Centrifuge modeling of jacked pile in clay
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摘要: 采用西澳大学室内鼓轮式离心机,在预先固结的高岭黏土中开展不同离心力场(50g,125g及250g,g为重力加速度)条件下的模型压桩试验、T-bar试验和静力触探试验,分析了模型桩在贯入过程、静置稳定过程中桩身径向应力(σr)的变化规律,并对后期桩体拉伸载荷阶段的径向应力变化值(Δσr)及桩侧摩阻力变化情况行了探讨,揭示了在不同超固结比(OCRs)黏土中静压桩侧摩阻力的演变特性.在此基础上,通过两种经验公式方法对桩侧摩承载力进行了预测计算和对比分析.研究结果表明:沉桩过程中桩端相对高度(h/B)对桩身径向应力的发展变化有很大的影响,桩身不同位置(h/B)的总径向应力对同一贯入深度而言,存在桩侧径向应力退化现象;基于静力触探试验提出的经验方法,能有效考虑静力触探锥端阻力(qt)和桩端相对高度(h/B)因素的影响,将其应用于黏土沉桩时桩侧摩阻力的预测,可取得与试验实测结果较吻合的结果.研究成果对软土地区静压桩施工与承载力设计具有一定的工程指导意义.Abstract: Notable contributions to our improved understanding of jacked pile behavior in sand have been achieved through instrumented model pile tests in laboratory test chambers, at elevated g-levels in the centrifuge, and in the field. In recent years, research focusing on pile behavior in clay has declined. Consequently, predictive methods for pile capacity have not advanced beyond those provided in American Petroleum Institute (API 2000) recommendations, which were based on research conducted in the early 1980s. This paper re-focuses attention on the shaft capacity of jacked piles in clay. Three centrifuge scale pile tests were performed in kaolin clay in the drum centrifuge at the University of Western Australia. The tests were performed in pre-consolidated blocks of kaolin, and were subsequently spun in the centrifuge at three different g-levels of 50g, 125g and 250g respectively. The piles were equipped with total pressure sensors located at different depths and were installed by jacking into samples of reconstituted kaolin clay. The kaolin clay samples were prepared to measure the range of the cone penetration test end resistance (qt), undrained strengths (su-Tbar), and overconsolidation ratios (OCRs). These pile tests were used to investigate the lateral stress changes (σr) developed along the pile shafts during pile installation and equalization. In addition, the change in the value of lateral stress (Δσr) and the changes in pile shaft resistance during the pile tension test were discussed. The characteristics of the jacked pile in the clay with different over-consolidation ratios (OCRs) were revealed. Furthermore, the centrifuge data were subsequently used to examine the current design methods for the evaluation of the shaft capacity of displacement piles in clay. The centrifuge test results show that during the pile penetration, a strong dependence of lateral stress on the relative depth of the pile tip (h/B) develops, and the total radial stress, as measured in a particular soil horizon, is observed to decrease as the relative depth of the pile tip (h/B) increases (where h is the height of the sensor above the pile tip, and B is the diameter of the pile). Based on the cone penetration test during the investigation, it is observed that the lateral stress developed on a displacement pile is strongly depended on the cone penetration test end resistance (qt) and the relative depth of the pile tip (h/B). It is shown that the empirical method allowing for a dependence of shaft friction on qt, and h provides good estimates of the shaft capacities measured in centrifuge experiments. The research results have certain theoretical and engineering significance for pile construction and bearing capacity design in the soft soil region.
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Keywords:
- jacked pile /
- radial stresses /
- clay /
- shaft capacity /
- centrifugal model
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