Xueyi Zhang

In the post-earthquake engineering setting, tunnel water inrush is controlled not only by water sources, but also by how fault and fracture pathways, excavation boundaries, and operation conditions reshape groundwater pressure. Here, we proposed a source-pathway constrained framework integrating an Almon-type distributed lag model, hydrochemical and isotopic constraints, mineral saturation screening, and three-dimensional groundwater simulations to characterize post-earthquake groundwater hydraulic behavior in the Mozi–Chayuan Tunnel, Southwest China. The event-adjusted distributed-lag model identified a primary significant rainfall-drainage response window of 35–76 d in tunnel 1/tunnel 1', whereas tunnel 2 showed no significant delayed response, confirming that rainfall acted mainly as delayed recharge rather than an immediate trigger. Hydrochemical facies, isotope-derived recharge elevations, and saturation-index patterns indicate the inrush water was predominantly meteoric-derived water supplied by adjacent catchments and shallow fractured aquifers, without evidence for a dominant deep saline or hydrothermal contribution. The simulations reveal a clear fluid-mechanical transition: excavation converted the tunnel into a near-zero-pressure drainage boundary, whereas lining and 8 m full-section grouting established a pressure-bearing boundary with pressure heads of approximately 70–110 m in the route-change section during operation. The fault-related damage zone and route-change section remained the main zones of pressure-head redistribution across all simulated conditions. Local piezometer validation at 4100 m yielded relative errors of 2.09% and 5.45%, supporting the reliability of the framework for post-seismic tunnel groundwater response assessment. This framework supports the safe operation and pressure-control design of the Mozi–Chayuan Tunnel and provides a transferable reference for assessing tunnel groundwater responses in earthquake-prone regions.

Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy’s law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i₀) below which there is no flow, and a critical hydraulic gradient (i_cr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i₀, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i₀ < i < i_cr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state’s change.

Adsorption reaction in unsaturated porous media is of great importance for soil and groundwater remediation. In this study, the influence of the Peclet number (Pe) and water saturation on adsorption behavior at liquid-liquid interfaces was quantitatively investigated. The pore-scale reactive transport in unsaturated porous media was directly simulated. The Navier-Stokes equations, the surface transfer and adsorption reaction equations, and the advection-diffusion equation (ADE) were coupled to obtain the flow and concentration fields. The results showed that water saturation had a significant influence on the complexity of the flow field. A nonmonotonic relationship was found between water saturation and the uniformity of the flow field. Peclet number had little influence on the maximum adsorption. On the other hand, the adsorption time showed a nearly linear relationship with the Peclet number and increased with increasing Peclet number. Additionally, a nonlinear relationship was found between water saturation and the maximum adsorption. As water saturation increased, the maximum adsorption tended to increase to a peak and then decrease. The peak of the maximum adsorption occurred at , , and the shortest adsorption time was observed at . However, the difference in adsorption times for saturations of 0.458 and 0.698 was not significant and was only about 20 PV difference.

Fractured rocks in the subsurface are ubiquitous, and the dynamics of mass transfer in fractured rocks plays an important role in understanding the problem in engineering geology and environmental geology. In this study, the influence of shear displacement on fluid flow and solute transport in a 3D rough fracture was investigated. A 3D self-affine rough fracture was generated using the modified successive random addition (SRA) technology, and three sheared fractures with different shear displacements were constructed based on the mechanistic model. A direct numerical model based on the Navier-Stokes equation and the advection-diffusion equation was developed to solve the fluid flow and the solute transport. The results showed that shear displacement had a significant influence not only on the fluid flow but also on the solute transport. A global measure of the spatial variability of the flow velocity showed that the heterogeneity became weaker with decreasing shear displacement. All measured BTCs deviated from the Gaussian profile and exhibited the typical anomalous behaviors, such as the long tail and the early arrival. Although the best-fitted results of the advection-dispersion equation (ADE) model and mobile-immobile model (MIM) were generally consistent with those of the BTCs, the MIM was more capable than the ADE model for characterizing the shear-induced anomalous behavior of the BTCs. It was found that the mass exchange process between the immobile and mobile domains was enhanced in the sheared fractures while the fraction of the advection-dominant mobile domain decreased as the shear displacement increased. Furthermore, the deviation of the Taylor dispersion coefficient from the fitted dispersion coefficient by the ADE model and MIM in the sheared fractures was confirmed due to the influence of shear displacement.

Although numerous studies have been paid much attention to rainfall-induced instability of multilayered slopes, the interface between layers is generally considered to be “zero thickness”, and the layer transition zone between layers is neglected. In this study, the influence of the layer transition zone on the rainfall-induced instability of multilayered slope was investigated. A model was developed to simulate the rainfall infiltration process, the distribution of pore water pressure, and the stability of multilayered slope by coupling the unsaturated seepage model and the slope stability analysis method. Based on the analysis of the multilayered slopes with the different thickness ratios of the layer transition zone, a method for determining the critical thickness of the layer transition zone was proposed. The results showed that the layer transition zone had a significant influence on the stability of multilayered slope. It was found that the presence of the layer transition zone in the multilayered slope reduced the hydraulic conductivity of the slope and increased the rate of formation of transient saturated zone, which contributed to excess pore water pressure at the toe of the slope. The analysis of the local factor of safety (LFS) showed that when the thickness ratios of the layer transition zone were between 2.5% and 5%, the corresponding hydraulic conductivity of the slope decreased by 1%-2.5% and the maximum failure area of the slope during the rainfall was 25% of the slope. Our study highlighted the importance of the layer transition zone for the rainfall-induced instability of the multilayered slope.

In this study, four homogeneous porous media (HPM1-HPM4), consisting of distinct proportions of sand-sized and clay-sized solid beads, were prepared and used as single fracture infills. Flow and nonreactive solute transport experiments in HPM1-HPM4 under three flow rates were conducted, and the measured breakthrough curves (BTCs) were quantified using conventional advection-dispersion equation (ADE), mobile-immobile model (MIM), and continuous time random walk (CTRW) model with truncated power law transition time distribution. The measured BTCs showed stronger non-Fickian behaviour in HPM2-HPM4 (which had clay) than in HPM1 (which had no clay), implying that clay enhanced the non-Fickian transport. As the fraction of clay increased, the global error of ADE fits also increased, affirming the inefficiency of ADE in capturing the clay-induced non-Fickian behaviour. MIM and CTRW performed better in capturing the non-Fickian behaviour. Nonetheless, CTRW’s performance was robust. 12.5% and 25% of clay in HPM2 and HPM3, respectively, decreased the flowing fluid region and increased the solute exchange rate between the flowing and stagnant fluid regions in MIM. For CTRW, the power law exponent () values were 1.96, 1.75, and 1.63 in HPM1-HPM3, respectively, implying enhanced non-Fickian behaviour. However, for HPM4, whose clay fraction was 50%, the value was 1.87, implying a deviation in the trend of non-Fickian enhancement with increasing clay fraction. This deviation indicated that non-Fickian behaviour enhancement depended on the fraction of clay present. Moreover, increasing flow rate enhanced the non-Fickian transport based on .