单向冻结条件下扩底桩抗冻拔能力试验研究

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

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,分别选取不同扩角(无扩角直桩、45°、60°、70°)和不同埋深(240、290、340 mm),以及不同扩底直径(74、98、114 mm)的扩底桩开展室内试验研究。根据试验及计算结果,3组试验的最终冻结深度分别为19.51、14.36、14.80 cm。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。桩的冻胀量变幅随冻结深度的增大呈先增大后减小的趋势,且与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,扩底桩的冻拔速率随冻结深度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。根据冻结过程中土的冻结速率和桩的冻拔量的变化规律,提出将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,埋深和扩底直径的增大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。

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, 45°, 60° and 70°), different embedded depths(240 mm, 290 mm and 340 mm), and different bell diameters(74 mm, 98 mm and 114 mm). Based on the test and calculated results, the ultimate freezing depths of the three test groups were 19.51 cm, 14.36 cm and 14.80 cm, respectively. The ultimate frost jacking displacements of belled piles with different bell angles were almost the same, which were merely 28.3% of the straight pile. The ultimate frost jacking displacements of belled piles didn’t decrease in proportion with the increase of embedded depth, but it decreased in proportion with the increase of enlarged base diameter. With the increase of freezing depth, the frost jacking amount amplitude of the piles increased first and then decreased, and it was negatively correlated with the soil freezing rate. The frost jacking rate of straight pile increased first and then decreased rapidly with the increase of freezing depth, while the frost jacking rate of belled pile increased first and then decreased steadily with the increase of freezing depth.The frost heave of soil had 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 amount of pile in the freezing process, the concept of anti-frost jacking factor of pile was proposed as the criterion of antifrost 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 angle, but it could be improved by increasing the embedded depth and diameter of enlarged base. Comprehensive comparison showed that the diameter of enlarged base was the most important factor affecting the antifrost jacking ability of pile, followed by the buried depth of the pile, and the smallest influence was the bell angle. The test results and analyses can provide references for designing and theoretical calculation of belled piles in seasonal frozen soil regions.