既有建筑地下开挖工程中在役桩竖向承载特性分析

Vertical bearing characteristics of in-service pile in underground excavation engineering of existing building

如何评价土体开挖条件下在役桩的竖向承载能力,是桩支承的既有建筑地下开挖工程中亟待解决的关键问题,围绕上述问题,明确土体开挖条件下土体应力状态改变、回弹等因素对桩侧和桩端承载特性的影响,建立考虑土体开挖效应影响的桩侧荷载传递双曲线模型和桩端荷载传递双折线模型,考虑开挖卸荷效应引起的土体超固结状态提出土体开挖条件下桩侧极限摩阻力的计算方法,基于有限差分法和荷载传递法形成既有建筑地下开挖条件下在役桩基竖向承载特性的计算方法。通过与现有试验结果比对分析,验证计算方法的合理性。结果表明,对地下增层开挖条件下单桩进行分析时,应考虑开挖引起的土体回弹、桩周土超固结状态等因素对桩侧和桩端承载特性的影响,当计算的归一化土体回弹量较小时(<1),沉降分析中可不考虑回弹效应。

In the project of underground excavation beneath existing building supported by piles,there is a need to evaluate the vertical bearing capacity of the in-service piles under the condition of soil excavation.The influence of variation of soil stress and soil rebound on the bearing characteristics of soil around pile and at pile base was determined.Considering the effect of soil excavation,a hyperbolic model of load transfer along pile shaft and a bilinear model of load transfer at pile end were established.In view of the over-consolidation state of soil induced by the unloading effect of excavation,the calculation method was established to evaluate the ultimate unit skin friction of pile shaft under the condition of excavation.On the basis of the finite difference method and the load transfer method,the calculation method was proposed for predicting the vertical bearing characteristics of in-service pile under the condition of underground excavation beneath existing building.The validity of the proposed method was verified by comparing with the experimental results.Results show that the influence of soil rebound and the overconsolidation state of soil due to excavation on the pile shaft resistance and pile base resistance should be taken into account for the analysis of the pile behavior subject to underground excavation.If the calculated normalized soil rebound is relatively small(e.g.less than 1),the soil rebound can be neglected in the settlement analysis.