考虑软黏土流变效应的静压桩长期承载力计算方法

Long-term bearing capacity of driven pile considering creep effect of soft clay

摘要:

针对软黏土流变效应引起土体的体积压密,使其强度随时间逐渐提高,从而软黏土中的静压桩在桩周超孔隙水压力消散后承载力仍持续增长的问题,在考虑静压桩的沉桩效应和桩周土初始固结的基础上,采用双曲线型函数描述桩侧和桩端的荷载传递特性,结合荷载传递法推导承载条件下桩周土的应力状态.在此基础上,根据流变模型和桩周土体的受力状态,采用拟似超固结比的变化模拟桩周土体的流变行为,分析桩周土体长期流变过程中力学特性的变化规律,提出软黏土中静压桩长期承载力的理论预测方法.通过现场静载荷试验对本文解答进行验证,研究软黏土中静压桩长期承载力随时间的变化规律,并探讨土体参数对静压桩长期承载力的影响.结果表明:软黏土中静压桩承载力的长期时效性主要来源于桩身下部侧摩阻力随桩周土流变的发展,承载力在流变初期增长迅速,随流变时间发展速率逐渐变缓,且流变系数和回弹系数越大的土层中承载力的增长较为显著.该计算方法可为上部结构拆除后旧桩的承载力确定及其再利用提供参考.

Abstract:

Due to the volume compaction induced by creep effect of soft clay, the strength of soft clay increases over time, which leads to the growth of the bearing capacity of the driven pile installed in soft clay, even though the excess pore water pressure has been fully dissipated. Considering the installation of pile and the primary consolidation of the surrounding soil, the hyperbolic load transfer curves were employed in this study to model the load transfer characteristics of pile shaft and pile base, and the stress state of the soil around the pile was deduced by load transfer method. On this basis, taking advantage of an advanced elastic-viscoplastic model, the creep behavior of soil adjacent to the pile was evaluated by the quasi-overconsolidation ratio. Then, the evolution of soil strength during creep was analyzed, and a theoretical method was proposed for the long-term bearing capacity of driven pile in soft clay. The proposed theoretical method was verified by static load test. The variation of long-term bearing capacity of driven pile in soft clay with time was investigated, and the effect of soil parameters on long-term bearing capacity of driven pile was further analyzed. Results show that the long-term bearing capacity of driven pile in soft clay was primarily attributed to the increase of shear stress on lower pile shaft. The bearing capacity increased rapidly and then gradually slowed down with service time of pile. It was found that the evolution of long-term bearing capacity of driven pile was more significant in soil with higher creep index and swelling index. The theorical method presented in this study is expected to provide guidance and suggestion for determining and reusing driven piles whose superstructures have been demolished.