湿陷性黄土层桩基侧摩阻力的试验研究

本文依托西安市东北二环立交工程实际,通过现场黄土层浸水试验以及桩基静载试验,研究在湿陷性黄土层中桩基侧摩阻力和桩端阻力的发挥情况、桩基负摩阻力取值问题及桩周土的沉降变化情况,并分析了其实验结果。在总结国内工程界多次试验结果的基础上,给出了湿陷性黄土层负摩阻取值的科学合理方法及应考虑的因素,并分析了影响负摩阻大小取值的因素和引起桩周土沉降差异的因素。研究结果表明：①本工程桩基提供负摩阻力段控制在3～9 m之间,取值建议为-2.0～-5.0 kPa;②桩基上的负摩阻值与特定桩长和地质环境有关,同时与湿陷下沉量及土的粘聚力大小也有关系。从而建议在西安平原地区,黄土层较小时（桩长20 m以上,湿陷层5 m以内）,可不考虑湿陷性对桩基承栽力的影响。本次试验为未来工程实践中湿陷性黄土层的沉降的研究及桩基负摩阻力的取值提供了参考依据和工程实践经验。

Tests in Side Friction around the Pile Collapsible Loess Layers

Based on the actual project of northeast Second Ring Interchange of Xi＇ an and through immersion test of soil layer and pile load test,this paper studies the side friction of pile foundation in collapsible loess layers,the tip resistance effect,the value selection of negative friction of pile foundation and the change of soil settlement around the pile.And it analyzes the experimental results to guide this project for pile foundation design.After summarizing several results of domestic engineering tests,the paper gives reasonable and scientific methods to select the value of negative friction in collapsible loess layer and also provides some factors which should be considered.It analyzes the factors that influence the value of negative friction and that cause differences in settlement of soil around the pile.The result show that;（1） the side friction distribution around the pile is about 3~9 m,and its value is -2.0～-5.0 kPa.（2） the side friction value around the pile is related to piles length and geological environment,also to the settlement of collapsible loess and soil cohesion.This paper suggests that the collapsible loess has no influence on the bearing capacity if the collapsible loess layer is within 5 m and the pile lengths are greater than 20 m.This paper can provide references and practical experience for the future study of settlement of collapsible loess layers and the value of negative friction for pile foundation.