Seeing Through the Soil: A Review of Transparent Soil Technology for Non-Intrusive Full-Field Deformation Measurement in Geotechnical Modeling

Transparent soil technology (TST) provides a non-intrusive visualization approach for geotechnical model experiments by matching the refractive indices of transparent particles and pore fluids. This review systematically examines recent advances in TST, from material preparation to measurement methods and engineering applications. The preparation of transparent sand and transparent clay, pore-fluid matching, degassing treatment, and mechanical similarity with natural soils are summarized. The development of optical measurement and image analysis methods, including laser slicing, particle image velocimetry, digital image correlation, speckle imaging, and three-dimensional reconstruction, is then discussed to clarify the transition of TST from qualitative observation to quantitative full-field deformation measurement. Bibliometric keyword evolution further indicates that research has shifted from early material feasibility toward engineering-oriented applications and intelligent visualization. Current applications in pile–soil interaction, tunneling, slope instability, seepage, and erosion demonstrate the value of TST for revealing displacement-field evolution, strain localization, seepage-path development, particle migration, and shear-band propagation. Remaining challenges include limited mechanical similarity, optical stability, large-scale applicability, and efficient data processing. Future work should emphasize standardized material systems, high-resolution three-dimensional visualization, multiphysics coupling, and integration with artificial intelligence.