Development of Graded Transparent Soft Clay and Its Application in Visualization Tests of Soft Soil Foundations

Looking at the demand for precise matching between the mechanical properties of similar materials and natural soft clay in soft ground visual model tests, this paper proposes a preparation method for graded transparent soil based on optical testing principles and geotechnical similarity theory. Using n-dodecane and 15# white oil as pore fluids, combined with nano-fumed silica and optimally graded fused quartz sand, a graded transparent soil material system with accurately matched refractive indices was constructed. The synergistic effect of nano-fumed silica and graded quartz sand enables dual optimization of material transparency and mechanical performance. The feasibility of simulating soft ground was verified through load-plate tests, jacked pile penetration model tests, and consolidated-drained (CD) triaxial tests. The consolidation compression and direct shear test results show that the material has a compression coefficient of 1.43 MPa^-1, a compression modulus of 1.72 MPa, a total internal friction angle φ ranging from 16.53° to 23.21°, and a total cohesion c ranging from 5.42 to 11.34 kPa. Further CD triaxial test results verified that the developed transparent soil exhibits effective shear strength parameters (c' = 9.65 kPa, φ' = 16.01°), which are highly consistent with the effective strength indices of natural Shaoxing silty clay (c' = 12.94 kPa, φ' = 13.52°), with a relative error of ultimate deviator stress less than 5% under various confining pressures. Moreover, different strength levels can be obtained by adjusting the pre-consolidation pressure. During shear loading, the material presents stable volumetric contraction without dilatancy, conforming to the typical contractive mechanical characteristics of normally consolidated soft clay. PIV velocity-field analysis of the indoor plate load test under constant-settlement-rate conditions revealed that the transparent soil displays a typical Prandtl failure mode, consistent with the failure characteristics of natural soft clay. The model test of jacked-pile penetration further confirmed that this material can effectively simulate the stress-deformation behavior of soft ground under the test conditions adopted in this study. The developed material offers a viable alternative simulant and relevant test reference for laboratory visual model investigations into soft ground deformation, pile-soil interaction and failure evolution in geotechnical model tests.