软土地区大吨位超长试桩试验设计与分析

温州350 m超高层中超长桩加载2800 t的试桩静载试验设计与分析表明：在地表土质承载力较低场地进行大吨位堆载试验时，可选择桩梁式堆载支墩–反力架装置来完成试验。对超长桩来说，在最大加载条件下，实测桩端阻力仅为桩顶荷载的25%左右，超长桩表现为端承摩擦桩性状。在使用荷载下，桩顶沉降的90%以上来自桩身压缩，在进行超长桩设计时，要充分考虑桩身质量对试桩沉降的影响。同时，桩底沉渣清除的干净与否，也直接影响超长桩的沉降。超长桩桩侧上部土层摩阻力具有不同程度的软化现象，而中下部土层侧摩阻力具有较弱的强化效应，因此在超长桩承载力计算时，不同深度土层的桩侧阻力和桩端阻力都应乘以相应不同的修正系数。试验结果显示淤泥土、淤泥质黏土、淤泥夹粉砂土中极限侧阻充分发挥所需的桩土相对位移阀值分别约为5～7 mm、6～8 mm和8～10 mm。

Large tonnage tests on super-long piles in soft soil area

A 2800-t static load test on super-long bored piles is conducted for a building 350 m in height in Wenzhou, which indicates that the heaped load supported by piles and beam-reaction system can be chosen to complete the large tonnage static load test in soft soil area. About 25% of the load at pile head is shared by the axial force at pile end, and the super-long piles work as end-bearing friction piles under the maximum loading. The percentage of super-long pile top settlement caused by the compression of pile shaft is over 90% under the working load. Therefore, the influence of pile quality on the settlement of pile top must be fully taken into account in design. In addition, the settlement of super-long piles is closely associated with the compressibility of the sediment at pile end. The shaft resistance of piles has a slight weakened and enhanced effect in the upper and lower soils, respectively, which means that the shaft resistance in different soil layers and the tip resistance should multiply different modification coefficients in estimating the bearing capacity of super-long piles. The ultimate pile-soil relative displacements for completely mobilizing the shaft resistance are found to be about 5 to 7, 6 to 8 and 8 to 10 mm in silt, silt clay and silt mixed with silty sand, respectively.