EVALUATION OF DYNAMIC SOIL-PILE-STRUCTURE INTERACTION DURING SOIL LIQUEFACTION AND CYCLIC MOBILITY

研究了1995年小古根-南部地震中引起土壤液化和建筑物地基破坏的条件。结果表明，造成地基破坏的可能原因包括：(1)上部结构施加在地基上的水平力和倾覆力矩；(2)土体剪切变形作用在深地基上的运动学力；(3)液化等地基破坏导致地基承载力降低；(4)横向扩展。在1995年小谷县南部地震中，预应力混凝土桩经历了液化引起的地面位移，利用钻孔摄像机和斜坡指示器对其破坏和变形模式进行了检测。结果表明，破坏主要集中在液化层与非液化层交界面处，表明地面位移对桩的破坏影响显著。在土-桩-结构模型上进行了离心振动台试验，研究了土液化过程中地面位移和上部结构惯性力对桩内应力的影响。结果表明，当结构的自然周期小于土沉积物的自然周期时，地面位移和惯性力是同相的，而当结构的自然周期大于土沉积物的自然周期时，它们是不相的。在此基础上，提出了一组连接在同一基础上的桩在侧向地面位移和惯性力作用下的解析模型，并计算了桩的应力和位移。计算的破坏和变形模式与现场观测结果吻合较好，验证了所建立的分析模型可以有效地用于土体液化过程中桩内应力的估计。

The conditions causing soil liquefaction and resulting damage to building foundations in the 1995 Hyogoken-Nambu earthquake are examined. It is shown that possible causes fo the foundation damage include : (1) horizontal forces and overturning moments imposed on the foundation from the superstructures, (2) kinematic forces acting on deep foundations due to shear deformation of soils, (3) reduction in bearing capacity due to ground failures including liquefaction, and (4) lateral spreading.Both failure and deformation modes are examined using a borehole camera and a slope-indicator for prestressed concrete piles of buildings that experienced liquefaction-induced ground displacement during the 1995 Hyogoken-Nambu earthquake. It is shown that the damage is concentrated at the interface between liquefied and non-liquefied layrs, suggesting the significant effect of ground displacement on pile damage.Centrifuge shaking table tests are conducted on a soil-pile-structure model to investigate the effects of ground displacement and inertia force from the superstructure on stresses in piles during soil liquefaction. It is shown that the ground displacement and inertia force are in phase if the natural period of the structure is less than that of the soil deposit, whereas they are out of phase if the natural period of the structure is greater than that of the soil deposit.An analytical model is then proposed in which a group of piles connected with the same foundation is subjected to lateral ground displacement and inertia force, and the stress and displacement of the piles are calculated. The computed failure and deformation modes agree reasonably well with the field observations, confirming that the proposed analytical model could be effectively used to estimate stresses in piles during soil liquefaction.