基于极限载荷试验的扩底抗拔桩承载变形特性的分析

结合天津于家堡南、北地下车库项目开展了2组各3根扩底抗拔桩的极限承载力试验,其中一组试桩有效桩长19 m,另一组试桩有效桩长30 m,试桩均加载至极限破坏状态。载荷试验过程中,两组试桩均对桩顶、有效桩长桩顶以及桩端位置进行了位移测试;其中有效桩长为19 m的试桩还开展了桩身轴力测试。对2组试桩成果从荷载位移曲线、桩身轴力分布、桩侧摩阻力分布规律等方面进行了分析。结合有限元数值模拟分析表明,扩底抗拔桩的承载力由等截面段桩侧摩阻力与扩大头抗力共同组成,首先由等截面段桩土侧摩阻力提供抗拔力;当上拔荷载进一步增大后,扩大头开始逐渐发挥作用,并且扩大头抗力占总承载力的比例逐步上升。极限状态下,有效桩长为19,30 m的试桩,扩大头提供的抗力占总抗拔承载力的比例分别约为50%和35%。扩大头的存在对于等截面段桩侧摩阻力的发挥影响较小。扩大头受周边土体法向力的竖向分量是扩大头抗力的主要组成部分,极限状态下,其可占扩大头抗力的70%左右。

Ultimate load tests on bearing and deformation behavior of uplift piles with enlarged base

A series of ultimate loading tests on two types of uplift piles with an enlarged base with effective pile lengths of 19 m and 30 m are carried out. For each type, three test piles are loaded to failure. The deformations of the test piles at the top and toe are continuously monitored throughout the tests. For the 19 m-long piles, the axial force of pile shaft is measured as well. Load-displacement curves, distribution of axial force of pile shaft and shaft resistance are investigated. The monitoring data and numerical analysis results show that the uplift capacity of uplift piles with an enlarged base is composed of shaft resistance of the uniform section and uplift resistance of the under-reamed part. When a pile is pulled up, the shaft resistance of the uniform section works firstly, subsequently followed by the uplift resistance of the under-reamed part. At the ultimate state, the uplift resistance provided by the under-reamed part accounts for about 50% and 35% of the total uplift capacity for the 19 m-long and 35 m-long piles. It is also found that the influence of the under-reamed base on the development of shaft resistance is not significant. The uplift resistance of the under-reamed base mainly comes from the vertical component of the normal force of the soil around the base, which takes a proportion of approximately 70% at the ultimate state.