深开挖条件下抗拔桩分析与设计方法

Design and analysis of uplift pile under deep excavation

上海世博500kV地下变电站抗拔桩桩端埋深为82.3m,开挖深度约34m,且开挖面积大,开挖土方量达450000m3。结合工程实际提出了深开挖条件下抗拔桩的分析与设计问题。对深开挖条件下抗拔桩开挖段侧摩阻力的扣除、开挖卸荷桩周土体围压减小引起的抗拔桩承载力损失及开挖卸荷基底土体回弹对桩产生预拉力等问题进行了分析。提出了一种双套管法实现开挖段侧摩阻力的扣除,得到有效桩长的抗拔承载力;采用有限元法和基于M indlin解的简化分析方法预估大面积深开挖土体卸荷引起抗拔桩承载力损失比例;采用有限单元法分析基坑开挖卸荷土体回弹对桩身产生预拉力,并指出深开挖的桩身设计应进行开挖过程及正常使用阶段两个方面的抗拉强度的验算。

The development of underground space has the tendency of more depth.Shanghai World Exposition 500kV underground substation has the architectural diameter of 130 meters and buried depth of 34 meters.The cast-in-place pile with the depth of 82.3 meters was used to resist water buoyancy.After subtracting the excavation depth of 34 meters,the pile length was about 48.3 meters.At the soft soil area,the piles were constructed on ground,and the test of uplift bearing capacity of the test piles which was longer than actual piles were carried out on ground.This brought out the key issue of how to analyze and design the uplift pile to reflect the influence of the deep excavation.Analyses were focused on the loss of side friction of the uplift piles in the excavated segment,the bearing capacity reduction of uplift piles because of the off-loading of excavation which lead to the pressure loss of soil around the piles,and the tension axial force of piles caused by the foundation rebound after the excavation.It was found that the bearing capacities of the uplift piles were reduced evidently and tension stresses appeared in the middle and lower part of the pile shafts due to unloading.A discount of the bearing capacities of the uplift piles was adopted in the design stage considering the decrease of bearing capacities of uplift piles due to unloading.The quantity of reinforcement of the uplift pile was checked by the sum of the axial force caused by the uplift load at the service stage and that caused by unloading at the excavation stage.