桩身残余应力的机理分析与测试方法

Mechanisms of Residual Sress on Piles and Its Measurement Methods

桩基静载试验中,忽略桩身残余应力,虽然不会影响桩基极限承载力的大小,但会影响桩侧摩阻力和端阻力的真实值。分析了残余应力产生的原因、机理和效果,分析和总结了残余应力的现场测试方法,并通过现场试桩实例分析了忽略桩身残余荷载对桩侧摩阻力和桩端阻力的影响。结果表明,忽略残余荷载的存在,竖向抗压桩静载试验结果会高估中性点以上桩段侧摩阻力和低估中性点以下桩段侧摩阻力和桩端阻力,会高估桩的竖向刚度。为了很好地了解和洞察桩基的荷载传递机理,在桩基静载试验中,建议在试桩前后系统地采集桩身测试元件读数,不能在试桩前人为地将测试元件读数设置为零。

Neglecting the residual stress in a pile static test (e.g., tension, compression, or O-cell test) will not affect ultimate pile load capacity (tensile or compressive) but will undoubtedly affect pile shaft and toe resistances. In this paper, the causes and mechanisms of residual stress are described and discussed. Field measured pile residual loads are summarized and compared. Specifically, two static loading tests in field were cited to demonstrate the effects of pile residual loads on skin friction and toe resistance of the piles. The compression loading tests indicate that the shaft resistance in the upper pile portion above the neutral plane and the vertical compressive stiffness of the test piles are overestimated, while the shaft resistance in the lower pile portion below the neutral plane and the toe resistance are underestimated if the residual loads. To better understand the load transfer mechanism of piles, it is suggested to systematically measure the pile strains before and after the test rather than assign zero strains initially.