盾尾刷更换时液氮冻结温度场及冻结参数影响的数值模拟分析

Numerical Simulation of Influence of Liquid Nitrogen Freezing Temperature Field and Parameters When Replacing Shield Tail Brush

为合理确定高水压下液氮冻结止水更换盾尾刷的冻结设计参数及掌握温度场变化规律,结合某过江通道长距离盾构掘进过程中盾尾刷更换时的液氮冻结止水工程,利用ADINA大型有限元分析软件建立液氮冻结止水及盾尾刷更换数值模型,模拟温度-时间变化曲线与现场实测数据对比,验证模型的合理性,并对土层、去路液氮温度、冻结管长度、冻结管间距、冻结方式进行敏感性因素分析。结果表明:1)冻结效果随土层的不同而发生改变,在卵石层、砾砂层、粉细砂层3种土层中,粉细砂层的冻结效果最差,卵石层最好;2)在有限的冻结时间内,去路液氮温度的不同,只影响土体从开始冻结至越过0℃完成相变这期间的降温速度,不影响完成冻结以后的土体降温速率;3)冻结管长度对土层冻结的影响较小,可以采用较短的冻结管,以降低施工成本和液氮消耗量,但不能过短,应保证其纵向有足够的支撑范围,经计算分析,当冻结管长度为1.52 m时,能在轴面处满足2.0 m的冻结壁厚度要求;4)冻结管间距对冻结效果影响大,冻结管越密,冻结速度越快,效果越好;5)在相同条件下,双环预埋冻结管液氮冻结效果优于管片上直接打孔液氮冻结。

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.