohnke H. Thermal conductivity and diffusivity of soils as related to moisture tension and other physical properties. Agricultural Meteorology 1965;2(4):271e9. Albanna M, Warith M, Fernandes L. Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers. Waste Management 2010;30(2):219e27. Albanna M, Warith M, Fernandes L. The application of artificial neural networks for simulation of biological CH4 oxidation in landfill biocovers. International Journal of Sustainable Water and Environmental Systems 2011;3(1):37e44. ASTM D2435/D2435M-11. Standard test methods for one-dimensional consolidation properties of soils using incremental loading. West Conshohocken, USA: American Society of Testing and Materials (ASTM) International; 2011. ASTM D2974-14. Standard test methods for moisture, ash, and organic matter of peat and other organic soils. West Conshohocken, USA: ASTM International; 2014. ASTM D422-63(1998). Standard test method for particle-size analysis of soils. West Conshohocken, USA: ASTM International; 1998. ASTM D5084-10. Standard test methods for measurement of hydraulic conductivity of saturated porous materials using a flexible wall permeameter. West Conshohocken, USA: ASTM International; 2010. ASTM D698-12. Standard test methods for laboratory compaction characteristics of soil using standard effort (12 400 ft-lbf/ft3 (600 kN-m/m3)). West Conshohocken, USA: ASTM International; 2012. Bajwa TM. Experimental characterization of the thermal, hydraulic and mechanical (THM) properties of compost based landfill covers. MS Thesis. Ottawa, Canada: University of Ottawa; 2012. p. 47e101.

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Dr. Mamadou Fall is a Full Professor in the Department of Civil Engineering at the University of Ottawa (Canada) and the Director of the Ottawa-Carleton Institute for Environmental Engineering. He obtained his PhD degree in Geotechnical Engineering from the Freiberg University in Germany. He was the Coordinator of the German Research Chair of Environmental Geosciences and Geotechnics. Dr. Fall has over 20 years’ experience on fundamental and applied research in geotechnical engineering. He has been leading several major research projects that are related to mine waste management, geotechnical hazards and risks, underground disposal of nuclear wastes, carbon sequestration and engineered landfill technology. He is currently supervising a large research team of postdoctoral researchers and graduate students (PhD and Masters). His team is performing leading edge research in the geotechnical and geoenvironmental fields in close collaboration with the industry, major federal and provincial governmental institutions, and international partners. Dr. Fall has over 150 publications to his credit. He has been involved in the organization of numerous workshops, seminars, and national and international conferences. He has been repeatedly invited as keynote speaker or lecturer, and regularly acts as a consultant as well as a reviewer for several scientific committees, peer review journals, and funding agencies. He also serves on the editorial board of several international journals.

Please cite this article in press as: Khoshand A, Fall M, Geotechnical characterization of peat-based landfill cover materials, Journal of Rock Mechanics and Geotechnical Engineering (2016), http://dx.doi.org/10.1016/j.jrmge.2016.05.007