Deformation monitoring of dam model test by optical fiber sensors

Abstract

In consideration of the difficulty of monitoring internal deformation in dam model tests, a new optical fiber sensor based on fiber Bragg grating (FBG) technology has been developed. Quasi-distributed FBGs are installed on a slender bar (circular cross-section) along axial direction to make an FBG sensing bar. For the dam physical model in this study, FBG sensing bars can be embedded in predrilled holes within the dam structure and the dam foundation. When the dam is loaded by hydraulic jacks and dam deformations occur, the sensing bar works like an elastic beam under arbitrary transverse and axial loading. According to the bending beam theory, the distribution of both horizontal and vertical displacements can be calculated from the strains measured by the FBGs. The two sensing bars used in the test were calibrated in the laboratory before testing. The calibration results have shown that the displacements measured by the sensing bars were in good agreement with those obtained using other conventional gauges. Recently a two-dimensional gravity dam model was constructed and instrumented with two embedded FBG sensing bars, together with surface mounted displacement transducers and strain rosettes. In the overload test, all the sensors were used to perform real-time monitoring of the model dam deformation. Major monitoring results are presented, compared and analyzed. The displacements measured by the FBG sensing bars in the dam model test are in good agreement with those from displacement transducers. The test results also reveal the deformation mechanism and failure mode of the gravity dam. 考虑到大坝模型试验中内部变形难以监测的现状,研制了一种新型的基于光纤布拉格光栅(FBG)技术的光纤传感器。这种传感器为圆棒式结构,其表面沿轴向安装了准分布式的光纤布拉格光栅。对于大坝物理模型,该传感器可预埋入坝体和坝基的内部。当大坝受到油压千斤顶荷载产生变形时,该传感器类似于一根一端固定并同时受轴向拉、压和横向弯曲的弹性梁。根据弯梁原理,由光纤布拉格光栅测得的应变结果可反算出大坝沿水平向和竖向的位移分布。室内标定试验结果表明,该传感器测得的变形量与其他常规传感器的读数一致。在一个二维混凝土重力坝物理模型的超载破坏试验中,采用预埋于坝体、坝基内部的光纤传感器以及表面安装式位移计、电阻应变花对该模型进行实时的变形监测。试验结果验证了该光纤传感器在大坝模型内部变形监测中的有效性,同时也揭示了大坝在超载作用下的变形机制和破坏形态。