Thermal conductivity (TC) characterizes heat transfer ability from geomaterials which is important in many engineering fields dealing with geothermal applications, waste disposal of steel factories, waste storage facilities, thermal improvement of the ground, etc. [1, 2, 3]. In this field, there are many studies for heat transfer and heat insulation through geomaterials. Adding reinforcement materials such as geotextile to geomaterials can improve the conditions of heat transmission or isolation [4].
Geotextiles are used, for improving the properties of soil and geomaterials such as shallow foundations [5], separation and filtration of coarse and fine soil aggregates in road construction [6, 7], for drainage in embankment dams and tunnels [8], for slope stabilization [9, 10] and in mechanically stabilized earth walls [11]. In these geotextile-reinforced geomaterials, heat transfer and heat insulation through reinforced geomaterials have particular importance.
Geothermal energy is the second most renewable energy source after solar energy [12]. This kind of energy reduces the current dependence on non-renewable energy sources and can be used for various local and large-scale purposes. Geothermal systems are divided into shallow geothermal systems and deep geothermal systems. The temperature of shallow depth soil layers is affected by seasonal temperature changes. However, typically for depths greater than 6–7 meters the temperature of the ground remains almost constant during the year and therefore, they are suitable for storing and extracting heat, as they are an unlimited source of energy. The system that provides this environment is called "shallow geothermal" [13]. Thermal investigation of soil around shallow geothermal systems contributes to the optimization and growth of this industry. The soil TC is one of the essential parameters in thermally geological research fields. The TC determines the ability to transfer heat, which is important in many fields such as sedimentary basin, geothermal utilization and geothermal engineering [14].
The TC as a vital soil property is the material ability to conduct heat and is based on the material structure [15]. It appears primarily in Fourier's Law for heat conduction. Non-metals can pass heat using phonons, which are waves in the lattice (Fig. 1). The TC is appropriate for conductivity, convection, and radiation in granular materials. Conduction is affected by mineral properties and contact between particles. Convection is related to the properties of the pore-filling fluid. The distribution of heat defines radiation without the need for material [16]. Typical applications of the soil TC utilization are in buried pipe projects, Geothermal Energy Piles (GEP), Ground Source Heat Pumps (GSHP), Borehole Thermal Energy Storage (BTES), shallow geothermal, very shallow geothermal, and electrical line insulation.
The soil TC value depends on many factors divided into two main groups. The first group is related to the soil properties such as texture, particle size, dry density, water content, porosity, saturation degree, mineralogical composition, and particle shape. The second group depends on other factors that are controlled or changed environmentally or intentionally. These include temperature changes, the number of freezing-thawing cycles, and so on [17–21]. There are two basic methods to measure the soil TC directly: the transient method and the steady-state method [22]. In general, steady-state techniques are helpful when the temperature of the material does not change with time. A simple steady-state method is the hot plate method, which contains a constant temperature gradient at a constant sample thickness. A circulating water system controls heat flow from one side to the other. The device consists of two parallel aluminum plates, including a thin plate heating element; the sample is located between two plates. The disadvantage is that a well-engineered experimental setup is usually needed, it takes a long time, and the temperature does not change with time. In this research, transient methods have been used because it takes less time to perform each test. In addition, it is practicable where the temperature changes with time. The advantages of this method are quicker measurements and measuring of the heating up process [23]. For places with no access to on-site or laboratory testing, indirect TC tests are suitable. An indirect method of determining the correlation between the TC and physical parameters such as wave velocity can be measured with geophysical methods [24]. Blázquez [25] showed that the TC of three geophysical formations is associated with the velocity of S and P waves by multiple seismic and surface wave analyses. Therefore, the TC of soil and rock is estimated from seismic explorations without previous measurements of the geological surface.
The TC in frozen soil samples with various water amounts has been investigated in previous studies. For example, Li et al. [20] investigated the TC of frozen clay samples having different water contents and showed uncertainty and variability for the TC of the clay samples. In addition, they proved that the TC of frozen samples was more prominent than in thawed samples, and it increased with decreasing the soil temperature in the frozen samples. The soil particle shape affects the physical properties of granular soils, Xiao et al. [26] investigated the effect of particle shape on the TC of five granular soil mixtures by using thermal needle tests. Particle shape is assumed by the "overall regularity" parameter, which is the average of sphericity, convexity, and aspect ratio. The overall regularity increases with decreasing the void ratio, resulting in an increased TC ratio [27]. Also, increasing the soil density at a given overall regularity increases the TC. Xu et al. [28] investigated the effect of temperature, soil moisture, and density on the conduction behavior of silty clay in the Genhe region of China. This study was performed in an area with a frozen layer on the ground. The experiments were performed on nine groups of soil samples and temperatures varying from − 40 to 20oC. The results show that the TC decreases linearly with decreasing the temperature at positive temperatures, and at negative temperatures, the TC increases with reducing the temperature. Chen [29] investigated the effect of porosity and degree of saturation of sands on their TC by performing laboratory experiments and used a laboratory thermal probe and measured the TC of medium-size, large-size, silty and fine-size sand. He observed that the TC increases with decreasing the soil porosity and with increasing the saturation degree. Tokoro et al. [30] estimated the effect of soil water content on the TC of three soil types by using a thermal probe method and proposed an empirical equation. They also performed electrical conductivity tests on the mentioned soils and calculated the TC by knowing the electrical resistance of the soils. They showed that the water dependence of TC is strongly correlated with electrical resistance, and the TC and moisture content are nonlinearly related. Barry-Macaulay et al. [31] compared the TC values of 27 laboratory test data with 4 TC empirical equations proposed by [32–35]. This comparison showed that the empirical equations presented by Lu et al. [35] and Côté and Konrad [34] are more suitable for fine-grained and coarse-grained soils, respectively.
