高低承台桩基地震行为差异研究

DIFFERENCE OF SEISMIC BEHAVIOR OF HIGH AND LOW CAPS OF PILE FOUNDATIONS

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、低承台桩基形式。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形式。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;并且,基础(承台)周期明显延长,充分表明高、低承台桩基形式地震行为差异迥然,高承台桩基形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。

The pile foundation could be classified into the high or low types according to the position of cap relative to the surface of grounds. In many engineering practice, the settlement of unconsolidated clay or backfilled soil after construction,the soil rheology or the seismic field subsidence may lead to the separation of the cap from the clay surface, i.e., the cap was embedded originally into clay to a new state of high cap pile foundation. For the static condition,this kind of separation may result in the negative skin friction force along the piles and reduce the vertical bearing capacity of piles. However, under the seismic shaking condition, the centrifuge shaking table experiments and simulation with ABAQUS on both the cases indicated that piles underwent a higher maximum bending moment and a much larger active depth under high cap case than those at low cap case. This suggested that the embedding condition played an important role in the seismic response of pile-cap foundation. It becomes more disadvantageous when the embedding condition of the cap changes from the low cap case to the high cap case.