Staff Research Highlight - Improving precision in regional scale numerical simulations of groundwater flow into underground openings
“The applicability of the new boundary condition was then demonstrated by using a hypothetical deep geologic repository system located in a hypothetical coastal crystalline aquifer. This example demonstrated that the inflow estimation could be significantly improved by applying the tunnel boundary condition and also that it could help to design the engineering solutions to control the inflow.”
Park, Y.-J., Hwang, H.-T., Suzuki, S., Saegusa, H., Nojiri, K., Tanaka, T., Bruines, P., Abumi, K., Morita, Y., & Illman, W. A. (2020). Improving precision in regional scale numerical simulations of groundwater flow into underground openings. Engineering Geology, 274, 105727. https://doi.org/10.1016/j.enggeo.2020.105727
The study presents a novel numerical framework to improve the accuracy of regional-scale groundwater flow simulations into underground openings, such as tunnels and deep geological repositories. Traditionally, simulating groundwater inflows into engineered underground structures has involved significant simplifications, often treating tunnels as drain features or imposing boundary conditions that fail to fully capture the physical behavior of fluid flow around these voids. This research addresses those limitations by introducing a new numerical boundary condition to simulate groundwater flow into underground openings more accurately.
Figure 9: Distribution of the maximum principal hydraulic conductivity within the domain. It is noted that the equivalent hydraulic conductivity for the finite elements were generated from a random fracture network.
In deep geological settings, accurately predicting groundwater flow into underground excavations is critical for both performance assessment and operational planning. However, capturing the true hydraulic behavior near excavated openings requires accounting for complex interactions, including changes in permeability due to excavation damage, heterogeneous rock properties, and the presence of engineered sealing zones. The researchers developed a flexible tunnel boundary condition that enables simulation of both unsaturated and saturated flow into discrete tunnel surfaces, factoring in construction-related hydraulic changes such as grout seals and damage zones.
This advancement holds particular value for nuclear waste management organizations, hydrogeological consultants, and engineers working on underground infrastructure projects in fractured rock environments. By moving beyond traditional boundary conditions and integrating excavation-specific hydrologic behavior, the study provides a more robust tool for assessing tunnel inflows, optimizing excavation design, and evaluating long-term environmental impacts.
HydroGeoSphere (HGS) played a central role in this study. The newly implemented tunnel boundary condition was designed within HGS’s fully integrated surface-subsurface flow framework, allowing for simultaneous simulation of unsaturated flow, pressure-driven saturated flow, and seepage into underground openings. HGS’s flexibility in assigning boundary conditions to complex geometries made it ideal for representing the evolving hydraulic behavior around the repository tunnels. This integration enabled the researchers to capture the influence of grouted zones, excavation damage, and the surrounding rock matrix on groundwater flow with high spatial resolution, validating the tool’s effectiveness for regional-scale site characterization and performance assessment.
Abstract:
Dewatering is a common engineering practice to secure the accessibility during the construction and operational phases of tunnels. It is thus important to accurately estimate the amount of groundwater flow into underground openings for the design and safe operation of tunnels. Numerical models need to be used to estimate groundwater flow into openings for given heterogeneity and regional hydrologic boundary conditions typically by assuming that the atmospheric pressure is maintained along open wall faces. As the scale of openings can be small as tens of centimeters, while models need to incorporate the regional hydrologic conditions, it is often practically impossible to explicitly represent the three-dimensional (3D) geometry of openings within numerical models. For the safety assessment of deep geological repositories of spent nuclear fuels, for example, complex configurations and geometry of various tunnels such as construction, access, ventilation, and deposition tunnels often need to be approximated as one-dimensional (1D) lines in 3D models. The application of a simple Dirichlet boundary condition along a set of tunnel nodes may yield inaccurate solutions due to geometric simplifications and coarse discretization. This study derives an appropriate boundary condition that can be applied to 1D tunnel segments to improve the accuracy when simulating inflow into openings. A third-type tunnel boundary condition is suggested to correct the difference between known wall pressure and the pressure simulated at tunnel nodes. Improvement in the precision of the solutions is demonstrated by comparing the numerical solutions using the new boundary condition with analytic solutions, and with numerical solutions when the 3D tunnel geometry is explicitly simulated. It is also shown that the formulation can be easily extended to incorporate the high permeability excavation damaged zone and the low permeability grouted zone in the tunnel vicinity. The applicability of the tunnel boundary condition is demonstrated using an example of a hypothetical deep geologic repository system consisting of various types of tunnels located in a hypothetical crystalline coastal aquifer.