基于改进西原模型的深井冻结压力数值计算分析

A numerical analysis of freezing pressure in frozen soils around a deep shaft based on the modified Nishihara model

深厚冲积层冻结压力取值大小是冻结法凿井外层井壁设计计算的重要依据。为此,基于符合深井冻土蠕变特性的改进西原模型,利用ABAQUS软件的用户子程序接口,实现该模型的UMAT开发。考虑土体冻结过程中的热-力耦合作用获得井筒开挖前土体冻胀应力分布规律,在此基础上,计算分析了深部冻结井的掘砌过程,获得了作用于外层井壁的冻结压力发展变化规律。计算结果表明:土体埋深、冻结壁温度、土体冻胀率等因素均影响冻结压力的大小。在其他条件不变的情况下,当埋深由400 m增加到500 m时,冻结压力增加21%;当冻结壁平均温度由-16℃降低至-18℃时,冻结压力减小10%;当土体冻胀率由2%增加到3%时,冻结压力增加3.8%。冻结压力随层位深度及土体冻胀率的增加而增加,而降低冻结壁温度则有利于冻结壁的稳定。数值计算结果与实测值的误差小于15%,比理论计算更有利于实际工程中深井冻结压力的计算预测。

Determination of the freezing pressure in deep alluvium is crucial in designing the outer shaft lining. Using the UMAT of ABAQUS subroutine, the modified Nishihara model for creep of frozen soil is implemented into ABAQUS. With considering the effect of thermo-mechanical coupling during freezing, the freezing pressure distribution of frozen soil before excavation is obtained. Based on these results, the variation of freezing pressure on the outer shaft lining is determined by analyzing the process of deep frozen shaft construction. The results show that the values of freezing pressure are related to the soil depth, soil frost heave ratio and the temperature of the frozen wall. The freezing pressure increases 21% when the depth increases from 400 m to 500 m, and decreases 10% when the average temperature of frozen wall decreases from -16 ℃ to -18℃, and increases 3.8% when the frost heave ratio increases from 2% to 3%. The freezing pressure increases with the increase of depth and frost heave ratio of soil. The lower temperature of the frozen wall benefits the stabilization of the frozen wall. The error between numerical and measured results is less than 15%, showing that the proposed method can reasonably predict freezing pressure of deep shaft in practice.