Liu et al. [36] investigated the TC of crumb rubber sand mixtures and showed that the mixture TC increases for sand fractions less than 20%. A significant immediate increase in the TC was found when the sand fraction exceeded 80%. Zhao et al. [37] investigated the TC of frozen soil at temperatures close to 0° C by transient and steady-state methods. The results show that the TC for undistributed frozen soils increases with increasing the soil density and water moisture content. Zhang et al. [38] investigated the TC of sand-kaolin-clay mixture with different properties by thermo-time domain reflectometry probe. The results show that the new laboratory experimental soil TC generated equation for estimating the TC is approximately 85%, similar to the laboratory results. Schj [39] used nine soil types and measured their TC at depths from 5 to 85 cm by heat pulse method, and then the soil samples with different saturation degrees were used for laboratory tests. The identified equation can predict the TC values with 87% accuracy compared to those obtained from laboratory results.
Urresta et al. [40] evaluated the relationship between the TC and temperature by using thermal response tests with Horizontal Ground Heat Exchangers (HGHEs) and determined the efficiency of the ground TC at shallow depths. The results show that at depths of HGHE, the earth temperature is intact and the short period of the thermal response test prevents heat interference between the ground surface and HGHEs. Malek et al. [41] explored the relationship between soil type, soil water content, and soil salinity with the TC values and showed that soil water content had the most significant effect on soil TC. The impact of salinity on the TC cannot be ignored. The impact of soil type, silty-loam, has lower TC than sandy-loam and sand.
In cold regions, the effect of freezing-thawing cycles on soils is an inherent climatic phenomenon [42]. Zhang et al. [43] indicated the impact of the freezing-thawing process on the TC of silty clay. The results show that the TC rises as the initial water content increases. Therefore, the TC decreases in soils with higher initial dry density after the thawing and freezing process. Ghoreshizadeh et al. [44] used the transient method to measure the TC of a sandy soil with various porosities, water moisture contents, and water levels and found that the TC decreases with increasing the soil porosity. In addition, they reported that at a constant porosity, the TC increases with increasing the soil saturation degree. Table 1 presents some research studies on the determination of the soil TC.
Table.1. A summary of some research studies conducted on soil thermal conductivity (TC).
Ref. | Item/Parameter examined | Soil type | Main result | Test method |
Al Nakshabandi and Kohnke [17]. | density | Fine sand, silt loam, clay | The TC of dry soils increases with increasing the bulk density of the grain size. | Transient method |
Abu-Hamdeh, N. H., and Reeder, R. C [47]. | density, moisture, salt concentration, and organic matter | Sand, sandy loam, loam, and clay loam. | Increasing the bulk density at a given moisture content for all soils studied increased the TC. Increasing the moisture content at a given bulk density increased the TC. | Transient method |
Barry-Macaulay et al. [46]. | Moisture contents and densities | silty clay- sandy clay- Sand- Clayey Sand- Basaltic Clay- Residual Siltstone | Coarse grained soils were observed to have a larger thermal conductivity than fine grained soils. In addition, the thermal conductivity of soils increased with an increase in dry density and moisture content. | Steady-state method |
Zhang et al. [43]. Xu et al. [45]. | Freeze-thaw Density- aiging time | Silty clay Compacted bentonite | The TC increases with increasing the initial water content. Thus, the soil TC decreases with higher initial dry density after the thawing and freezing process. Thermal conductivity decreased with increasing aging time for both of the compacted bentonites. | Transient method Transient method |
Li et al. [20]. | Freezing | Clay | The TC of frozen samples was more prominent than in thawed samples, and it increased with decreasing the soil temperature in the frozen samples. | Transient method |
Zhao et al. [37]. | Freezing | Undisturbed frozen soil samples | The TC of undistributed frozen soils increases with increasing the soil density and water content. | Steady-state method |
Xiao et al. [26]. | Particle shape | Granular soil | The void ratio decreases with increasing overall regularity, and the TC increases. Also, with increasing the soil density at a given overall regularity, the TC increases. | Transient method |
Liu, Cai, and Liu [36]. | Crumb rubber-soil mixtures | Sand | The TC of mixtures increases for sand fractions less than 20%. | Transient method |
Xu et al. [28]. | Temperature | Silty clay | The TC decreases linearly with decreasing the temperature at positive temperatures, and at negative temperatures, the TC increases with reducing the temperature. | Transient method |
Urresta et al. [40]. | Horizontal ground heat exchanger | Sandy loam | At depths of HGHE, the earth temperature is intact and the short period of the thermal response test prevents heat interference between the ground surface and horizontal ground heat exchangers. | Transient method |
Malek, Malek and Khanmohammadi [41]. | Soil type, salinity, moisture, water content | Sand, sandy loam, and silt loam | The impact of salinity on the TC cannot be ignored. The effect of soil type, silty-loam, has lower TC than sandy-loam and sand. | Transient method |
Ghoreshizadeh et al. [44]. | Porosity- water level | Sand | The TC decreases with increasing porosity. At a constant porosity, the TC increases with increasing the saturation degree. | Transient method |
In this study, laboratory tests were performed to focus on the TC measurement of fine-grained sand, medium-grained sand, and mixed fine and medium-grained sands. For this purpose, a TC device was manufactured by the authors to find out the TC values of granular soils. The noveltyof this study may be summarized as:
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Manufacturing a standard TC device with the thermal needle probe method.
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Placing geotextile layers in the sandy soil and measuring the TC values for various gotextile layer locations in various types of soils.
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Using different sizes of containers and measurements of infilled sand TC with different porosities for laboratory investigation of the radial effect.