竖向受压下考虑安装扰动螺旋钢桩数值模拟分析与现场载荷试验

Field test and numerical analysis of a helical steel pile under vertical compression loads considering installation effects

为研究螺旋钢桩在砂质土地区竖向受压荷载下承载特性,首先,基于螺旋钢桩现场静载试验,分别讨论国内外确定其极限承载力差异,其次,根据现场静力触探结果评价土层参数建立考虑安装扰动效应的静载试验数值计算模型,计算桩顶荷载位移曲线与现场实测对比验证,最后在数值计算结果基础上,分析不同荷载等级下桩沿深度范围轴力和侧摩阻力变化规律以及侧阻和端阻承担荷载大小与比例。结果表明:国内lg P-s方法较国外Livneh和ElNaggar方法与修正Davisson方法确定极限承载力更大,国外2种方法偏保守;考虑安装扰动效应的荷载位移曲线较未考虑安装扰动效应的计算结果更接近现场实测;竖向荷载作用下,螺旋钢桩中心钢轴侧阻与上中部螺旋叶片端阻基本同时发挥作用,随着荷载增加,下部螺旋叶片与钢轴底部端阻继续发挥,比重逐渐增加,且当桩顶位移超过约4%叶片直径后,基本保持在71%左右。

An in-situ vertical compression load test of a helical steel pile was conducted in silty sand and sand in order to study the bearing characteristics of helical piles in sandy soils under vertical compression loads. The difference of the existing methods for determining the ultimate bearing capacity of the helical pile tested was discussed. A finite element model of the helical pile under a vertical load considering installing effects was established based on soil layer parameters obtained from field cone penetration tests,and comparison of the pile load-displacement curves between the calculation and measuring results was performed. Based on the numerical simulation,the distributions of the axial force and shaft friction force of the pile along the depth under different load levels were explored,and the magnitudes of the shaft and end resistances and their ratio were investigated. The results show that the ultimate bearing capacity calculated by lg P-s method is larger than those obtained by Livneh & E1 Naggar and modified Davisson approaches,which indicates the later two methods over-conservative and that the simulation P-s curve considering installation effects gives better agreement with field measurement than that without considering installation effects. Under vertical compressive loading,the shaft resistance of the pile and the end resistance of middle-upper plates of the pile take effect almost simultaneously,and with increasing the vertical load,the end resistance provided by lower plates comes into play and increases gradually up to a peak value about 71% of the ultimate load with a displacement of the pile head exceeding about 4% of the diameter of plates.