不同堆载形式对群桩负摩阻力的影响

在既有桥墩桩基础周围土体堆载时，会引起土体的沉降和侧向变形，进而在桩身产生负摩阻力，对桩基变形及承载性能有很大影响。为研究围载和单侧边载作用下群桩中不同位置桩基的受力差异，以3×3群桩基础为研究对象，进行围载和单侧边载作用下的模型试验，分析了不同位置桩基轴力、侧摩阻力、中性点位置和基桩承载力安全系数等的变化规律及差异。研究结果表明：围载工况下，角桩的轴力、侧摩阻力最大，边桩次之，中心桩最小；中性点位置角桩最深，边桩略高，中心桩距桩顶最近。边载工况下，靠近边载侧和中间一排桩基轴力、侧摩阻力与围载时的变化规律类似，远离边载侧的一排桩基受边载影响较小，无负摩阻力；各桩基中性点位置变化规律类似于围载工况。与围载工况相比，边载时同一位置桩身轴力、负摩阻力均较小，中性点位置较高。与单侧边载工况相比，围载时各基桩承载力安全系数FS均较小且随荷载的增大衰减梯度较大。该研究成果为不同堆载形式下群桩基础的设计提供参考。

Effect of different surcharge loading forms on negative skin friction of pile groups

When a load is applied on soil around the pile group foundation of the existing bridge pier, it can result in the settlement and lateral deformation of soil and negative skin friction, which would have a great influence on the deformation and bearing capacity of the piles. To investigate the stress difference among piles at different locations under one-side load and surrounding load, model tests of 3×3 pile group foundation were performed to determine the change rules and the differences of axial force, negative skin friction, neutral point position, and safety factors of bearing capacity of piles among different piles. The results show that under the surrounding load, the axial force and skin friction values of the piles at the corner of pile group reach the maximum, those of side piles are the second, and those of the pile in the centre reach the minimum. In addition, the neutral points of the piles in the corner locate at the deepest, those of the side piles locate slightly higher, and those of the pile in the centre are closest to the pile tops. Under one-side load, the axial force and skin friction of the piles in the row near the side load and in the middle row change similarly to those under the surrounding load, and piles in the row far away from the side load are less affected by the load, and there is no negative skin friction acting on the piles. The change rule of the neutral point position in each pile is also similar to that under the surrounding load. Under the side load, the axial force, negative skin friction of the piles at the same location are smaller, but the neutral point position is higher than those under the surrounding load. The safety factor FS of bearing capacity of each pile under the surrounding load condition is less and decreases faster than that under the one-side load condition. This study could provide references for design of pile-group foundation under different loading modes.