Analysis of macro and micro freezing characteristics of gravelly soil.

To investigate the mechanism of effect of different gravel contents on freezing characteristics of gravelly soil, macroscopic and microscopic experiments were combined. Unidirectional freezing experiment and NMR experiment were conducted on gravelly soil, with gravel contents of 0%, 10%, 20%, 30%, 40% and 50%. In unidirectional freezing experiment, final temperature, stratified moisture content and frost swelling of the samples showed a parabolic pattern with an upward opening about the gravel admixture. Among that, it was at the lowest point of 30% content with the closest space between gravel and soil grains, the least consumption of heat conduction, the largest compression of water migration, making the slowest development of frost-heaving and water supply. NMR experiment results showed that freezing characteristic curve was divided into four stages of subcooling-fast-slow-stable freezing, and gravel content was proportional to the subcooling temperature and value of sudden drop of unfrozen water. Combined with the T spectrum, it could be seen that larger pore structure of 30% content was relatively stable with well-distributed water migration during freezing process, and order of water freezing was free water went ahead of bound water, leading to a reduction rate of water migration was from fast to slow, with a consistent pattern of the evolution rate of macro parameters. In summary, the addition of a reasonable gravel proportion in engineering applications of cold zone could ensure relatively well-distributed and stable soil properties and pore structure, and relative low difference in temperature, moisture pooling, and frost swelling. Ultimately improving the safety and reliability of the overall operation of the project.