An image-based method for evaluating local deformations of saturated sand in undrained torsional shear tests

An image-based measurement system was developed to evaluate the induced local deformations of saturated sand specimens in undrained torsional shear tests. The images of specimens taken by a camera in front of a pressure chamber were noticeably distorted. To correct the errors caused by the curvature of the specimens and by the refraction of rays traveling through water, acrylic resin, and the atmosphere, a coordinate correction method was proposed to convert the apparent coordinates obtained from the distorted specimen image into the real coordinates under planar conditions. This method was verified using a rigid dummy specimen and was illustrated using a sand specimen as an example. The results obtained indicate that the resolution of displacement measurements is approximately 0.01 mm, representing approximately 3.3e-5 of the specimen height. Moreover, high measurement accuracy is obtained even if the dummy specimen has undergone a rotation of up to 26.4 degrees. This measurement system was further extended to directly evaluate the local deformation of sand specimens through a transparent membrane. The direct and indirect evaluations indicate that the direct evaluation is more effective for quantitatively representing specimen deformation than indirect evaluation since membrane wrinkles and vertical slippage between the specimen and membrane occurred during liquefaction. (C) 2020 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.