GNGTS 2013 - Atti del 32° Convegno Nazionale

can be analyzed for a known porous medium to validate the influences that material characteristics and water content have on them. Within the box, three parts were separated by permeable membranes. For the analyses, the central part, approximately 0.6 m in length, was filled by the porous medium for a thickness of about 0.3 m. In the outer sectors, two PVC pipes were placed surrounded by high porosity material for adjusting the water level and establishing a water flux if needed. In the central sector within the analyzing medium, a thermal resistor, controlled by a thermometer and rheostat to ensure a constant temperature, served as a heat source. Four resistance thermometers (Pt-100) were placed at specified distances from the heat source, while four watermark sensors continuously monitored the soil moisture, assuring a constant water content. A total of more than 50 experiments were carried out. They differ in time of heat up, static or dynamic conditions of water fluxes, number, position, temperature and geometric configuration of the heat sources, position of the T-sensors. During some of these temperature-recording tests, electrical surveys were carried out with different configurations. Fig. 1 reports the configuration adopted during the tests presented in this paper. The heat source was placed in the center of the box and the T-sensors were aligned, aside of the heat source, along the major axis of the box. For monitoring the electrical resistivity changes in two perpendicular directions, 24 electrodes were placed in a network mode all around the source with a 9 cm spacing. The variation of the apparent resistivity as a function of temperature and time was therefore recorded in different locations. This kind of configuration was useful for checking the heat spreading in a plan view located at about 10 cm depth, which is also the depth of the temperature sensors. The tested porous medium presented in this paper has 91% vol. of sand and 9% vol. of silt, compacted to a porosity of 0.46 and at complete saturation. Results and discussions. The results of the electrical surveys showed the expected correlation of decreasing electrical resistivity values with the increasing temperature induced by the heat source, the opposite is true after the source’s turn off. The electrical resistivity decreases more slowly with increasing distance from the heat source, as temperature does, and is dependent upon the heat flux within the material. Fig. 2 shows the comparison between the temperature recorded by each sensor and the apparent resistivity measured close to it. The resistivity changes achieved show an inverse trend with the temperature data. More specifically, a good agreement is clear in the heating period, while a less marked increase in resistivity is noticeable when the source was turned off. The electrical resistivity shows also a slower return to the initial conditions than temperature. As an average, it has been observed that a 10% positive variation in temperature generates a 2.5% negative change in electrical resistivity. Fig. 3 presents the time-lapse sequence of the apparent resistivity during the heating and the cooling period imaged in the plan view Fig. 1 – Laboratory device built for temperature and resistivity monitoring. A picture on the left and an explanatory sketch on the right, with the progressive electrode number and the position of the heat source and temperature sensors (T). 125 GNGTS 2013 S essione 3.2