Model Test of Liquid Nitrogen Freezing-Temperature Field of Improved Plastic Freezing Pipe

Abstract

Liquid nitrogen freezing technology is widely applied to reinforce the water-bearing ground around launching and receiving shafts of shield machines in subway tunnel construction projects. An improved liquid nitrogen freezing pipe that can be cut by a shield machine is proposed in this paper. The improved pipe features a plastic outer pipe instead of a stainless steel outer pipe into which small-diameter holes are symmetrically drilled at equal distances along the wall of the inlet pipe to ensure the uniformity of the wall temperature. A model test was conducted to examine the liquid nitrogen freezing-temperature field of the improved plastic freezing pipe. Model test results showed that the wall temperature is unevenly distributed when the traditional freezing pipe (i.e., one that is open only at the bottom of the inlet pipe) is used for liquid nitrogen freezing. By contrast, a relatively uniform wall temperature distribution was obtained when the improved freezing pipe (i.e., with equidistant holes drilled on the wall of the inlet pipe) was used. Similarity ratios indicated that, if a 100-mm-diameter improved plastic freezing pipe is used for on-site liquid nitrogen freezing, the growth velocity of the frozen soil is approximately 75–150 mm/day.