This paper presents a series of flume-scale experiments to investigate suffusion at the bottom of a cutoff wall in an internally unstable sandy gravel alluvium. The initiation, progression, and potential failure of suffusion and the interactive effects of geomechanical and hydraulic conditions with the evolution of suffusion were investigated in this particular application. Temporal and spatial development of pore pressure, earth pressure, and settlement demonstrated suffusion was a multi-phase (involving pore water, fine and coarse fractions) and multi-field (involving seepage, seepage-induced fine-fraction variation, and stress–deformation) coupling phenomenon. Suffusion initiated at the downstream side of the tip of the cutoff wall and then generally progressed backward to the upstream side. The monitored earth pressure provided an evidence of the heterogeneous stress distribution in internally unstable soil. Two linear empirical formulas for average hydraulic gradients at the initiation of suffusion and at blowout were derived based on the flume-scale model experiments.

The concrete–rock interfacial transition zone (ITZ) is generally considered the weak layer in hydraulic engineering, for it is more permeable than the intact concrete or rocks. The water permeability of the ITZ is a critical parameter concerned with structural safety and durability. However, the permeability and pore structure of the ITZ has not been investigated previously, and the mathematical model of ITZ permeability has not been established. This study performed multi-scale experiments on the concrete–rock ITZ with various rock types (limestone, granite, and sandstone). A series of quantitative and qualitative analysis techniques, including NMR, SEM-EDS, and XRD, characterize the ITZ pore structures. The controlled constant flow method was used to determine the permeability of the concrete, rock, and ITZ. The mathematical model of ITZ permeability was proposed using the fractal theory. The consistency between the experimental data and the proposed model indicates the reliability of this study. The results of the experiment show that ITZ permeability is between 4.08 × 10⁻¹⁸ m² and 5.74 × 10⁻¹⁸ m². The results of the experiment and the proposed model could determine ITZ permeability in hydraulic structure safety and durability analysis.