3 In municipal solid waste (MSW) landfills, biodegradation of the organic MSW fraction 4 results in elevated waste and basal liner temperatures which have the potential to cause the clay 5 component of the basal liner to experience severe moisture loss over time and eventually 6 undergo desiccation cracking. Cracking of the basal liner’s clay component would result in an 7 uncontrolled release of contaminants into the surrounding environment an d ultimately give rise 8 to a variety of major environmental concern s. Accordingly, this study examined the variation 9 of temperature -moisture profiles along the depth of a compacted clay liner (CCL) exposed to 10 different constant elevated waste temperatures ( CETs) in the absence and presence of two heat 11 reduction techniques , respectively. Rockwool insulation layers with varying thickness es and 12 galvanized steel cooling pipes with varying flowrate s were introduced separately as the two 13 heat reduction techniques . Introduction of both techniques led to a significant attenuat ion of 14 the temperature rise and desiccation experienced by the CCL in the face of different CETs. An 15 increase in rockwool thickness increments led to a progressive red uction of CCL temperature , 16 while an increase in flow rate under turbulen t condition did not have a significant influence on 17 the temperature and desiccation reduction of the CCL . Nevertheless, the present study certainly 18 highlights the potential of the two proposed heat reduction techniques to minimize desiccation 19 and consequently increase the service life of CCLs exposed to different elevated temperatures 20 in MSW landfills. 21 Keywords : Waste ; compacted clay liner ; temperature; desiccation; rockwool; cooling pipe 22 23 24 2 1. Introduction 25 The waste stored within a municipal solid waste landfill (MSWL ) is typically isolated 26 from the surrounding environment using a liner built into the base of the landfill, which 27 essentially functions as a barrier between the environment and the municipal solid waste 28 (MSW ). The prime purpose of a basal liner is to prevent the migration of leachate present 29 within the MSW into groundwater and protect it from contamination . Over time, the 30 exothermic reactions related to aluminum production waste, incinerator ash, landfilled hot 31 wastes, iron waste, bottom ash, tires, flue gas desulfurization gypsum, steel mill slag, petroleum 32 coke, lime kiln dust, fluidized bed combustion resi dues, dried wastewater sludge and 33 biodegradation of the organic content generates heat , which eventually leads to elevated waste 34 and basal liner temperature [1, 2] . Researchers have reported w aste temperatures of 35°C - 35 135°C and basal liner temperatures of 30°C -70°C in the past [2-9]. Meanwhile, the 36 groundwater table is known to possess a relatively cool temperature of 5 – 25°C [10-12]. 37 Consequently, t he significant tempe rature difference between the heated basal liner and the 38 cool groundwater table leads to the inevitable development of thermal gradients across the liner 39 and the subsoil as heat flows downward from the liner ( high- temperature region) to the 40 groundwater table ( low temperature region) as per the second law of thermodynamics. 41 Thermal gradients in turn trigger the downward diffusion of water vapour from the 42 warm ed basal liner towards the cooler groundwater table (where it condenses), owing to the 43 vapour density dependence on temperature . The m igration of water vapour results in a 44 significant reduction of moisture content in the warmed liner and leads to the development of 45 capillary pressure gradients across the liner . The capillary pressure gradient then causes the 46 liquid water to flow upwards in an attempt to balance the downward flux of water vapour. The 47 reduced water content owing to downward vapour diffusion results in reduced unsaturated 48 hydraulic conductivity which ultimately limits the volume of liquid wa ter that reach es the warm 49 3 liner. Ultimately, the inability of the upward water flux to balance the downward vapour 50 diffusion flux causes the clay component of the liner to experience desiccation cracking . This 51 process is accelerated if the basal liner comp rises a geomembrane, since the geomembrane ( due 52 to its impermeability ) limits the transport of water vapour to the liner’s clay component from 53 the top portion of the landfill [13-15]. Cracking of the basal liner’s clay componen t would 54 result in an uncontrolled release of leachate to the surrounding environment , which would 55 ultimately give rise to a variety of major environmental concerns such as groundwater 56 pollution, soil contamination, vegetation damage, and unpleasant odours etc. [16-19]. Fig. 1 57 illustrates the basic desiccation mechanism of a liner exposed to elevated waste temperatures 58 in a typical MSW landfill (note that the diagram does not include all landfill components for 59 the sake of clarity and simplicity). 60 61 Fig. 1. Desiccation mechanism of liner exposed to elevated waste temperature 62 Deployment of landfill liners comprising geosynthetic components (such as 63 geomembranes , geonets , geotextiles etc.) in “low- and middle -income ” countries is often found 64 to be economically unfeasible due to the high material and installation costs involved. 65 Final cover Subsoil Groundwater Heat flow MSW (35°C -135°C) Highest temperature at waste centre Liquid water capillary rise Water vapour diffusion Liner (30°C -70°C ) Desiccation cracking 4 Moreover, the installation of liners comprising geosynthetic materials require s a very high level 66 of quality control with regard to site preparation, installation, and protection control . Hence, 67 the use of geosynthetic lining materials is generally not recommended in low- and middle - 68 income countries [20-23]. As a result , in such countries, compacted clay liners (CCLs) are 69 found to be the most commonly used landfill liner system. While CCL is less expensive, it also 70 possesses the required qualities of a suitable liner, such as good attenuation capacity and low 71 permeability. Moreover , the deployment of CCLs as landfill liners in low - and middle -income 72 countries usually enables the utilization of locally available materials, workmanship, 73 equipment, and technology [20, 23, 24] . 74 A total of 125 countries out of 179 surveyed countries in the world (i.e., nearly 70%) 75 fall under the low - and middle -income categor ies, when classified by the development level of 76 gross national income per capita (GNI) [25]. Hence, it is safe to presume that at present, a large 77 portion of the world most likely still employ s CCLs as the ir default landfill liner. Yet , 78 surprisingly, studies that have examined the effect of elevated waste temperature s on CCL 79 temperature -moisture dis tributions in landfills were found to be incredibly scarce within the 80 literature. Holzlöhner [26] applied a temperature gradient to a heavy clay column compacted 81 within a polyvinyl chloride tube and observed that the clay column shrank as a result of 82 desiccation to such an extent that it was separat ed from the top and sides of the tube . Holzlöhner 83 et al. [27] subjected a range of soils (from sand -silt to high -plasticity clay) to temperature 84 gradients to calculate diffusion coefficients under the effect of temperature and moisture 85 gradient s. Abdulr ahman [28] exposed compacted clay samples to elevated temperatures for 86 over two months and found that the hydraulic conductivity rises with increasing temperature. 87 Abdulrahman [28] attributed the observed permeability rise to micro -crack generation (caused 88 due to desiccation) and increased viscosity of the permeant that aided its movement through 89 the clay matrix . However, none of the studies conducted by Abdulrahman [28] ; Holzlöhner 90 5 [26]; and Holzlöhner et al. [27] involved a comprehensive investigation into the variation of 91 temperature -moisture profiles along the depth of the clay liner when exposed to different 92 elevated waste temperatures within a MSW landfill. 93 Over the past few years, researchers have also proposed several methods to reduce 94 heating and desiccation of basal lining systems when exposed to elevated temperatures in 95 various waste containment facilities . El-Zein et al. [29] and Yu et al. [30] showe d that 96 increasing the bentonite mass per unit area of a GCL (by approximately 10%) leads to a 97 reduc tion in its desiccation crack area. Bouazza et al. [31] ; El-Zein et al. [29] ; and Yu, El -Zein 98 [32] discovered that the existence of an air -filled gap between the heat source and the liner 99 leads to an attenuated heat flow across the liner system. El-Zein et al. [29] ; and Yu, El -Zein 100 [32] specifically observed that the presence of an air -filled gap le ads to a reduction in 101 desiccation crack width in a GCL exposed to elevated temperatures. Yu et al. [33] found that 102 while polymer -enhancement reduces the desiccation cracking of a GCL exposed to moderate 103 temperatures, it is ineffective at higher temperatures. Yu, El -Zein [34] achieved GCL hydration 104 via artificial irrigation and hydraulic separation of the liner system from the underlying subsoil , 105 which consequently led to complete prevention of its desiccation when exposed to elevated 106 temperatures. Hoor [35] ; Hoor et al . [4]; Rowe et al. [36] ; and Rowe et al. [37] numerically 107 examined the installation of a horizontal cooling pipe array beneath the waste at the base of the 108 landfill and found that it leads to a reduction in liner surface temperature. Moreover, numerical 109 studies conducted by Hoor [35] ; Hoor, Rowe [38] ; Hoor, Rowe [39] ; and Rowe, Hoor [40] 110 showed that the theoretical introduction of a suitable insulation layer of either sand, geofoam , 111 shredded tires, tire chips or soil within a liner system could substantially reduce its temperature 112 when exposed to high waste temperatures . 