分层地基中扩底桩抗拔承载力的计算方法研究

Research on computation methods of uplift capacity of enlarged base pile in layered soils

采用极限平衡法,利用幂函数形式的滑移面假设,推导出扩底抗拔桩在分层地基中极限承载力的简化计算公式,并将计算结果和其他方法计算结果分别与室内模型试验和原位试桩结果作对比,分析后发现文中计算公式是几种破裂面假设及其计算公式集成的一种简化计算方法;密砂中取幂指数N=10,松砂中取N→∞的计算结果和室内模型试验结果是一致的;分层地基中的原位试桩具有明显的尺寸效应,应区分短桩(L/d<20)和中长桩(L/d>20);文中计算方法取N→∞,在短桩的计算结果和原位试桩结果一致,在深基础中长桩则需引入分层折减系数进行修正,分层折减系数在砂土中取0.5,黏土中取0.6是合适的。

A new simplified computing formula of uplift capacity of enlarged base pile in layered soils is deduced according to power law slip surface premises and based on limit equilibrium method. The comparison between theoretical predictions which is in accordance with the simplified formula and other theoretical methods and centrifuge tests and in situ test for enlarged base pile is illustrated in detail. According to the comparative analysis, the following conclusions are gained： the deduced theoretical formula is an integrated simplified method of several failure surface premises and corresponding formulas. The theoretical prediction by simplified method while power N=10 in dense sand and N→∞ in loose sand is consistent with centrifuge tests results. Short pile（L/d〈20） and long pile（L/d〉20） should be distinguished because in situ test has obvious measurement effect in layered soils. The simplified method is accurate for short pile while power N→∞; on the other hand, this formula needs to be revised using layered discount coefficient for long pile. It is reasonable that layered discount coefficient equals to 0.5 in sand and 0.6 in clay.