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.