113 However, the above works that examined the effect of cooling pipes and insulation 114 mechanisms on heat reduction in landfill basal liners were strictly numerical and require 115 6 experimental evidence to confirm their respective efficacies. Thus far, to the authors’ 116 knowledge, no experimental work has been carried out to examine the heat reduction effect of 117 introducing an insulation layer or a cooling pipe system directly on top of a landfill basal lining 118 system when exposed to elevated waste temperatures. Therefore, the objective of this study 119 was to conduct a laboratory investigation into the variation of temperature -moisture profiles 120 along the depth of a CCL exposed to elevated waste temperatures and examine the potential 121 effectiveness of utilizing either cooling pipes or an insulation layer on the reduction of the 122 CCL’s temperature and consequently, its desiccation . Accordingly, a series of laboratory 123 experiments were conducted, where a CCL block was exposed to elevated temperatures of 124 40°C -70°C . Variation s of temperature -moisture profiles within the CCL in the absence and 125 presence of heat reduction techniques were then assessed. 126 2. Materials 127 2.1. Compacted Clay Liner (CCL) 128 Marine clay , which was employed as the CCL for t he present study, is generally known 129 to occur in the form of deposits along many coastal regions around the world [41]. In Malaysia, 130 marine clay is abundantly available along the coastal regions of the Peninsular, particularly in 131 regions such as Johor, Malacca, Klang, Jeram, Penang, and Alor Star [42]. Several researchers 132 have recommended the use of marine clay as a liner material based on its physical properties, 133 low hydraulic conductivity, heavy metal adsorption capacity, natural desiccation cracking 134 resistance, heavy metal retention capability and the potential to function as a geosorbent that 135 retain s pathogenic bacteria in landfill leachate [43-48]. At present , marine clay is utilized as a 136 landfill liner in Malaysia, Singapore and Japan [49-51]. 137 For the present study, marine clay samples were obtained from the vicinity of the 138 coastal region located in Kampung Dungun, 45800 Jeram, Selangor, Malaysia ( 3°11'53.7"N , 139 101°18'23.4"E ). The basic geotechnical and thermal properties of the collected marine clay are 140 7 provided in Table 1. It should be mentioned here that all water contents mentioned in Table 1 141 were calculated using the gravimetric method, and that the thermal propert ies reported were 142 calculated for marine clay compacted (using the mini- compaction apparatus) at a mois ture 143 content that was 2% wet of optimum, which was the moisture content that was used to prepare 144 marine clay for all experiments in this research [52]. In accordance with the British Standar d 145 Classification System (BSCS) , marine clay was classified as a clay of high plasticity (CH). An 146 image of crushed marine clay is provided in Fig. 2(a). 147 Table 1. Basic geotechnical and thermal properties of marine clay 148 Soil Properties Value Natural Water Content (%) 60.46 Specific Gravity 2.48 Atterberg Limits Liquid limit (%) 66 Plastic limit (%) 31 Plasticity Index (%) 35 Organic Content (%) 3.79 Activity 0.81 British Standard Classification System (BSCS) CH Maximum Dry Density (MDD) (g/cm3) 1.54 Optimum Moisture Content (OMC) (%) 24.20 Linear shrinkage (%) 14.2 Particle size distribution (%) Gravel - Sand 12 Silt 45 Clay 43 Fines Content (Clay + Silt) 88 Unconfined Compressive Strength (UCS) (kPa) 704 Thermal Properties Thermal conductivity (W/m°C) 1.160 Volumetric Heat capacity (MJ/m3°C) 1.893 Thermal Diffusivity (mm2/s2) 0.613 Thermal Resistivity (°C.cm/W) 86.23 0 8 2.2. Cooling pipe 149 A few essential factors were carefully considered prior to selecting the material that 150 was used to fabricate the cooling pipe network. Firstly, as mentioned previously, majority of 151 the nations that utilize compacted clay as their landfill barrier system without the use of 152 geosynthetic component (s) fall under the low - and middle -income category. Hence for such 153 nations to successfully implement the cooling pipe technique as a viable solution for heat 154 reduction in CCLs, it is vital that the pipe material be of low cost. Secondly, it is crucial that 155 the chosen pipe material be readily available in th e market for the construction stage of the 156 cooling pipe system and also for emergency instances where certain pipes are found to be 157 damaged beyond repair and thus require immediate replacement. Thirdly, it is essential that the 158 chosen pipe material possessess a high thermal conductivity for an effective heat exchange 159 between the coolant and the surrounding medium that the pipe network would be buried in. 160 Finally, it is required that the pipe material be resistant to corrosion. Since galvanized steel 161 (GS) meets all of the above -mentioned criteria , it was thus chosen to fabricat e the cooling pipe 162 network [53-57]. Fig. 2(b) shows the heat exchange component of the cooling pipe network 163 fabricated using G S. 164 2.3. Thermal insulator 165 In this study, rockwool is proposed as a potential thermal insulation material that c an 166 be safely deployed in MSW landfills to minimize CCL desiccation in the face of elevated waste 167 temperatures due to three major reasons . Firstly it is a sustainable nature- based insulation 168 material made of renewable volcanic stone. Secondly, it possesses a thermal con ductivity 3 – 169 26 times lower than those of other liner insulation materials previously proposed by Hoor [35]; 170 Hoor, Rowe [38]; and Hoor, Rowe [39] . Finally, rockwool is moisture resistant yet vapour 171 permeable, meaning that while it resists water absorption by default, its vapour permeability 172 permits water vapour to pass through and hydrate the CCL from the top, thus minimizing its 173 9 desiccation under elevated waste temperatures. Basic properties of the purchased rockwool 174 boards are tabulated in Table 2. 175 Fig. 2. Images of: (a) marine clay (crushed); (b) galvanized iron cooling pipe; (c) rockwool 176 Table 2 Basic properties of rockwool 177 Property Value Reference Density (kg/m3) 54.957 Measured (laboratory) Thermal Conductivity (W/m .K) 0.028 Measured ( KD2 Pro Analyzer ) Volumetric heat capacity (MJ/m3.K) 0.127 Measured ( KD2 Pro Analyzer ) Thermal diffusivity (mm2/s) 0.218 Measured ( KD2 Pro A nalyzer ) Melting Point (°C) > 1000°C Manufacturer’s specifications R-value at 50mm thickness ( m2.K/W) 1.389 Manufacturer’s specifications Water Absorption (partial immersion) (kg/m2) 1 Manufacturer’s specifications Water Vapor Absorption < 0.04 Vol % Manufacturer’s specifications 3. Methodology 178 3.1. Cooling pipe design concept 179 The proposed cooling pipe system in the present study was inspired by heat exchanger 180 designs suggested by Coccia et al. [58] ; Hoor [35] ; Hoor et al. [4] ; Rowe et al. [36] ; and Rowe 181 et al. [37] . The concept entails the installation of a n arra y of closed -loop horizontal pipes 182 embedded within a thick sand protection layer (henceforth referred to as pipe sand layer or 183 PSL) located directly above the CCL . A coolant being circulat ed through the pipe network at 184 a designated flowrate will intersect t he heat flow towards the CCL and absorb the heat from 185 the sand . Fine sand particles with high thermal conductivity will ensure good contact with pipes 186 (a) (b) (c) 10 and facilitate the heat exchange process. Consequently, the magnitude of temperature 187 experienced by the CCL will be significantly reduced. The heated coolant will then be 188 transported to facilities outside the landfill where the heat energy will once again be extracted 189 from the coolant and used for various applications, such as onsite use, preheating of domestic 190 hot water, heating of greenhouses in winter, intensifying the evaporation of leachate collected 191 in leachate ponds etc. [4, 35- 37, 59] . Following heat extr action outside the landfill, the coolant 192 will then be recirculated through the pipe system. Water is proposed as the coolant due to its 193 low cost, low viscosity, availability and non-hazardous nature in the event of a leak. 194 195 Fig. 3. Three -dimensional view of p roposed cooling pipe configuration in a MSW landfill 196 (s = intermodular/intramodular spacing ) 197 Based on heat exchange principles, a pipe configuration that comprises alternating entry 198 and return pipes is considered the most efficient arrangement [4]. Accordingly, a hypothetical 199 three -dimensional illustration of the proposed pipe configuration (comprising a 4- pass module 200 arrangement) installed within a MSW landfill is provided in Fig. 3. The design proposed in is 201 purely conceptual in nature and the illustration in Figs. 3 shows only a portion of the landfill 202 in terms of length . Hence, a detailed design analysis would later be required to fully assess the 203 practicality of installing the proposed pipe array system within a landfill. It was decided that 204 Heated fluid Cold fluid Heated fluid flow Cold fluid flow Sand CCL Single module s s 11 the pipe array (embedde d within the sand layer ) will be located below the leachate collection 205 system (LCS) due to two main reasons . Firstly, if placed below the LCS, the sand layer will 206 provide protection to the CCL from physical damages [36]. Secondly, i f the sand layer with 207 the pipes were located above the LCS, significant volumes of leachate may ent er the sand layer 208 and interfere with the heat excha nge process. However, w hen located below the LCS, the 209 majority of the leachate will be collected by the leachate collection pipes and the leachate effect 210 on the heat exchange process will be minimized . A schematic cross -section diagram of a 211 landfill profile following installation of the cooling pipe system is provided in Fig. 4(a) . 