(1. College of Architecture and Civil Engineering，Xi′an University of Science and Technology，Xi′an，Shaanxi 710054，China；
2. School of Civil Engineer，Southwest Jiaotong University，Chengdu，Sichuan 610031，China；3. College of Geology and Environment，Xi′an University of Science and Technology，Xi′an，Shaanxi 710054，China；
4. Earthquake Agency of the Xinjiang Uygur Autonomous Region，Urumqi，Xinjiang 830011，China)
Abstract：With the deepening of globalization and the development of polar resources，a large number of engineering activities will be carried out in the permafrost regions. In addition, in engineering that employing the artificial freezing method，intervention in the thawing of frozen layer by artificial thawing means is a possible way to eliminate thawing settlement. Microwave irradiation has high efficiency in thawing frozen soil，and can be used for rapid thawing and auxiliary-excavation method in cold regions. In this paper，the melting and softening laws of frozen soil under microwave irradiation and the corresponding microscopic mechanism are studied. The results show that：(1) the frozen soil sample with initial temperature of －15 ℃ completely melted after microwave irradiation for 85 s，the internal and surface of the sample melted simultaneously within the depth of microwave penetration；(2) the thermal efficiency of microwave irradiation is much higher than that of traditional heat conduction thawing method；(3) the temperature change of frozen soil during microwave heating has obvious thermal relaxation stage；(4) the unconfined compressive strength of frozen soil dropped from 1.43 MPa to 0.05 MPa after continuous exposure to microwave for 30 s. According to the results of nuclear magnetic resonance (NMR) test on frozen soil during microwave irradiation，combining the above experimental results，following conclusions can be drawn：(1) the fundamental reason for the rapid melting of frozen soil under microwave irradiation is that it contains a certain amount of unfrozen water，thermal movement of those water molecules is enhanced after they absorb microwave，and thus the unfrozen water is heated，meanwhile heat is transferred to the adjacent ice and soil particles，resulting in the continuous melting of the ice and the increase of the overall temperature；(2) the melting and softening process of the frozen soil under microwave irradiation are controlled primarily by the unfrozen water content. In summary，microwave irradiation is a promising artificial thawing method that could be used in various engineering scenarios in cold regions.
2020, Vol. 39 
