大直径杯型冻土壁温度场数值分析

Numerical analysis of temperature field within large-diameter cup-shaped frozen soil wall

结合南京地铁10号线过江隧道盾构始发工程,运用有限元软件建立三维数值模型,对大直径杯型冻土壁温度场的发展与分布规律进行研究,分析不同因素对该温度场的影响规律,比较研究不同土层下该温度场的降温规律。数值计算表明:在设计冻结方案下,杯型冻土帷幕厚度满足加固范围要求,开始交圈时间由早到迟依次为外圈管>中圈管>内圈管,形成闭合大直径杯型冻结帷幕的时间为12 d;冻结壁交圈时间随导热系数的增大而线性减小,随容积热容量和原始地温的增大而线性增大,原始地温每升高5℃,冻结壁交圈时间增加约1 d;相变潜热变化对冻结初期和后期土体降温过程几乎没有影响;不同土层降温速度由快到慢分别为砂土水泥土>黏土水泥土,砂土>黏土;砂土水泥土与砂土、黏土水泥土与黏土几乎同时达到相变阶段;无论水泥改良与否,砂土总比黏土的开始交圈时间早4 d。所得结果为今后类似工程设计提供了理论依据。

This paper presents a comprehensive study of the generation and distribution patterns of the temperature field within a large-diameter cup-shaped frozen soil wall used in shield construction of Nanjing Subway Line 10. Three dimensional finite element analyses are taken to elucidate various influential factors for the temperature field. It is found that, if the thicknesses of cup-shaped frozen soil walls can satisfy the designed amounts needed for the purpose of ground reinforcement, frozen wall forms in a clear sequence： firstly, in the outer tubes, then the intermediate tubes, and lastly, the inner tubes. The ＂closure time＂ of the frozen wall, which means the time period required for the full formation of the frozen soil wall, is heavily dependent on soil＇s thermal conductivity as well as the original ground temperature. The closure time would decrease linearly with the increase of the thermal conductivity; it would increase linearly with the increase of original ground temperature of soil and volumetric heat capacity. For the concerned Nanjing shield tunneling site, it took around 12 days. This would have increased by approximate every one more day if the original ground temperature increases by every 5℃. The latent heat is found to be of no apparent influence in the cooling process of soil. Notwithstanding, the cooling speed varies much with different types of soil. Generally, the cooling speed of sandy cemented soil and sand is faster than that of clay cemented soil and clay respectively. However, they reach the phase-transitional-stage almost simultaneously. It is also noted that the frozen walls in sand are always formed 4 days earlier than those in clay regardless of whether the cement is in place or not. Results and conclusions from this research may play a useful role to aid designs of similar projects in the future.