212 213 Fig. 4. Schematic landfill cross -section following installation of a : (a) cooling pipe system ; 214 (b) insulation layer 215 3.2. Insulation design concept 216 The insulation design involves the installation of a rockwool layer directly above the 217 CCL and below the LCS. When located below the LCS, the rockwool layer will provide 218 protection to the CCL from physical damages [36]. Moreover, since the leachate collection 219 system is above the rockwool layer, the leachate will not accumulate on top of the rockwool 220 layer and disrupt its functionality. Fig. 4(b) presents the schematic diagram of a landfill cross - 221 Sand Gravel Entry pipes Return pipes Insulation (a) CCL MSW LCS (b) CCL MSW LCS 12 section following installation of the rockwool insulation layer. The insulation design is merely 222 conceptual in nature and will require the conduction of a detailed design analysis later, if 223 deemed worthy of being implemented within a MSW landfill. 224 3.3. Sample preparation 225 Marine clay collected from the site possessed a natural (gravimetric) water content of 226 60.5% which rendered immediate crushing impossible. Hence, the clay was oven dried at 227 105°C for over 24 hours and then crushed using a mechanical grinder into fine particles. The 228 crushed clay was passed through a 2.36 mm sieve and stored at room temperature in sealed 229 polythene bags . 230 3.4. Experimental program 231 The experimental program examined the effect of implementing different heat 232 reduction techniques on the temperature and desiccation reduction of a CCL block subjected 233 to constant elevated waste temperatures. Ac cordingly, the experimental program was divided 234 into three main subprograms , viz. base case, cooling case and insulation case. The base case 235 involve d subjecting the CCL to constant elevated waste temperatures in the absence of any heat 236 reduction mechanisms, which simulated the present situation in MSW landfills. The cooling 237 and insulation cases entailed the introduction of a cooling pipe network and a rockwool layer 238 directly above a CCL subje cted to constant elevated temperatures, respectively. For all cases, 239 the required waste temperatures above the CCL were generated with the aid of an electric heat 240 pad circui t. The conditions that were adopted for each experiment are tabulated in Table 3 and 241 the depicted via a flowchart in Fig. 5. 242 Table 3. Summary of experimental conditions 243 ID Experiment CET (°C) Pipe material Coolant flowrate (m3/h) Insulation material Insulation thickness (mm) 1 Base case 40 - - - - 2 Base case 55 - - - - 13 3 Base case 70 - - - - 4 Cooling case 40 GS 0.75 - - 5 Cooling case 70 GS 0.75 - - 6 Cooling case 70 GS 1.5 - - 7 Insulation case 40 - - Rockwool 20 8 Insulation case 40 - - Rockwool 35 9 Insulation case 40 - - Rockwool 50 10 Insulation case 70 - - Rockwool 20 11 Insulation case 70 - - Rockwool 35 12 Insulation case 70 - - Rockwool 50 CET = Constant elevated temperature ; GS = Galvanizeed steel 244 245 Fig. 5. Experimental flowchart 246 3.4.1. Heat generation mechanism 247 The circuit employed for the application of heat comprised a mica flat heater as the 248 electric heat pad, a PT100 Resistance Temperature Detector (RTD) probe, a GT8 ATS 210 249 Proportional Integral Derivative (PID) controller and a 40 Amp Solid -State Relay (SSR ). All 250 circuit components were purchased from the company, Elmec Heaters and Controllers based 251 in Chennai, India. As the first step of heat application, a fine dry sand layer , henceforth referred 252 to as the temperature detection sand layer (TDSL), was place d over either the CCL, pipe sand 253 layer (PSL) or rockwool insulation layer, depending on the experimental case. The heat pad 254 was then placed on top of the TDSL and the RTD probe was inserted into the mid-region of 255 Determination of geotechnical and thermal properties of compacted clay Cooling case 0.75 m3/h 1.5 m3/h 40°C 70°C 70°C 20 mm 50 mm 35 mm Insulation case 40°C 70°C Base case 40°C 55°C 70°C 40°C 70°C 40°C 70°C 14 the TDSL. Subsequently, the heat pad and RTD probe were connected to the PID controller 256 and the SSR to form a closed circuit. Once the desired temperature was entered into the PID 257 controller, the TDSL temperature was continuously monitored by the PID controller via the 258 RTD sensor and maintai ned at the required temperature by switching the heat pad on and off 259 with the aid of the SSR. The top surface of the heat pad was insulated (referred to as heat pad 260 insulation or HPI) in order to minimize heat loss to the external environment . Since the TD SL 261 (a) was assigned a low thickness (25 mm), (b) was dry with minimal moisture variation and (c) 262 possessed a reasonably high thermal conductivity, it was presumed that the entire TDSL layer 263 was uniformly heated to the desired temperature during all experimental cases. Accordingly, 264 the TDSL served as the heated waste layer above the CCL. 265 3.4.2. Base case 266 Crushed and oven- dried clay was mixed with adequate water until a moisture content 267 that was 2% wet of its optimum moisture content (OMC) was attained , as typical of current 268 practice in the field [60, 61] . For compaction of the clay, it was decided that a compaction 269 energy equivalent to that of the reduced proctor test be delivered to the clay in order to be 270 conservative. The reduced proctor effort (356.7 kJ/m3) corresponds to the minimum level of 271 compactive energy required for a typical soil liner [61, 62] . The l ower the degree of soil 272 compaction, the higher the number of water -filled pores and the higher the water content 273 gradient. Consequently, the higher the potential for desiccation cracking [63]. During 274 compaction, a heavy metal plate was placed on the moist clay and the standard proctor rammer 275 was dropped onto the plate. The compaction energy delivered was approximately 383 kJ/m3 276 which was slightly higher tha n that of the reduced proctor test in order to allow for energy 277 losses that occur when the hammer contacts the metal plate [52]. 278 The clay was compacted in five lifts to a total height of 130 mm within a custom - 279 designed wooden tank that possessed a length and width of 300 mm each. The present study 280 15 thus employed a CCL volume which was 2.5 to 5 times the volume of soil columns subjected 281 to elevated waste temperatures by researchers in the past [13, 28, 31, 33, 64, 65] . Despite the 282 typical thickness of a CCL installed at the base of a landfill being equivalent to approximately 283 1 m, previous studies have repeatedly shown that when soil columns were subjected to high 284 temperatures, it was the top region of the soil (particularly the upper 100 mm) that served as 285 the critical zone for moisture and heat variation [31, 63, 69, 70]. Therefore, for the present 286 study, only the top 110 mm of the CCL depth was examined for varia tion in temperature and 287 moisture profiles along the depth. Accordingly, the CCL’s top surface (i.e., the interface 288 between TDSL and CCL) served as the reference datum for CCL depth and corresponded to a 289 depth of 0 mm. Moreover, in accordance with the gener al sign convention, all depths below 290 the CCL top surface were considered negative. Meanwhile, the 20 mm of excess clay below 291 the first 110 mm of the CCL depth was simply included to simulate downward continuity of 292 the CCL (similar to field conditions) and to prevent the base of the wooden tank from 293 interfering with the temperature and moisture readings measured at a depth of 110 mm from 294 the CCL’s top surface. 