Accurate determination of soil stiffness at small strain (0.001 % -0.1 % strain) is very important as it portrayed the stiffness of soil underneath geotechnical structures. To evaluate stiffness at small strain, it is important to achieve a minimum strain measurement accuracy of 10-4 %, this is attained using transducers, strain gauges and sensors which are attached on the specimen locally inside the triaxial cell. Several local strains measuring techniques have emerged with the intention of developing a seamless system which is easy, accurate and less expensive. This study epitomizes the existing types of small strain measuring instrumentation, their trend of development and technology. Those that can measure both axial and radial strain, axial strain alone and radial strain alone are distinguished and described. Also, the accuracy, features, merits, and demerits of each type of device have been discussed accordingly. This paper provides information that enables selection of a suitable device that will best fit a particular application. It is anticipated that the study will inspire further researches in the area.
The paper explores the possibility of using high-resolution fiber Bragg grating (FBG) sensing technology for on-specimen strain measurement in the laboratory. The approach provides a means to assess the surface deformation of the specimen, both the axial and radial, through a chain of FBG sensor (C-FBG), in a basic setup of a uniaxial compression test. The method is cost-effective, straightforward and can be commercialized. Two C-FBG; one was applied directly to the sample (FBGBare), and the other was packaged (FBGPack) for ease of application. The approach measures the local strain with high-resolution and accuracy levels that match up to the existing local strain measuring sensors. The approach enables the evaluation of small-strain properties of the specimen intelligently. The finite element model analysis deployed has proven the adaptability of the technique for measuring material deformation. The adhesive thickness and packaging technique have been shown to influence the sensitivity of the FBG sensors. Owing to the relative ease and low-cost of instrumentation, the suggested method has a great potential to be routinely applied for elemental testing in the laboratory.
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