Development of Experimental Setup for Measuring Thermal Conductivity Characteristics of Soil

Thermal conductivity displays a key role in design of engineering structures where, thermal stresses resulting from heat and temperatures are of concern. Significant efforts were made to measure the thermal conductivity of different materials. For thermal conductivity characterization of soil samples it is essential to have very flexible set-up. Hence, this paper provides details about indigenously developed experimental setup for thermal conductivity measurement. The design of this newly developed setup is based on the basic principle of steady state heat flow. This experimental setup is designed in order to measure the thermal conductivity of various materials such as soils, rocks, concrete and any type of unbonded and bonded materials. In this paper, initially the theoretical background of the measurement techniques and the principle of heat flow are described, followed by design description and working procedure. The design has been kept very simple, adjustable for varying type and size of specimens and easy to operate with excellent level of accuracy as evident from system calibration. The accuracy and precision of the newly developed setup was verified by testing reference materials of known thermal conductivity and in the test results a high correlation coefficient (R 2 = 0.999) between experimental data and fitting curve was achieved.


INTRODUCTION
temperature are of concerns [1]. It is more accurately  [3]. Each of these methods has certain advantages and limitations.
The thermal properties of soil and other composite materials alters due to modifications of their properties such as the emerging soil stabilization technology which has a significant role in the ground improvement where different soil stabilizing agents are used [4]. Therefore, it In addition, the current experimental setup can easily be modified and optimized for testing samples of different materials; such as, soil, concrete, ceramic, cemented materials and un-cemented materials etc. Different shape of samples can also be tested; such as, cylindrical and cubical shapes. Various sizes of samples ranging from 10-100 mm thickness and 36-140 mm diameter may be used for testing.

Theoretical Background
The thermal conductivity of soil may be defined as the quantity of heat transmitted through the unit length of substance per unit cross-section, per unit time under a unit temperature gradient. Thermal properties are of great interest to designing of many of the problems associated with geotechnical engineering. Several studies have been conducted to study the impact of various parameters on the thermal performance of the insulating materials [5][6][7].
The heat flow that occurs due to three processes i.e.
conduction, convection, and radiation; their rate of flow through material could be delayed by applying insulating materials [8]. Thermal insulation materials decrease the flow of heat within the building due to their low thermal conductivity [8]. The thermal conductivity in these applications is the most challenging task faced by the civil engineering professionals; whereas, temperature changes across the material play a significant role to affect the conductivity characteristics.
Factors that affect the rate of heat flow include the conductivity, temperature difference, sample thickness and the contact area of the sample. The thermal properties of the soil are also affected by the density and water content of the specimen [13]. From material to material, the thermal conductivity or resistivity characteristics vary to make these good conductors or an insulator [9]. Moreover, the factors responsible to affect the rate of transfer of heat conduction from one medium to another medium in any material consist of the material conductivity, for instance, some materials are highly conductive and some are less conductive. Moreover, the material size and thickness also matters. The effect of the sizes was explored by [10][11][12].
The heat flow mechanism mainly depends on the temperature gradient and its transfer through a medium either by conduction, convection or radiation. The heat flow mechanism is shown in Fig. 1(a-b) where it is being transferred from the heat source to the heat sink by the conduction process [14].
The amount of heat energy (Q) transferred across a material is directly proportional to the area of cross section (A) and the temperature gradient T and inversely proportional to the sample thickness (L) and can be expressed as given in Equation (1), and therefore, the coefficient of thermal conductivity (K) can be expressed as shown in Equation (2).
Where in Equations (1-2) Q is heat energy, L is sample thickness, T is the temperature gradient, K is the coefficient of thermal conductivity and A is the area of cross section.
Guarded heat flow is the most effective method which is usually used for the determination of thermal conductivity of materials [3]. Guarded heat flow technique is more appropriate to characterize low thermal conductivity materials for building insulation materials. It works on the principle of steady-state transfer of heat through the known thickness of the test specimen between a hot plate and a cold plate. However, there are certain limitations to the equipment that is the longer measurement time is needed to perform the experiment; hence it takes a longer time to achieve a steady-state temperature gradient [15].
Other potential error in measurements might cause due to the improper contact between the surfaces of the test specimen.
The list of the most commonly used thermal conductivity measurement methods is provided in Table 1

Experimental Apparatus
A schematic diagram of the experimental setup is shown in Fig. 2 and the photographs of the various components are shown in Fig. 3. The schematic diagram and photographs are labelled appropriately and listed in  1(b). ILLUSTRATION IS THE HEAT TRANSFER MECHANISM temperature of 200°C. The heat sink temperature is maintained through water circulation in the heat sink chamber using a motor as shown in Fig. 2. The system operates at 110 volts AC power supply. The system is provided with the provision of surcharge weight for an appropriate docking. In order to avoid the heat transfer from the sides of the specimen, a vacuum is created inside the chamber through the suction pump. The heat source temperature and heat sink temperature is shown on LCD display. A Complete setup of the experiment in operation mode is shown in Fig. 4.

Sample Assembling
The sample of the required size and well-polished (for appropriate docking) is placed on the heat sink plate as shown in Fig. 2

Testing procedure
After completing the system assembly the heat sink water circulation motor is started to keep the heat sink

CALIBRATION OF THE SETUP
The calibration and accuracy of the conductivity measuring device are done by performing the tests on standard samples of known conductivity. The calibration setup is shown in Fig. 5 and the calibration chart developed is shown in Fig. 6.

Sample Preparation
In the present study clayey soil was used as a base material; while lime and wheat straw were used as soil

RESULTS AND DISCUSSION
A variety of tests were conducted on different materials. Typical results of some of the tests conducted on clayey samples are presented below: Clayey samples added with the various percent of wheat straw are shown in Fig. 7. All samples were prepared by dry mixing the soil with the requiredpercent of wheat straw and thereafter added with water to prepare a workable paste. The samples were sun dried for a week. The sundried samples were oven dried before thermal conductivity tests. The tests results are given in Table   3.The effect of wheat straw on the thermal conductivity   From the experimental results, it may be seen that there is a gradual decrease in the thermal conductivity and increase in the thermal resistivity due to the increase in the wheat straw content.
Similarly, the effect of lime on the thermal conductivity/ resistivity of clayey soil was also examined. The samples were prepared by dry mixing the lime with clayey soil and thereafter added with water to prepare workable pastes. The samples were then sun-dried for a week and oven dried before testing. The samples prepared with the various percent of lime added in the clayey soil are shown in Fig. 10. The results are given in Table 4.The effect of lime on the thermal conductivity and thermal resistivity are shown in Figs. 11-12 respectively. From Figs. 11-12 it can be seen that there is a decrease in the thermal conductivity and increase in the thermal resistivity of clayey soils due to the increase in the lime content. The comparison of the effect of lime and wheat straw on the thermal conductivity of clayey soils is shown in Fig. 13. From Fig. 13 it can be seen that both lime and wheat straw resulted to decrease in the thermal conductivity; however, the effect of wheat straw is dominant as compared to the effect of lime on the thermal conductivity characteristics, which could be because of the low specific density of wheat straw as compared to the lime. It is also reported in [18][19] that the thermal conductivity is decreasing as the density is decreasing [19] performed thermal conductivity experiments on Hemp shiv mixed with clay and found that the addition of Hemp shiv resulted to the lowest thermal conductivity due to the low-density samples.