295 Following compaction of each layer, samples from the CCL’s mid -region were 296 acquired for the determination of initial water content. The wooden tank was assigned a 297 significant wall thickness of 2 cm in order to ensure that it provided the CCL with good 298 insulat ion that minimize d heat transfer between the internal contents of the tank and the 299 external envi ronment . Moreover, t he tank was coated with automotive paint to minimize the 300 absorption of soil moisture by wood. Type T and J t hermocouple probes of 1.5 mm diameter 301 were inserted into the CCL at designated depths through pre -drilled holes in the tank , with the 302 three uppermost thermocouples being more closely spaced to capture the steep variation of 303 temperature close to the top surface. The thermoocouples were purchased from OMEGA 304 Engineering Singapore and RS Components Sdn Bhd (Malaysia). Following inse rtion, a ll 305 16 thermocouples were connected to a datalogger to record the CCL temperatures at 4- second 306 intervals. The heat pad and the TDSL were placed directly on top of the CCL . A small 307 overburden stress of approximately 1.5 kPa was applied by placing metal weights on top of the 308 heat pad in order to ensure good contact with the TDSL. Southen, Rowe [15] reported that the 309 overburden stress has an insignificant effect on the desiccation of liners and moisture 310 redistribution. Therefore, the effect of a varying overburden stress on liner desiccation was not 311 analysed in this study. Moreover, a LCS was not included in order to be conservative, sinc e the 312 presence of LCS above CCL reduce s the heat flow to CCL due to the low thermal conductivity 313 of the LCS’s gravel medium [36]. Fig. 6(a) provides a schematic diagram of the liner 314 configuration set up for the base case . 315 3.4.3. Cooling case 316 In the cooling case, a cooling pipe system that comprised a single four -pass module 317 embedded within a fine sand layer (referred to as pipe sand layer or PSL) was placed on top of 318 the CCL. Figs. 6(b) and 7 show the liner configuration of the cooling case and plan view of 319 the lab oratory cooling pipe network, respectively. The cooling pipes were connected to a 320 SUNSUN HJ -1500 submersible water pump (immersed in a cool water supply tank) and a 321 cooling tank to form a closed loop. The submersible pump was used to pump water into the 322 entry pipe and the cooling tank was used to reduce the temperature of the heated water flowing 323 through the return pipe before it was directed back to the water supply tank. A scaling factor 324 of 10 was used for the PSL thickness, pipe diameter, pipe spacing and coolant flow rate. The 325 PSL thickness was simply assumed to be 0.35 m in the field in order to be conservative , since 326 the lower the sand layer thickness between the heated waste and the CCL, the higher the heat 327 flow to the CCL [38] . Having considered previous works conducted by [35] ; and [36], the 328 (inner) pipe diameter and pipe spacing (between the entry and return pipes within a module) in 329 the field were assumed to be equivalent to 150 mm and 2.6 m, respectively. 330 17 Accordingly , an inner diameter of 15 mm and a n intermodular pipe spacing of 260 mm 331 were chosen for the laboratory pipe network . Moreover, the effect of doubling the coolant flow 332 rate on the reduction of CCL temperature and desiccation were tested. The two flow rates 333 chosen for the laboratory cooling pipe system were 0.75 m3/h (Re = 21275) and 1.5 m3/h (Re 334 = 43857) , both of which ensured turbulent flow conditions, since prior studies have shown that 335 turbulent flow results in a higher heat extraction rate and a lower liner temperature [4, 66, 67] . 336 When scaled up while preserving the Reynold’s number, the laboratory flowrates of 0.75 m3/h 337 and 1.5 m3/h correspond to approximately 7.5 m3/h and 15 m3/h in the field, respectively. 338 339 Fig. 6. Schematic diagrams of laboratory liner configurations for: (a) base case; (b) cooling 340 case; (c) insulation case (all dimensions in mm) 341 PSL CCL TDSL HPI Thermocouple Heat pad Insulation Wood Entry pipe Return pipe RTD (a) 17.5 17.5 20 20 20 20 130 300 25 1.5 kPa (b) 35 1.5 kPa (c) 1.5 kPa B A C 18 342 Fig. 7. Top view of laboratory cooling pipe network (all dimensions in mm) 343 3.4.4. Insulation case 344 In the insulation case, the pipe array and sand layer were replaced with a rockwool 345 insulation layer above the CCL . Accordingly, t he effect of varying thickness of the rockwool 346 insulation layer on the reduction of CCL temperature and desiccation was investigated through 347 three rockwool thicknesses of 20 mm, 35 mm and 50 mm. Since previous studies that utilized 348 rockwool for insulation within buildings showed that the utilization of a rockwool thickness 349 greater than approximately 50 mm either led to a decline or an insignificant rise in net energy 350 savings , the maximum rockwool thickness employed for the present study was limited to 50 351 mm [68 -70]. 352 3.4.5. Operat ion of the test program 353 A wooden lid was placed on top of the wooden tank following assemblage of the 354 experimental setup to minimize heat transfer between the external environment and internal 355 contents of the tank during operation . The heat pad and datalogger were then switched on and 356 allowed to run continuously until the varia tion of temperature at various depths within the CCL 357 became stable over time. Once the temperature readings were observed to be come relatively 358 stable, the experimental program was terminated. Subsequently, coring tubes were inserted into 359 the mid -region of the CCL and core samples were obtained via extrusion . The extruded core 360 PSL Cold water Water flow Heated water Cold tank water Pump Cooling tank 300 Water supply tank 300 260 19 samples were then sliced and the final gravimetric water content of each slice (located at a 361 certain depth within the CCL) was d etermined [27]. 362 4. Results and Discussion 363 4.1. Base case 364 4.1.1. Temporal variation of CCL temperature 365 During the base case, the CCL was subjected to constant elevated temperatures ( CETs) 366 of 70°C, 55°C and 40°C , respectively until the temperature distributions within the CCL 367 stabilised over time. Figs. 8(a-c) show s the temporal variation of the soil temperature at 368 different depths within the CCL ’s mid -region for all three CETs . It is clear from Fig s. 8(a-c) 369 that upon the application of heat (regardless of the magnitude), the CCL temperature increased 370 dramatically within the first 12 to 18 hours and then gradually achieved a stabilised state over 371 time. Since heat and moisture transport is known to be a couple d process, it was presumed that 372 the temperature and moisture variation along the CCL depth stabilised concurrently . A similar 373 observation was reported by Southen, Rowe [15] when they exposed a composite liner -subsoil 374 profile to an elevated temperature , where significant temperature changes in the subsoil column 375 were observed within the first 20 hours of heat application . 376 377 (a) 20 378 379 Fig. 8. Variation of temperature over time at different depths within the CCL when subjected 380 to a constant elevated temperature (CETs) of (a) 40°C ; (b) 55°C ; (c) 7 0°C 381 4.1.2. CCL temperature profiles 382 Fig. 9 depict s the vertical profiles of the initial and final temperature s recorded along 383 the depth of a CCL subjected to C ETs of 70°C, 55°C and 40°C, respectively. The vertical 384 profile s of the initial temperature (IT) and final temperature (FT) correspond to the temperature 385 readings that were obtained along the depth of the CCL ’s mid -region before heat application 386 and at the end of the experiment when the temperatures were found to stabilise, respectively . 387 Fig. 9 show s that all final thermal profiles were non-linear and the temperature reduced with 388 increasing CCL depth regardless of the CET magnitude . In order to better analyse the thermal 389 distribution within the CCL, spline interpolation was performed between the experimental data 390 points (along the CCL depth) and OneWay splines (OWS) were fitted using the SRS1 Splines 391 software add-in installed in Microsoft excel [ 71]. The OneWay spline is similar to the Cubic 392 (b) (c) 21 spline but uses a parabolic fit internally for its interpolation and is more constrained . The fitted 393 OW splines ensured continuity of the experimental results by filling the gaps between the 394 experimental da ta points, which in turn enabled the calculation of constant elevated 395 temperature attenuation percentages (CETAP s) along the CCL depth. 396 397 Fig. 9. Initial (IT) and final temperature ( FT) variation along the depth of a CCL subjected to 398 constant elevated temperatures ( CETs) of 70°C , 55°C and 40°C 399 With the aid of OW spline interpolation, the CETAP s were calculated along the depth 400 of the CCL’s mid -region at intervals of 0.2 mm ( for over 550 locations ) using Eq. (1) , where 401 𝑇𝑇𝐶𝐶𝐶𝐶𝐶𝐶 and 𝑇𝑇𝑓𝑓 refer to the applied CET and the FT at different depth s along the CCL’s mid - 402 region, respectively. Accordingly, Fig. 10 displays the calculated CETAP S along the CCL 403 depth for all three C ETs. It is evident from Fig. 10 that as the C ET magnitude increased, the 404 CETAP increased as well. The CCL subjected to a C ET of 70°C experienced a total 405 temperature drop of approximately 30.7% at a depth of 110mm, while the CCLs subjected to 406 CETs of 55°C and 40°C experienced temperature losses of 28.1% and 1 6.5%, respectively. 