基于载荷试验的振沉钢管桩承载力研究

Investigation on Axial Capacity of Vibratory Driven Steel Pipe Piles Based on Static Load Tests

液压免共振锤沉钢管桩施工工艺在城市桥梁工程建设中逐步得到了较广泛的应用,其施工设备轻便、施工速度快,很好地满足了桥梁预制拼装施工的要求。针对液压免共振锤沉钢管桩竖向抗压极限承载力的预测问题,对比了现行各规范计算方法的差异。针对两根长度为50 m、外径700 mm的振沉钢管桩开展了静载荷试验,从试桩结果可以看出,振沉桩休止期42 d对应的实测极限承载力并未达到所有规范计算的极限承载力;休止期72 d对应的实测极限承载力相较42 d时增长了至少16%,达到了公路规范和上海岩土规范计算值。通过不同休止期试桩结果的对比分析,说明振沉桩的休止期不应小于42d,该值高于目前规范对静压桩和打入桩的推荐休止期;同时也说明现有振沉钢管桩极限承载力计算方法有值得改进之处;上海地区,当桩端持力层为密实砂土时,采用公路规范和上海岩土规范的计算方法是较合理的。

Steel pipe pile driven with resonance-free hydraulic vibratory hammer is popular in urban bridgeengineering,which only requires potable equipment and shows a higher efficiency. For axial compressioncapacities of vibratory piles,comparison was made between those predicted by current Chinese codes. Fieldfull scale tests were performed on two identical steel pipe piles with length of 50 m and outer diameter of700 mm. Test results show that these codes overestimate the bearing capacity of vibratory pile tested at 42-dayafter end of driven(EOD). After another 30 days(72-day after EOD),measured capacity for the second testpile(virgin compression)increases not less than 16% compared the one measured at 42-day after EOD. The72-day measured capacity meets the requirements of Specifications for Design of Foundation of Highway Bridge and Culvert and Shanghai Code for Investigation of Geotechnical Engineering. Load tests results showthat,to achieve a higher measured capacity than current codes designed values,vibratory driven piles shouldbe loaded no earlier than 42-day after EOD,which is larger than 28-day recommended by current codes. Thatis to say,vibratory driven piles need longer time for aging to produce a higher capacity. This also implies thatcapacity prediction methods given by current codes could be refined for vibratory driven piles. In Shanghai,forvibratory driven piles toed in dense sand,both Specifications for Design of Foundation of Highway Bridge andShanghai Code for Investigation of Geotechnical Engineering produce well prediction.