一种新型光纤光栅局部位移计在小应变测量中的应用

小应变下的土体模量的变化对基坑周围既有建筑物的变形和路基沉降分析具有重要意义。土体模量会随土体的应变呈非线性变化,特别是应变范围为10~(-5)~10~(-3),土体模量随应变的增加显著降低。由于土体应变的变化范围广,传统的单一测量仪器无法连续量测土体从微小应变到小应变的模量变化情况。通常测量土体小应变模量的方法有弯曲元和局部位移计方法。弯曲元方法能有效测量土体微小应变的模量,但不能得到土体模量在小应变情况下变化规律,因为弯曲元方法无法得到和控制土体的应变。设计了基于布拉格光纤光栅传感技术的局部位移计。通过设计和标定试验,成功将布拉格光纤光栅局部位移计应用于改装的三轴试验仪器中。为进一步的对比分析,在改进的三轴试验装置中安装了弯曲元试验系统。基于改装的三轴试验仪器,进行了不同围压下的固结不排水试验,得到了土体从微小应变到小应变下模量的变化曲线。试验结果验证了光纤光栅局部位移计能有效的测量土体在小应变情况下的模量非线性变化特征。

New fiber Bragg grating sensor-based local displacement transducer for small strain measurements of soil specimens

The small strain stiffness changing with soil strains is significant for the performance analysis of the existing structures subjected to the nearby excavations and subgrade settlement analysis. The soil stiffness has a nonlinear relationship with the soil strains. Especially in the strain range of 10^-5 to 10^-3, the soil stiffness dramatically decreases with the increasing soil strain. Due to the large ranges of soil strains, there is a lack of single transducer to measure the soil strains from very small strain to small strain. Two commonly used small strain measurement methods are the bender element method and the local displacement measurement method. The bender element method can obtain the soil stiffness at the very small strain level. However, it cannot obtain the relationship between the stiffness and the soil strain as it cannot control the soil strain during measurement. In this study, a new fiber Bragg grating (FBG) sensor-based local displacement transducer (LDT) is developed. After the design and calibration tests, the newly developed transducer is installed in a modified triaxial apparatus. In order to further verify the feasibility of the newly developed transducer, the bender element sensors are also incorporated in the modified apparatus. Three consolidation undrained compression tests are conducted. The test results indicate that the FBG-based LDT can be used to effectively measure the local small strains of soil specimens.