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.

Cut-off walls have been widely used to control seepage in dams and their foundations for over 100 years. The soil at the bottom of a cut-off wall is usually under complex stress and composed of various particles with different contents. In this study, the effects of the stress state and grain composition on the evolution of internal erosion were investigated in sandy gravel foundations containing a suspended cut-off wall. The following observations were made based on the experimental results: (1) The fine-particle content of the soil sample considerably influences the evolution of internal erosion. For high fine-particle content, particles are likely to clog under the downward flow; however, for low fine-particle content, the downward flow assists the migration of fine particles. (2) Deviatoric stress considerably influences the soil structure. The soil sample is compressed under low deviatoric stress; however, it undergoes dilatancy under high deviatoric stress. (3) Dilatant behavior is also controlled by confining stress. The higher the confining stress, the higher is the deviatoric stress applied on the soil sample, which produces dilatancy, and the smaller is the volumetric expansion. The obtained results will enhance the understanding of the progression mechanism of internal erosion around suspended cut-off walls.

Permeability evolution in remolded fault gouge creates critical uncertainties in geotechnical parameterization for dam foundations. However, the underlying multi-scale mechanisms, including mineral migration and pore structure changes, remain insufficiently understood. This study investigates these mechanisms using remolded plastic-thrust fault gouge from the Yulong Kashi hydropower project in China. We developed an innovative sample preparation method that combines in situ mineral self-cementation and directional compaction. The study integrated multidisciplinary tests including field in situ permeability tests; seepage–stress coupling tests; and micro-scale NMR/XRD/SEM-EDS analyses. Results demonstrate that remolded samples exhibit 1–2 orders of magnitude lower permeability (10⁻⁷ cm/s) than in situ samples (10⁻⁵ cm/s). This significant reduction is primarily caused by the loss of cementing agents and the uniform compaction of remolded samples, which leads to degraded pore connectivity. SEM-EDS analysis highlighted the leaching of cementing materials (such as K⁺, Ca²⁺ ions), while XRD revealed changes in mineral composition, with chlorite dissolution being the primary mineral alteration associated with permeability decay. Additionally, artificially enhanced cohesion distorted the mechanical behavior of the samples. These findings provide an explanation for why conventional laboratory tests tend to underestimate in situ permeability and overestimate shear strength in fault zones. This study establishes microstructure-informed correction frameworks for hydraulic and mechanical parameters in fault-crossing hydraulic engineering applications