扩底抗拔桩承载力计算方法与工程应用

上海地区扩底抗拔桩具有中长度、小扩展角度的特点。基于现场足尺试验与工程实践,提出了适合该桩型抗拔承载力计算的两种方法:圆柱面剪切法和扩大系数法。其中圆柱面剪切法假定桩端扩大头以上一定范围内的土体剪切面直径等于扩大头最大直径,超过此范围的桩等截面部分的侧摩阻力不受影响。扩大系数法通过扩底扩大系数和旁压扩大系数来反映扩大头段抗拔阻力的提高。这两种方法分段套用现有规范的抗拔桩计算公式,参数选取容易,公式简洁,便于工程设计人员采用。这种新型的扩底抗拔桩及其承载力计算方法在瑞金医院地下车库和上海铁路南站南广场中得到应用,试验结果皆大于计算值,其中上海铁路南站南广场工程计算值与试验结果非常接近,进一步验证了计算方法的合理性,并为认识扩底抗拔桩的承载特性提供了参考。

Analytical method of bearing capacity and its engineering application of pedestal uplift piles

Considering the characteristic of medium length and small angle of the pedestal uplift piles in Shanghai area,based on full-scale field test and engineering practice and combined with calculation principal of theoretical method,this paper suggests two calculation methods of the withdrawal resistance of pedestal uplift piles: cylindrical face shear method and amplification factor method.Cylindrical face shear method assumes that the diameter of shear plane of definite range soil above pedestal sector equal to the maximum diameter of pedestal sector,and the side friction resistance of constant cross-section part is not impacted.Amplification factor method uses the pedestal and lateral pressure amplification factor to reflect the increase of uplift resistance on the pedestal sector.The parameters of these two methods are same with conventional calculation method of bearing capacity of driven cast-in-place piles.Meanwhile,two actual application projects for the Underground Garage of Ruijin Hospital and South Square of Shanghai South Railway Station are introduced in this paper,it is indicated that the test value are greater than calculation result,and specially calculation result and test value of piles in the Shanghai South Railway Station are in good agreement,demonstrating that that the proposed methods are applicable to pedestal uplift piles,and providing valuable reference to realize the loadbearing characteristic of pedestal uplift piles.