407 Besides CETAP , the final temperature (FT) gradient along the CCL depth was also calculated 408 0 20 40 60 80 1000 10 20 30 40 50 60 70 80(-) Depth (mm)Temperature ( °C) IT (Exp) 40°C (Exp) 55°C (Exp) 70°C (Exp) IT (OWS) 40°C (OWS) 55°C (OWS) 70°C (OWS) 22 using Eq. (2) , where 𝑇𝑇𝑓𝑓1, 𝑇𝑇𝑓𝑓2 and 𝐻𝐻 refer to the FT at 0 mm depth, FT at - 110 mm depth and the 409 110 mm height of the analyzed CCL region, respectively [13, 15, 65, 72] . Accordingly, the 410 calculated vertical FT gradients for CETs of 70°C, 55°C and 40°C, were approximately 411 1.9°C/cm, 1.4°C/cm and 0.6°C/cm, respectively. The obtained values indicate that the vertical 412 FT gradient (or the rate of CET attenuation) along the CCL depth rose with increasing CET. 413 𝐶𝐶𝐶𝐶𝑇𝑇𝐶𝐶𝐶𝐶 (%)= 𝑇𝑇𝐶𝐶𝐶𝐶𝐶𝐶− 𝑇𝑇𝑓𝑓 𝑇𝑇𝐶𝐶𝐶𝐶𝐶𝐶×100 Eq. (1) 𝐹𝐹𝑇𝑇 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 (°𝐶𝐶/𝑐𝑐𝑐𝑐)= 𝑇𝑇𝑓𝑓1− 𝑇𝑇𝑓𝑓2 𝐻𝐻 Eq. (2) 414 Fig. 10. C onstant elevated temperature attenuation percentage ( CETAP) along the depth of a 415 CCL subjected to constant elevated temperatures ( CETs) of 70°C, 55°C and 40°C 416 The nonlinear temperature attenuation observed in each F T profile was primarily 417 governed by the water content reduction that occurred along the CCL depth upon exposure to 418 CETs. Heat transport through soil occurs almost entirely via conduction. Heat transfer through 419 convection is considered to be significant only under circumstances w here a high flow rate 420 water or air is present, while heat transfer through radiation is considered to be significant for 421 only surface soils [73]. Heat flow through soil via conduction can be explained using Fourier’s 422 0 5 10 15 20 25 30 350 20 40 6080 1000 5 10 15 20 25 30 35(-) Depth (mm)Constant elevated temperature attenuation percentage (%) 40°C 55°C 70°C 23 law, expressed in Eq. (3 ) in one -dimensional form , where the 𝑄𝑄, 𝑘𝑘, 𝐶𝐶, 𝑇𝑇, 𝑧𝑧 and 𝑔𝑔𝑇𝑇/𝑔𝑔𝑧𝑧 refer to 423 heat transfer rate, thermal conductivity, surface area normal to the direction of heat transfer, 424 soil temperature, soil depth and vertical temperature gradient, respectively [74, 75] . Provided 425 that the soil surface area 𝐶𝐶 remains constant throughout the analyzed soil depth 𝑧𝑧, it is clear 426 from Eq. (3) that the rate of heat transfer through the soil is directly proportional to both the 427 soil’s thermal conductivity and its vertical temperature gradient. Thermal conductivity of the 428 soil matrix is heavily influenced by the thermal conductivity of its pore fluid(s). Since the pore 429 fluid fills the voids within the soil matrix and functions as a bridge between soil particles, its 430 thermal conductivity directly affects the magnitude of heat transfer between soil particles. 431 Therefore , the higher the thermal conductivity of the pore fluid, the higher the rate of heat 432 transfer between soil particles, and vice versa [31] . 433 𝑄𝑄 (𝐽𝐽/𝑠𝑠)= 𝑘𝑘𝐶𝐶𝑔𝑔𝑇𝑇 𝑔𝑔𝑧𝑧 Eq. ( 3) In the present study, when a CET was applied to the CCL’s top surface, the heat flow 434 through the CCL would have induced evaporation of the liquid pore water and converted it into 435 pore water vapour. Subsequently, the pore water vapour would have either (a) remained within 436 the clay voids, or (b) diffused to cooler regions and enabled the voids it initially occupied to be 437 replaced with air. Since the thermal conductivity values of water vapour and air are known to 438 be significantly lower than that of liquid wat er [76, 77] , the replacement of liquid pore water 439 in clay voids with either pore water vapour or air would have significantly hindered the heat 440 flow between clay particles. Accordingly, the evaporation of liquid pore water a long the CCL 441 depth would have led to reduced downward heat flow and consequently, attenuation of the 442 applied CET along the CCL . Furthermore, a rise in the CET would have lead to an increased 443 vertical temperature gradient and consequently, an increased rate of heat transfer along the 444 CCL depth (according to Eq. (3) ). This would have in turn led to an increased evaporation of 445 24 liquid pore water along the CCL depth (particularly in the uppermost portion) and ultimately 446 an increased reduction of CET , i.e., an increased CETAP as clearly visible in Fig. 10. 447 4.1.3. CCL water content profiles 448 Figs. 11(a-c) portray the vertical profiles of the initial and final gravimetric water 449 content s measured along the depth of the CCL subjected to C ETs of 70°C, 55°C and 40°C, 450 respectively . The vertical profile s of the initial water content s (IWC ) and final water contents 451 (FWC) correspond to the water contents measured along the depth of the CCL ’s mid -region 452 prior to the application of heat and following termination of the experiment, respectively. As 453 expected, Figs. 11(a-c) show that the application of heat led to a reduction of water content 454 along the entire depth of the CCL . Moreover , the magnitude of water content reduction 455 declined with increasing depth of the CCL. Similar to the case with thermal profiles, OneWay 456 splines were fitted to the the experimental data points to ensure continuity along the CCL depth 457 and enable the calculation of water content reduction percentage (WCRP) along t he depth of 458 the CCL’s mid region when subjected to C ETs of 70°C, 55°C and 40°C, respectively. The 459 WCRP (with respect to the initial water contents) was calculated along the depth of the CCL’s 460 mid-region at intervals of 0.2 mm using Eq. ( 4), where 𝑤𝑤 𝑖𝑖 and 𝑤𝑤𝑓𝑓 refer to the initial and final 461 water contents at different depths along the CCL’s mid -region, respectively. Accordingly, Fig. 462 12 depicts t he calculated WCRPs along the CCL depth for the three CETs. 463 𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶 (%)= 𝑤𝑤𝑓𝑓− 𝑤𝑤𝑖𝑖 𝑤𝑤𝑖𝑖×100 Eq. ( 4) 𝐹𝐹𝑊𝑊𝐶𝐶 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 (% per cm)= 𝑤𝑤𝑓𝑓1−𝑤𝑤𝑓𝑓2 𝐻𝐻 Eq. ( 5) 464 25 Fig. 11. Initial (I WC) and final water content (FWC ) variation along the depth of a CCL 465 subjected to constant elevated temperatures ( CETs) of: (a) 70°C ; (b) 55°C ; (c) 40°C 466 Fig. 12 shows that the WCRP declined with increasing CCL depth for all C ETs and the 467 rate of water content reduction was generally higher in the uppermost region of the CCL for 468 (a) 0 20 40 60 80 1000 10 20 30(-) Depth (mm)Water content (%) IWC (Exp) FWC (Exp) IWC (OWS) FWC (OWS) (b) 0 20 406080 1000 10 20 30(-) Depth (mm)Water content (%) IWC (Exp) FWC (Exp) IWC (OWS) FWC (OWS) 0 20 40 60 80 1000 10 20 30(-) Depth (mm)Water content (%) IWC (Exp) FWC (Exp) IWC (OWS) FWC (OWS) (c) 26 all CETs. Moreover, the WCRP rose as the CET increased . The CCL that was subjected to a 469 CET of 70°C suffered a maximum water content reduction of approximately 41.1% at the CCL 470 top surface (where the highest heat was experienced), while the CCLs subjected to C ETs of 471 55°C and 40°C experienced water content losses of 29.4% and 15.7% at the same location, 472 respectively. The final water content (FWC) gradient formed along the CCL depth was 473 calculated using Eq. ( 5), where 𝑤𝑤𝑓𝑓1, 𝑤𝑤𝑓𝑓1 and 𝐻𝐻 refer to the FWC at -110 mm depth, FWC at 0 474 mm depth and the 110 mm height of the analyzed CCL region, respectively. Accordingly, the 475 calculated vertical FWC gradients for C ETs of 70°C, 55°C and 40°C, were 0.8% per cm, 0.5% 476 per cm and 0.2% per cm , respectively. The obtained values indicate that the FWC gradient 477 along the CCL depth rose as the CET increased . 478 479 Fig. 12. Water content reduction percentage ( WCRP) along the depth of a CCL subjected to 480 constant elevated temperatures ( CETs) of 70°C, 55°C and 40°C 481 The attenuation of WCRP with increasing CCL de pth could be explained by the reduced 482 downward heat flow through the CCL. A s the magnitude of heat flowing through the CCL 483 reduced with increasing depth, the magnitude of liquid pore water that evaporated as a result 484 of the heat would have also decreased, which would have in turn lessened the reduction of 485 water content with increasing CCL depth. This would also explain why the rate of water content 486 0 20 40 6080 1000 5 10 15 20 25 30 35 40 45(-) Depth (mm)Water content reduction (%) 40°C (CET) 55°C (CET) 70°C (CET) 27 reduction was significantly greater in the uppermost region of the CCL where the heat flow 487 was higher . Moreove r, an increase in the magnitude of CET applied on the CCL surface would 488 have evidently le d to a corresponding increase in the magnitude of downward heat flow through 489 the CCL . Increased downward heat flow would have caused a rise in liquid pore water 490 evaporation and led to an increased water content reduction along the CCL. This would explain 491 the rise in WCRP observed with increasing CET. 492 4.2. Insulation case 493 4.2.1. CCL temperature profiles 494 Figs. 13(a) and (b) depict the final temperature profile s measured along the depth of a 495 CCL subjected to constant elevated temperatures (CET s) of 40°C and 70°C, respectively, in 496 the presence of 0 mm, 20 mm, 35 mm and 50 mm thick rockwool layers . Similar to the base 497 case, OneWay (OW) splines were fitted to experimental data points along the CCL depth. 