竖向荷载下群桩-土-承台相互作用数值分析

在大桩径、小桩距的群桩条件下，不仅有来自桩侧、桩端和承台传递的多重应力叠加，还有群桩对桩间土的夹持作用影响，桩-土-承台之间作用更加复杂。用有限差分软件模拟固定桩距、桩径，变化竖向荷载下桩-土-承台的相互作用。从各层土的侧摩阻力、不同位置桩的桩顶荷载、荷载-沉降关系、桩间土体位移等方面的计算结果分析桩-土-承台之间的相互影响。结果表明，荷载超出117.8 MN（略大于Pu/2，Pu为群桩极限承载力）后，群桩对上部桩间土的夹持作用开始减小，桩侧上部侧摩阻力增大；桩侧下部侧摩阻力在多重应力叠加作用下呈减小趋势，不同位置的桩侧摩阻力影响范围有差异；用群桩沉降达到5%倍桩径时的荷载作为群桩的竖向极限承载力是可取的；当沉降与桩径的比值超出1%后，承台分担荷载的比例逐渐增大，群桩分担荷载的比例减小。

Numerical analysis of interaction of pile group-soil-raft under vertical load

Besides the interaction of the multiple stress superposition from piles shaft friction, pile bottom resistance and contact stress between low caps and soil, the clamping action makes the cap-pile-soil interaction more complicated in pile foundation with large pile diameter and small spacing. 3D finite difference program is employed to evaluate the pile-soil-cap interaction with the change of vertical load, while the pile spacing and pile diameter remain constant in the analysis procedure. Analysis is carried out to explain the interaction of cap-pile-soil from the following aspects such as the lateral friction of soil in each layer, load on the top of characteristic piles, load-settlement relationship, vertical displacement of soil between piles, etc. The results show that: the clamping action on the upper soil among piles will decrease, and the pile shaft friction in the upper part will increase when the load exceeds 117.8 MN which is slightly larger than Pu/2 (Pu means ultimate bearing capacity of piles group); the lower part of pile shaft friction shows a trend of decrease due to the influence of the multiple stress superposition, and the decrease trend in characteristic piles has different influence scope; the load, which makes settlement of piles group achieve 5% of pile diameter, can be regarded as the vertical ultimate bearing capacity of piles group, and it is reasonable; when the ratio of piles group’s settlement to pile diameter exceeds 1%, the proportion of the load carried by piles decreases because of the cap contribution.