考虑时间效应的群桩负摩阻力模型试验研究

Model tests on negative skin friction for pile groups considering time effect

基桩负摩阻力是桩基础设计中必须考虑的重要问题之一,但是针对考虑土体固结时间效应的群桩负摩阻力研究却相对较少。进行了在地面堆载固结条件下,黏性土层中3×3群桩负摩阻力性状的室内模型试验研究,测得了不同桩间距(3d,4d,6d)条件下各位置桩(角桩、边桩、中心桩)的桩侧负摩阻力、桩端阻力以及桩周土体分层沉降随固结时间的变化情况,分析了桩身下拽力和中性点位置随时间的变化规律;并进行了同等条件下单桩及2×2群桩(4d)试验作为比较分析。试验结果表明,群桩中桩侧负摩阻力引起的下拽力和中性点位置都存在明显的时间效应和群桩效应,其数值与基桩数、基桩的布置位置及桩间距等因素有关;在本文试验情况下,当桩–土相对位移达到2 mm时,桩侧负摩阻力将达到其最大值的80%～90%。

Negative skin friction is one of important problems in design of pile foundation, but the negative skin friction of pile groups considering time effect is less studied. The negative skin friction of 3 × 3 pile groups （3d, 4d, 6d） on the viscosous soil layer under surface load consolidation conditions is studied by model tests. The negative skin friction of pile side, tip resistance of pile and variation of settlement of each soil layer with consolidated time are measured. The position of neutral point and dragload are analyzed. For comparative analysis, a single pile and 2 × 2 pile groups （4d） are tested under the same conditions. The results show that the dragload and position of neutral point caused by the negative skin friction of pile side exhibit significant time effect and pile group effect for both the single pile and pile groups, and they are affected by the number of piles, the position of piles in group and the spacing of piles, it is also shown that the negative skin friction of pile side reaches 80%- 90 % of the maximum one when the relative displacement of pile-soil reaches 2 mm under the present model test conditions.