城市轨道交通深厚软土地层地下车站超长钻孔灌注桩承载-沉降特性分析

Analysis of Bearing-settlement Characteristics of Extra-long Bored Piles in URT Deep Soft Stratum Underground Station

[目的]为有效控制软土地层深基坑开挖及使用过程中车站结构的变形,同时增大坑底承载力,需对上部含有空桩的超长钻孔灌注桩(以下简称“超长桩”)承载-沉降特性进行深入研究。[方法]以某城市轨道交通双线L型交叉换乘车站为例,采用自平衡法静载试验和数值模型对超长桩承载-沉降特性的影响因素进行分析,并探究超长桩承载特性与桩身长度、桩身刚度及桩身直径之间的关系。[结果及结论]静载试验表明:上部含有空桩的超长桩在荷载箱等量增荷加载过程中,桩顶位移及其增速均逐渐增大,在卸荷过程中桩顶出现位移回弹现象。数值模拟分析表明:超长桩的桩身轴力从桩顶到桩端呈逐渐减小趋势,在桩顶极限荷载作用下,桩身上部的轴力远大于桩端;桩身长度和桩身刚度变化会影响上部含有空桩的超长桩承载-沉降特性,桩身刚度改变对其造成的影响很小。

[Objective]To effectively control the deformation of station structure during the excavation and usage of deep foundation pit in soft soil stratum,and to increase the bearing capacity of foundation pit bottom,it is necessary to conduct an in-depth study of the bearing-settlement characteristics of extra-long bored piles(hereinafter as EBP)with void in the upper part.[Method]Using an urban rail transit double-line L-shaped cross transfer station as example,the influencing factors of the bearing-settlement characteristics of EBP are analyzed through static load tests by self-balanced method and numerical modeling.The relationship between the bearing characteristics of EBP and the pile length,pile stiffness,and pile diameter is explored.[Result&Conclusion]The static load test results indicate that,during the incremental loading in load box of EBP with void in upper part,both the pile top displacement and the displacement growth rate gradually increase,with pile top exhibiting displacement rebound phenomenon during unloading.Numerical simulation analysis shows that the axial force of EBP shaft decreases gradually from pile top to pile end.When the pile top is under ultimate load,the axial force at pile shaft upper part is significantly greater than at pile end.Changes in pile shaft length and stiffness affect the bearing-settlement characteristics of extra-long piles with void in upper part,while changes in pile shaft stiffness have little impact.