竖向循环荷载下钙质砂单桩沉降特性模型试验研究

钙质砂中的桩基础时常需要承受构筑物上的竖向循环荷载。研制了新型循环加载桩基模型试验系统,通过电位移计、微型土压力盒等传感器,研究了加载次数、循环荷载幅值对桩顶累计沉降、沉降速率、桩端和桩侧阻力变化的影响,分析了循环荷载对钙质砂单桩沉降规律和机理。试验结果表明:①桩顶循环累计沉降随循环次数增加而增大,二者满足对数函数关系;在不同动荷载比(动荷载幅值与桩基极限承载力比值)下,可分为稳定型、渐进型、破坏型3种沉降形式,有着显著的“门槛效应”。②动荷载比越大,桩顶循环累计沉降终值越大,存在一个临界循环次数,达到临界循环次数后,桩顶循环沉降速率(沉降量增量与循环次数比)随循环次数的增加而减小。③循环加载时,桩端和桩侧分担上部荷载比例不断变化,循环桩侧平均摩阻力随循环次数增加逐渐减小,存在“累积损伤”现象,致使循环桩端平均阻力增大,循环荷载加剧桩端钙质砂颗粒破碎和密实,对桩顶累计沉降增加有弱化趋势,动荷载比与桩侧平均摩阻力弱化系数满足Logistic函数关系。研究成果对钙质砂桩基工程设计、施工具有指导意义。

Model Tests on Accumulated Settlement of Single Pile in Calcareous Sand under Vertical Cyclic Loads

Calcareous sand pile foundation is often required to withstand cyclic vertical load on the structure. Model tests of vertical cyclic load on single pile in calcareous sand were carried out with a self-designed test device. The effects of dynamic load amplitude and cycle number on the accumulated settlement on pile top, the settlement rate, and the pile-end and pile-side frictional resistance were examined by sensors (e.g., soil pressure cell and electric displacement meter). The law and mechanism of the settlement of single pile in calcareous sand under cyclic loading were analyzed. As revealed from the test results, 1) more cycle number resulted in larger accumulated settlement of pile top, displaying a logarithmic functional relationship. Under different dynamic load ratios (ratio of dynamic load amplitude to limit bearing capacity of pile foundation), the accumulated settlement of pile could be divided into stable type, progressive type and destructive type, indicating an evident “threshold effect”. (2) Lager dynamic load ratio led to correspondingly larger final accumulated settlement of pile top. However, once undergone a critical number of loading cycle, the cyclic settlement rate (ratio of settlement increment to cycle number) of the model pile tended to reduce and stabilize. 3) The proportion of the upper load shared by the pile end and the pile side varied constantly during cyclic loading. A phenomenon of “progressive damage” was identified. With the rise in the number of cycles, the pile-side frictional resistance reduced progressively, while the cyclic average resistance of the pile top enhanced slowly. Under cyclic loading, the calcareous sand particles broke around pile end, thereby hindering the settlement tendency. The dynamic load ratio and the attenuation coefficient of pile-side frictional resistance pertain to Logistic function relationship.