Abstract
A measurement approach for small deformations of soil specimens has been proposed in this study. The proposed approach consists of a small deformation transducer (SDT) based on fiber Bragg grating sensors which could provide an alternative tool to measure local small deformations of a soil specimen with high accuracy. The working principle, design procedures, calibrations and applications of the SDT are presented. An analytical solution is derived to obtain the relationship between the small deformation of the transducer and the wavelength shift of the FBG sensor, which was further evident in the laboratory calibration tests. The measurement range and resolution of the SDT can be adjusted by choosing different length and thickness of the material. The SDT can achieve a strain resolution of 4.45 micro-strains for a soil specimen with 80 mm in height. Measurement errors and stability were also examined and the results show that the maximum measurement error was around 0.01 mm. The designed SDT was further installed in a modified triaxial apparatus. Three shearing tests under different confining pressures were conducted. Results measured by the newly developed SDT are analyzed with comparisons to the results using external linear variable differential transformer (LVDT) transducers. The results provide evidence that this measurement approach is suitable for measuring the local deformations of soil specimens with high accuracy and stability.
Highlights
Knowledge about the small deformation behavior of gravel materials is essential for the design and safety control of geotechnical structures
An analytical solution has been developed to obtain the relationship between the wavelength shifts of the fiber Bragg grating (FBG) sensors and the local deformations of the small deformation transducer (SDT)
The SDT was employed in a modified triaxial test for measuring the small deformation behavior of soil specimens under different confining pressures
Summary
Knowledge about the small deformation behavior of gravel materials is essential for the design and safety control of geotechnical structures. Many researchers have highlighted the importance of accurate measurements of the small strains of gravel material, such as soil specimens and aggregate [1,2,3]. In the past few decades, various methods and techniques have been developed to measure small strain behavior, such as bender elements, the resonant column method, proximity transducers, miniature linear variable differential transformer sensors (mini-LVDTs), mini-inclinometers, Hall-effect transducers, local deformation transducers (LDTs), etc. The bender element and LDT are the most commonly used methods to measure soil small strain behavior. The bender element is an effective method to measure very small strain behavior of soil specimens by using shear waves [6]. The soil strain induced by the shear wave can only be estimated, for example, Dyvik and Madshus [7] regarded the strain value is in the range of 10−5 , while Leong et al [8] and Pennington [9]
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