动力沉桩桩基拉应力分布及其施工控制试验研究

Experimental research on distribution of tensile stress in pile foundation by dynamic piling and construction control

采用"现场沉桩试验、室内桩基模型试验、现场沉桩实例验证"的技术路线,通过对16根桩的动测试验结果的全面介绍和分析,揭示了打桩过程中桩身拉应力的分布规律,即桩身最大拉应力通常发生在桩的上部,特别是打桩初期,桩尖位于软土层或桩周为软土地基时,桩身最大拉应力点发生在距桩顶1/4l(l为桩长)附近,拉应力占同点压应力的50%左右,这是桩身产生垂直桩轴线方向横向裂缝乃至发生桩身断裂现象的根源。试验亦表明,采用碟簧桩帽沉桩能有效地减小锤击拉应力,克服了采用老式桩帽辅助沉桩时预应力混凝土(管)桩桩身拉应力过大而造成桩身断裂这一缺陷,控制实测拉应力在允许拉应力(5 MPa)范围之内,以确保沉桩桩身完整,这是防治桩身拉裂的有效措施。

Adopting the field piling test, laboratory test of model pile and engineering examples of field piling, the principle of distribution of tensile stress in pile body is discussed by the introduction and analysis of dynamic measurement of sixteen piles. The results show that the maximum tensile stress always occurs at the top of pile, especially when pile end is embedded into the soft soil; the maximum tensile stress is located at 1/41 （I is pile length） to the top during the beginning of piling, which leads to occur transverse cracks which are perpendicular to axis of pile and the rupture of pile body. The results also show that the disk-spring pile cap can decrease hammer tensile stress, diminish the phenomenon of over-tensile stress when piling aided by conventional pile cap and make measured stress below allowable one （below 5 MPa which is considering effective method to prevent pile body from rupture）.