Temperature measurements of long-distance liquid nitrogen freezing in high-confined aquifer reinforcement

Based on the Qingchun Road cross-river tunnel of Hangzhou, frozen parameters and process of long-distance liquid nitrogen freezing in high-confined aquifer are obtained, and field measurements of freezing temperature are performed. The results show that the long-distance liquid nitrogen freezing temperature should be guaranteed in the way when the temperature reaches the working liquid nitrogen below -100℃, and the exit temperature after cycling should be held between - 50℃ ～ - 70℃. By using three 1 cm-thick layers of polyethylene insulating materials and three layers of sealed film alternately, the heat preservation effect is obvious, the package temperature has a rise of 0.070 ℃ / m, and the longest distance for the liquid nitrogen freezing can reach 1000 m. The development speed of permafrost from large to small order is gravel layer> silt sand layer> silt clay layer. The temperature of segment is affected by external environment, and the temperatures at segment-shield shell interface before, during and after welding of shield tail brushes are 5.4℃ ～ - 3.2℃, 19.3℃ ～ 8.7℃ and 12.7℃ ～ 4.8℃ so as to ensure the thawing and welding quality of frozen walls. Compulsory thawing only needs 2 days to achieve the condition for the shield, and this procedure has a total duration of 23 days, which solves the problem of replacing shield tail brushes in confined aquifer.