特殊浸水下桩基负摩阻力试验研究

为妥善解决湿陷性黄土地区负摩阻力引起的桩基承载力下降问题，以消除桩周黄土湿陷性为出发点进行新途径和新方法的探索，采用非传统的特殊浸水方式分别对混凝土灌注桩、微型钢管砂浆复合桩进行了现场试验。结果表明:桩顶受荷时采用注水孔与试坑相结合浸水量较大，浸水段桩侧以负摩阻力为主，最大值达319.62 kPa，局部出现正摩阻力；成桩过程中采用泥浆循环浸水量较小，桩周土体在桩顶受荷前完成部分湿陷，受荷后桩侧以正摩阻力为主，局部出现负摩阻力，且数值较小，最大值为32 kPa；特殊浸水条件下桩周土体沿桩身分段湿陷，桩侧出现多个负摩阻力峰值及中性点，正、负摩阻力交错分布；桩基负摩阻力的大小受浸水方式、加载方式、浸水固结时间的综合影响，建议在桩基施工过程中采用合理的施工工艺对桩周土体预先进行微量浸水，消除部分黄土湿陷，以避免由于地下水环境改变而引发的桩基承载力下降。

Test Research on Negative Skin Friction of Pile Foundation Under Special Immersion

In order to solve the problem of bearing capacity decrease of pile foundation caused by negative skin friction in collapsible loess areas, taking the elimination of collapsibility of loess around the pile as the starting point and exploring with new ways and methods, the field tests of concrete filled pile and micro steel pipe mortar composite pile were carried out respectively by using non-traditional special immersion methods. The results show that the immersion mode of water injection hole combined with test pit has a large amount of soaking water when the pile top is loaded. The negative skin friction of the pile side of soaking section is dominant, the max value is 319.62 kPa, and the local positive frictional resistance appears. The immersion mode of slurry circulation has a small amount of soaking water in the process of pile formation. The soil around the pile is partially collapsed before the pile top is loaded. The positive frictional resistance of the pile side is dominant after the pile top is loaded, the negative skin friction occurs on part of the pile side, and the value is smaller, the maximum value is 32 kPa. Under the special immersion condition, the soil around the pile will collapse discontinuously along the pile body. Multiple negative skin friction peaks and neutral points occur on the pile side, and the positive and negative frictional resistances are staggered. The negative skin friction of pile foundation is mainly affected by the mode of immersion, the mode of loading and the time of immersion consolidation. In order to avoid the decrease of the bearing capacity of the pile caused by the change of the groundwater environment, it is suggested to use reasonable construction technology to presoak the soil slightly around the pile to eliminate some loess collapsibility during the construction of pile foundation.