基于群桩基础改进的变径微型桩承载特征研究

Bearing characteristics of variable diameter micropile based on the improvement of group pile foundation

为改善输电杆塔微型桩基础的承载特性、降低用料成本,提出基于微型群桩改进的变径微型桩基础型式。通过高原山区等截面微型群桩原型试验与数值反演模型相互验证,对3种变径微型单桩进行模拟,并与微型群桩承载效果进行对比分析,揭示变径微型桩的承载特征与变形机理。结果表明:微型群桩基础(2×2单桩)由于承台和群桩效应的影响,其抗压和抗拔承载力分别大于和小于对应的单桩承载力总和;双扩径微型桩(扩径部分直径=2×等截面桩身直径)的极限抗拔和抗压承载力约为等截面微型桩的3.8倍和2.7倍,土体变形集中于扩径部位,且变形量较大,表现为多支点摩擦-端承桩特性;于桩顶增设承台可有效改善双扩径单桩的下压及水平承载性能,与未增设承台相比,承载力分别提高了2.1倍和2.2倍。研究表明,通过扩展部分桩径并增设承台使微型单桩性能提升至等截面群桩水平的方法可行,对送变电建设具有工程意义。

To improve the load-bearing of transmission tower micropile foundation and reduce the material cost,a modified variable diameter micropile foundation type based on group micropile is proposed in this study.The bearing characteristics and deformation mechanism of variable diameter miniature pile were revealed by simulation with three kinds of variable diameter miniature micropiles and comparison with the bearing effect of micropile group pile,using the prototype test and numerical inversion model of micropile group pile with equal section in the highland mountainous area.The results show that the compressive and pullout bearing capacities of micropile group foundation(2×2 single pile)are greater and less than the total of corresponding single pile bearing capacity,respectively,due to the effect of bearing platform and group pile effect.The ultimate pullout and compressive bearing capacities of double diameter-extended micropile(the diameter of the dilated part=2×the diameter of the equal)is approximately 3.8 times and 2.7 times of the equal section micropile.The soil deformation is concentrated in the dilated part with large deformation value,which presents the characteristics of multi-fulcrum friction-end bearing pile.The addition of cap can improve the downward pressure and horizontal bearing capacity of double-expanded micropile effectively,which increase 2.1 times and 2.2 times compared to that in the situation of no cap,respectively.This study indicates that improving the performance of single micropile to the level of equal section group micropiles by extending part of the pile diameter is feasible and has engineering significance for the transmission and substation construction.