Experimental Investigation of Nail as a Geothermal Structure and Heat Exchange Effects on its Pull-Out Behavior

Using geothermal energy by extracting heat from the ground has been widely accepted as a renewable energy source in place of fossil energy. Typically, extracting geothermal energy includes drilling deep wells (over 1000 m) into the ground which tends to be costly. To decrease total costs of this method, using structural foundations is suggested. Ground-embedded structures such as foundations, retaining walls, tunnel linings, etc. can be used as absorber elements for ground heat exchange. The heat exchange system includes absorbing and transporting ground thermal energy to buildings via a fluid that circulates in pipes placed within the ground structure. This paper examines the use of nails as a thermal–structural element, termed thermal nails, to serve the dual purpose of bearing load and exchanging heat with the ground. The soil–nail interaction and the effects of heat exchange on its pull-out capacity are investigated. For this purpose, a laboratory experimental setup is built to simulate both heat injection and extraction through the thermal nails. The results show that heat transfer through the thermal nail doesn’t decrease the pull-out strength and in some cases can lead to an increase in pull-out capacity by up to 10%. The slope of load–displacement curves also show an improvement by about 40% when the soil temperature is increased by 80%. Furthermore, the measured rate of temperature change in the sandy soil shows the feasibility of using thermal nails.