增层开挖引起既有预制桩残余应力释放分析

Release of residual stress in existing preformed pile due to further excavation beneath pile raft

为了更好地理解地下室增层开挖后既有工程桩的应力状态和承载性状,以采用预制桩基础的高层建筑为研究对象,分析增层开挖对既有预制桩施工残余应力的影响.从施工残余应力简化模型和能量法概念出发,提出桩残余应力的计算模型,再结合增层开挖前后桩侧摩阻力和桩端阻力的计算模型以及回弹量的计算方法,确定计算残余应力所需的模型参数,模拟出增层开挖前后桩身残余应力的变化情况.结果表明,增层开挖会造成桩侧摩阻力及桩端阻力的损失,并引起更高幅度的桩身残余应力释放.同时,桩残余应力的释放程度与计算点到开挖面的距离有关,距离越远释放幅度越小,这说明残余应力的释放与桩-土间的相对位移有关.

This study aims to improve the understanding on the stress state and bearing behavior of existing pile subjected to excavation beneath the raft or cap of the pile. Concerning high-rise buildings supported by preformed piles, analysis was carried out to observe the change of post-installation residual stress locked in the piles due to such excavation. A proposal for residual stress calculation was proposed by using a simpli- fied simulation model and the principle of energy conservation. The parameters for calculating the residual stress were determined based on the models for pile＇s shaft and base resistances and the model for soil heave subjected to excavation. The change of residual stress due to excavation was thus derived. The results in- dicate that excavation leads to the loss of shaft and base capacities of the piles, but the rate of reduction is smaller than that for the residual stress. The release degree of the residual stress is related to the distance between the calculated point and the excavation surface, i. e. , farther distance corresponds to smaller de- gree of release. This finding is an indicator that the release of residual stress is closely associated with the relative pile-soil displacement.