高低承台桩基地震行为差异研究

 对于采用桩基的各类结构，按承台埋没于地基土体与否，可区分之高、低承台桩基型式。在许多实际工程中，因回填土或欠固结土地基的继续沉降，或因土体流变或场地震陷等后沉降效应，均可致使承台与土体接触解除，即先前按设计采用的低承台桩基可能转为高承台型式。在静力条件下，这将导致桩身产生负摩阻力而降低其部分竖向承载能力；然而，在地震作用下，通过开展离心机地震模型试验和ABAQUS计算分析发现：当承台与地基土体脱离时，桩基最大弯矩设计值将会增大，且弯矩有效深度也将变深；更且，基础(承台)周期明显延长，充分表明高、低承台桩基型式地震行为差异迥然，高承台桩基型式较之相应低承台情况更为不利，在抗震设计时应充分予以认识、考虑。

STUDY ON DIFFERENCE OF SEISMIC BEHAVIOR BETWEEN HIGH AND LOW RAFT PATTERN OF PILE FOUNDATION

For structures constructed onto pile foundations，according to embedded condition of raft，pile foundation could be distinguished into high and low raft pattern. In many practical engineering，Due to post stage settlement of unconsolidated clay or backfilled soil after construction，or other cases such as soil rheology and seismic field subsidence，all of these may lead to a separation of raft from clay surface，i.e. from original design state which raft was embedded into clay to a new state of high raft pile foundation. For static condition，this kind of separation may disadvantageously result in negative skin friction force along piles，and in turn to reduce vertical bearing capacity of piles. However，during seismic shaking condition，this study，by conducting centrifuge shaking table experiments and ABAQUS simulation on both cases，indicates that pile will undergo a higher maximum bending moment and a much larger active depth under high raft case than those of low raft case. This suggests that embedding condition plays an important role in seismic response of pile-raft foundation，it will become more disadvantageous when raft embedding condition changes from low raft case to high raft case，and should be taken into enough account on practical engineering design.