Experimental Study of Artificial Ground Freezing Using Liquid CO2 as New Cold Transfer Medium

The artificial ground freezing (AGF) has been using CaCl2 brine as cold medium for a long time, but for the new requirements of underground engineering on the high strength and short duration of ground reinforcement, the traditional brine medium is often difficult to meet the new conditions, and deep freezing with ultra-low temperature mass as the refrigerant is theoretically considered to be an important direction for the AGF method to solve this problem. In order to provide guidance for the design and construction of deep freezing, this paper completed the first in-situ test of liquid CO2 medium with double-row freezing pipes by using the new refrigeration equipment, and comprehensively analyzed the freezing effect and compared with the traditional brine medium. The results show that: the new refrigerant temperature decreases very quickly, and can be reduced to -30℃ in 1 day after starting the machine, and reaches below -40℃ in 3 days, then gradually remains stable. It takes only 3 days for the freezing wall to be partially closed, and 7 days for the vertical formation of continuous and reliable freezing wall, which saves 3 times of time compared with traditional brine freezing. In the whole process of freezing, the development rate of frozen soil of the new CO2 medium is greater than that of brine freezing, and the longer the freezing time, the greater the increase, the rate of 20 days, 30 days, 40 days increased by 31%, 35% and 40% respectively. The freezing wall formed by the "plum blossom " double-row pipe is roughly "V" shaped distribution, the vertical frozen soil exceeds the lower pipe end by about 600mm, and the total thickness of frozen soil can reach 3.06m in the horizontal direction for 36 days freezing, while the average temperature of freezing wall can drop to -20.1°C. In the insulation section near the ground surface, the freezing wall thickness on one side at the end of the test was 350 mm, compared with 1103 mm in the standard freezing section, the extension was reduced by about 2/3, and the soil temperature increases more than 10 ℃, the outer wall insulation of freezing pipe could effectively reduce the cold loss of the new cold transfer medium.