贵州贵定尖山营特大桥基础异常升降成因探讨

尖山营特大桥桥墩每年均出现雨季抬升，旱季沉降的异常现象。本文针对桥墩异常升降问题，采用野外地质调绘、现场监测、钻探、物探以及室内试验等方法相结合，分析造成尖山营特大桥墩体异常升降的主要原因。通过现场地质调绘和钻孔岩芯发育情况获取研究区地质构造分布，分析构造作用对异常升降的影响；基于天然音频大地电法勘探与对研究区地表水出露情况的勘察，研究承压水的赋存情况；将钻孔岩芯进行室内膨胀特性试验，研究了桥址区地基岩土体的吸水膨胀特性对桥墩升降的潜在作用。结果表明:排除地质构造和承压水原因，引起桥墩异常升降的主要原因是膨胀岩土体与地下水物理化学反应带动上部结构抬升及沉降引起的。桥址区二叠系吴家坪组（P2w）地层岩土体的膨胀力试验预估值在3～59 MPa之间，上部总荷载1.38～13.5 MPa，膨胀力大于上部结构重力载荷；原状膨胀岩试样在1.05 MPa的轴压作用下吸水，存在0.35%的膨胀应变，结合已有研究可确定130 m厚的膨胀性地层可造成上部结构抬升45.5 mm。二叠系吴家坪组（P2w）地层的吸水膨胀、失水收缩性能，与地下水的补给与排泄相结合，是引发桥墩异常升降的主要原因。

A study of the reasons of abnormal lifting characteristics of foundation near the Jianshanying Large Bridge in Guiding county of Guizhou Province

Jianshanying bridge is a control station on the newly-built high-speed railway line from Guiyang to Guangzhou. This paper combined the methods of field geological survey, field monitoring, drilling, geophysical exploration and laboratory test to analyze the main reasons for the abnormal lifting of the great bridge piers. The geological structure distribution in the study area was obtained by geological mapping and drilling core development. The developmental state of groundwater in the bridge site is deduced from the survey of surface water and audio magnetotelluric exploration (AMT). Indoor expansion tests of drill core were carried out to study the water-absorbing expansion characteristics of the foundation rock soil in the bridge site area. The results show that excluding the reasons of geological structure and groundwater in the confined aquifer, the superstructure rises and falls due to the physical and chemical reaction between expansive rock mass and groundwater. The Permian Wujiaping formation (P2w) in the bridge site area has the potential swelling force estimated to be between 3 MPa and 59 MPa, which is much greater than the structural loads (ranging from 1.38 MPa to 13.5 MPa). Under the action of axial pressure of 1.05 MPa, the undisturbed expansive rock sample absorbs water and has an expansion strain of 0.35%. Combined with the existing studies, it is found that the expansion formation 130 m thick can cause the uplift of the superstructure of 45.5 mm. Therefore, the Permian Wujiaping formation (P2w) is mainly responsible for the abnormal rise and fall of the bridge piers due to its water absorption expansion and water loss shrinkage, combined with groundwater recharge and discharge.