单向冻结条件下扩底桩抗冻拔能力试验研究

采用单向冻结的方法，针对季节冻土区扩底桩的抗冻拔能力开展了室内试验研究。试验分别选取不同扩角（无扩角直桩、45o，60o 和70o）， 不同埋深（240mm，290mm和340mm），及不同扩底直径（74mm，98mm和114mm）的扩底桩进行。实验结果表明，三组试验的最终冻结深度分别为19.51cm，14.36cm和14.80cm。不同扩角扩底桩的最终冻拔量基本相同，且仅为直桩的28.3%，扩底桩的最终冻拔量随着埋深的增大并非按照比例减小，但随扩底直径的增大基本按比例减小。桩的冻拔位移幅度随冻结深度的增大呈先增大后减小的趋势，且和土体的冻结速率呈负相关关系。直桩的冻拔速率随冻结深度先增大后迅速减小，而扩底桩的冻拔速率随冻深呈先增大后稳定减小的趋势。土体冻胀在桩的约束下呈漏斗形，扩底桩对桩侧土的约束冻胀明显强于直桩，间接说明了扩底桩的抗冻拔能力优于直桩。考虑冻结过程中土的冻结速率和桩的冻拔量变化规律，提出了桩的抗冻拔因子作为桩抗冻拔性能的评判标准，对不同影响因素下扩底桩的抗冻拔效果进行分析，结果表明扩角对桩的抗冻拔能力影响较小，增大桩的埋深和扩底直径可提高桩的抗冻拔能力。综合比较，影响扩底桩抗冻拔性能最主要的因素是扩底直径，其次为桩的埋深，影响最小的是桩的扩角大小。试验结果及分析可为季节冻土区扩底桩的抗冻拔设计及理论计算提供参考。

Experimental Study on the Anti-frost Jacking Ability of Belled Pile Under Unidirectional Freezing Condition

By applying the method of unidirectional freezing, a series of laboratory tests were carried out to study the anti-frost jacking ability of belled piles in seasonal frozen soil regions. The tests were conducted on piles with different bell angles (non-bell, 45o, 60o and 70o), different embedded depths (240mm, 290mm and 340mm), and different bell diameters (74mm, 98mm and 114mm). The results indicated that the ultimate freezing depths of the three tests were 19.51cm, 14.36cm and 14.80cm, respectively. The ultimate frost jacking displacements of belled piles with different bell angles were almost the same, which were merely 28.3% of straight pile. The ultimate frost jacking displacements of belled piles were not decrease in proportion with increase of embedded depth, but decrease in proportion with increase of diameter of enlarged base. With the increase of the freezing depth, the frost jacking displacements amplitude of the piles increases first and then decreases, and is negatively correlated with the freezing rate of the soil. The frost jacking rate of straight pile increases first and then decreases rapidly with increase of the freezing depth, while the frost jacking rate of belled pile increases first and then decreases steadily with increase of the freezing depth. The frost heave of soil was a funnel shape under the constraint of pile. Further, the constraint effect of belled pile was stronger than that of straight pile, which indirectly indicated that the anti-frost jacking ability of belled pile was better than that of straight pile. Considering the freezing rate of soil and frost jacking displacement of pile in the freezing process, the concept of anti-frost jacking factor of pile was proposed as the criterion of anti-frost jacking ability of pile, and anti-frost jacking effects of belled piles were also analyzed under different influencing factors. The analysis showed that the anti-frost jacking ability of pile was less affected by bell angles, but it could be improved by decreasing the bell angle and increasing the embedded depth and diameter of enlarged base. The test results and analysis can provide reference in designing and theoretical calculation of belled piles in seasonal frozen soil regions.