Detection of the freezing state and frozen section thickness of fine sand by ultrasonic testing

Determining the freezing state and frozen section thickness is fundamental to assessing the development of artificial frozen walls but is commonly difficult or inaccurate because of a limited number and fixed position of thermometer holes under complex field conditions. We report a novel experimental design that measures soil temperature, water content, and ultrasonic properties to monitor movement of the cryofront (0°C isotherm), water migration, and acoustic parameters during progressive upward freezing of fine sand under laboratory conditions. Ultrasonic testing during different stages of freezing revealed changes in three acoustic parameters (wave velocity, wave amplitude, and frequency spectrum). As the cryofront ascended through the sand at different water contents, wave velocity continually increased, whereas wave amplitude initially decreased and then increased. Wave velocity measurements revealed the cryofront position during freezing, but measurements of wave amplitude did not. The frequency components indicated the frequency of different evolving freezing regions during upward freezing and the freezing state of fine sand during later stages of freezing. The freezing state can be evaluated on the basis of single vs multiple peaks and the kurtosis of frequency spectrum change. An equation developed to predict the thickness of the frozen section and tested against measured values in the laboratory and field showed accuracies of 86.84–99.33%. The equation is used successfully to estimate frozen wall thickness in artificially frozen fine sand in Guangzhou, China.