Abstract:

To reasonably determine the freezing design parameters of liquid nitrogen and variation patterns of the temperature field when replacing a shield tail brush under high water pressure, a case study is conducted on the waterstop by liquid nitrogen freezing when replacing a shield tail brush in a shield tunneling project. The ADINA finite element analysis software is used to model the liquid nitrogen freezing and sealing and shield tail brush replacement. The rationality of the model is validated by comparing the simulated temperaturetime results with the fieldmeasured data. Furthermore, the sensitivity factors of the soil layer, liquid nitrogen temperature, freezing tube length, freezing tube spacing, and freezing method are analyzed. The results are summarized as follows: (1) The freezing effect varies with soil layers, and the freezing effect of liquid nitrogen is the worst in fine silt sand and the best in pebbles compared with gravel. (2) In a limited freezing time, different liquid nitrogen temperatures only affect the cooling rate from the start of freezing to over 0 °C. It does not affect the cooling rate under 0 °C. (3) The freezing pipe length slightly affects the soil freezing effect. A freezing pipe length as short as 1.52 m can meet the requirement of a freezing wall thickness of 2.0 m and save the cost. (4) The freezing pipe spacing considerably affects the freezing effect; the closer the pipes, the faster the freezing speed, and the better the freezing effect. (5) Under the same conditions, the freezing effect of doubleloop preembedded freezing pipe liquid nitrogen freezing is better than that of liquid nitrogen freezing through holedrilling on a segment.

Key words: high waterpressure sandy layer, shield tail brush replacement, liquid nitrogen freezing method, numerical analysis, temperature field