498 Final temperature (FT) profiles measured during the insulation case show that similar to the 499 base case results, the applied CET reduced with increasing CCL depth regardless of the CET 500 or rockwool layer thickness. This observation is attributed to the coupled heat -moisture 501 transport theory suggested previously in section 4.1.2. More importantly, Fig s. 13(a) and (b) 502 show that the FT profiles measured along the CCL depth reduced as the applied rockwool 503 thickness increased. Rockwool possesses a high thermal resistance since it contains a large 504 quantity of microscopic dead air cells. Heat transfer occurs via three modes, viz. conduction, 505 convection and radiation. Since air is a poor thermal conductor, the heat transfer through 506 rockwool via conduction is reduced substantially. Moreover, since the air particles are 507 essentially trapped inside tiny pockets within rockwool, their movement is significantly 508 restricted, which results in reduced heat transfer via co nvection. Finally, the presence of tiny 509 dead air cells also causes the radiation paths to break up into small distance segments so that 510 the long- wave infrared radiation is absorbed and/or scattered by rockwool, which ultimately 511 28 leads to the reduction of he at transfer via radiation as well [78]. Therefore, in general, rockwool 512 retards heat flow that occurs through all three heat transfer modes. 513 514 515 Fig. 13. Final temperature variation along CCL depth for different rockwool thicknesses : (a) 516 40°C CET; (b) 70°C CET 517 (a) 0 20 40 60 80 1000 10 20 30 40 50(-) Depth (mm)Temperature ( °C) Initial (Exp) 0 mm (Exp) 20 mm (Exp) 35 mm (Exp) 50 mm (Exp) 0 mm (OWS) 20 mm (OWS) 35 mm (OWS) 50 mm (OWS) Initial (OWS) (b) 0 20 406080 1000 10 20 30 40 50 60 70 80(-) Depth (mm)Temperature ( °C) Initial (Exp) 0 mm (Exp) 20 mm (Exp) 35 mm (Exp) 50 mm (Exp) 0 mm (OWS) 20 mm (OWS) 35 mm (OWS) 50 mm (OWS) Initial (OWS) 29 Fig. 13(a) shows that in the face of 40°C CET, all three rockwool thicknesses ensured 518 that the CCL temperature remained below 35°C , which is generally cons idered the elevated 519 temperature limit for the liner, since temperatures above 35°C possess the ability to adversely 520 affect the liner’s performance [8, 9, 13, 36, 79, 80]. In the face of 70°C however , only 35 and 521 50 mm rockwool thicknesses maintained the CCL temperature below 35°C , as depicted in Fig. 522 13(b) . In order to further analyse the effectiveness of rockwool application on CCL temperature 523 reduction, the FT attenuation percentages (FTAPs) were calculated for different rockwool 524 thicknes ses using Eq. ( 6), where the terms, 𝑇𝑇𝑏𝑏𝑏𝑏 and 𝑇𝑇𝑖𝑖𝑏𝑏 refer to the base case FT and insulation 525 case FT, respectively. The FTAP values indicate the percentage by which the base case FT 526 profile reduced when a specific rockwool thickness was applied on top of the CCL , 527 respectively. Fig. 14(a) display s the calculated FTAPs for varying rockwool thicknesses along 528 the depth of a CCL subjected to a 40°C CET. I t can be seen from Fig. 14(a) that the introduction 529 of a 20 mm rockwool thickness immediately led to a si gnificant reduction of the base case FT 530 by 14.7- 23.8% . However, while a rise in rockwool thickness led to a continuous increase in the 531 FTAP, the rate at which the FTAP rose , dropped steeply as the rockwool thickness increased. 532 𝐹𝐹𝑇𝑇𝐶𝐶𝐶𝐶 (%)= 𝑇𝑇𝑏𝑏𝑏𝑏− 𝑇𝑇𝑖𝑖𝑏𝑏 𝑇𝑇𝑏𝑏𝑏𝑏×100 Eq. ( 6) For example, the computed FTAP growth rate for rockwool thickness increment from 533 0 to 20 mm was 1.2% per mm, whereas that calculated for rockwool thickness increment from 534 35 to 50 mm was 0.03% per mm. From these results, it is perceived that for an applied CET of 535 40°C, increasing the insulation thickness was truly effective only until a certain thickness was 536 reached (which in this case, was 35 mm), following which, the additional heat flow retardation 537 achieved by further increasing the rockwool thickness was negligible. Taking into 538 consideration the effectiveness of the three thic knesses in reducing CCL temperature and their 539 individual material /installation costs (which are major factors in large scale applications ), the 540 30 authors recommend that a 20 mm rockwool insulation thickness be employed in MSW landfills 541 where the ETs encounte red by CCL are equal to or lower than 40°C, in order to achieve a 542 satisfactory temperature reduction at a reasonable cost. 543 Fig. 14. Final temperature attenuation percentage (FTAP) variation along CCL depth for 544 different rockwool thicknesses: (a) 40°C CET; (b) 70°C CET 545 Meanwhile, Fig. 14(b) portray s the calculated FTAPs along the depth of a CCL 546 subjected to a 70°C CET for varying rockwool thicknesses . Similar to the 40°C CET case, 547 while rockwool thickness increments led to a progre ssive reduction of CCL temperature, the 548 temperature reduction rate reduced with increasing rockwool thickness. However, unlike in the 549 case of 40°C, the additional temperature reduction achieved by increasing the thickness from 550 35 mm to 50 mm was still sign ificant. As per the second law of thermodynamics, heat flows 551 from hot to cold regions. The higher the temperature difference between the hot and cold 552 regions (i.e., temperature gradient), the higher the rate of heat transfer (as expressed in Eq. (3)). 553 When 70°C ET was applied above the CCL by the heat source, the temperature difference 554 between the initially cool CCL bottom and the heat source was much higher than in the case of 555 (a) 0 20 40 60 80 10010 20 30 40(-) Depth (mm)Final temperature attenuation (%) 20 mm 35 mm 50 mm (b) 0 20 40 60 80 10010 30 50 70(-) Depth (mm)Final temperature attenuation (%) 20 mm 35 mm 50 mm 31 40°C CET application. As a result, the heat flow triggered by 70°C CET was so high that even 556 after the application of 35 mm thickness, a significant heat flow was still present for the 50 mm 557 thickness to retard. Nevertheless, the 35 mm rockwool thickness was still able to attenuate the 558 CCL temperature from a high range of 48.5- 70°C to a low range of 30.5- 34.5°C. This 559 corresponded to a FTAP range of 37.1- 50.7%. Accordingly, the authors recommend that a 35 560 mm rockwool thickness be deployed in MSW landfills that encounter ETs equal to or lower 561 than 70°C . 562 4.2.2. CCL water content profiles 563 Figs. 15(a) and (b) depict the final gravimetric water content profiles measured along 564 the depth of a CCL subjected to CETs of 40°C and 70°C , respectively, in the presence of 565 different rockwool thicknesses. It can be observed that the FWC profiles generally decreased 566 along the CCL depth regardless of the CET or insulation thickness . This observation is 567 attributed to the reduced downward heat flow along the CCL depth, explained in section 4.1.3. 568 In order to analyse the effect of rockwool insulation on the desiccation reduction of the CCL, 569 water content growth percentages (WCGPs) were calculated using Eq. (7) , where the terms 570 𝑤𝑤𝑏𝑏𝑏𝑏 and 𝑤𝑤𝑖𝑖𝑏𝑏 refer to the FWCs of base case and insulation case experiments, res pectively. The 571 WCGP values indicate the percentage by which the base case FWC profile increased when a 572 specific insulation thickness was applied on top of the CCL. Fig. 16(a) portrays the variation 573 of WCGPs calculated for different rockwool thicknesses along the depth of a CCL subjected 574 to a 40°C CET . From b oth Figs. 15(a) and 16(a), it can be observed that as the insulation 575 thickness increased, the FWC generally rose as well (particularly within the CCL’s upper 576 region) . 577 𝑊𝑊𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑐𝑐𝑐𝑐𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑔𝑔𝑔𝑔𝑐𝑐𝑤𝑤𝑔𝑔ℎ 𝑝𝑝𝑔𝑔𝑔𝑔𝑐𝑐𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 (%)= 𝑤𝑤𝑖𝑖𝑏𝑏− 𝑤𝑤𝑏𝑏𝑏𝑏 𝑤𝑤𝑏𝑏𝑏𝑏×100 Eq. (7) 32 Fig. 16(b) depicts the variation of WCGPs calculated for different rockwool 578 thicknesses along the depth of a CCL subjected to a 70°C CET . Unlike in the case of 40°C 579 CET, an apparent rise in the FWC throughout the CCL depth following rockwool application 580 can be observed in Fig s. 15(b) and 16(b). Since the desiccation that CCL experienced 581 throughout its depth in the face of 70°C CET was much higher than what it experienced when 582 exposed to 40°C CET during the base phase, the desiccation reduction along the CCL depth 583 when exposed to 70°C CET was more apparent in the presence of insulation. F rom Figs. 15(b) 584 and 16(b) , it can also be observed that the FWC along the CCL depth generally rose with 585 increasing insulation thickness , especially in the CCL’s uppermost region . Moreover, the 586 magnitude of desiccation reduction achieved by each insulation thickness reduced with 587 increasing CCL depth. This is because the desiccation magnitude suffered by the CCL during 588 the 70°C CET base case experiment reduced with increasing CCL depth. 589 Fig. 15. Final water content profiles along CCL depth for different rockwool thicknesses : (a) 590 40°C CET; (b) 70°C CET 591 (a) 0 20 40 60 80 10020 22 24 26 28(-) Depth (mm)Water content (%) 0 mm 20 mm 35 mm 50 mm (b) 0 20 4060 80 10014 18 22 26(-) Depth (mm)Water content (%) 0 mm 20 mm 35 mm 50 mm 33 Fig. 16. Water content growth percentage variation along CCL depth for different rockwool 592 thicknesses : (a) 40°C CET; (b) 70°C CET 593 4.3. Cooling case 594 4.3.1. CCL temperature profiles 595 4.3.1.1. Effect of cooling pipes 596 Figs. 17(a) and (b) depict the final temperature profiles measured along the depth of a 597 CCL subjected to constant elevated temperatures (CETs) of 40°C and 70°C, respectively, in 598 the presence of galvanized steel cooling pipes . Similar to the base and insulation cases , 599 OneWay (OW) splines were fitted to experimental data points along the CCL depth. Final 600 temperature (FT) profiles measured during the cooling case show that, similar to the base case 601 results, the applied CET reduced with increasing CCL depth regardless of the CET or coolant 602 flowrate employed applied. This observation is attributed to the coupled heat -moisture 603 transport theory suggested previously in section 4.1.2. More importantly, Figs. 17(a) and (b) 604 show that the FT profiles measured along the CCL depth reduced w hen cooling pipes were 605 introduced, regardless of the CET that was applied . This observation is attributed to the heat 606 exchange that would have occurred between the coolant, cooling pipe wall and the sand layer 607 (a) 0 20 40 60 80 100-10 -5 0 5 10(-) Depth (mm)Water content growth (%) 20 mm 35 mm 50 mm (b) 0 20 40 6080 1000 10 20 30 40 50(-) Depth (mm)Water content growth (%) 20 mm 35 mm 50mm 34 that surrounded the cooling pipe (i.e., the PS L). Heat is known to flow from hot to cold regions , 608 as described by the second law of thermodynamics . When a constant elevated temperature 609 (CET) was applied to the top surface of the PSL, the heat would have flowed downwards 610 through the fine sand grains via conduction towards the CCL and the cooling pipes would have 611 intersected the heat flow before it reached the CCL . Subsequently, the heat would have 612 transfer red from the sand grains to the pipe wall via conduction and from the pipe wall to the 613 coolant via convection. 