铁路路基下膨胀土长短微型桩抗隆起研究

Study on resisting upheaval of long-short micropiles of expansive soils under railway subgrade

在非饱和膨胀土地区,地基隆起变形严重威胁着高速无砟铁路安全运营。为减弱或消除地基隆起变形,降低高速铁路工程投资,结合非饱和膨胀土地基气候影响深度特征,将传统微型桩改进为一种抗隆起的长短微型桩结构。再由若干基本假定、工作机制及应力路径分析,推导长短微型桩加固膨胀土地基后隆起量的计算方法。依托云桂铁路路基基床现场湿干试验,开展实际工程案例及设计关键参数研究,结果表明,相比天然地基,长短微型桩可有效减弱地基隆起量,减幅约60%;增大长短微型桩桩径、缩小桩间距、优化线路标高及适当增加桩长均可减弱甚至消除地基隆起变形。基于参数敏感性、影响权重比及布桩方案分析,提出膨胀土区域无砟轨道路基基底长短微型桩的抗隆起设计原则,即推荐采用"小桩径、小桩间距"长短微型桩加固无砟轨道路基基底膨胀土,长微型桩桩长宜略大于临界桩长且不应小于大气影响深度,短微型桩桩长不宜小于大气急剧影响层深度。

In unsaturated expansive soil regions,ground upheaval threats the safe operation of high-speed ballastless railway. To reduce or eliminate the ground upheaval and to cut down the engineering investment of high-speed railway,a long-short micropile structure considering atmospheric influence depth of unsaturated expansive soil foundations was proposed on the basis of the traditional micropiles. In accordance with several basic assumptions,an upheaval calculation methodology of expansive soil foundations reinforced by long-short micropiles was derived based on the analysis of working mechanics and stress path. Field tests of swelling-shrinkage responses of an expansive soil foundation under the subgrade bed of Yun-Gui high-speed railway were performed,and key design parameters were investigated. The results show that long-short micropiles can significantly reduce the upheaval of expansive soil foundations by about 60%. The ground upheaval can be reduced or even eliminated by increasing the pile diameter of long-short micropiles,reducing the pile spacing,optimizing the elevation of the railway line or appropriately increasing the pile length. Based on analysis of parameter sensitivity,influence weight ratio and pile setting scheme,a design principle of long-short micropiles to control ground upheaval under ballastless track subgrades in expansive soil regions was proposed,namely that small pile diameter and small pile spacing is recommended to reinforce expansive soils under ballastless track subgrade,that the length of long micropiles should be slightly larger than the critical pile length and not less than the atmospheric influence depth,and that the length of short micropiles should be greater than the depth of soil layer affected by atmosphere dramatically.