Reliability Analysis of Vertical Capacity for Necking Piles Assisted with Support Vector Machine
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摘要: 针对常用的一次可靠度方法（FORM）计算基桩承载力可靠度指标的精度不高，将支持向量机（SVM）与FORM相结合计算缺陷桩承载力可靠度指标.以缩径缺陷桩为例，开展了1根完整桩及5根缩径桩的竖向加载试验.采用随机加权法对缺陷桩承载力折减系数的均值进行估计.算例表明缩径处被土体填充，致使此处桩的侧摩阻力转化为土体界面摩擦力，削弱了桩的承载能力.缩径长度越大，缩径处被土体填充的面积就越大，桩承载力折减系数和可靠度指标越小.缩径位置距离桩顶越近，缩径限制侧摩阻力发挥的程度越大.桩端阻力弥补了部分损失的侧摩阻力，使得缩径位于桩身浅部和中部时，桩承载力折减系数及可靠度指标大致相同；位于深部时，承载力损失最小，可靠度指标最大.Abstract: Since commonly used first order reliability method (FORM) often yields inaccurate reliability index of pile bearing capacity, support vector machine (SVM) is combined with FORM to calculate the reliability index of bearing capacity of defective piles in this paper. Taking the reduced-diameter defective pile as an example, the vertical loading test of one complete pile and five reduced-diameter piles was carried out. The random weighting method is used to estimate the mean value of the bearing capacity reduction coefficient of the defective piles. The example shows that the shrinkage is filled with soil, which converts the lateral friction resistance of the pile here into the friction of the soil interface, and weakens the bearing capacity of the pile. The larger the shrinkage length, the larger the area filled by the soil at the shrinkage, and the smaller the pile bearing capacity reduction coefficient and reliability index. The closer the shrinkage position is to the top of the pile, the greater the degree to which the shrinkage limits the resistance of the side friction. The pile end resistance compensates for part of the loss of lateral friction resistance, so that when the shrinkage diameter is located in the shallow and middle parts of the pile body, the pile bearing capacity reduction coefficient and reliability index are roughly the same; when located deep, the bearing capacity loss is minimal and the reliability index is the largest.
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图 4 模型桩
第一个字母S、M和D代表缩径底部到桩端距离的140 mm、100 mm和40 mm；第二个字母L、M和S分别代表缩径长度的20 mm、10 mm和5 mm；a和b分别表示缩径直径和缩径长度
Fig. 4. Model pile
图 8 完整桩、SM桩、MM桩及DM桩桩周土体位移矢量图
Fig. 8. Soil displacements around intact pile, SM pile, MM pile and DM pile
图 9 完整桩、SL桩、SM桩及SS桩桩周土体位移矢量图
Fig. 9. Soil displacements around intact pile, SL pile, SM pile and SS pile
表 1 透明土的基本参数
Table 1. Parameters of transparent soil
Cu Cc ρd (g/cm3) ρdmax(g/cm3) ρdmin (g/cm3) γ
(kN/m3)6 1.354 1.438 1.481 1.239 2.51 注：Cu为不均匀系数; Cc为曲率系数; ρd为干密度，ρdmax为最大干密度，ρdmin为最小干密度，密度单位为g/cm3; γ为重度，单位为kN/m3. 
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