614 Convection that occurs in a fluid when in motion, is classified as natural and forced 615 based on the motion ’s initiation mechanism . Simply stated, convection that occurs in a fluid 616 that is forced to flow by external means, is termed forced convection, whereas convection that 617 occurs in a fluid whose motion is caused via natural means (such as buoyancy effect) , is termed 618 natural convection. Accordingly, since the coolant was pumped into the pipe network via a 619 water pump in the present study, it is presumed that the heat transfer from the pipe wall to the 620 coolant would have occurred via forced convection. The transfer of a significant magnitude of 621 heat f rom the PSL to the coolant would have led to a reduced heat flow towards the CCL. This 622 would have ultimately led to the formation of a final temperature profile along the CCL depth 623 that was lower than that formed in the absence of cooling pipes (i.e., duri ng the base case ). 624 625 (a) 0 20 40 60 80 1000 5 10 15 20 25 30 35 40 45(-) Depth (mm)Temperature ( °C) Initial (Exp) 40°C (BP, Exp) 40°C (0.75 m³/h, Exp) Initial (OWS) 40°C (BP, OWS) 40°C (0.75 m³/h, OWS) 35 626 Fig. 17. Final temperature variation along CCL depth in the absence and presence of cooling 627 pipes: (a) 40°C CET; (b) 70°C CET 628 In order to further analyse the effectiveness of cooling pipe utilization on CCL 629 temperature reduction, the FT attenuation percentages (FTAPs) were calculated using Eq. ( 8) 630 where t he terms, 𝑇𝑇𝑏𝑏𝑏𝑏, 𝑇𝑇𝑏𝑏𝑏𝑏, refer to the base case FT and cooling case FT, respectively. The 631 FTAPs calculated along the CCL depth denote the percentages by which the base case FT 632 profile along the CCL depth reduced when cooling pipes were introduced. 633 𝐹𝐹𝑇𝑇𝐶𝐶𝐶𝐶 (%)= 𝑇𝑇𝑏𝑏𝑏𝑏− 𝑇𝑇𝑏𝑏𝑏𝑏 𝑇𝑇𝑏𝑏𝑏𝑏×100 Eq. (8) Fig. 18 portray the variation of calculated FTAPs along the depth of a CCL subjected 634 to CETs of 40°C and 70°C in the presence of cooling pipes. From Fig. 18, it is apparent that 635 under a CET of 40°C, the introduction of cooling pipes ( with a flow rate of 0.75 m3/h) led to a 636 significant reduction of the base case FT by 15.3-25.3%. The maximum temperature recorded 637 along the depth of the CCL for the 0.75 m3/h GS 40°C CET experiment was 29.9°C , which 638 occurred at the CCL’s top surface. Since this temperature was lower th an the adverse 35°C 639 elevated liner temperature limit , the proposed cooling pipe system in its current state was 640 (b) 0 20 406080 1000 10 20 30 40 50 60 70 80(-) Depth (mm)Temperature ( °C) Initial (Exp) 70°C (BP) 70°C (0.75 m³/h, Exp) 70°C (1.5 m³/h, Exp) Initial (OWS) 70°C (BP, OWS) 70°C (0.75 m³/h, OWS) 70°C (1.5 m³/h, OWS) 36 deemed appropriate to be employed in MSW landfills that encounter ETs equal to or lower 641 than 40°C . Meanwhile, the FTAP values calculated for the 0.75 m3/h GS 70°C CET 642 experiments were of the range 36.1- 40.9%. From Figs. 17(b) and 18, it is clear that the cooling 643 pipes (regardless of the flow rate employed) struggled to attenuate the heat that flowed towards 644 the CCL when a CET of 70°C was applied. This is because the application of a 70°C CET 645 triggered an intense downward heat flow due to the high temperature difference that existed 646 between the heat source and the cool CCL bottom at the time of heat application. The maximum 647 temperatures recorded along the depth of the CCL’s mid region for the 0.75 m3/h and 1.5 m3/h 648 GS 70°C CET experiments were 41.4°C and 37.8°C, respectively, both of which are higher 649 than the adverse 35°C elevated liner temperature limit adopted in the present study. 650 Accordingly, the proposed cooling pipe system in its present state is not recommended 651 to be deployed in MSW landfills that experience ETs close to 70°C , without e conomical 652 adjustments being made to enhance the heat exchange efficiency that the cooling pipe system 653 currently possesses. A few recommended adjustments include reducing the pipe spacing, 654 increasing the pipe diameter, or even changing the medium surrounding the pipes from pure 655 fine sand to materials with higher thermal conductivities such as mixtures of lime, concrete, 656 sand and cement [4, 36, 67, 79] . Fig. 19 displays the FTs measured at three specific points 657 located 15 mm below the CCL ’s top surface for the 0.75 m3/h GS experiments. The three 658 points , as shown in Fig. 6, were located below the entry pipe (point -A), below the centre of the 659 PSL (point -B) and below the return pipe (point -C), respectively. From Fig. 19 it is apparent 660 that for the 40°C CET experiment, the temperature difference between the three measured 661 locations was minimal, most likely due to the low heat flow associated with the applied 40°C 662 temp erature. 663 37 664 Fig. 18. Final temperature attenuation percentage (FTAP) variation along CCL depth in the 665 presence of cooling pipes 666 667 Fig. 19. Final temperature variation along the horizontal distance of a CCL subjected to 668 constant elevated temperatures of 40°C and 70°C i n the presence of cooling pipes 669 On the contrary, a clear temperature pattern is noticed for the 70°C CET experiment. 670 The CCL temperature measured under the inlet and outlet were lower than that measured in 671 the centre region. This is due to the absence of any cooling pipes in the immediate vicinity of 672 the CCL’s centre region. Moreover, for both 40°C and 70°C GS experiments, the CCL 673 temperature measured under the inlet was slightly lower than that measured under the outlet. 674 0 20 40 60 80 1000 5 10 15 20 25 30 35 40 45(-) Depth (mm)Final temperature attenuation (%) 40°C (0.75 m³/h) 70°C (0.75 m³/h) 70°C (1.5 m³/h) 22.6 22.6 22.629.429.9 29.933.737.2 33.9 1824303642 Below entry pipe (A) Below center (B) Below return pipe (C)Temperature ( °C) Temperature measurement location in CCLInitial 40°C (0.75 m³/h) 70°C (0.75 m³/h) 38 The coolant temperature would have been lower in the entry pipe than in the return pipe, since 675 the coolant would have absorbed a certain amount of heat from the PSL and become warm by 676 the time it reached the return pipe. Accordingly, the temperature difference between the PSL 677 and the coolant would have been higher in the entry pipe than in the return pipe. As a result, 678 the heat exchange rate would have been higher in the entry pipe than in the return pipe, which 679 would have resulted in the CCL located region underneath the entry pipe experiencing a higher 680 cooling than that located underneath the return pipe [79] . 681 4.3.1.2. Effect of coolant flow rate 682 In addition to investigating the general reduction of CCL temperature in the presence 683 of cooling pipes, the effect of doubling the coolant flow rate (under turbulent conditions) on 684 CCL temperature reduction was also examined in the present study. This was because 685 according to the energy balance relationship generally used for steady -flow systems, the fluid 686 flow rate is proportional to the heat transfer rate to or from the fluid . Therefor e, an increase in 687 fluid flow rate would lead to an increase in the net heat transfer rate between the cooling pipes 688 and the surrounding sand medium, which would in turn reduce CCL temperature. The energy 689 balance relationship is expressed in Eq. (9) , where the terms 𝑄𝑄̇, 𝑐𝑐̇, 𝑐𝑐𝑝𝑝, 𝑇𝑇𝑖𝑖 and 𝑇𝑇𝑒𝑒 refer to heat 690 transfer rate ( to or from the fluid ), mass flow rate, specific heat , mean fluid temperature at the 691 inlet and mean fluid temperature at the exit of the tube, respectively [66] . 