基于Geddes方法的既有基桩竖向承载力评估

Evaluation of Vertical Bearing Capacity for the Pile in Service by Geddes' s Solution

自实际的桩顶向下一定范围内的桩身视为长度不大于实际基桩的一根假想桩基(即"虚桩")。根据Geddes解,计算"虚桩"桩侧摩阻力引起的"虚桩"桩端平面下方土体附加应力。分析软土、粘土和强风化地层中基桩受荷后不同时期承载力试验结果,提出沉降稳定基桩总体承载力提高幅度限值。根据土体附加应力水平,结合相关规范对地基土受荷后承载力评价的规定,提出沉降稳定的桩基础桩侧摩阻力和桩端阻力抗力系数取值方法。进行了同一场地下的试桩和工程桩受力现场监测。试桩桩基采用静载荷试验方式加载,工程桩桩基通过上部结构施工加载,通过试桩确定桩侧摩阻力标准值。采用本文方法对工程桩桩基的侧摩阻力变化情况进行了评估,并与监测结果进行对比,结果表明本文方法偏于安全。对于沉降不稳定的桩基,亦根据Geddes法原理,提出了桩侧摩阻力折减系数的计算方法。

Virtual pile,which was shorter than and part of an actual pile,was applied to calculate the additional stress in soil under the toe of the virtual pile by Geddes' solution. Piles in soft soil,clay and strongly weathered rock were tested twice or more by static axial compressive method after they were constructed. The testing data showed that vertical bearing capacity of the piles with stable settlement would increase and its limit value was decided accordingly.According to the magnitude of the additional stress in soil,combined with the regulation of how to evaluate the soil bearing capacity under certain load,the amend factors for side resistance and tip resistance of an actual pile with stable settlement were put forward. Side friction of drilled pills in two groups in nearly the same region were monitored.Piles in Group 1 were tested by static loading test to decide the standard value of side friction. Piles in the other group were bridge piles and were loaded by dead load of superstructures. Side friction in bridge piles after superstructures had been built up and evaluated according to the amend factors and verified by side friction monitoring data. Besides,the amend factors for the piles with unstable settlement were also brought out.