嵌岩旋挖扩底抗拔桩工程应用研究

 对于单轴抗压强度在14.4 MPa以下风化程度不同的泥岩、泥质砂岩互层的特殊地质条件，按照现行设计规范，抗拔桩基础往往难以找到合适的终孔地层，利用旋挖钻机成孔、扩孔施工工艺形成扩底抗拔桩解决上述问题，并根据现场原型试验，对嵌岩旋挖扩底抗拔桩承载规律进行研究。根据桩顶静载试验和桩身应变测量试验数据，分析抗拔桩的桩身开裂、桩身变形规律。分析后认为，嵌岩扩底抗拔桩极限承载力主要由桩侧摩阻力、扩大头抗拔力提供，桩侧摩阻力是逐渐发挥作用的，计算桩的极限抗拔力时不宜考虑全部的桩侧摩阻力，扩大头抗拔力在整个抗拔力中占较大比例。嵌岩扩底抗拔桩极限承载力主要受桩顶位移控制。极限承载力是桩顶位移达到极限值(即容许上拔量)所对应的承载力，而不是抗拔桩真正所能发挥出来的最大承载力。当上部结构对抗拔桩桩顶位移比较敏感时，宜采取措施控制桩身变形，而不是单一提高桩的极限抗拔承载力。

APPLICATION TO BEARING CAPACITY OF ROCK-EMBEDDED UNDER-REAMED UPLIFT PILES

When different weathered sandstones and mudstones are inter-embedded at different buried depths,and the weathered rock maximum strength is smaller than 14.4 MPa,it is difficult to find a suitable pile-end depth according to technical code for building pile foundations. Under-reamed uplift piles are constructed by rotary rig in different weathered sandstone and mudstone in South China. Rock-embedded under-reamed uplift piles are instrumented with strain gauges to measure the axial load distribution at all load increments. Based on the testing data,the crack and the deformation of shaft are discussed. The results show that the ultimate uplift capacity of pile is affected by pile friction and enlarged base of pile. The pile friction could be calculated considering that it is variable with pile length. The ultimate uplift capacity of pile is mainly affected by enlarged base of pile. Testing data show that the ultimate uplift capacity of pile is also decided by the critical displacement which is limited by the above building structure. To get larger ultimate load capacity of pile,it is useful to decrease the cracks and the deformation of pile under the pulling force such as constructing a short and large-diameter pile. Some methods such as vertical prestressed anchor are suggested to reduce the crack of shafts.