Microscopic mechanisms of microwave irradiation thawing frozen soil and potential application in excavation of frozen ground

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 degrees 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.

Automated summary

The study found that microwave irradiation can rapidly thaw frozen soil, with the strength decreasing quickly, and the efficiency of microwave irradiation is 140 times higher than traditional conduction heating. Nuclear magnetic resonance tests revealed that the fundamental reason for thawing frozen soil is the presence of a certain amount of unfrozen water between ice and soil grains.