Abstract: Transparent soil techniques provide a visual medium and experimental approach for the testing of geotechnical tunnel models. However, most existing studies focus on individual material development or adaptations to specific working conditions, with systematic synthesis and comprehensive overviews remaining limited. This review discusses progress in transparent soil-based tunnel model testing from the perspectives of materials, optics, experimental applications, and evaluation. From the material perspective, the authors review the evolutionary development of transparent soil materials and summarize the compositional characteristics and physicomechanical parameter matching principles of sand-type, clay-type, and rock-like transparent soils, emphasizing key issues related to material preparation and parameter adjustment. From the perspective of optics, the authors clarify the optical fundamentals of transparent soil model testing and discuss core components such as transparency requirements, model box size constraints, laser parameter selection, optical measurement techniques, and artificial speckle preparation while comparing the applicable ranges of digital image correlation and particle image velocimetry. From the experimental perspective, the authors systematically review representative applications of transparent soil materials in tunnel geotechnical model tests. These applications cover tunnel face stability, grouting diffusion, and construction-induced ground deformation. On this basis, experimental observations with potential transferability are identified. From the perspective of evaluation and outlook, several limitations are addressed, including constraints related to material equivalence, restrictions on model scale imposed by optical path length, insufficient speckle stability, and the high cost of experimental systems. Practical approaches to mitigating these problems are proposed (such as three-step equivalence calibration based on reactivity evaluation, guidance for selecting optical measurement techniques, and directions for the development of new transparent soil materials) to enhance engineering comparability and extrapolation capability under unprecedented working conditions.

Key words: transparent soil, tunnel engineering, physical modeling test, particle image velocimetry, digital image correlation, speckle pattern preparation, stratum displacement field