Engineering activities are becoming increasingly frequent in seasonally frozen and permafrost regions. Artificially thawing frozen soil before excavation may significantly increase the efficiency of excavation and reduce cost. Microwave irradiation can thaw frozen soil very rapidly, but has not yet been widely used in the field. One important reason is that the mechanisms of thawing and softening of frozen soil by microwave irradiation are not fully understood yet, therefore, the practicability and efficiency of this method cannot be well evaluated. In this study, microwave irradiation tests of frozen soils with five moisture contents were conducted and changes in their temperature, unfrozen water content and strength were measured. The results showed the following: (1) Microwave irradiation heating thawed frozen soil with a temperature of −14.4 °C completely within 60 s. The interior and surface of frozen soil samples melted simultaneously. (2) Conduction heating required 8400 s to melt frozen soil with the same temperature, which is 140 times longer than microwave irradiation. (3) Strength of the frozen soil decreased rapidly during microwave irradiation. Combined with the results of nuclear magnetic resonance (NMR) tests, it was found that the fundamental cause of the rapid melting of frozen soil is that a certain amount of unfrozen water exists between ice and soil grains. Accordingly, the following microscopic mechanisms of microwave irradiation thawing frozen soil are revealed. Unfrozen water absorbs microwaves and its molecular thermal motion increases along with its temperature. Then heat is transferred to ice and soil grains and frozen soil is thawed further. Based on a careful discussion on the efficiency and penetration depth of microwave-induced thawing, it was suggested that microwave heating is a promising artificial thawing technology in excavation of frozen ground.

在季节性冻结和多年冻土地区，工程活动变得越来越频繁。开挖前人工解冻冻土可显着提高开挖效率并降低成本。微波辐射可以非常快速地融化冰冻的土壤，但尚未在该领域中广泛使用。一个重要的原因是，尚未充分了解微波辐射对冻土的融化和软化机理，因此，该方法的实用性和效率无法得到很好的评估。在这项研究中，进行了五种水分含量的冷冻土壤的微波辐射测试，并测量了它们的温度，未冷冻水分含量和强度的变化。结果表明：（1）微波辐射加热解冻的冻土温度为-14℃。60 s内完全达到4°C。冷冻土壤样品的内部和表面同时融化。（2）传导加热需要8400 s才能融化相同温度的冷冻土壤，这是微波辐射的140倍。（3）在微波辐射下，冻土的强度迅速下降。结合核磁共振（NMR）测试结果，发现冻结土壤快速融化的根本原因是冰与土壤颗粒之间存在一定量的未冻结水。因此，揭示了微波辐射解冻冻土的以下微观机理。未冷冻的水吸收微波，其分子热运动随温度升高而增加。然后热量被转移到冰和土壤中，冷冻的土壤被进一步融化。