692 𝑄𝑄̇ (𝑊𝑊)= 𝑐𝑐̇×𝑐𝑐𝑝𝑝(𝑇𝑇𝑒𝑒−𝑇𝑇𝑖𝑖) Eq. ( 9) Interestingly however, it can be observed from Figs. 17(a) and (b) that the FT profiles 693 obtained for 0.75 m3/h and 1.5 m3/h flow rates were very similar to each other. This observation 694 is confirmed by Fig. 18 which show s that doubling the flow rate merely increased the FTAP 695 range by 2.5- 5.2% . Even the trivial reduction of CCL temperature that was achieved by 696 doubling the flow rate was limited to the uppermost 16 mm of t he CCL. This indicates that 697 39 doubling the flowrate under turbulence conditions did not have a significant influence on CCL 698 temperature reduction or more specifically, the net heat transfer rate. In general, when the 699 velocity (or flow rate) of a fluid (i.e., the coolant in this case) is increased during heat exchange, 700 the warm and cool masses of the fluid are brought into contact with each other more frequently, 701 as a result of which heat is transferred via conduction at a greater number of sites within the 702 fluid and the overall heat transfer rate across the fluid is increased. Essentially, the higher the 703 fluid velocity (or flow rate), the higher the heat transfer rate. Accordingly, the rate of heat 704 transfer across a fluid is known to rise significantly as it t ransitions from laminar to turbulent 705 flow conditions (i.e., Re > 4000). This is because, unlike in the case of laminar flow conditions 706 where thermal energy is transferred across streamlines via molecular diffusion , the thermal 707 energy under turbulent flow c onditions, is transported across the fluid much more rapidly by 708 twirling eddies present in the flow which results in an increased rate of heat transfer. However, 709 the rate of heat transfer is known to achieve a maximum limit when the flow reaches fully 710 turbulent conditions, which corresponds to Reynolds number greater than 10,000 in most 711 practical situations [66]. 712 In the present study, the Reynolds numbers calculated for the two laboratory flowrates 713 of 0.75 m3/h and 1.5 m3/h were 21275 and 43857, respectively. Based on the calculated 714 Reynold numbers, it can be presumed that under a flow rate 0.75 m3/h, the coolant flow (within 715 the laboratory pipe network) would have already reached fully turbulent conditions, where the 716 rate of heat transfer between the PSL and the cooling pipe would have most likely already 717 reached its maximum limit. Therefore , increasing the coolant flow rate further would have led 718 to an insignificant change in the rate of heat transfer and final temperature profile formed along 719 the CCL depth. The same theory would apply to the proposed field- scale cooling pipe system 720 as well since the Reynold’s number is preserved when the laboratory flow rates are scaled up. 721 It can thus be concluded that flow rate variation would have an insignificant effect on CCL 722 40 temperature reduction as long as the flow rate employed ensures fully turbulen t conditions 723 within the pipe network. 724 4.3.2. CCL water content profiles 725 Fig. 20(a) depicts the final gravimetric water content profiles measured along the depth 726 of a CCL subjected to CETs of 40°C and 70°C in the presence of cooling pipes . It can be 727 observed that the FWC profiles generally decreased along the CCL depth regardless of the CET 728 or coolant flow rate . This observation is attributed to the reduced downward heat flow along 729 the CCL depth, explained in section 4.1.3. In order to analyse the effect of cooling pipes on the 730 desiccation reduction of the CCL, water content growth percentages (WCGPs) were calculated 731 using Eq. ( 10), where the terms 𝑤𝑤𝑏𝑏𝑏𝑏 and 𝑤𝑤𝑏𝑏𝑏𝑏 referred to the FWCs of base case and cooling 732 case experiments , respectively. WCGP values indicate the percentage by which the base case 733 final WC profile along the CCL depth increased when cooling pipes were employed. Fig. 20(b) 734 shows the variation of calculated WCGPs along the depth of a CCL subjected to CETs of 40°C 735 and 70°C in the presence of cooling pipes . It is evident from Fig s. 20(a) and Fig. 20(b) that 736 regardless of the CET applied, the final water contents measured along the CCL depth were 737 generally higher in the presence of cooling pipes than in the absence. This is attributed to the 738 temperature reduction of the CCL achieved by the heat extraction of cooling pipes. 739 𝑊𝑊𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑐𝑐𝑐𝑐𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑔𝑔𝑔𝑔𝑐𝑐𝑤𝑤𝑔𝑔ℎ 𝑝𝑝𝑔𝑔𝑔𝑔𝑐𝑐𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 (%)= 𝑤𝑤𝑏𝑏𝑏𝑏− 𝑤𝑤𝑏𝑏𝑏𝑏 𝑤𝑤𝑏𝑏𝑏𝑏×100 Eq. ( 10) It can be observed that for the 40°C CET , only the uppermost region of the CCL 740 experienced a rise in the base case FWC when cooling pipes were introduced, whereas a 741 significant rise in base case FWC was observed throughout the entire depth of the CCL for the 742 70°C CET (regardless of the flow rate employed). This was because the heat flow triggered by 743 the application of 70°C CET during the base case caused the CCL to experience desiccation 744 consistently throughout its entire 110 mm depth, unlike in the case of 40°C CET , where the 745 41 CCL’s desiccation was limited to its uppermost region (as shown in Fig. 11(c) ). Finally, Figs. 746 20(a) and (b) show that doubling of flow rate led to an insignificant rise in the base case FWC 747 measured along the CCL depth in general. This is expected since it was observed in section 748 4.3.1.2 that increasing the flow rate under fully turbulent conditions had an insignificant e ffect 749 on CCL temperature reduction. 750 Fig. 20. (a) Final water content profiles along CCL depth in the presence of cooling pipes; (b) 751 water content growth percentage along CCL depth in the presence of cooling pipes 752 5. Conclusion 753 The present study examined the variation of temperature -moisture profiles along the 754 depth of a compacted clay liner ( CCL ) exposed to constant elevated waste temperatures (CETs) 755 of the range 40°C -70°C in the absence and presence of two heat reduction techni ques, viz. 756 cooling pipes and rockwool insulation. Following analysis of the results obtained in this study, 757 the following conclusions were drawn: 758 (a) 0 20 4060 80 1000 10 20 30(-) Depth (mm)Final water content (%) 40°C (BP) 40°C (GS, 0.75 m³/h) 70°C (BP) 70°C (GS, 0.75 m³/h) 70°C (GS, 1.5 m³/h) (b) 0 20 4060 80 100-10 10 30 50(-) Depth (mm)Water content growth (%) 40°C (GS, 0.75 m³/h) 70°C (GS, 0.75 m³/h) 70°C (GS, 1.5 m³/h) 42 1. In the absence of any heat reduction techniques (i.e., in the base case), application of CETs 759 led to an augmentation of temperature and reduction of water content along the entire depth 760 of the CCL. The rise in temperature and reduction of water content along the CCL depth 761 increased as the applied CET rose. 762 2. Introduction of rockwool insulation (in the insulation case) led to significant temperature 763 reductions along the CCL depth under CETs of 40°C and 70°C. However, while rockwool 764 thickness increments led to a progressive reduction of CCL temperature, the rate of 765 temperature reduction attenuated with increasing thickness. 766 3. Introduction of rockwool insulation led to a significant reduction of the desiccation 767 experienced by the CCL exposed to CETs of 40°C and 70°C. Desiccation of the CCL 768 generally reduced as the insulation thickness increased . 769 4. Rockw ool thicknesses of 20 mm and 35 mm were recommended to be employed for liner 770 insulation in MSW landfills that encounter temperatures near 40°C and 70°C, 771 respectively .The recommended thicknesses were the minimum thicknesses (from those 772 tested) that were fou nd to be sufficient to maintain the CCL ’s temperature below the 35°C 773 elevated temperature limit, above which temperatures can adversely affect the liner ’s 774 performance . 775 5. Introduction of cooling pipes (in the cooling case) led to significant reductions in 776 temperature rise and desiccation along the CCL depth under CETs of 40°C and 70°C. The 777 CCL region located underneath the entry pipe carrying cool water , experienced a higher 778 cooling than that located underneath the return p ipe carrying warm water. 779 6. Increasing the flowrate under fully turbulent conditions (Re > 10,000) did not have a 780 significant influence on the reduction of CCL temperature and desiccation. 781 7. The proposed cooling pipe system in its current state was deemed appropriate to be 782 employed in MSW landfills that encounter temperatures close to 40°C , but was not 783 43 recommended to be employed in those that encounter temperatures close to 70°C without 784 economical adjustments being made to enhance the system’s current heat exchange 785 efficiency . This was because the maximum CCL temperatures recorded for the 70°C CET 786 cooling pipe experiments exceeded the adverse 35°C elevated liner temperature limit 787 adopted in the present study. 788 In general, the two proposed heat reduction techniques were found to be effective in 789 successfully reducing the temperature rise and desiccation experienced by a CCL that is 790 subjected to elevated temperatures in MSW landfills. 791 6. Credit author statement 792 Vihan Jayawardane: Conceptualization, Methodology, Experimentation , Results analysis; 793 Writing - Original Draft 794 Vivi Anggraini : Writing - Review & Editing , Supervision, Funding 795 Manh -Vu Tran : Methodology, Supervision 796 Mehdi Mirzababaei : Results analysis, Writing - Review & Edit ing, Supervision 797 Agusril Syamsir : Funding 798 7. Declaration of Competing Interest 799 The authors declare that they have no known competing financial interests or personal 800 relationships that could have inappropriately influence d the work reported in this paper 801 8. Acknowledgement 802 Financial support for this study is provided by the Ministry of Higher Education in Malaysia 803 with grant number FRGS/1/2021/TK02/MUSM/03/1. This support is gratefully acknowledged. 804 9. Data availability 805 All data generated or analysed during this study are included in this published article . 806 10.